Bespoke Photonic Devices Using Ultrafast Laser Driven Ion Migration in Glasses ⇑ T.T

Progress in Materials Science 94 (2018) 68–113

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Progress in Materials Science

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Bespoke photonic devices using ultrafast laser driven ion migration in glasses ⇑ T.T. Fernandez a,1, M. Sakakura b,1, S.M. Eaton a, B. Sotillo a, , J. Siegel c, J. Solis c, Y. Shimotsuma d, K. Miura d a Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche (IFN-CNR), Milano, Italy b Optoelectronics Research Centre, University of Southampton, Southampton, United Kingdom c Laser Processing Group, Instituto de Optica, Consejo Superior de Investigaciones Cientíﬁcas (IO,CSIC), Madrid, Spain d Department of Materials Chemistry, Kyoto University, Kyoto, Japan article info abstract

Article history: This Review provides an exhaustive and detailed description of ion migration phenomena Received 16 August 2016 which occur inside transparent dielectric media due to the interaction with intense ultra- Accepted 20 December 2017 short pulses. The paper differentiates various processes underlying the ion migration influ- Available online 29 December 2017 enced by simultaneous heat accumulation and diffusion. The femtosecond laser induced temperature distribution, the major driving force of ions in dielectrics, is described in detail. Keywords: This discussion is based on three meticulous analysis methods including the thermal modi- Femtosecond laser micromachining fication of transparent dielectrics at various ambient temperatures, numerical simulations Ion-migration and comparison with direct observation of the light-matter interaction and micro-Raman Scanning electron microscope Glass spectroscopy. The ion migration phenomena studied have been triggered in four different Waveguides configurations: at low repetition and high repetition rates, and observations perpendicular Photonic devices and parallel to the laser irradiation direction. Inspired by this research, potential applications are highlighted including space-selective phase separation, a laser-based ion exchange fabrication method and optical micropipetting by tailoring the plasma profile. Ó 2018 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 70 2. Background...... 70 2.1. Femtosecond laser sources ...... 70 2.2. Femtosecond laser microfabrication ...... 71 2.2.1. Nonlinear absorption ...... 71 2.2.2. Relaxation and material modification ...... 72 2.3. Dielectric materials used for fabrication ...... 74 2.4. Characterization techniques ...... 74 3. Basic principle of heat accumulation and ion migration by fs laser irradiation...... 75 3.1. Observation of thermal diffusion after single fs laser irradiation inside a glass ...... 77 3.1.1. Observation of thermal diffusion by a transient lens (TrL) method ...... 77

⇑ Corresponding author at: Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche(IFN-CNR), Milano, Italy (B. Sotillo). E-mail address: [email protected] (B. Sotillo). 1 Co-ﬁrst authors. https://doi.org/10.1016/j.pmatsci.2017.12.002 0079-6425/Ó 2018 Elsevier Ltd. All rights reserved. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 69

Nomenclature

Acronym Meaning AR Aspect Ratio Bi Bidirectional pumping BSE Back Scattered Electrons CCD Charge-Coupled Device CW Continuous-Wave DIC Differential Interference Contrast EBSD Electron Back Scattered Diffraction EDS, EDX Energy Dispersive X-ray Spectroscopy EELS Electron Energy Loss Spectroscopy EPMA Electron Probe Microanalysis FT Fourier Transform HAADF High Angle Annular Dark Field HRR High Repetition Rate ICCD Intensiﬁed Charge-Coupled Device IFT Iterative Fourier transform IG Internal Gain LCOS Liquid Crystal on Silicon LRR Low Repetition Rate NA Numerical Aperture OM Optical Microscope ORA Optimal Rotation Angle ppmw Parts Per Million Weight R Repetition rate RE Rare-Earth RIC Refractive Index Contrast SA Spherical Aberration SEM Scanning Electron Microscopy SLM Spatial Light Modulator STEM Scanning Transmission Electron Microscopy

Tout Threshold Temperature TEM Transmission Electron Microscopy TL Thermal Lens TrL transient Lens ULE Ultra-Low Expansion Uni Unidirectional pumping WDS Wavelength Dispersive Spectroscopy m-PL Micro-Photoluminescence m-Raman Micro-Raman

3.1.2. Observation of thermal diffusion by micro Raman measurement...... 79 3.2. Heat accumulation with a high repetition rate laser ...... 80 3.2.1. Simulation of heat accumulation...... 80 3.2.2. Evaluation of the temperature distribution during high repetition rate laser irradiation ...... 81 3.2.3. Mechanism of thermal modification inside glasses ...... 84

3.2.4. Interpretation of different Tout by the viscoelastic model ...... 86 3.3. Absorptivity and photoexcitation mechanism...... 87 3.4. Trends of ion migration observed perpendicular to the incident laser ...... 89 4. High repetition rate laser irradiation ...... 91 4.1. Observation of ion migration with high repetition rate lasers ...... 91 4.1.1. Ion migration in phosphate glass waveguides ...... 94 4.1.2. Ion migration in tellurite glass waveguides ...... 94 4.1.3. Dual regimes of ion migration ...... 95 4.2. Controlling the directionality of ion migrations ...... 97 5. Application of ion migration ...... 100 5.1. Space-selective phase separation ...... 100 5.2. Replacing the ion exchange method ...... 101 5.3. Optical micropipette through plasma shaping ...... 103 6. Modification of ion migration with multiple spot irradiation ...... 103 6.1. Modification of ion migration...... 103 70 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

6.2. Principle of a holographic laser irradiation ...... 104 6.3. Modification of ion migration perpendicular to the laser propagation ...... 105 6.4. Modification of ion migration parallel to the laser propagation ...... 106 6.5. Possible applications of ion migration with multiple spot irradiation...... 109 7. Conclusion ...... 109 References ...... 109

1. Introduction

Studying the underlying physical mechanisms from its smallest possible dimension provides extraordinary precision and control for any experimental technique. One such method that has benefited from new physical insights is ultrafast laser waveguide writing in glasses. Here, femtosecond laser pulses are focused beneath the surface of glass, with light nonlinearly absorbed to yield a permanent and localized refractive index modification. By translating the sample relative to the laser, optical waveguides can be inscribed along 3D trajectories, enabling novel integrated optical devices ranging from UV [1] to Terahertz [2]. However, controlling the refractive index modification, which affects the quality of photonic circuits, is a challenge that has yet to be met by researchers. Since the pioneering work by Miura’s group in 1996, bulk modification of glasses by ultrafast laser irradiation has been studied extensively, with over two thousand papers citing the seminal paper [3]. Several mechanisms have been proposed to explain the ultrafast laser induced refractive index change, including heat accumulation [4,5], thermal quenching [6,7], structural modification [8], color center formation [9], shock wave propagation and nanograting formation [10]. All of these effects influence the resulting morphological change, but their prominence depends on the laser processing conditions and glass type. The crucial role of ion migration during femtosecond laser writing has been disregarded until only recently. The first ever report of ion migration was from Miura’s group while studying space selective crystallization in amorphous dielectrics using a 800 nm, 130 fs, 200 kHz femtosecond laser [11]. The authors suggested that crystal nuclei are formed at points where the crystallization temperature is exceeded, due to atomic diffusion and the microstructure rearrangement. As described in this Review, ion migration is always present during ultrafast laser processing of glasses, and provides unprecedented control over the refractive index modification, enabling bespoke photonic devices. This Review provides a coherent overview of the numerous reports of ion migration during ultrafast bulk laser microfabrication, giving a more complete understanding of the underlying laser-material interaction physics. The results are classified into low and high repetition rate regimes – at high repetition rates (HRR, >200 kHz), there is an accumulation of heat between successive pulses [4]. Although there is not sufficient thermal accumulation at lower repetition rates (<10 kHz), the spatial distributions of heat buildup and ion migration can be modulated with a hybrid approach based on parallel irradiation with high and low repetition rate sources. Additionally, the observation of migration of ions is performed in two different configurations (a) parallel and (b) perpendicular to the incident laser direction. This Review gives insight into the strong difference observed in ions migrating in both configurations, and analyzes the dependence of ion migration on the glass composition. This Review is set to stimulate new research to better comprehend fundamental light matter interactions, which will lead to a further improvement in the performance of integrated optical circuits applicable to sensing [12,13], fiber to the premise [14], astrophotonics [15,16] and quantum information [17].

2. Background

2.1. Femtosecond laser sources

When the first optical waveguide was demonstrated using femtosecond laser writing in 1996 by Miura’s group [3], there was a limited range of suitable ultrafast laser sources. To achieve the necessary peak intensity (1013 W/cm2 [18]) to drive nonlinear absorption in glasses, focused femtosecond laser pulses from high energy (>1 lJ) amplified Ti:Sapphire lasers with pulse durations of 100 fs were employed. However, Ti:Sapphire laser systems have several disadvantages for femtosecond laser processing. First their complexity of alignment and mode locking and sensitivity to environmental conditions make them unsuitable for stable long-term operation. Schaffer and Mazur were the first to demonstrate that optical waveguides could be formed with lower energy (10 nJ), high repetition rate (10 MHz) femtosecond laser oscillators [19]. Please see Sections 2.2.2.1 and 2.2.2.2 for detailed discussions. Although high repetition rate femtosecond laser oscillators are promising for waveguide writing, the low nanojoule pulse energies require a high NA (1.0–1.4 NA) oil immersion microscope objective to induce nonlinear absorption. Since the working distance of such objectives are about 150 lm, this limits the possibility to form out of plane waveguides, hindering the most compelling advantage of femtosecond laser writing, i.e. 3D microfabrication. In the past ten years, high-repetition rate bulk solid state [20] and fiber [21] femtosecond lasers providing pulse energies of 1 lJ have been developed. The higher pulse energies available from these sources allow for a weaker focusing condition, enabling true 3D processing, while still maintaining nearly circular waveguide cross sections and mode shapes, for efficient coupling with external fibers. Generally, femtosecond fiber lasers are favored over bulk solid state systems based on Yb:KYW, T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 71

Yb:KGW or Yb:YAG due to their higher average power, less-stringent cooling requirements, higher beam quality, and more stable operation. Table 2-1 summarizes commercial femtosecond laser systems suitable for optical waveguide writing in transparent materials.

2.2. Femtosecond laser microfabrication

Focused femtosecond laser pulses yield peak intensities greater than 10 TW/cm2, which cause strong nonlinear absorption and localized energy deposition in the bulk of transparent materials such as glass. After several picoseconds, the laser- excited electrons transfer their energy to the lattice, leading to a permanent modiﬁcation. Depending on the laser and material properties, this modiﬁcation may result in damaged and irregular scattering centers, or smooth structures with a positive refractive index alteration.

2.2.1. Nonlinear absorption Focused femtosecond laser pulses, with wavelengths typically in the visible or near-infrared, do not have enough photon energy to be linearly absorbed in glasses. Instead, valence electrons may be promoted to the conduction band through nonlinear photoionization, which proceeds by multiphoton ionization and/or tunneling photoionization pathways depending on the laser and glass properties [22]. In addition to nonlinear photoionization, avalanche photoionization also occurs, explain- ing the small variation in threshold intensity for breakdown with bandgap [23]. Because of this low dependence of the breakdown threshold on the bandgap energy, femtosecond laser nanofabrication can be applied to a wide range of glasses and other transparent materials.

For multiphoton absorption the number of photons m required to bridge the bandgap must satisfy mhm > Egap, where Egap is the bandgap and m is the laser frequency. For high laser intensity and low frequency, the strong laser ﬁeld distorts the band structure and reduces the energy barrier between the valence and conduction bands, allowing for direct band-to-band tran- sitions by quantum tunneling. Nonlinear photoionization is usually a combination of both tunneling and multiphoton ionization for typical femtosecond laser waveguide writing conditions in glass [24]. The advantage of using femtosecond laser pulses is that it offers a deterministic breakdown, since nonlinear photoionization can seed the electron avalanche. This is in contrast to the stochastic breakdown with longer pulses which rely on the low concentration of impurities (about 1 impurity electron in conduction band per focal volume in glass), randomly distributed in the substrate to seed an electron avalanche [25]. For subpicosecond laser pulses, absorption is faster than energy coupling to the lattice, decoupling the absorption and lattice heating processes [24]. Seeded by nonlinear photoionization, the electron density in the conduction band increases via avalanche ionization until the plasma frequency approaches the laser frequency, at which point the plasma becomes strongly absorbing. For a typical femtosecond laser with 1-lm wavelength, the plasma frequency equals the laser frequency when the free carrier density is 1021 cm3, the critical density for optical breakdown. In terms of intensity, this breakdown threshold is 1013 W/cm2 in glasses. Laser wavelength and the material bandgap together decide the number of photons required for multiphoton absorption, which causes electrons to be promoted from the valence to the conduction band. Several different types of transparent materials are available with bandgaps varying from 1.5 to 12 eV, whereas the commer- cially available lasers have photon energies between 1.1 and 2.4 eV. It was demonstrated that the variation of the breakdown threshold intensity varies only by a factor of two for the whole span of materials ranging from 3 to 12 eV using an 800-nm wavelength (1.55-eV photon energy) laser [23], indicating the importance of avalanche ionization, which depends linearly on intensity. Because of this low dependence of the breakdown threshold intensity on the bandgap energy, femtosecond laser nanoprocessing can be used in a broad range of transparent materials.

Table 2-1 Laser model, technology, pulse duration, repetition rate, average power and wavelength of commercial systems for femtosecond laser writing of optical waveguides (speciﬁcations obtained from respective company web sites, July 2016). In the table, s is the laser pulse duration, R is the repetition rate, P is the average power and k is the wavelength.

Laser model Technology s (fs) RP(W) k (nm) Spectra Physics Spitfire Ti:Sapphire reg. amp. 40 1–10 kHz 14 800 Coherent RegA 9050 Ti:Sapphire reg. amp. 40 250 kHz 1.5 800 High Q Spirit HE Yb:KYW amplifier 400 1 MHz 16 1040 Clark MXR Impulse Yb fiber ampliflier 250 25 MHz 20 1030 Menlo Yb fiber ampliflier 300 1 MHz 10 1030 BlueCut Amplitude Satsuma HP Yb fiber ampliflier 400 2 MHz 50 1030 IMRA mJewel DE2020 Yb fiber amplifier 350 1 MHz 20 1045 Light Conversion Pharos PH1-20 Yb:KGW amplifier 290 1 MHz 20 1030 Cazadero Yb or Er doped fiber 370 4 MHz 4 1030 or 1550 Calmar Laser 72 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

2.2.2. Relaxation and material modification Although it is well accepted that nonlinear photoionization and avalanche ionization are responsible for the creation of a free electron plasma, the physics are less clear when the electrons have transferred their energy to the lattice and the material is modified. In the nearly thousand published articles on optical waveguide writing citing the first work by the Miura group [3], the reported morphological changes can be generally classified into three types of modifications: a smooth refractive index change [26], a form birefringent refractive index modification [27–30] and microexplosions resulting in empty voids [31]. The type of modification depends on many exposure parameters such as energy, pulse duration, repetition rate, wavelength, polarization, focal length, and scan velocity, but also on material properties such as bandgap, thermal con- ductivity and the glass composition. In pure fused silica glass, the most commonly processed material for waveguide writing, these three different morphologies can be observed by simply changing the incident laser energy [32].

2.2.2.1. Effect of pulse energy. An isotropic regime of modification is useful for optical waveguides, where smooth and uniform refractive index modification is required for low propagation loss. At low pulse energies just above the modification threshold (100 nJ for typical femtosecond laser focusing conditions [19]), a smooth refractive index modification has been observed in fused silica [32], which Krol’s group has attributed to densification from rapid quenching of the melted glass in the focal volume [33]. In fused silica, the density and hence refractive index increases when glass is quickly cooled from a higher temperature [34]. Micro-Raman spectroscopy experimentally confirmed an increase in the concentration of 3 and 4 member rings in the silica network in the laser exposed region, signaling a densification of the glass [32]. Shock waves generated by focused fs laser pulses giving rise to stress have been shown to play a role in driving densification under certain conditions [35]. A small contribution to morphological changes produced by a focused femtosecond laser may be due to color centers, which alter the absorption spectra and hence the refractive index due to the Kramers- Kronig relation [36]. Waveguides formed in fused silica [37] were found to exhibit photo-induced absorption peaks at 213 nm and 260 nm that correspond to positively charged oxygen vacancies and non-bridging oxygen hole centers defects, respectively. However, the color centers were completely erased after annealing at 400 °C, even though waveguide behavior was observed up to an annealing temperature of 900 °C. It is therefore unlikely that color centers played a significant role in the refractive index change. Other research in borosilicate glasses has supported this claim [38]. In Yb-doped-phosphate glasses, Withford’s group has shown that laser induced color centers contribute about 15% to the observed refractive index increase [39]. In glasses with structures that are more complex than fused silica, one must also consider the ion exchange between network formers and network modifiers [40,41]. Such observations by the Miura and Solis groups give a new parameter for optimizing the glass composition to improve laser-written waveguide structures, and is the motivation for this Review. For pulse energies higher than those suitable for waveguide writing (150 to 500 nJ for typical femtosecond laser focusing conditions), birefringent refractive index changes have been observed in fused silica [28] and borosilicate glasses [42]. Kazansky’s group suggested that the birefringence was due to periodic nanostructures caused by interference of the laser field and the induced electron plasma wave [29]. These nanogratings develop after multiple laser pulses [27] and are always oriented perpendicularly to the laser polarization [43] as shown in Fig. 2-1. Their structural properties can be controlled with the laser processing parameters, allowing for precise tuning of their birefringent properties [44]. However, the mechanisms responsible for the self-organization of nanogratings are not yet fully understood. In 2008, Taylor’s group discovered that nanogratings consist of self-aligned nanocracks [45]. They proposed that inhomogeneous dielectric breakdown results in the formation of a nanoplasma resulting in the growth and self-organization of nanoplanes [30]. The model was found to accurately predict the experimentally measured nanograting period for a certain range of experimental conditions in fused silica. The Nolte group recently published an important review paper, giving futher experimental insight into the formation nanogratings [27]. They applied non-destructive small angle X-ray scattering and found that the characteristic size of the smallest features were nanocavities of dimensions 30 200 300 nm3. The dimensions of these nanocavities were indepen- dent of exposure parameters, whereas exposure to multiple laser pulses led to an increase in their total number. They then applied focused ion beam milling to dissect a portion of the sample and found that hollow cavities are the primary constituents of nanogratings and that their sheet-like arrangement gives rise to their periodicity. The large index contrast between the cavities and the surrounding material was found to be the cause of the high birefringence [46] despite the small feature size. Continuous grating planes emerge as adjacent cavities link due to their close proximity, whereas the material in between remains devoid of pores and is therefore more resilient. The presence of hollow cavities facilitates the anisotropic etch rate of HF acid parallel and perpendicular to the laser polarization [43]. This effect can be exploited to fabricate buried microchannels for microfluidic applications (Fig. 2-1(c)). These nanogratings are not usually suitable for waveguide devices as birefringence is often seen as a detriment. However there are optical applications where nanogratings are useful such as rewritable optical memory [43], birefringent waveplates [47] and integrated polarization beam splitters [48]. Nolte’s group [42] applied a tunable pulse duration femtosecond laser to study nanograting formation in Corning ultra- low expansion (ULE) TiO2-doped silicate glass and Schott Borofloat 33 borosilicate glass. The birefringence of nanogratings in ULE is comparable to those in fused silica, while the nanostructures in borosilicate glass show much lower birefringence. Interestingly, the period of the nanogratings is also dependent on the type of the glass, being 250 nm for ULE (similar to fused silica) but only 60 nm in case of Borofloat 33. As the properties of nanogratings in ULE and borosilicate differ significantly T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 73

Fig. 2-1. Scanning electron microscope image of buried nanogratings (sample cleaved and polished at writing depth) with polarization parallel (a) and perpendicular (b) to the scan direction. Overhead view (c) of etched microchannels demonstrating polarization selective etching with parallel (top), 45° (middle) and perpendicular (bottom) linear polarizations [45]. from those in fused silica, a more general model of nanograting formation must account for this differing behavior amongst glasses. At even higher pulse energies (>500 nJ for typical femtosecond laser focusing conditions), pressures greater than Young’s modulus are generated in the focal volume, creating a shockwave after the electrons have coupled their energy to the ions (10 ps) [32]. The shockwave leaves behind a less dense or even hollow core, depending on the laser and material properties [49]. By conservation of mass, this core is surrounded by a shell of higher refractive index. Such voids may be exploited for 3D memory storage [50] or photonic bandgap materials [51].

2.2.2.2. Effect of repetition rate. The above interpretations for the structural changes induced by focused femtosecond lasers typically assumed single pulse interactions, but can likely be extended to explain the modification from multipulse interactions during waveguide writing, assuming the repetition rate is low enough that thermal diffusion has carried the heat away from the focus before the next pulse arrives [32]. In this case, the following pulses may add to the overall modification, but still act independently of one another. For high repetition rates (>100 kHz), the time between laser pulses is less than the time for heat to diffuse away, giving rise to a buildup of temperature in the focal volume. For sufficiently high pulse energy, the glass near the focus is melted and as more laser pulses are absorbed, this melted volume continues to expand until the laser is removed, and due to rapid cooling and ion migration, produces a region of altered refractive index. The reader is referred to Section 3 for a more detailed discussion of the heat accumulation effect and corresponding elemental redistribution when processing glasses with high repetition rate femtosecond laser pulses.

2.2.2.3. Linear propagation. Neglecting spherical aberration [52] and nonlinear effects [53], the spatial intensity proﬁle of a focused femtosecond laser beam can be well represented by the paraxial wave equation and Gaussian optics. The 2 diffraction-limited minimum waist radius w0 (1/e intensity radius) for a collimated Gaussian beam focused inside a transparent material is:

M2k w ¼ ð2-1Þ 0 pNA where M2 is the Gaussian beam propagation factor [54], NA is the numerical aperture of the focusing objective and k is the free space wavelength. The Rayleigh range z0 inside a dielectric of refractive index n is given by:

M2nk z ¼ ð2-2Þ 0 pNA Chromatic and spherical aberration alter the intensity distribution near the focus so that Eqs. (2-1) and (2-2) are no longer valid. Chromatic aberration as the result of dispersion in the lens can be corrected by using chromatic aberration-corrected microscope objectives for the wavelength spectrum of interest. For lenses made with easily-formed spherical shapes, light rays that are parallel to the optic axis but at different distances from the optic axis do not converge to the same point, result- 74 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 ing in spherical aberration. This can be addressed by using multiple lenses such as those found in microscope objectives or using an aspheric focusing lens. In waveguide writing, the index mismatch at the air-dielectric interface introduces addi- tional spherical aberration. As a result, there is a strong depth dependence for femtosecond-laser written buried structures [52,55], which is even more pronounced for higher NA objectives [24] except for oil-immersion lenses [56] or dry objectives with collars that can correct for spherical aberration at different focusing depths [52]. Dispersion from mirror reﬂection and transmission through materials can broaden the pulse width which can reduce the peak intensity and alter the energy dissipation at the focus. However, it is only for short pulse <40-fs oscillators with large bandwidths that dispersion becomes an issue. In this case, precompensation of the dispersion through the microscope objective is required to obtain the shortest pulse at the focus [57].

2.2.2.4. Nonlinear propagation. The spatially varying intensity of a Gaussian laser beam can create a spatially varying refractive index in glasses. As the nonlinear refractive index n2 is positive in glasses, the refractive index is higher at the center of the beam compared to the wings. This variation in refractive index creates a positive lens which focuses the beam inside a dielectric with a strength dependent on the peak power. If the peak power of the femtosecond laser pulses exceeds the critical power for self-focusing [24]:

3:77k2 Pc ¼ ð2-3Þ 8pn0n2 a collapse of the pulse to a focal point is predicted. However, as the beam self focuses, the increased intensity drives nonlinear ionization which creates a free electron plasma, which acts as a diverging lens that counters self-focusing. A balance between self-focusing and plasma defocusing leads to ﬁlamentary propagation, which results in vertically elongated refractive index structures, which are undesirable for transversely written waveguide structures, the standard geometry for waveguide fabrication. Filaments can be exploited to fabricate waveguide devices by scanning the sample vertically along the beam axis. Self-focusing can be avoided in waveguide fabrication by tightly focusing the laser beam with a microscope objective to reach the intensity for optical breakdown without exceeding the critical power. 20 2 In fused silica, n0 = 1.45 and n2 = 3.5 10 m /W [58] so that for k = 1030 nm, the critical power is 3 MW. From Eq. (2-3), the critical power is proportional to the wavelength squared, therefore, lower critical powers result when working with the second harmonic of femtosecond lasers. Also, the critical power is inversely related to the nonlinear (and linear) 18 2 refractive index, making it difﬁcult to form waveguides in heavy metal oxide glasses with n0 2, n2 10 m /W [59] 17 2 and chalcogenide glasses with n0 2.5, n2 10 m /W [60].

2.3. Dielectric materials used for fabrication

To date, a wide variety of materials has been used for femtosecond laser microfabrication, including glasses [23], crystals [61], ceramics [62], and polymers [63,64]. However, the focus of most studies has been on glasses and crystals, mainly because of their wide applications forming the backbone of photonic devices. Even though there is a large inventory of glasses that are of potential interest for photonic devices such as fused silica, silicates, germanates, phosphates, tellurites, borates, ZBLAN and chalcogenides [65–73], fused silica is generally favored for passive applications due to its high transparency from the deep UV to NIR and its excellent thermal, chemical and mechanical stability. For active photonic devices, there was a surge of femtosecond laser written waveguides in silicate, phosphate, germanate and tellurite glasses doped with rare-earth ions such as Nd3+,Yb3+,Er3+,Tm3+ and Ho3+ [1,74]. Among crystalline materials studied are lithium niobate, Ti:Sapphire, YAG and KGW crystals [75–78] due to the possibility of combining their nonlinear properties in a highly integrated platform enabled by femtosecond laser inscription. Other than the in-house fabricated glasses, various commercial glasses used in the discussion for this review paper are Corning eagle 2000 [79] (Section 3) Corning 0211 Borosilicate (Sections 3.1 and 3.1.1) [80], Schott B270 Superwrite crown (Sections 3.2.2.1,3.2.3,6.4 and 6.5) [81], Schott AF37 alumino borosilicate (Section 6.3) [82], Schott D263 borosilicate (Section 3.2.2.2) [83], Schott B33 boroﬂoat (Section 3.2.2.3) [84], Schott Foturan II (Section 3.3, Fig. 3-14) [85], Matsunami glass S1111 crown (Section 3.4, Fig. 3-15) [86], Kigre QX and MM2 phosphate (Section 4.1.1,4.1.3 and 4.2) [87] and Schott IOG-1 phosphate (Section 4.1.1) [88].

2.4. Characterization techniques

Investigating the laser affected zone using various material characterization techniques will reveal valuable information regarding the glass structure, co-ordination of bonds and possible migration of ions/atoms. Ion migration is a relatively new information providing deeper insights which aids in tuning the laser-induced morphology in glasses (discussed in Sections 3.4–6.5). The main characterization technique to analyze the ion migration process during bulk femtosecond laser microfabrication relies mainly on electron microscopy [89] which reveals the elemental profile or its distribution. Micro-Raman (l-Raman), micro-photoluminescence (m-PL) spectroscopies and refracted near field profilometry are valuable supplementary characterization techniques deducing valuable information about atomic and molecular networks. In scanning electron microscopy (SEM), a Z-contrast study can be done by using backscattered electrons (BSE) from the sample, or a more precise T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 75 compositional analysis by means of wavelength dispersive spectroscopy (WDS) or energy dispersive X-ray spectroscopy (EDX). In the case of WDS, high spectral resolution and high throughput enable count rates greater than 105/s but EDX is faster and simpler to implement. EDS is the most commonly used technique for qualitative elemental analysis to demonstrate which elements are present and their relative abundance. When the sample is bombarded with a focused beam of electrons, spectral lines of X-rays are emitted due to the transition of electrons between pairs of K, L, M and N shells of the atom between two high and low energy levels. It is quite easy to qualitatively identify elements having atomic number greater than 10 with this technique. The photon counting of X-ray intensitites can provide a precision that is limited by sta- tistical error. For major elements a precision of 2r is possible but due to the uncertainties in the compositions of the stan- dards and errors in the various corrections which need to be applied to the raw data, overall analytical accuracy is near ±2%. The precision can be improved by using longer counting times. Spatial resolution is governed by the penetration and spread- ing of the electron beam in the specimen, hence the spatial resolution for low atomic number ranges between 1 and 5 lm3 and for higher atomic numbers this could be improved to 0.2–1 lm3. Since the electrons penetrate an approximately constant mass, spatial resolution is a function of density. The size of the interaction volume could also be increased by increasing the accelerating voltage. WDS uses the Bragg diffraction principle of an analyzing crystal to preferentially diffract the wavelength of interest to the detector. WDS is a non-destructive quantitative analyses with only a few micrometer spot sizes, at detection levels as low as a few tens of parts per million weight (ppmw), and for light elements down to atomic number 5 (boron). In this technique a beam of electrons, typically 15–20 kV, is accelerated in an evacuated electron column of a SEM to the sample surface to generate characteristic X-rays for the elements to be analyzed. It is then selectively identified using an analytical crystal with specific lattice spacings. When X-rays reach the crystal at a specific angle only those X-rays that satisfy Bragg’s law are reflected and a single wavelength is passed through to the detector. The X-ray source-crystal distance is a linear function of the wavelength. Hence only one element could be measured at a time unlike in EDS. When compared to EDS, WDS exhibits superior peak resolution of elements and sensitivity of trace elements. SEM techniques are not as destructive as transmission electron microscopy (TEM), with sample preparation typically con- sisting of metallization to avoid charging or heating effects. Analysing a femtosecond laser irradiated zone in SEM could reveal a densification or a rarefied zone by direct visualization of the heavy atom segregation or the Z-contrast, Z, being the atomic weight. The contrast arises from the enrichment or the depletion of heavy elements with respect to the un- irradiated/pristine sample. One difficulty in the analysis is distinguishing between a change in the local valence state of glass constituents and ion migration. The X-ray microanalysis system used for compositional analysis in the SEM has a resolution of about 125 eV, which enables identification of the different elements present in the sample, but not their oxidation states that require a resolution of 1 eV. The system integrates the number of counts associated to the characteristic emission of a given element, yielding only local compositional variations. Additionally, if the prevalent local valence of multi-valence impurities is changed upon solidification, then the glass network shows a larger distribution of bonding characteristics that could be easily distinguished using supplementary characterizations like l-PL and l-Raman. Employing a TEM can increase the resolution by one order of magnitude but at the cost of destroying the sample [90]. The elemental analysis in TEM has a larger range, including WDS, EDX, high angle annular dark field (HAADF) imaging or electron energy loss spectroscopy (EELS). The advantage of EELS over WDS or EDX is that it is more sensitive to light elements and the energy resolution is higher, thus extracting more data such as chemical bonding or valence states. To date, there are only a few reports in which TEM was being used to probe laser written waveguides. In one notable paper, Gorelik et al. applied TEM to characterize a very thin slice of a micrometer-sized waveguide fabricated in crystalline quartz [91]. Both static single pulse exposures and scanned line structures revealed an amorphous core surrounded by a disturbed crystalline structure. TEM was also applied by Juodkazis et al. to observe a transformation of crystalline to amorphous sapphire [92]. Cao et al. [93,94] also used TEM to study the crystallization within the laser irradiated region which is discussed in Section 4.1.

3. Basic principle of heat accumulation and ion migration by fs laser irradiation

High repetition rate femtosecond laser irradiation [4–6,95] causes localized melting of glasses resulting in ion migration [41,72,96–108]. Because the shape of the molten region and distributions of ions are influenced by the heat accumulation, the temperature distribution is essential information to elucidate the underlying physics and to control the ion migration. Evidence of heat accumulation during bulk femtosecond laser irradiation of glasses was first reported by Schaffer et al. [95]. During high repetition rate (25 MHz) femtosecond laser irradiation of zinc-doped borosilicate glass (Corning 0211), they found that the modification size increased with the number of laser pulses (Fig. 3-1(a)). They explained the pulse- number dependence of the modification volume was due to an accumulation of heat from consecutive laser pulses, and successfully predicted the modification size using a simple thermal diffusion model (Fig. 3-1(b)) [6]. Later, Eaton et al. investigated the effect of heat accumulation by femtosecond laser irradiation at different repetition rates (0.1–5 MHz), and accurately predicted the modification volume from the simulated temperature increase due to heat accumulation (Fig. 3- 2(a)) [4]. They found that heat accumulation becomes pronounced inside an alkali-free borosilicate glass above 200 kHz repetition rate (Fig. 3-2(b)), when the time between pulses is shorter than the time for heat to diffuse out of the focal volume. In a later work [5], they clarified that the laser pulse energy, which controls the strength of thermal diffusion from individual laser pulses, is also crucial in driving heat accumulation. 76 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 3-1. (a) Optical microscope image of modifications inside a zinc-doped borosilicate glass (Corning 0211) by irradiation with 30 fs laser pulses of 5 nJ at 25 MHz, focused with a 1.4NA objective. The modifications became larger as the number of laser pulses increased from 102 to 105. (b) Plot of the radius of the modifications against the number of laser pulses. The curve is the radius calculated by a thermal diffusion model.

The modification by heat accumulation, which we will refer to as ‘‘thermal modification” in this Review, produces nearly spherical modifications with two clear boundaries as shown in Fig. 3-2(a). Inside the inner boundary (darker region of the modification by 1 MHz irradiation in Fig. 3-2(a)), the flow of glass melt can be observed during laser irradiation [104–106,108]. This is the region where ion migration occurs [41,96–101,103,107]. In the region between the inner and outermost boundaries (outer modified region), changes of elemental distributions and glass structures have not been observed. The mechanism in forming the outer modified regions is not completely understood. Several researchers have made the assumption that there is a threshold temperature of the outermost boundary and defined the threshold temperature hypothetically to determine the temperature distribution during heat accumulation [5,6]. Fig. 3-2(b) plots the temperature versus pulse shot number at different repetition rates, as simulated by a thermal diffusion equation [4]. The dotted black line at 1225 °C is the working point, which was used to estimate the melting threshold of the borosilicate glass. At 100 kHz repetition rate, the temperature drops below the melting threshold before the next pulse arrives and therefore the modified volume is defined by single pulse thermal diffusion alone. However at high repetition rates, where there is less time between successive pulses, the temperature remains above this melting threshold. Therefore as more pulses are absorbed, the temperature and size of the modification increases. Because the ion migration is influenced by heat accumulation during high repetition rate (HRR) femtosecond laser irradiation, the important information for understanding the mechanism of ion migrations are (i) the time scale of thermal diffusion inside glass after a single photoexcitation, (ii) the theoretical and experimental estimation of the temperature distribution during heat accumulation, (iii) the mechanism of thermal modification and the threshold temperature at which thermal modification occurs, and (iv) the tendency of ion migration and the driving force of ion migration. In this section, the fundamental studies of heat accumulation and ion migration by HRR femtosecond laser irradiation are reviewed. First, several observation reports of the temperature distribution during single femtosecond laser irradiation will

Fig. 3-2. (a) Optical microscope image of modiﬁcations from static femtosecond laser exposures in a borosilicate for different pulse number and repetition rates (focusing lens 0.65NA, pulse energy 450 nJ). (b) The plots of the temperature against pulse shot number at different repetition rates, which were simulated by a thermal diffusion equation. The dotted black line is the working point of the glass which was used to estimate the melting threshold. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 77 be reviewed [109–113]. These studies elucidated the temperature elevation and cooling rate after the photoexcitation, which will help us in studying the heat accumulation from a train of femtosecond laser pulses. Next, the simplest method [114– 119] for the simulation of the temperature distribution during heat accumulation will be discussed. Finally, the general tendency of the ion migration will be reviewed and one simulation study of ion migration based on a thermo-diffusion model will be discussed.

3.1. Observation of thermal diffusion after single fs laser irradiation inside a glass

After a femtosecond laser pulse is focused inside glass, excitation of electrons occurs by multiphoton or tunneling ionization and subsequent avalanche ionization [95,120,121]. The energy of excited electrons (plasma) is transferred to the lattice in the electron-phonon collision time, which is as fast as several picoseconds [122–124]. Due to the fast energy transfer from hot electrons to the lattice, the temperature in the photoexcited region is elevated much faster than the elastic relaxation time of the glass. As a result, large thermal stress is generated in the photoexcited region just after the photoexcitation, and the relaxation of thermal stress generates a strong stress wave [122–126]. After the stress wave propagates far away from the photoexcited region, the temperature in the photoexcited region decreases by thermal diffusion over a much a longer time scale. The stress relaxation time (tac) and thermal diffusion time (tth) can be determined from the speed of sound (cs), thermal diffusion coefﬁcient (Dth), and the smallest dimension of the photoexcited region (dh) [127]:

dh tac ¼ ð3-1Þ cs

2 dh tth ¼ ð3-2Þ 4Dth 3 Using typical values for femtosecond laser bulk processing inside glass with a high NA objective, cs =510 m/s, Dth = 7 2 1 6 510 m s [128] and dh =10 m (the laser spot size), tac = 200 ps and tth = 0.5 ls are obtained. Therefore, in considering the heat accumulation by HRR femtosecond laser irradiation, the stress relaxation in the photoexcited region is so fast that only the effect of thermal diffusion is taken into consideration. To experimentally observe thermal diffusion inside glass due to photoexcitation from femtosecond laser irradiation, the transient lens method and l-Raman spectroscopy have been applied. Below we provide an overview of these two methods.

3.1.1. Observation of thermal diffusion by a transient lens (TrL) method The thermal lens (TL) method [129,130] is used to observe the temperature change due to photoexcitation. The similar transient lens (TrL) method provides even further information on the refractive index dynamics including changes in density, temperature and chemistry [129,130]. In both the TL and TrL method, the lens effect due to the refractive index modification in the photoexcited volume is detected as the intensity pattern change of the transmitted probe beam. In many cases, a TrL signal is detected as the transmission through a pinhole as shown in Fig. 3-3(a) [111]. Because the probe beam on the pinhole is expanded or contracted by the transient lens, the change of the transmittance through the pinhole reflects the transient lens effect. Because the TrL comes from the thermal expansion and molecular polarizability change due to the temperature change in the photoexcited volume, the temperature change can be estimated by detecting the TrL signal. In addition, the thermal diffusion, i.e. cooling of photoexcited glass, can be measured from the decay of the TrL signal. Sakakura et al. applied the TrL method to the observation of the temperature change inside glass after photoexcitation by a focused femtosecond laser pulse [109,111]. The optical setup for TrL measurement in their study is shown in Fig. 3-3(b). A femtosecond laser pulse was focused inside glass with a 20 objective lens to induce nonlinear photoexcitation in the focal volume. At the same time, a CW probe beam was passed through the photoexcited region. When the probe beam transmits through the photoexcited region, the phase distribution of the probe beam is modified by the refractive index distribution, which is originated from photoinduced carriers, stress and temperature changes around the photoexcited region. The modified phase distribution results in the intensity distribution change of the probe beam at the far field. In the method, the intensity distribution change was monitored by detecting the probe beam after a pinhole with a diameter of 1 mm using a photomultiplier. The temporal change of the intensity was acquired by a digital oscilloscope as a ‘‘TrL signal”. The observed TrL signals after photoexcitation in a zinc-borosilicate glass is shown in Fig. 3-3(c). The TrL signal rose in several hundred nanoseconds and decayed in several microseconds. The amplitude of the rise and the peak intensity of the TrL signal increased with increasing excitation pulse energy. The rise and decay components in the TrL signals can be explained only by the thermal diffusion. The authors simulated the TrL signal based on the thermal diffusion model and compared them with the observed signals. They assumed that the initial temperature distribution can be expressed by [109]: "# r 2 z 2 DTðt ¼ 0; r; zÞ¼DT0 exp ; ð3-3Þ wth=2 lz where r is the radial coordinate, z is the axial position, DT0 is the initial temperature change at the peak, wth is the width of the heated region, and lz is the length of the heated region. This initial temperature distribution gives the following equation of the temporal evolution of the temperature distribution by solving the thermal diffusion equation [109]: 78 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 3-3. (a) Principle of the measurement of a TrL signal. (b) Optical setup for TrL measurement of tightly focused fs laser-induced dynamics inside a glass. (c) TrL signals observed inside a zinc-doped borosilicate glass after irradiation with a 220 fs laser pulse focused with a 0.45NA objective. (d) TrL signals simulated by diffraction theory and Eq. (3-5) with different D/th (maximum phase change due to the change in temperature). (e) Fitting of the observed TrL signals by those simulated by Eq. (3-5) with a best ﬁt D/th. (f) The temperature change at different positions from the center of the photoexcited region obtained by the ﬁtting of the TrL signals in (e).

!"# 1=2 ðw =2Þ2 l2 r2 z2 DTðt; r; zÞ¼DT th z exp : ð3-4Þ 0 = 2 2 = 2 2 ðwth 2Þ þ 4Dtht lz þ 4Dtht ðwth 2Þ þ 4Dtht lz þ 4Dtht where t is the time after the photoexcitation and Dth is the thermal diffusivity. TrL signals come from the modification of the phase distribution of the probe beam after passing through the photoexcited region. Therefore, to simulate TrL singals due to thermal diffusion, the modification of the phase distribution should be derived using the temperature change of Eq. (3-4). If it is assumed that the refractive index change is proportional to temperature change, the modification of the phase distribution of the probe beam after passing through the heated region was calculated by integrating Eq. (3-3) along the beam propagation direction, z: "# = 2 2 ; ðwth 2Þ r D/ðt rÞ¼D/th 2 exp 2 ð3-5Þ ðwth=2Þ þ 4Dtht ðwth=2Þ þ 4Dtht where D/th is the phase change at the beam center, which is given by 3=2 2p dn : D/th ¼ DT0lz ð3-6Þ n0k dT The intensity distribution of the probe beam at the far field can be calculated by substituting Eq. (3-5) into the Fresnel diffraction equation [131]. Because the TrL signal is the light intensity through the pinhole (Fig. 3-3(a)), the TrL signal intensity can be obtained by calculating the intensity at the central part of the probe beam.

Fig. 3-3(d) shows the TrL signals simulated using Eq. (3-5) with different D/th and wth = 2.0 mm. As can be seen, the TrL signals simulated with D/th = 6.0, 4.0 and 2.0 reproduced the TrL signals for pulse energies of 0.8, 0.6 and 0.4 mJ, respectively. The initial temperature change just after the photoexcitation, DT0, can be estimated by ﬁtting experimental TrL signals as shown in Fig. 3-3(e). The TrL signals observed at various conditions could be reproduced by D/th = 5.8, wth = 1.7 mm and 2 6 1 Dth = 0.75 mm /ls. Assuming that lz =50lm and using n0 = 1.5 and dn/dT = 3.4 10 K for borosilicate glass, the temperature elevation was estimated to be DT0 = 1790 K. The temperature evolution inside glass using this estimated value is shown in Fig. 3-3(f). The temperature decays monotonically for r < wth, while the temperature for r > wth is elevated tran- siently and then decays. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 79

The important ﬁnding from the TrL measurement is that the thermal diffusion can be calculated approximately by the thermal diffusion model using the room temperate thermal diffusion coefﬁcient. Because the TrL measurement showed that the temperature decreased below 50 °Cat10ls and reached the room temperature at 100 ls, the TrL measurement means that repetition rates greater than 100 kHz are necessary for heat accumulation.

3.1.2. Observation of thermal diffusion by micro Raman measurement Another method to observe the temperature distribution change is a measurement of temperature-dependent Raman spectrum by a confocal Raman microscope [112]. Because the intensity of the anti-Stokes band in the Raman spectrum depends on the population of excited vibrational states, the ratio between the intensities of Stokes and anti-Stokes bands can be used to measure the temperature. If the material is in thermal equilibrium, the ratio can be expressed in terms of the Boltzmann distribution [132]: I hm AS ¼ exp R ð3-7Þ IS kBT where IS and IAS are the intensities of the Stokes and anti-Stokes Raman bands, respectively at the frequency mR. This equation means that the intensity of an anti-Stokes band becomes larger relative to that of the Stokes band as the temperature increases, as shown in Fig. 3-4(a). Because the Raman frequency, mR, is known from the Raman spectra, the temperature T can be obtained from the observed intensity ratio. Yoshino et al. observed the time-resolved Raman spectra from the photoexcited region inside a glass after femtosecond laser irradiation and obtained the temporal evolution of the temperature by analyzing the intensity ratio given by Eq. (3-7) [112,113]. The optical setup for the time-resolved Raman temperature measurement in a glass after fs laser irradiation is shown in Fig. 3-4(b). A femtosecond laser pulse was focused inside a glass sample (B33 glass or silica glass) with an objective lens to induce nonlinear photoexcitation at the focus. A Raman excitation pulse from the frequency doubled Nd:YAG nanosecond laser (532 nm) was input into the same optical path as the fs laser pulse and focused at the photoexcited region to induce Raman scattering. The Raman scattering was collected by the same objective lens and detected by a gated ICCD after passing through a pinhole and polychrometer. The time of the observation was changed by controlling the trigger pulse to the YAG laser for Raman excitation by a pulse delay generator. The temporal evolution of the temperature in the photoexcited region from the analysis of the observed Raman spectra is shown in Fig. 3-4(c) and (d). Both in fused silica and B33 glasses, the cooling times were from 0.1 lsto10ls and the initial temperature increases were higher than 1000 K, consistent with TrL measurements [109,111]. Interestingly, the pulse energy-dependence of the temperature increase and cooling time were different between fused silica and B33 glasses (Fig. 3-4(e) and (f)). As shown in Fig. 3-4(e), the initial temperature increase became larger with increasing pulse energy in a fused silica, while the pulse energy dependence of the temperature was much smaller in B33 glass. The authors argued that the difference could be from the varied laser-induced morphologies in fused silica and B33 glass (Fig. 3-4(g)); void structures were formed in a fused silica, while filamentary material modification occurred in B33 glass. The filamentation of the photoexcited region affects the cooling time, because the longitudinal length of the photoexcited region (lz in Eq. (3-3))is included in the diffusion term in the equation governing thermal diffusion (Eq. (3-3)). The pulse energy-dependence of the time constants in B33 glass suggests that lz becomes larger with increasing pulse energy and larger lz makes the cooling time longer.

Fig. 3-4. (a) Schematic illustration of the Raman spectra change due to the temperature increase. The black and red lines are Raman spectra before and after temperature increase, respectively. (b) Optical setup for a spatial-temporal-resolved Raman measurement. (c),(d) Temporal evolutions of the temperatures obtained from the Raman spectra observed in fused silica and B33 glass after fs laser irradiation, respectively. (e), (f) The initial temperature increases and time constants plotted against pulse energies analyzed by the temperature changes in (c) and (d). (g) Femtosecond laser induced modiﬁcations along the beam propagation direction in two glasses. 80 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

3.2. Heat accumulation with a high repetition rate laser

The typical femtosecond laser-induced modifications inside a glass with and without heat accumulation are shown in Fig. 3-5(a) and (b) [115]. In the case of 1 kHz irradiation, the modification is as small as the diameter of the laser focal spot (Fig. 3-5(a)). On the other hand, the modification at 250 kHz is much larger than that at 1 kHz (Fig. 3-5(b)). The simulated temperature change at 250 kHz clearly shows the accumulation of thermal energy (Fig. 3-5(c)), so the larger modification is attributed to a thermal modification induced by the heat accumulation. As discussed before, the thermal modifications inside most glasses have two boundaries, with the inner region due to the flow of the glass melt [108,130]. In this region, both a change in the elemental distribution (i.e. ion migration) and the precipitation of crystal have been observed. On the other hand, there is no change in elemental distributions between the inner and outermost boundaries, although a refractive index change is observed in this region. In this section, we review the method to simulate the temperature distribution during HRR laser irradiation.

3.2.1. Simulation of heat accumulation Here, we explain the simplest method of the simulating heat accumulation. The temporal evolution of the temperature distribution during repeated laser irradiation can be calculated by the equation of thermal diffusion with a time-dependent heat source [115]: @Tðt; x; y; zÞ 1 @Qðt; x; y; zÞ ¼ r½DthrTðt; x; y; zÞ þ ð3-8Þ @t qCp @t where T(t,x,y,z) and Q(t,x,y,z) are the time-dependent temperature and heat source by repeated laser irradiation, respectively. Dth, q and Cp are the thermal diffusivity, density and heat capacity of the material, respectively. When the repetition rate of laser irradiation is R =1/tL, the time-dependent heat source can be written as: "# XN1 @Qðt; rÞ r2 z2 ¼ Q dðt nt Þ exp ð3-9Þ @t 0 L = 2 = 2 n¼0 ðwth 2Þ ðlz 2Þ where we assumed that the distribution of the thermal energy by a single laser irradiation is given by Eq. (3-4), and Q0 is the density of the thermal energy at the center of the photoexcitation and N is the number of pulses. In this equation, the generation of the thermal energy is expressed as a delta function, because the temperature increase occurs much faster (<10 ps) than that of the thermal diffusion (>10 ns). Because the differential equation of the thermal diffusion equation (Eq. (3-8))is linear if Dth is constant, the temperature distribution after the Nth pulse can be calculated from the sum of the temperature change after each pulse:

XN1 Tðt; x; y; zÞ¼ DTðt nDtL; x; y; zÞþTa ð3-10Þ n¼0

2 2 0.5 where DT(t,x,y,z) is the same as DT(t,r,z) in Eq. (3-3), r =(x + y ) and Ta is the ambient temperature. Fig. 3-6(a) shows the temporal evolution of the temperature at various positions during 250 kHz laser irradiation calculated by Eq. (3-10). In this calculation, the temperature change at the center by a single laser irradiation is DT0 = 1000 K. The temperature at the center of the photoexcited region rises and decays repeatedly at every pulse. This drastic temperature

Fig. 3-5. (a), (b) Optical microscope images of modiﬁcations inside a sodalime glass by irradiation with focused 100 fs laser pulses at 1 kHz (a) and 250 kHz (b). Two boundaries are observed in (b). Within the inner boundary, glass was melted during laser irradiation and ion migration occurred. In the outer region, no melting and ion migration occurred. (c) Simulation of temperature change during laser irradiation at 1 kHz (red dotted line) and 250 kHz (black solid line). T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 81

Fig. 3-6. (a) Temperature change at different radial positions from the laser focus simulated using Eq. (3-10). The parameters are DtL =4ls, Ta =20°C, DT0 2 1 = 1000 K, wth = 1.1 lm, lz = 9.0 lm, and Dth = 0.46 lm ls . (b) Temperature changes at different z positions simulated by an equation with more a complex distribution of the heat source. The repetition rate is 300 kHz. (c) Simulated temperature distributions along the scanning axis during laser irradiation at 500 kHz for 100 mm/s writing speed. oscillation at each photoexcitation become less evident further from the center. The temperature for r >10mm became almost constant after 10 ms (2500 pulses). This temperature profile with time follows a similar trend as slightly more complex models proposed by other researchers [3,116,118,119]. For example, Miyamoto et al. simulated the temperature change during heat accumulation by solving the thermal diffusion equation with a heat source with a more a complex distribution [118]. Also in this case, the temperature oscillation became smaller further away from the center (Fig. 3-6(b)). Therefore, the simplified numerical calculation in Eq. (3-10) provides a reasonable estimation of the heat accumulation. To simulate the heat accumulation in many applications of femtosecond laser processing, the movement of the heat source must be taken into account. In this case, the equation for the simulation of the heat accumulation, Eq. (3-10), must be modified to include the sample translation [116–118]. Fig. 3-6(c) shows the simulated temperature distributions along the scanning axis inside a glass during laser irradiation and scanning at 100 mm/s. This simulation shows that the temperature distribution becomes asymmetric in the transverse y direction at a fast scanning speed [117]. The simulation of the temperature distribution at different scanning velocity gives us valuable information for understanding the fs laser- induced crystallization inside a glass [133], because the heating and cooling rates, which are determined by the asymmetric temperature distribution in Fig. 3-6(c), are important for the seeding and growing of crystals in the heated region.

3.2.2. Evaluation of the temperature distribution during high repetition rate laser irradiation Several researchers have evaluated the temperature distributions inside glass during HRR femtosecond laser irradiation [114–119,134]. Some of the approaches are based on the comparison between the shape of the thermal modification and the simulated temperature distributions. Shimizu et al. investigated how the radius of the thermal modification depends on the ambient temperature and obtained the temperature distribution by analyzing the relation between the radius and ambient temperature [117]. Miyamoto et al. compared the isothermal lines in the simulated temperature distribution with the boundaries of the thermal modifications [117,118]. On the other hand, some researchers investigated the temperature distribution more directly by a measurement of Raman spectra during laser irradiation. Their methods for the simulation of temperature distributions are essentially the same as those described in Section 3.1.2. The temperature distributions evaluated by these researchers are different in several ways. Firstly, different definitions were used for the threshold temperature (Tout) at which the outermost boundary of the thermal modification is formed. Shimizu et al. proposed that Tout depends on the laser exposure time and is equivalent to the glass transition temperature at 1 s exposure time [114,115]. On the other hand, Miyamoto et al. assumed that Tout should be higher than the glass forming temperature based on the observation of laser welding of glasses [116,118]. Because the threshold temperature is important for determining the temperature distribution and evaluation of the light absorptivity during HRR laser irradiation, this temperature must be accurately determined. Below we review the various methods for calculating the temperature distribution and discuss the origin of the discrepancies between the various works.

3.2.2.1. Method by analysis of thermal modification at various ambient temperatures. Shimizu et al. proposed a unique method to estimate the temperature distribution during HRR femtosecond laser irradiation [114,115]. A glass sample was placed inside a temperature controllable enclosure so that laser writing could be performed at varying ambient temperatures. As the temperature of glass increased, the radius of the modification became larger as shown in Fig. 3-7(a). This temperature dependence suggests that the volume of the modification is determined by the characteristic threshold temperature, Tout, because the temperature of the glass before irradiation enlarges the region in which the temperature exceeds Tout during laser irradiation. Fig. 3-7(b) shows that the thermal modification occurs in the region where the temperature during laser 82 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 3-7. (a) Thermal modifications inside a soda lime glass by 250 kHz femtosecond laser irradiation at different pulse energies and different ambient temperatures. (b) Optical microscope images of the modification at various temperatures (Ta). The expected temperature distributions during laser irradiation are shown below, with the broken lines indicating the characteristic temperature, Tout for the modification.

irradiation was above the threshold temperature Tout and the volume where the temperature was above Tout becomes larger with increasing ambient temperature, Ta. Based on the idea, they measured the radii (Rb) of the thermal modifications, which had been induced by focusing 2.0 lJ and 80 fs laser pulses inside a sodalime glass with a 20 objective lens at 250 kHz, versus ambient temperatures, Ta (Fig. 3-8 (a)). Then, they analyzed the plot of Rb versus Ta to obtain the temperature distribution during laser irradiation and Tout. Fig. 3-8(a) shows that Rb became larger with increasing Ta. The first step of their analysis was to obtain the relation between Rb, Ta and Tout based on the model shown in Fig. 3-7(b). They expressed the temperature distribution change during laser irradiation by DT(r), where r is the distance from the center of a thermal modification. Because the model shown in Fig. 3-7(b) assumed that the thermal modification should occur in the region where the temperature during laser irradiation has been above Tout, the temperature at the boundary (r = Rb where r is the radial position from the laser beam axis) of the modification must be equal to Tout. In addition, when the ambient temperature is Ta, the temperature at r is DT(r) in addition to Ta. Therefore, the obtained the relation between Rb, Ta and Tout:

DTðr ¼ RbÞþTa ¼ Tout: ð3-11Þ

This equation implies that Rb becomes larger at higher Ta, because DT(r) is a decaying function. At the next step, the authors obtained a simple function that can express simulated temperature distributions during laser irradiation at 250 kHz. They found that the simulated temperature distributions can be ﬁt with a simple function [35,115]. A DTðrÞ¼ 2 ð3-12Þ ðr R0Þ þ B where the parameters A, R0 and B are just for expressing temperature distributions. By substituting Eq. (3-14) into Eq. (3-13), the following equation can be obtained:

= A 1 2 RbðTaÞ¼R0 þ B : ð3-13Þ Tout Ta

This equation expresses the dependence of Rb on the ambient temperature Ta. Therefore, at the final step, the authors fit the plots in Fig. 3-8(a) with Eq. (3-13) to obtain A, B, R0 and Tout. The fits are shown as solid curves in Fig. 3-8(a). Because the parameters A, B, R0 and Tout can be obtained by the fitting procedure, one can determine the temperature distribution during laser irradiation and the threshold temperature (Tout). The temperature distributions in Fig. 3-8(b) gradually becomes larger for longer exposures but surprisingly, for exposures greater than 1 s, the temperature profiles are nearly identical. The subtle difference in the temperature distributions at 1 s and 10 s exposure times seems curious, because the thermal modification of the 10 s exposure was larger than that of the 1 s exposure. The difference in the modification volumes by similar temperature distributions can be explained by the exposure time-dependence of Tout: Tout is 580 °C at 1 s and decreases with increasing exposure time (Fig. 3-8(c)).

The variation in Tout with exposure time was explained as due to the viscoelastic modiﬁcation of glass at high temperature under pressure loading. Further insight into this effect will be given in Section 3.2.3.

3.2.2.2. Method by comparison of the simulation with the observation. Miyamoto et al. observed the cross sections of laser- induced modiﬁcations lines, formed by translating the glass sample perpendicular to the incident laser beam [118]. Fig. 3-9(a) shows optical microscope images of the cross sections of modiﬁcation lines inside a borosilicate glass written with T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 83

Fig. 3-8. The analysis of thermal modifications that had been induced by focusing 2.0 lJ and 80 fs laser pulses inside a sodalime glass with a 20 objective lens. (a) Radius of thermal modification (Rb) as a function of the ambient temperature (Ta). (b) The temperature distributions at different irradiation times is obtained by fitting of the plots in (a). (c) Tout at different exposure time (open circles). The shaded rectangles indicate the temperature range at which the percentage of the visco-elastic relaxation is between 1% and 99%.

10 ps laser pulses (1 MHz, 0.5 mJ/pulse, NA = 0.65) at different scanning velocities. The modiﬁcation became larger at slower speeds, which corresponds to a longer dwell time (more incident pulses per spot size). The structures were formed upstream of the geometrical focus, which can be explained by the enhanced absorptivity due to the high temperatures driven by heat accumulation [116,118]. The simulated temperature distributions were compared with the observed cross-sections of the modiﬁcations. The heat source was expressed as follows [118]: 2wðzÞ 2r2 qðr; zÞ¼ exp ; 0 6 z 6 l ð3-14Þ px2ðzÞf x2ðzÞ

Fig. 3-9. (a) Optical microscope images of the cross sections of thermal modification lines by 10 ps laser irradiation at 1 MHz at different scanning velocities. (b) The area of the heat source in the simulation (left) and the shape of the average intensity distribution w(z) (right). (c) Simulated isothermal lines of Tin (red) and Tout (blue) to reproduce the structural boundaries in (a). These isothermal lines were simulated by choosing the parameters in w(z). (d) The cross section of the modification line written by focusing the laser at the interface between two Schott D263 glass plates. 84 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u 2 u 2 2 t M kz ; M k xðzÞ¼x0 1 þ 2 x0 ¼ ð3-15Þ px0ng pNA where q(r, z) is the heat distribution from each laser pulse, w(z) is the time-averaged laser power integrated in (x, y) plane, f is the repetition rate, x(z) is the waist radius, l is the vertical length of the interaction region, M2 is the beam quality factor, k is the laser wavelength and ng is the refractive index of the glass. Because the thermal modification occurred above the focus, they expressed w(z) as: wðzÞ¼azm þ b; 0 6 z 6 l ð3-16Þ This equation implies that the absorbed laser power decreases as the beam approaches the focus. The typical shape of w (z) is shown in Fig. 3-9(b). Fig. 3-9(c) shows the isothermal lines that fit the boundaries of the observed modifications. The red and blue lines in the

ﬁgure represent the threshold temperature of the inner boundaries (Tin) and that of the outermost boundaries (Tout), respectively. The blue lines corresponds to Tout in the previous Section 3.2.2.1. The authors determined Tout by femtosecond laser welding two borosilicate glass plates, as shown in Fig. 3-9(d). Based on the disappearance of the interface in the modiﬁed region, they concluded that the temperature at the outermost boundary should be equal to the forming temperature of 4 the glass, at which the viscosity is 10 dPas. In Schott D263 glass, this corresponds to a temperature of Tout = 1051 °C. The temperature is higher than the one proposed by Shimizu et al. at which the viscosity is 107–1012 dPas and this discrepancy will be discussed later. Miyamoto et al. also determined the temperature at the inner boundary (red lines in Fig. 3-9(c)), which corresponded to

Tin = 3500 °C [116,118]. They proposed that this high temperature generated thermally excited free electrons which became seeds for avalanche ionization. This proposal was consistent with the measurement of the light absorptivity which increased with repetition rate [118]. The measurement of the absorptivity will be described in the Sections 3 and 4.

3.2.2.3. Method by micro-Raman measurement. In Section 3.1.2, the measurement of the temperature distribution by microRa- man was described. By a similar method, Hashimoto et al. measured the temperature distribution around the photoexcitation during heat accumulation by 1 MHz femtosecond laser irradiation inside a borofloat glass (Schott B33) [134]. The principle of the measurement is essentially the same as that described in Section 3.1.2, but here the Raman spectra were recorded while the sample was scanned perpendicular to the laser propagation direction. During laser irradiation and heat accumulation, a bright light emission near the photoexcited region prevented the temperature measurement. The measurement of Stokes and anti-Stokes lines was possible in the region 6 mm away from the center of the photoexcited region (Fig. 3- 10(a)). The measured temperatures in the outer region (the region between dotted yellow and red lines in Fig. 3-10(b)) were fitted by the theoretically calculated temperature distributions, while those in the inner region (the region inside dotted red lines in Fig. 3-10(b)) were higher than the theoretically calculated temperatures. From the measurement, the temperature at the outermost boundary of the thermal modification from the Raman measurement was found to be Tout = 820 °C, which is close to the softening temperature. This temperature is higher than that measured by Shimizu et al. but lower than the value predicted by Miyamoto et al. In the present Raman measurement, the glass was translated at 10 mm/s, while the glass was held stationary in the experiment by Shimizu et al. Therefore, the heating time in the Raman measurement was shorter than that of Shimizu et al. and since the threshold temperature increases for shorter heating times, this could explain the discrepancy. This point will be discussed in the next section in detail based on the mechanism of the thermal modification in the outer region.

3.2.3. Mechanism of thermal modification inside glasses Within the inner boundary of thermal modifications, the flow of glass melt is observed during laser irradiation, and elemental distribution changes occur. Therefore, the viscosity of the glass during laser irradiation must be low enough for ions to diffuse inside the inner region. The mechanism of the generation of the outer region in thermal modifications is more dif- ficult to comprehend, as no elementary distribution changes or changes in the Raman spectra are observed. In this section, a mechanism for the modification in the outer region is proposed, which also provides insight into the different threshold temperatures found in the literature. Shimizu et al. found that the threshold temperature depended on the exposure time of laser pulses (Fig. 3-8), and the dependence can be explained by the visco-elastic deformation of a glass [49,50]. During laser irradiation, thermal expansion occurs in the heated region, and the thermal stress loads the outer region because of the temperature gradient. Therefore, the deformation in the outer region should be modeled as a deformation of a glass under heating and stress loading. The viscoelastic deformation of a heated glass under high stress can be expressed by the Voigt-Kelvin element which includes vis- cous and elastic deformations [128]. Based on the model, the time-dependent strain, e(t), under a stress S can be expressed by eðtÞ¼ðS=2GÞ½1 expðGt=gÞ ð3-17Þ where G and g are the shear modulus and viscosity, respectively. Because the viscosity of the glass decreases drastically with increasing temperature (Table 3-1), the amount of deformation during laser irradiation depends largely on the temperature. They expressed how much relaxation of visco-elastic deformation occurred during laser exposure by a relaxation ratio: T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 85

Fig. 3-10. (a) Raman spectra at different displacements from the center of the photoexcited region during laser irradiation at 1 MHz. (b) Temperature distributions obtained by analysis of Raman spectra at different positions and different times after photoexcitation. The image shown below is an optical microscope image of the cross section of a modiﬁcation. The dotted red and yellow lines between the temperature distributions and the image indicate the boundaries of the inner and outer regions, respectively.

Table 3-1 Viscosities, time constants of visco-elastic deformation, and temperature of some glasses at reference points of three glasses (Schott B-270, D 263 and D33). The time constants were calculated by the viscosities, typical shear modulus (10 GPa) and Eq. (3-21). The temperatures with * were determined by interpolation using other temperatures. The temperatures were obtained from the Schott web site.

log10(g [Pa s]) sve Temperature of Schott B-270 Temperature of Schott D263 Temperature of Schott D33 Strain point 13.5 3160 s 511 °C 529 °C 518 °C Annealing point 12.0 100 s 541 °C 557 °C 560 °C Softening point 6.6 4 104 s 724 °C 736 °C 820 °C – 5.0 1 105 s *827 °C *838 °C *968 °C Flow point 4.0 1 106 s *915 °C *928 °C *1096 °C Working point 3.0 1 107 s 1033 °C 1051 °C 1270 °C

Z tex Prelax ¼ 1 exp fGt=g½Tðt; rÞgdt ð3-18Þ 0

where T(t,r) is the time-dependent temperature distribution and tex is the laser exposure time. The relaxation ratio was calculated at various temperatures using the temperature dependence of the viscosity of a sodalime glass. Fig. 3-11(a) shows the application of Voigt–Kelvin model to calculate the relaxation ratio of viscoelastic deformation under a stress loading. The plot of the relaxation ratio against the temperature just after laser exposure is shown in Fig. 3-11(b), where Prelax = 100% means that the relaxation of visco-elastic deformation has completed during the laser exposure time. The simulation of visco-elastic deformation showed that 99% of stress relaxation occurs at 610 °C in heated glass during a 1 s laser exposure time, while only 1% of stress is relieved at 540 °C. The temperature range between 540 °C and 610 °C overlaps with the glass transition temperature of soda lime glass (560 °C). The transition temperature range and the experimentally determined threshold temperature are plotted against the exposure time in Fig. 3-8(c), revealing that experimentally determined threshold temperature falls within the transition temperature range predicted by the visco- elastic model. This implies that visco-elastic relaxation had completed inside the outermost boundary during laser exposure, while the visco-elastic relaxation outside the outermost boundary was too small to observe. The different relaxation ratio inside (>99%) and outside of (<1%) the outermost boundary could generate large stress in the outer region, which can be detected using a polarization microscope. The birefringence distribution in the thermal modification in Fig. 3-11(c) shows that large birefringence appeared only in the outer modified region. Because the birefringence originates mainly from stress, this birefringence distribution suggests that larger stress is generated in the outer modified region after laser irradiation. Because the rapid cooling after the laser irradiation prevents further visco-elastic deformation, the frozen visco-elastic deformation gives rise to a larger stress in the outer modified region. 86 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 3-11. (a) Application of Voigt–Kelvin model to calculate the relaxation ratio of viscoelastic deformation under a stress loading. (b) Prelax plotted against the temperature at different laser exposure times. (c) Birefringence distributions in the thermal modiﬁcation. The top image shows the slow axis distribution and the bottom one is the retardance distribution.

3.2.4. Interpretation of different Tout by the viscoelastic model The viscoelastic relaxation can account for different threshold temperatures for varied heating times because the time constant of viscoelastic deformation of heated glass depends on the viscosity:

sve ¼ gðTÞ=G; ð3-19Þ Table 3-1 gives the time constants at different viscosities using a shear modulus of 10 GPa. The threshold temperature measured by Hashimoto et al. using micro Raman measurement, which has been described in the Sections 3.1.2 and 3.2.2.3 [134] can be explained by this table. They used Schott D33 and measured a threshold temperature of 820 °C, equal to the softening point of the glass. The heating time in the Raman measurement was 0.2 ms, calculated by the effective number of pulses (200) for a scan speed of 10 mm/s at 1 MHz repetition rate. According to Table 3-1, the time constant of the visco-elastic deformation at the softening point is 0.4 ms, which is comparable to the heating time in the Raman measurement. Therefore, the higher threshold temperatures by the Raman measurement compared to other studies can be explained by the visco-elastic model. In the experiment by Miyamoto et al., thermal modiﬁcations were induced by translating Schott D263 glass at 20 mm/s during laser irradiation [117], so the heating time was 0.1 ms. According to the time constant of the visco-elastic deformation in Table 3-1, the threshold temperature is between 736 °C and 838 °C. However, the threshold temperature reported by

Miyamoto et al. was Tout = 1051 °C. One possible reason for this discrepancy may be their deﬁnition of Tout, which they assumed to be at the working temperature (g =103 Pa s), when the interface between two glasses after laser welding was no longer visible after laser irradiation (Fig. 3-9(d)). However, there is no evidence that welding of two glasses needs to be heated above the working temperature. The working point is the viscosity at which a mass of glass is delivered to a machine to be worked on, and the softening point is the minimum viscosity that it will deform under its own weight [135]. Therefore, it is possible that two glasses can be welded for temperatures below the working point. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 87

Miyamoto et al. examined the validity of the threshold temperature by evaluating the thermal energies by photoexcitation using Tout = 1051 °C and comparing them with the measured light absorptivities at various irradiation conditions. They found that the evaluated thermal energies can explain the measured light absorptivities with a maximum uncertainty of ±3% over a wide range of laser parameters [116,118]. This agreement seems to support the validity of their deﬁnition of Tout. However, the temperature dependent heat capacity was not taken into consideration in the evaluation of the thermal energy. The heat capacity of a glass becomes more than 1.3 times larger above the glass transition temperature compared to its room temperature value. Therefore, if the temperature dependent heat capacity is taken into consideration in their evaluation, Tout will be as low as the softening temperature. A thorough understanding of the thermal modiﬁcation in the outer region is important for application of ion migration inside glass for the following reasons: Firstly, the temperature in the region of elemental distribution can be estimated from the determination of the temperature at the outermost boundary. Secondly, one can predict that elemental distributions sel- dom occur between the inner and outermost boundaries, because the viscosity in the outer region is not suitably low to per- mit ion migration during the laser exposure time. Thirdly, one must have knowledge of the stress due to viscoelastic relaxation at the outermost boundary to prevent crack generation at the boundary.

3.3. Absorptivity and photoexcitation mechanism

The absorption during bulk femtosecond laser irradiation inside various glasses has been measured by simple transmission loss measurements [116–118,136,137]. Miyamoto et al. measured the absorptivities as a function of repetition rate and pulse energy (Fig. 3-12(a)) by placing a power meter beneath the glass [116,118]. They found that the absorptivity increased at higher repetition rates, similar to a study by Eaton et al. [5]. Because the temperature in the photoexcited region increases at higher repetition rate due to heat accumulation, the enhancement of absorptivity at higher repetition rate was attributed to the linear absorption by thermally excited electrons. Miyamoto et al. [116] also evaluated the temperature distributions by comparing the simulated temperature distribution and the shape of the thermal modification (Fig. 3-13(a)), and calculated the thermally excited electron density (Fig. 3-13(b)) under the assumption that the energy distribution of electrons follows a Maxwell-Boltzmann profile. Their simulation includes the intensity distribution of focused laser pulse, and light absorption by thermally excited electrons and electrons in the valence bands, and thermal diffusion inside a borosilicate glass. The thermally excited electron density (free electron density) increases with increasing repetition rate and nearly saturates above 400 kHz. According to the calculated high free electron density at a high repetition rate, they concluded that avalanche ionization dominates because laser pulses are linearly absorbed quite efficiently by electrons already present in the conduction band. The enhancement in the absorptivity during HRR femtosecond laser is also possible using double femtosecond laser pulses. Sugioka et al. showed that the thermal modification becomes larger by double-pulse fs laser irradiation, in which two pulses with a time delay of 1 ps were focused inside glass [136–138]. Fig. 3-14 shows the diameter of the thermal modification versus the delay time between two pulses. In the double-pulse laser irradiation, the pulse width was 360 fs, the pulse energy of the two pulses was 0.8 mJ, and the repetition rate was 200 kHz. The diameter by single-pulse irradiation with the same total energy (1.6 mJ) is also shown. Clearly, the thermal modification by double-pulse irradiation is larger than

Fig. 3-12. Absorptivities measured by a transmission loss measurement using a power meter. (a) The dependence of the nonlinear absorptivity on repetition rate and pulse energy. 10 ps laser pulses were focused inside a Schott B263 glass with a 0.55-NA objective. During the measurement, the glass was scanned at 20 mm/s. (b) Absorptivity measured as a function of pulse energy for sodalime glass. 70 fs laser pulses were focused at 250 kHz for 1 s with a 0.45NA objective. 88 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 3-13. (a) Simulated temperatures just before laser irradiation during successive laser irradiation at 1 MHz as a function of repetition rate. The temperatures were measured at two different points on the beam axis. z is the position along the beam axis and the origin is the geometrical focus of the excitation laser pulse. l is the length of the absorbed region along the beam axis. In this simulation l was 72 lm. (b) Thermally excited free electron densities calculated from the temperature in (a).

Fig. 3-14. Heat accumulation by double-pulse fs laser irradiation. (a) The diameter of the thermal modification plotted against the delay time of the second pulse relative to the first pulse (first pulse p-polarization, second pulse s-polarization). (b) The absorption of the second pulse in the double-pulse irradiation plotted against the delay time.

those by single pulse irradiation. The largest modification occurred for a delay time of 12.5 ps and decreased significantly for delays between 15 and 30 ps. The dependence of the diameter on the delay time was also studied by measuring the absorptivity. The authors observed the absorptivity of the second pulse in double-pulse laser irradiation with the energy of the first pulse held at 0.8 mJ and that of the second pulse varied. Fig. 3-14(b) shows the absorption of the second pulse plotted against the delay time. The absorption of the second pulse has a similar dependence on the delay time as the diameter of the thermal modification. Because the absorption of the second pulse was maximum at 12.5 ps, which is longer than the lifetime of the photoexcited electrons, they attributed the enhancement of the absorption by double-pulse irradiation to the creation of self-trapped excitons. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 89

Fig. 3-15. EPMA mapping perpendicular to femtosecond laser irradiation at 200 kHz repetition in (a) 67SiO2-7CaO-9K2O-9Na2O-4ZnO glass and (b) GeO-

Na2O glass. (c) Schematic of the tendency of femtosecond laser induced ion migration (perpendicular to laser irradiation direction) inside a silicate glass, as proposed by Kanehira et al.

Fig. 3-16. (a)–(c) Optical microscope images (OM) of the modiﬁcations and transverse EPMA mappings of Ca, Si and O after 250 kHz fs laser irradiation inside CaO-SiO2 glass. In (b) and (c), 250 kHz laser pulses were focused at two points simultaneously. The red arrows in the OM images indicate the boundaries of the molten regions. (d)–(f) Temperature distributions for the simulation of the thermo-diffusion, and the simulated distributions of CaO and

SiO2 after 10 ms of irradiation.

3.4. Trends of ion migration observed perpendicular to the incident laser

According to the evaluated temperature distribution, the viscosity of the glass inside the inner boundary is low enough for elements to migrate to form different elemental distributions during laser irradiation. We refer to the region inside the inner boundary as the ‘‘molten region”. The next question is what determines the direction of ion migration and which elements migrate in the molten region. The elements in glasses can be classified roughly into three groups: glass network formers, glass network modifier, and intermediates [139]. According to Sun’s single bond strength criterion for oxide glasses, the oxide with larger single bond strength is the network former while the network modifier has a weaker single bond strength

[140]. Intermediates have bond strengths in between those of formers and modifiers [140]. For example, in the SiO2-Al2O3- CaO-Na2O glass system, Si is the glass network former, CaO and Na2O are modifiers, and Al2O3 is the intermediate. When the elemental distributions inside silicate glasses are observed perpendicular to the incident laser direction, the network former, Si, is always concentrated in the central region of the molten region, and modifiers, such as Ca and Na, tend to be concentrated at the periphery of the molten region (Fig. 3-15(a)). In glass containing GeO-Na2O, Ge is concentrated in the center while Na diffuses out of the molten region (Fig. 3-15(b)). Based on this tendency, Kanehira et al. proposed, in the first detailed investigation of ion migration [40], that the network formers of a glass tend to migrate in the center of the molten region while the modifiers tend to migrate out of the central region (Fig. 3-15(c)). Comparing this tendency of the ion migration with the temperature distribution during heat accumulation, it can be expected that the network formers should migrate to the higher temperature region while the modifiers should migrate to the lower temperature region. The diffusion of elements driven by a temperature gradient is known as the Soret effect [141–143]. Because the temperature gradient is as large as 100 K lm1 in the region of ion migration (for example, Fig. 3-8) there must be a strong Soret effect there. In silicate glasses, it has been found that the network former, SiO2, migrates to the higher temperature region, but the migration directions of modifiers and intermediates depend on the composition and kind of element [143]. 90 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Therefore, the investigation of the Soret effect on the ion migration in glass during heat accumulation is crucial to understand the resulting modiﬁcation. Shimizu et al. simulated the ion migration inside glass during HRR femtosecond laser irradiation by solving the diffusion equation numerically [144,145]. They selected a simple glass, which has only two oxides, CaO and

SiO2, and compared the distributions of Ca, Si and O after femtosecond laser irradiation and those according to the thermo- diffusion equation. Fig. 3-16 shows the elemental distributions after femtosecond laser irradiation at 250 kHz for both single spot (Fig. 3-16 (a)) and double-spot (Fig. 3-16(b) and (c)) irradiation. For all cases, the elemental distribution change occurred in the molten region. For single spot irradiation, Ca was concentrated near the periphery of the molten region, while the Si concentration became higher in the center. For the double spot irradiation in Fig. 3-16(b), the higher concentration regions of Si were tear- drop shaped, and the Ca concentration did not increase between the two inner modiﬁcations. When the space between the two spots was reduced as shown in Fig. 3-16(c), the two molten regions were connected during laser irradiation, but the Si concentration between the molten regions was lower and the Ca concentration increased near the boundary of the connected molten region.

In the numerical simulation, Shimizu et al. assumed that there are only two diffusion species, CaO and SiO2 and that their self-diffusion coefficients are equivalent to those of Ca and Si. The diffusion of CaO and SiO2 in the temperature distribution were based on the fluxes of the species, which depend on the concentration distribution and temperature gradient [144,145]: DCaQ Ca ; v ; JCa ¼DCar rTðx yÞþ CCaðx yÞð3-20Þ RT2 DSiQ Si ; v ; JSi ¼DSir rTðx yÞþ CSiðx yÞð3-21Þ RT2 where Ji is the flux of CaO and SiO2 (i = Ca or Si), Ci(x, y) is the molar concentration, Di is the self-diffusion coefficient, Qi is the heat of transport, R is the gas constant, v is the mass flow velocity, and T(x, y) is the temperature distribution. From these equations and the equation of continuity, the time-derivative of CaO concentration under constant volume is [144,145]: @ @ CCa CCa DSiQ Si DCaQ Ca ¼ r fð1 XCaCCaÞDCa þ XCaCCaDCag r CCað1 XCaÞ rT ð3-22Þ @t @x RT2 where Xi the partial molar volume. This equation shows that the ion migration is driven by the gradient of the temperature distribution.

To simulate the observed distributions of CaO and SiO2, the diffusion equations were solved under the temperature distributions shown in Fig. 3-16(d)–(f). The simulated concentration distributions of CaO and SiO2 after 10 ms of irradiation are

Fig. 3-17. The experimental data that show different ion migrations with and without mixed alkali effect. They are EPMA mapping of elements after fs laser irradiation at 250 kHz inside three calcium silicate glass (KCS, CCS and KCCS). KCS contains only K ions, CCS contains only Cs ions, and KCCS contains both ions. The distributions of Cs and Si depend on the co-existence of K ions in the glasses. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 91

shown in the lowest panel of Fig. 3-16(d)–(f). The simulated CaO and SiO2 distributions reproduced the observed Electron Probe Microanalysis (EPMA) mapping of Ca and Si very well for both single and double spot exposures. From an atomic point of view, what determines the direction of ion migration? The authors speculated the factor to determine the direction of the diffusion from Eq. (3-22) [145]. The diffusion direction of CaO by the temperature gradient is determined by the sign of DSiQSi - DCaQCa. Because CaO diffused to the lower temperature regions (Fig. 3-16(a)–(c)), the sign of DSiQSi -DCaQCa must be negative and DCaQCa must be larger than DSiQSi. If it is assumed that QSi is positive and comparable to QCa, DCa must be larger than DSi, which implies that CaO can diffuse faster than SiO2 in the glass melt. The larger diffusion coefficient of CaO means that the bond strength of Ca with oxygen ion is weaker than that of Si. Therefore, the direction of ion migration should be determined by the bond strength of the species with oxygen. Although the simulation of thermo-diffusion suggests that the ion migration could be predicted by the transient temperature distribution and Soret effect, not all the ion migration can be explained by them. As described in Section 4, the elemental distributions become complicated under a strong flow of glass melt, and the ion migration is no longer correlated with the transient temperature distributions. In addition, since the mobility of ions can change with the coexistence with other ions, ion migration depends on the composition of the glass. For example, in a silicate glass which contains cesium (Cs) but no potassium (K), Cs ions are distributed in the central region after fs laser irradiation. On the other hand, in a silicate glass with coexistence of Cs and K, Cs ions are distributed at the periphery of the molten region (Fig. 3-17) [107]. This could be due to a mixed-alkali effect; the self diffusivity of one alkali ion depends on the concentration of other alkali ions [128]. Therefore, to simulate the ion migration under strong melt flow or with a mixed-alkali effect, a more sophisticated molecular dynamics simulation to describe the motion of ions are needed. In this Section, we described the fundamentals of heat accumulation and ion migration by focused femtosecond laser pulses. We started with the observation of the temperature change for the case of single pulse bulk irradiation of glass. In this case, the Transient Lens (TrL) method can be applied to gain information on the refractive index dynamics due to the temperature change. The TrL signal intensity and its decay showed that the initial temperature change just after the photoexcitation was about 1800 K for standard fabrication conditions in borosilicate glass and the temperature decreased near the room temperature within 10 ls. The temperature increase and decrease imply that laser repetition rates greater than 100 kHz are needed to drive heat accumulation in this glass. These findings were in agreement with time-resolved lRaman spectroscopy, in which the intensity of the Stokes and anti-Stokes Raman bands were monitored to infer the temperature change within the laser- modified melt. Next, the effect of multi pulse irradiation was described. At high repetition rates (>100 kHz), the time between consecutive laser pulses is less than the time for heat from the absorbed laser pulses to diffuse out of the focal volume. As a con- sequence, there is a buildup of heat near the focus which produces a molten region which increases in size as more pulses are absorbed. In this situation, heat accumulation and thermal diffusion from individual pulses act in concert to produce the modification, which is typically larger than the focal volume and consists of a concentric shell-like structure with an inner and outer boundary. At low repetition rates, the laser-induced melt cools before the next pulse arrives and as a result, the modification is due to single pulse thermal diffusion, with a size comparable to that of the focal volume. To study heat accumulation during high repetition rate femtosecond laser irradiation, it is important to find the threshold temperature (Tout) at which the outermost boundary of the thermal modification is formed. Tout was determined from the radius of the thermal modification induced at different ambient temperatures. The determined Tout increased with laser exposure time, being equivalent to the glass transition temperature at 1 s exposure time. The increase in Tout with exposure time was explained as due to the viscoelastic modification of glass at high temperature under pressure loading. The simulation of visco-elastic deformation showed that 99% of stress relaxation occurs at 610 °C in heated glass during a 1 s laser exposure time, while only 1% of stress is relieved at 540 °C. The temperature range between 540 °C and 610 °C overlaps with the glass transition temperature of soda lime glass (560 °C) and the experimentally determined threshold temperature. From the temperature distribution, the viscosity of the glass inside the inner boundary was found to be low enough for elements to migrate to form different elemental distributions during laser irradiation. When observed perpendicular to the laser direction, the glass formers segregate in the epicenter whereas the intermediate and the network modifiers segregate in outer concentric regions. This elemental redistribution can be partially explained from the transient temperature distribution and Soret effect but the situation becomes complex when there is a strong flow of glass melt at high repetition rates and net fluences, requiring more sophisticated simulations. The next Section will provide a more detailed description of ion migration during high repetition rate femtosecond laser irradiation, including trends in the axial direction.

4. High repetition rate laser irradiation

4.1. Observation of ion migration with high repetition rate lasers

In the heat accumulation regime, where the temperature can easily rise over the glass transition temperature and even melting points of the dielectric, the laws of transport are no longer completely governed by diffusion in solids or molten liquids. The discussion regarding the general theory and hypothesis can be read in Section 4.2. The ﬁrst proposal for the possibility of ion migration during high repetition rate femtosecond laser irradiation was put forward by Miura et al. [11].In 92 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

their work corresponding to a space selective growth of b-BaB2O4 SHG crystals in an amorphous environment (BaO-Al2O3- B2O3), they deduced that due to the burst of ionization at the focal spot, where the temperature exceeds Tg and crystallization (Tc), highly localized nucleation happens due to atomic diffusion and microstructure rearrangement. The experiment was carried out using an 800 nm Ti:Sapphire laser with 130 fs pulse width and 200 kHz repetition rate. The proof of elemental migration came out in a similar experiment using the same laser to produce LiNbO3 and BaTiO3 localized crystals in Li2O- Nb2O5-SiO2,Na2O-BaO-TiO2-SiO2 and BaO-TiO2-SiO2 bulk glasses from the same group [101]. In the latter paper they used EDX integrated to a SEM for elemental mapping. Fig. 4-1 shows the corresponding images of the ion migrations observed in

32.5Li2O-27.5Nb2O5-40SiO2 in which Si is enriched in the center, which is presumed to be migrated from its immediate radial perimeter where there is a reduction of Si. The vice versa was observed for Nb element. The white dotted line in

Fig. 4-1a shows the scanned distance corresponding to the x-axis of Fig. 4-1b. In the 47.5BaO-5Al2O3-47.5B2O3 glass sample, enrichment of Aluminium at the center which migrated from its radial perimeter and Barium was seen migrating in the opposite direction. In this paper, they have used this observation of ion migration to explain the onset of localized crystallization and con- clude that the thermal and chemical gradients generated by femtosecond laser irradiation is responsible for space selective crystallization. The studies followed by these ﬁrst reports were based on the observation and characterization of ion migration perpendicular to the laser beam irradiation and majority of the structures were circular in cross section owing to the isotropic radial heat transport. The work from Yonesaki [101] was revisited by Cao et al. [93,94]. Instead of a static exposure [101] they wrote scanned microstructures using a femtosecond laser with 1030 nm wavelength, 300 fs pulse width and 300 kHz repetition rate. The structures were characterized parallel to the laser beam irradiation as opposed to [101]. They also used more powerful and high resolution scanning transmission electron microscopy-high-angle annular dark-ﬁeld (STEM- HAADF) and TEM. STEM-HAADF characterization revealed a lamella like nano-structure with dark (25 nm) and bright zones (100 nm). The dark zones were rich in Si and the bright zones were rich in Nb with their contents varying inversely.

TEM further conﬁrmed this observation and additionally provided the information about the SiO2 amorphous phase separation and LiNiO3 crystalline phase. They concluded that although the initial idea was that LiNiO3 crystallization occurred with Si migrating outside, higher resolution characterization methods revealed that self-organized nanostructures composed of lamellas of SiO2 were observed with embedded LiNbO3 crystalline plates at the center of the laser-modiﬁed region.

He et al. also has studied the ion migration related to precipitation of Sr2TiSi2O8 phase in 33.3SrO-16.7TiO2-50SiO2 glass samples [146]. The structures were characterized in the plane parallel to the irradiation direction. The authors identiﬁed three zones with an amorphous phase in the middle of the inverted tear drop structure, with a circumferential crystallized region and a weakly crystallized region just outside the damage zone. In the EDX analysis, the central zone was divided into top and bottom halves, with the top half showing evidence of crystallization from the electron backscattered diffraction (EBSD) scans. The Ti concentration variation was almost constant throughout the EDX scans, whereas the Sr concentration increased in the circumferential crystallized region in the top and bottom halves. At the zones of Sr enrichment, Si was observed to migrate away. Solid evidence of cross migration between Si and Sr was seen only in the central lower half. In conclusion the authors inferred that the glass requires an increase of Sr but less Si content to promote the desired crystallization.

Recently a search for elemental redistribution in silicate glasses (33Li2O-33Nb2O5-34SiO2) was reported in waveguide structures written using a 1030 nm, 300 fs, 300 kHz laser source [80]. In this study they mapped the migration, using

Fig. 4-1. (a) SEM image showing the Z contrast proﬁle of the elements, white dots show the region probed to plot the graph, (b) graph showing the intensity distribution of the elements along the dotted line. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 93

EDX and WDS, both in transverse and longitudinal sections of the waveguides. However in both cases, they could not find evidence of ion elemental redistribution. An equally important application of HRR femtosecond lasers is based on their capability to produce waveguides for photonic devices like amplifiers, couplers, splitters, and sensors. In such cases it is crucial to understand the ion migration profile parallel to the laser beam irradiation since the wave front of the light that is propagating through the waveguide encounters such a profile. With such a motivation, several reports came out from the group of Denise Krol. Two of their reports [147,148] were aimed to precipitate metal or semiconductor nanocrystals embedded in a dielectric for photonic device applications. In both these cases, they used the same femtosecond laser, with repetition rates of 473 kHz and 1 MHz. The dielectric used for this work was a remelted and quenched Schott borosilicate glass containing CdSxSe1-x nanocrystals (OG570). The base glass contained B, Na, K, Zn and Si as the glass constituents. Before doing the laser inscription, a thermal treatment was performed to the commercial glass to eliminate the presence of semiconductor nanocrystals. This was the first work to rule out the Ludwig-Soret effect which results in separation of ions based on their respective mobilities and expects migration of network modifiers (Na, K, Zn) which are faster compared to network formers (Si). In the laser modified area, Na and K migrated towards the lower part of the heat accumulated zone whereas Zn migrated towards the heat diffused zone. Si in turn appears to migrate towards the upper part of the heat accumulated zone but due to the weak migration, the images have a low contrast. They hypothesized this result as a feedback loop of the composition dependent light absorption and energy distribution dependent diffusion. A more detailed and exhaustive study was published [149] where the same laser at 1 MHz repetition rate was applied to write waveguides on the same re-melted and quenched OG570 borosilicate glass. Quite similar to the report in 473 kHz repetition rate, the Na and K elements migrate away from the beam towards the bottom half of the heat accumulated zone, zinc migrates towards the upper half of the heat diffused zone while Si clearly gets segregated in the top half of the heat accumulated zone (Fig. 4-2). Direct formation of Bi nanoparticles inside bismuth germanate glass was demonstrated using a 1030 nm, 370 fs, 500 kHz laser [81]. The ion migration was characterized perpendicular to the laser irradiation direction inside a bismuth germanate glass and it was found that the central dark region was enriched with Bi with a cross migration of Germanium. The presence of bismuth nanoparticles was confirmed by TEM and the mechanism of elemental distribution was explained using [144].

A very similar previous study on 40Bi2O3-60GeO2 (wt%) glass was reported with elemental migration but no precipitation of nanoparticles was observed. A 800 nm, 120 fs, 250 kHz laser was used in this study with the lack of heat accumulation was predicted to be the reason for no precipitation. Instead the authors observed that the central dark region to be a Ge rich zone (low Bi) whereas the white concentric shell was rich in Bi (low Ge). PbS quantum dots were also reported recently to be produced by direct precipitation in borosilicate glasses using fs pulses and was presented as a feasibility study of a PbS quantum dot waveguide[82] . Troy et al. [150] reported ion migration in Zinc Phosphate (ZnP) and Magnesium Zinc Phosphate (MgZnP) glass waveguides. They used a 1030 nm, 750 fs, 1 MHz fiber laser for inscribing the waveguides. The waveguides showed a slightly irregular morphology along their longitudinal direction, but had cross sections that were in the shape of a tear drop. The elemental mapping of both ZnP and MgZnP revealed that Phosphorous migrate towards the tail of the tear drop whereas the network modifiers, Zn and Mg migrate towards the head. From the microscope images, it is quite clear that the Phospho- rous rich zone is the densified region and the modifier rich zone is the rarefied one. They report that in the network modifier rich zones there is a change towards a Q1 species from the Q2 as opposed to the phosphorus-rich regions, which exhibit a change towards Q2 species from the Q1. The superscript numbers represent the number of bridging oxygens attached to the phosphate. Even though no waveguiding was reported, their observations provided insight to the role of ion migration in the densification mechanism for laser written waveguides. The most distinctive results from these reports is that the monovalent ions (K, Na) prefers to migrate towards the negative index change zones and there is densification where heavy elements tend to segregate. Quite recently Zhang et al. [83] has put forward the idea of polarization dependent microstructures using high repetition rate femtosecond laser irradiation in glass. In this study they used a 1030 nm, 370 fs, 500 kHz femtosecond laser to write

Fig. 4-2. (a) Optical microscope image showing the waveguide cross section (the writing laser was incident from the bottom). The other images show the Z contrast of the waveguide (BSE) and the corresponding elemental migrations. 94 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

static structures inside a 64SiO2-17Al2O3-5B2O3-15CaO, (wt%) glass. 0.2–50 s long exposures were used to produce the structures and the structures were characterized by EPMA in the direction parallel to the laser irradiation direction (cross section of the structure). They found that there is a strong migration of network formers like Si4+,O2 and Al3+ in the central region of the structure, whereas the network modiﬁer Ca2+ cross migrates to the top and the bottom regions relative to the central region. They use this observation to discuss the formation of silicon nanocrystals in the central region. Due to the heat accumulation in the center, Ca2+ ions were driven off to the bottom, where glass network was strongly disrupted due to the arrival of new network modiﬁers. Si4+ ions in turn migrate to the central region to aggregate there and with proper temperature crystallization was triggered. They also try to put forward explanations in the formation of polarization dependent micro bubbles with respect to the ion migrations.

4.1.1. Ion migration in phosphate glass waveguides Until now, no work was dedicated to make use of the effect of ion migration in an optical waveguide, the backbone of photonic devices. Until recently, the optimization of femtosecond laser written waveguides was quite restricted, with the tuning performed by varying only the net ﬂuence. The ﬁrst time femtosecond laser written waveguide optimization was performed with an entirely different perspective was achieved by Fernandez et al. [41] by tuning the constituents in a phosphate glass, ultimately increasing the refractive index by one order magnitude above previous reports.

Among various commercial phosphate glass compositions, a set of glasses with and without La2O3 was identified and iso- lated to demonstrate the importance of optimizing the glass matrix composition for femtosecond laser writing. It was shown that the presence of La2O3 enables a large positive refractive index contrast (RIC) in the written structures. The responsible mechanism was identified as the migration of La to form a region of increased refractive index accompanied by the out- diffusion of K. The compositional changes unambiguously correlate to positive and negative refractive index modifications. Indeed, the refractive index changes observed via La migration were far beyond what can be attributed to changes in the glass structure. The first glass had a composition of 10 mol% La2O3 and 8 mol% of K2O (La-rich sample) and the second glass had only 0.4 mol% La2O3 (La-less glass). The Al2O3 content of both glasses is 10 mol%. Waveguides were inscribed using a fiber-based femtosecond laser amplifier operating at a wavelength of 1030 nm with a pulse width of 400 fs and a repetition rate of 500 kHz. The compositional map of the waveguides produced in the La-rich sample evidenced a quite homogeneous increase of La concentration (Fig. 4-3) in the high refractive index region that looks white in contrast in the secondary electrons SEM image due to Z-contrast. The observed increase of La is 25% relative to the La concentration in the pristine glass. This large enrichment in La is accompanied by the cross migration of K to the lower refractive index zone that increases its local concentration by a similar amount. For the observed relative increase of 25% in the La content, a Dn = 1.2 102 was estimated. In the La-less sample, there is a P depletion accompanied by the cross migration of Al which is enriched by 10% in the high refractive index zone but the Dn values observed were much smaller and can be attributed to changes in the glass matrix density rather than to compositional ones. The results show that apparently single valence ions (K+,Na+,3+) move in the opposite direction as multivalence ones (Si2+,4+,Zn2+) [148]. The tendency observed in the case of the phosphate glass is similar (single valence ions (K+,P+), multivalence ions (La3+,Al3+)). As described in Section 6.3, other factors such as thermal gradients, viscosity, and diffusion coefficients also play a role in defining the migration direction.

4.1.2. Ion migration in tellurite glass waveguides The ﬁrst femtosecond laser written waveguide ampliﬁer in tellurite glass was reported in 2010 [96]. After the discovery of ion migration in phosphate glass, the tellurite waveguide sample was characterized for ion migration, in search of a better perspective on the origin of the refractive index change. This was also motivated by the increasing interest of low-phonon energy glasses (tellurites, tellurides and chalcogenides) boosted by manufacturers in the mid-infrared optical device sector [151] for applications such as medical, environmental sensing/monitoring, military surveillance and high sensitivity gas detection. The preparation of the tellurite based glass (50TeO2:20P2O5:20Na2O:5ZnO:5ZnF2 with 1 wt% Er2O3, 1 wt% CeO2

Fig. 4-3. (a) Secondary SEM image of the waveguide fabricated in the La-rich phosphate glass. The yellow arrow corresponds to the scan distance in the (c) line scan, (b) EDX false color compositional mapping showing the distribution of La (red) and K (cyan) in the irradiated region. (c) and (d) line scans of combined ion migrations in La-rich and La-less glass. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 95

and 2 wt% Yb2O3) and the waveguide writing procedure is described elsewhere [72]. The waveguides were written using a femtosecond fiber laser (1-MHz rep. rate, 1040-nm wavelength, 400-fs pulse duration) with a high numerical aperture (1.4NA microscope objective. Fig. 4-4(a1),(b1) show two SEM secondary electron images of waveguides written with 2 and 4 mm/s scan speeds, respectively. Waveguide A (Fig. 4-4(a1)) shows a single positive and negative index change regions while waveguide B (Fig. 4-4 (b1)) has a positive index region between two depressed index, dark contrasted zones. As in the case of the phosphate glass, the higher refractive index zone is located in the region where the SEM images show a positive Z-contrast. The positive index change of both waveguides is around 3 103 [41] slightly higher for waveguide A due to the higher dose. The corresponding EDX compositional profiles for Te and Na are shown in Fig. 4-4(a) and (b). The correlation between EDX compositional profiles, SEM images and refractive index measurements evidences that the increase of the local concentration of Te is responsible for the refractive index increase (densification). On the contrary, Na moves out from the densified region, something similar to what has been above described regarding K and La in the case of phosphate glass. This clearly indicates that alkaline element migration plays an important role in the process, as will be further discussed in Section 4.1.3. This is the first time strong ion migration has been reported in a tellurite based glass to form optimized waveguides for optical amplification [152–154].

The migration of Te can be described using a structural model where two TeO4-bipyramid units decompose in two TeO3-pyramid units plus two non-bridging oxygen sites. This model is supported by micro-Raman measurements showing that Te-enrichment in the high index zones is related to the conversion of trigonal bipyramids to trigonal pyramids causing an increase of the packing fraction that aids densification. The migration of Na towards the low density region can be then explained in the framework of several models developed to explain ionic diffusion in glasses [155,156] where monovalent ions easily diffuse via defect assisted transport. Na+ occupies preferentially the non-bridging oxygen sites [155,156] due to the Te migration from its initial position to the densified one, leaving behind a high defect rich region. This picture is again consistent with a situation where monovalent ions (Na+) migrate towards the rarefied zone while multivalence ones (Te2+,4+,6+) move to the higher refractive index, densified region [41]. The term ‘valency’ is used here just to catalogue ions through their common valence values. The spatial distribution of the other elements of the glass composition (Zn, P, Er, Yb, Ce, F and O) shows minor changes with respect to the non-irradiated zone.

4.1.3. Dual regimes of ion migration The role of pulse energy in the local compositional changes induced in phosphate-based glasses has been analyzed in detail in [97] showing how element migration is strongly conditioned by the element atomic mass, with an activation energy threshold for individual elements in a multicomponent glass. For the experiment a phosphate glass with similar composition as the La-rich phosphate glass described in Section 4.1.1 was used to produce densified structures upon irradiation with pulse energies in the 520–700 nJ range (parameters similar to [41]). Illustrative SEM secondary electron images and EDX compositional cross sections along the laser incidence axis are shown in Fig. 4-5. The appearance of two different ions migration regimes can be clearly seen. Within regime-I (low energies <600 nJ), the comparison of element distribution and Z-contrast in the SEM images indicates that Al and Si migration define the densified regions while K+ suffers a large concentration increase (>80%) in the low density zone (Fig. 4-5(a)). Interest- ingly, when the pulse energy increases, (Fig. 4-5(b)) the K+ concentration increase is much smaller (30%). This is consistent with the expected higher defect density near threshold and the K+ migration enabled by point defects, as discussed above. On the other hand, in the low energy regime, heavy elements (La) also move, but without the well-defined trend that is observed at higher energies within regime-II (>600 nJ). In this case, light elements except K show little local compositional changes.

Fig. 4-4. Line scans showing strong ion migration in (a) waveguide A (2 mm/s), (b) waveguide B (4 mm/s) along with respective (a1) & (b1) secondary electron images. 96 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 4-5. Ion migration observed in two waveguides written at (a) 520 nJ (b, c) 700 nJ. Laser is incident from the left side of the page.

Fig. 4-6. Simulation of internal gain vs length showing the effect of ion migration. (IG – internal gain, Uni – Unidirectional pumping 210 mW, Bi- Bidirectional pumping, 420 mW, 0% – No ion migration, 24% – local concentration increase of Er3+).

The activation of heavy elements in regime-II would be consistent with the existence of an element-dependent activation energy, as mentioned earlier. The migration of rare earth ions (Fig. 4-5(c)), that are present in the glass composition (Er, Yb), was reported upon high repetition rate femtosecond laser writing [97]. The enrichment of Er3+ and Yb3+ in the guiding region can be considered a priori as a detrimental collateral effect of the waveguide formation mechanism in the case of pre-designed active photonic devices. In the same work, the impact of this effect in a real optical amplifier was analyzed in by assessing the performance of active waveguides with different Er3+ and Yb3+ content. The optical amplifier was configured using 2.1 cm-long waveguides with bidirectional pumping (420 mW) at 976 nm. As expected from the increase of Er3+ concentration (see Fig. 4-5(c)), the absorption, enhancement and internal gain strongly increased, the latter reaching a value of 9.4 dB. This value is in the range for the needs in miniature loss-less splitters, amplifiers or rare-earth-based sensors. It should be noted that the indicated waveguide length was not optimized. Fig. 4-6 shows simulations of the internal gain as a function of length for an Er3+ enrichment in the guiding region of 24% (the Yb3+ enrichment is similar, as shown in Fig. 4- 5(c)). The initial doping level of the bulk glass is 2 wt% Er3+ and 4 wt% Yb3+, which would show an optimal length of 3.6 cm for bidirectional pumping and a maximum internal gain of 10.5 dB. The Er3+ enriched waveguide would show a shorter optimal length (3 cm) and an internal gain of 12 dB. The rare earth (RE) enrichment has in this case a positive impact in terms of amplifier performance. In general however, RE ion-migration has to be carefully treated for device optimization purposes. If we consider for instance a 3.5 wt% Er3+ and 7 wt% Yb3+ phosphate glass [157], a similar enrichment in the guiding region would lead to an absorption of 0.85 dB/cm (equivalent to increase the absorption in 4–5 dB for a length change of 5 mm). The side effect in low phonon energy glass (tellurites, tellurides, chaclogenides) having poor energy transfer rate coefficient [158] can be even more severe, while in doubly or triply doped [159] ones for cascaded energy transfer, the RE migration effect might prove challenging in terms of stable amplifier or laser operation. As a result, RE-ion migration has to be carefully treated when producing active devices by femtosecond laser writing at HRR. An alternative would be working in regime-I, in which only light elements contribute to densification. The penalty in this case would be a lower index contrast, an acceptable compromise for active devices. Alternatively, as shown in Fig. 4-6, modelling of the expected impact of the RE enrichment can be used to optimize the device performance. The experimental quantification of active ion migration for each waveguide is given in Table 4-1 along with the observed 4 4 optical absorption at 1534 nm ( I15/2 ? I9/2), signal enhancement and internal gain due to this effect. The first row corresponds to a simulated value calculated from the spectroscopic parameters of the un-irradiated bulk sample. The optical amplification experiments were carried out for bidirectional pumping (420 mW) in 2 cm-long waveguides [97]. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 97

Table 4-1 The experimental quantiﬁcation of active ion migration for each waveguide along with the observed optical absorption at 1534 nm signal enhancement and internal gain.

Energy (nJ) Relative increase of Er3+ (%) Absorption (dB) for 2.1 cm Enhancement (dB) Internal gain (dB) – 0 9.2 17.2 8 610 8.6 9.9 15.4 5.5 700 21.8 11 18.8 7.8 730 23.25 11.3 19.5 8.2 760 23.85 11.4 19.7 8.3 790 24 12.4 21.8 9.4

Fig. 4-7. (a) Normalized on-axial laser intensity distribution using water (red curve) and 1-bromonaphthalene (black curve) as immersion liquids. Resulting modiﬁcations written at 250 kHz, 700 nJ with varied exposure times using (b) water and (c) 1-bromonaphthalene.

4.2. Controlling the directionality of ion migrations

Ion migration can be influenced by controlling the intensity distribution of the focal volume of the laser, defining the region in which energy is deposited inside the material. A simple and cost effective way to control the ion migration direction without use complex adaptive optics is by exploiting the spherical aberration introduced into the laser beam by the refractive index mismatch at the air/glass interface. An inversion of the asymmetric cross section of transversally laser- written structures was first demonstrated by Luo et al. [103] in a silicate glass with a composition of 60SiO2-20Al2O3- 20CaF2-12YbF3 (mol.%). They focused the 40 fs, 250 kHz laser pulses using an oil immersion objective (100, NA = 1.25) and achieved an inversion of the on-axial intensity distribution by using immersion liquids with two different refractive indices; water (n1 = 1.33) and 1-bromonaphthalene (n1 = 1.658), having values less than and greater than that of silicate glass (n2 = 1.508), respectively. The calculated axial intensity profiles are shown in Fig. 4-7(a). Both profiles are noticeably elongated by aberration. The actual intensity peaks lie either after or before the geometrical focal point with the accompanying tails pointing in opposite directions, which is the direction of the of pointed end of the tear drop shape [160]. Besides reporting the shape inversion witnessed in optical microscopy in Fig. 4-7(b) and (c) the authors also demonstrated an inversion in the ion migration distributions. In both cases it was found that Al and Si move towards the point end of the tear drop whereas Ca and Yb move in the opposite direction. They attribute this behavior to the laser-induced temperature gradient, driving the free ions away from the exposure center and leading to a new elemental distribution. The driving force for elemental redistribution was investigated by Fernandez et al. utilizing combined plasma emission microscopy and optical transmission microscopy carried out in-situ [100]. A long working distance 50, 0.42NA microscope objective lens and a 12 bit charge-coupled device (CCD) camera was installed along the transverse writing direction (y-axis) to enable side-view microscopy images of both plasma emission and optical transmission in the focal region. Images of the plasma emission were acquired while translating the sample along the y-axis with optimum waveguide writing conditions. The camera recorded a video of the plasma distribution approaching the focal plane and the sharpest image was chosen, corresponding to the in-focus plasma distribution. After fabrication the laser beam was blocked and the sample was illuminated from the y direction and scanned again through the focal region of the microscope, recording the trans-illumination image that was best in focus. The glass used and the writing parameters were the same as discussed in Section 4.1.1 [41]. A comparison between the plasma images and the waveguide morphology taken by microscopy are shown in Fig. 4-8.It can be seen that the plasma filament is located approximately in the center of the written structure, which has an inverted 98 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 4-8. Transverse view images of the laser-modiﬁed region (laser directed from the top) at a depth of 100 mm inside the glass sample. The upper row shows merged in-situ images composed of trans-illumination images (grayscale) and plasma emission images (false color scale). The bottom row displays ex-situ recorded DIC micrographs of the same structures after end polishing.

Fig. 4-9. (left) Secondary electron image of the cross section waveguide written at 730 nJ superimposed by the plasma distribution profile. Profiles of the relative changes in the concentration of La and K are shown on the right and compared with the intensity profile of the plasma distribution. tear-drop shape. The thermal accumulation due to the high repetition rate of the laser leads to this considerable increase of the modified volume, starting from the narrow plasma streak. This was the first time that a precise experimental determination and overlap of the initial plasma distribution and the resulting waveguide had been performed. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 99

Fig. 4-9 displays an image of the waveguide written at 730 nJ recorded with a scanning electron microscope and detecting secondary electrons. A strong Z contrast is observed between the guiding, non-guiding and surrounding regions, which is consistent with the enrichment of the high refractive index region with a heavy element and its depletion in the low refractive index zone. The plot in the middle of Fig. 4-9 shows the results of EDX measurements of the local concentration of K and La of this waveguide along the z-direction. It confirms local La enrichment to be responsible for the refractive index increase and Z contrast. The plot to the right of Fig. 4-9 shows the plasma intensity profile along the z-direction. Interestingly, a comparison of the different profiles reveals that the plasma distribution is not confined to or centered in the region of La enrichment but extends well into the region of La depletion and K enrichment. This in-situ imaging strategy was applied by Luo et al. [103] to study the inversion of ion distribution profiles by adjusting the spherical aberration. A coverslip-corrected objective with a numerical aperture 0.85 was used to write waveguides at a depth of 100 mm, the experimentally determined depth for which spherical aberration was minimized. Other waveguides were written at a depth of 50 mm, where the effective spherical aberration (SA) was negative and also at a depth of 300 mm, corresponding to positive SA [100]. The resulting waveguide structures are shown in Fig. 4-10, showing plasma emission, trans-illumination and Differential Interference Contrast (DIC) microscopy images. At d =50mm (negative SA),

Fig. 4-10. Transverse view images of the laser-modified region (laser incident from the top) at different depths inside the glass sample. Left column d =50 mm, middle column d = 100 mm, right column d = 300 mm, all written at 60 mm/s. The upper row shows plasma emission images, the middle row merged in- situ images composed of trans-illumination images (grayscale) and plasma emission images (false color scale) and the lower row displays ex-situ recorded DIC micrographs of the same structures after polishing. 100 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 the main plasma peak is accompanied by a slightly weaker peak located closer to the surface, which is consistent with an intensity distribution being affected by negative SA. These two peaks are accompanied by an intensity tail extending into the sample. Interestingly, both peaks are located in the index-depressed region, as opposed to the previous results in which they were observed in the index-increased regions. Allow the results show that negative SA can be exploited to invert the ion migration direction, they also illustrate that the process is more complex than presumed. At d = 100 mm (zero SA), the absence of spherical aberration leads to an intensity profile that is expected to be axially symmetric. While this is indeed observed in the plasma distribution, the distribution is unexpectedly elongated transversally. This effect might be attributed to non-linear propagation and absorption, yielding a rotated plasma profile with an inverted longitudinal/transversal aspect ratio compared to the case of negative SA. The DIC image of the waveguide written demon- strates that the rotated plasma distribution causes a rotation of the ion migration direction, leading to a rotated index distribution. The merged image reveals that the position of the plasma distribution lies not in the high index region but close to the interface between the regions of increased and depressed refractive index. In this Section, we presented results of femtosecond laser induced ion migration when producing passive and active optical waveguides in phosphate and tellurite glasses for photonic applications. It was found that heavy elements migrated towards the laser-densified zone. In the active-doped glasses, active ions could also migrate and affect the gain properties of the photonic devices. The insights gained by the ion migration studies were used to simulate and optimize the active device performance. Further, we showed how spherical aberration, pulse front tilt and an oil immersion environment could be used to tailor the direction of ion migration.

5. Application of ion migration

5.1. Space-selective phase separation

The equilibrium structure of a glass at a given temperature depends on the composition [128]. At a given temperature, some glasses will have a homogeneous network, while others show phase separation and have inhomogeneous glass networks. Even if two glasses have the same composition but different ratios, phase separation occurs in one glass but not in the other glass at the same temperature. This composition-ratio dependence of phase separation suggests that the phase separation inside a glass can be controlled space-selectively using ion migration. Based on the idea, Shimizu et al. utilized ion migrations by HRR laser irradiation inside a Na2O-SiO2 glass to induce phase separation space-selectively [161]. The metastable immiscibility diagram for Na2O-SiO2 system is shown in Fig. 5-1(a) [141]. This diagram indicates how the spatial distribution of the phases depends on the temperature and the concentration of Na2O. In the diagram, outside of the ‘‘immiscibility boundary”, the composition of the glass is spatially homogeneous, while inside the boundary, ‘‘immiscible region”, phase separation occurs. In the regions ‘‘A” and ‘‘C”, nucleate droplet phase separation occurs, but the droplet is composed of Na-poor glass in ‘‘A” while Na-rich glass in ‘‘C”. In the region of ‘‘B”, spinodal decomposition occurs and the composition of glass becomes spatially ﬂuctuating [128]. As a result, random networks of Na-rich and Na-poor glasses are formed in the region ‘‘C”. In the space-selective phase separation by Shimizu et al., they used a Na2O-SiO2 glass of Na2O concentration of 21–30 mol%, of which composition the glass is spatially homogeneous according to the diagram in Fig. 5-1(a). When the Na2O concentration is 5–12 mol%, spinodal decomposition occurs and random networks are formed [162,163] ((Fig. 5-1 (a)). Therefore, if the glass of 5–12 mol% Na2O can be formed locally by ion migration inside the glass by HRR femtosecond laser irradiation (Fig. 5-1(b)), the phase-separated microstructures can be formed space-selectively.

Fig. 5-1. (a) Metastable immiscibility diagram for Na2O-SiO2system. In the images of the glass structures, the dark region is Na-poor glass and bright region is Na-rich glass. (b) Concept of a space-selective phase separation using fs laser irradiation inside Na2O-SiO2 glass. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 101

Fig. 5-2. (a) SEM image of the cross section of a modification line inside a Na2OSiO2 glass (upper), and Raman spectra at different positions (lower). The cross section was exposed by polishing the glass. (b) SEM images of the modified region after annealing and subsequent HF etching. (c) SEM images of the modified region after annealing and subsequent HF etching after optimization of the composition of a glass, annealing temperature and etching time. These SEM images show that sub-micron structures were formed successfully in all the area of the modified region.

The optical microscope images of the polished surface of the glass after fs laser irradiation at 250 kHz are shown in

Fig. 5-2(a). Just after the laser irradiation, no phase separation occurred, but the changes of the Na2O concentration were observed by the micro-Raman measurement in the modified region. The Raman spectrum at the modified and unmodified regions means that the Na2O concentrations decreased from the original concentration. No phase separation in the modified region regardless of the decrease of Na2O concentration suggests that the irradiation time was not long enough for the glass melt to decompose to two phases. Therefore, the glass after the laser irradiation was annealed near the glass transition temperature for several hours to induce phase separation. The SEM images of the 21Na2O-79SiO2 glass after annealing at 575 °C for 3 h and subsequent HF etching are shown in Fig. 5-2(b). After the HF etching, only SiO2 rich glass remained in the phase separated region. In this case, the sub-microstructures were formed apart from the center of the molten region. The phase separation region suggests that the Na2O concentration in the immiscible composition region was not formed in the center of the molten region. The authors demonstrated that it is possible to produce sub-micro structures in the whole molten region by fs laser irradiation (Fig. 5-2(c)) after optimizing the composition of the Na2O-SiO2 glass. This study is the first demonstra- tion of control of space-selective sub-micro structures inside a glass using elemental distribution change by femtosecond laser irradiation. Micro-channels can be fabricated inside a glass by femtosecond laser irradiation and post-chemical etching [45].By applying space-selective phase separation in glasses by ion migration with HRR fs laser irradiation, micro-network structures can be formed in a micro-channel space-selectively, serving as particle selection filters.

5.2. Replacing the ion exchange method

Slit beam shaping has been known for more than a decade to be a simple, yet powerful technique in low-repetition rate fabrication of photonic structures with cross sections which can be controlled by selecting the appropriate slit width and numerical aperture of the lens [164–166]. Only recently has it emerged that slit shaping also has a strong role in high repetition rate laser fabrication, influencing the shape of the structure despite the strong effect of heat accumulation [41]. The influence of slit reshaping during HRR femtosecond laser fabrication has been investigated by recording the plasma distribution with and without a slit in true writing conditions [100]. The diameter of plasma distributions produced by slit-shaped beams was observed to be significantly narrower than expected from the calculated intensity profile. In fact, the diameters of plasma distributions with and without a slit are comparable. This observation is a clear indication for non-linear and/or thermal effects determining the effective volume in which the energy is deposited. Possible mechanisms involved in this respect are self-focusing [167] and/or thermal lensing [109,168]. The experiments were carried out using the same configuration and parameters as in Section 4.1.1. The longitudinal and transversal cross sections of the plasma distributions have been measured, where their aspect ratio is AR = dz/dx, and the area they enclose is A = p dz dx/4. The results are summarized in Fig. 5-3. For a circular Gaussian beam, both the aspect ratio and area increase strongly above a threshold energy (500 nJ). 102 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 5-3. Pulse energy dependence of the aspect ratio and area of the plasma distributions for a circular input beam (‘‘no slit”) and a slit-shaped beam (‘‘slit”).

Fig. 5-4. Effect of slit beam shaping and ion migration in tuning the V-number of the waveguides.

It was found that large areas or aspect ratios, the waveguides did not show single mode propagation. In contrast, for a slit- 2 shaped beam both the aspect ratio and area have low values (ARslit 3 and Aslit 5 mm ), which are much less sensitive to a variation in pulse energy compared to a focused circular Gaussian beam. This result is useful in producing waveguides with precisely controlled properties or to mimic the state-of-the-art ion- exchange technique [169]. The concentration of the ions inside the solute and/or ion exchange time is used to control the refractive index change for the ion exchange. In femtosecond laser micromachining, one can tune the ion migration, which T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 103

Fig. 5-5. Cross sectional view of structures formed inside a phosphate glass using 500 kHz repetition rate femtosecond laser. (A1) fabrication conditions 0.65NA, 245 mW, 60 mm/s to drive cross migration between Al (blue) and K (orange), (A2) writing parameters 0.65NA, 335 mW, 60 mm/s to yield cross migration of La (magenta) and K (green). (B1) 0.68NA objective, 260 mW, 80 mm/s at 100 mm depth. (B2) 0.85NA aberration corrected objective, 100 mW, 100 mm/s at 100 mm depth. (C1) and (C2) written with 0.68NA without (395 mW, 60 mm/s) and with slit (365 mW, 60 mm/s, 1.4 mm slit width), respectively. is responsible for the refractive index change, via the net ﬂuence. The role of the metal negative masks (fabricated by sput- tering deposition and selective etching) in ion exchange can be replaced by laser beam slit shaping to control the size of the waveguide. We have mimicked this technique and put to application by tuning the V-number of the waveguides to match the parameters to a Corning SMF-28 ﬁber [170] for telecommunication applications. Fig. 5-4 shows the overall picture of the propagated mode characteristics in waveguides written with different energy and slits. The single mode propagation was extended over a wider energy window offering a larger degree of freedom in waveguide writing.

5.3. Optical micropipette through plasma shaping

Optical micropipette [171] is a possible high impact application that could find its use in the fields of materials processing, semiconductor technology, and medicine. With the help of high repetition rate short pulse irradiation, which leads to heat accumulation rather than pure diffusion, laser written structure dimensions can be tightly controlled. When combined with complementary strategies such as slit beam shaping, inducing negative, zero or positive spherical aberration, a novel light based tool could be formed to shape the plasma within a transparent dielectric medium, giving unprecedented control over ion migrating mechanisms and the resulting refractive index profile. Fig. 5-5 shows a series of refractive index distributions created in a phosphate glass. By controlling the total fluence and focusing condition, the position of the positive refractive index change zone and the size of the waveguide can be tailored. Fig. 5-5(A1) shows the cross migration of Aluminum and Potassium when irradiated using a lower fluence. This scenario could be changed by activating the heavy elements using a much higher fluence so that the cross migration will be between Lanthanum and Potassium as shown in Fig. 5-5(A2) [97]. Fig. 5-5(B1) and (B2) shows how varying the spherical aberration can invert the position of the structure produced inside glass [100]. Fig. 5-5(C1) and (C2) shows the image of two waveguides fabricated with the same fluence without and with a slit, respectively [41,172]. Combining these effects, we propose that an optical micropipette could be designed to relocate/remap/transfer ions from an area of interest to another. A proper tuning of laser fluence, spherical aberration and slit shaping offers a good control of the type and direction of migrating ions. Three possible potential application scenarios are sketched in Fig. 5-6. The central blue layer is the area of interest which could be an implanted layer, a material sandwich or even micro/nano crystallites. In this region, the easily tailored laser plasma can be used like a micropipette, to either draw elements out or inject elements, offering unprecedented control over the properties of photonic circuits.

6. Modiﬁcation of ion migration with multiple spot irradiation

6.1. Modiﬁcation of ion migration

Femtosecond laser induced ion migration inside glass depends not only on the composition of the glass but also on the irradiation parameters since the transient temperature profile is influenced by the distribution of photoexcited electrons. There have been several experiments in which the elemental distributions by ion migration are modulated by varying the laser irradiation parameters [97,100,102,103,105,106,108]. In particular, the elemental distributions parallel to the beam propagation have been altered, because the intensity profile along the propagation axis is strongly affected by spherical aberration and nonlinear light propagation. Other than the net fluence and the spherical aberration discussed in Section 5.3, another exposure parameter which can influence the elemental distributions after femtosecond laser writing is the pulse front tilt, in which the arrival time of an ultrashort pulse varies across the beam profile. The strong effect of the pulse front tilt on fs laser induced modifications was discovered inside fused silica glass by Kazansky et al. [173,174]. They found that the femtosecond laser-induced modification depends on the relation between the pulse front tilt and the translation direction of the glass. Later, it was found that not only modification lines but also the positions of micro-bubbles and defects can be affected by a pulse front tilt [175]. The influence of the pulse front tilt on the ion migration was also observed by Fernandez et al. [100]. They found that 104 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 5-6. Three generic scenarios with plasma shaping by tuning the spherical aberration and/or slit shaping.

Fig. 6-1. (a) Principle of a holographic laser irradiation with a spatial light modulator (SLM). (b) The cycle of FT and IFT calculation in the iterative Fourier transform (IFT). (c) The intensity distribution change in the process by the optimal rotation angle (ORA) method. the highly concentrated region of elements contributing higher refractive index depended on the translation direction, and interpreted that the plasma distribution, which affects ion migration, could be affected by the pulse front tilt. However, a pulse front tilt is not an ideal parameter to control the ion migration, because it is rather difﬁcult to control. A more powerful method to control ion migration in femtosecond laser microfabrication is by using the holographic laser irradiation technique, in which multiple focal spots of arbitrary distributions can be generated [176].

6.2. Principle of a holographic laser irradiation

Using a holographic method, multiple light spots of arbitrary distribution can be generated simultaneously. The principle of this method is shown in Fig. 6-1(a), where a spatially phase-modulated laser beam is focused to generate multiple focused spots. The spatial phase distribution of a laser beam is modulated by a spatial light modulator (SLM)[177]. The electric ﬁeld of a laser beam on the focal plane of the lens is related to the spatial phase distribution just before the lens by the following relation: ZZ "#() x2 þ y2 ðX xÞ2 þðY yÞ2 EðX; YÞ/ jE ðMx; MyÞj exp iDuðMx; MyÞþip exp ip dxdy ð5-1Þ SLM kf kf

where (X, Y) and (x, y) are the coordinates at the focal plane and SLM plane, respectively. E(X, Y) and ESLM(x, y) are the electric fields on the focal plane and the SLM plane, respectively. M is the magnification of the telescope, k is the wavelength of the laser beam, and f is the focal length of the objective lens. By selecting an appropriate phase distribution, multiple light spots of arbitrary distribution can be generated on the focus. This phase distribution is often called a phase hologram. In holographic laser processing, two methods are often used in calculation of a phase hologram: the iterative Fourier transform T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 105 method [176,178] and the optimal rotation angle (ORA) method [179]. In the iterative Fourier transform method, Eq. (5-1) is expressed by the Fourier transform, and the Fourier transform and the inverse-Fourier transform are repeated with the sub- stitution of amplitude distributions by desired distributions until a satisfactory intensity distribution is obtained (Fig. 6-1 (b)). In the ORA method, the phase hologram is modified to increase the sum of the light amplitudes (S in Fig. 6-1(c)) at the desired positions pixel by pixel. Fig. 6-1(c) shows how repeated modification of the phase hologram can be done to produce a more clear image of the word ‘‘light”. The principles of these methods are described in detail in other papers [176,178,179].

6.3. Modiﬁcation of ion migration perpendicular to the laser propagation

In the method by Sakakura et al. HRR femtosecond laser pulses were focused at a single spot to induce local melting around the photoexcited region, and spatially modulated LRR fs laser pulses were focused at multiple spots around the molten region to alter the transient temperature distributions and the shape of the molten region [105]. The optical setup for parallel laser irradiation is shown in Fig. 6-2(a). The polarizations of 250 kHz and 1 kHz fs laser pulses were perpendicular to each other. These pulse trains were mixed by a polarization beam splitter and reflected on the Liquid crystal on Silicon (LCOS)-SLM. Because the SLM is active only to the horizontally polarized light, only the 1 kHz laser pulses were modulated in this setup. Therefore, the unmodulated 250 kHz laser pulses were focused at a single spot after focusing with an objective lens, while the phase-modulated 1 kHz laser pulses were focused at four spots after the objective lens as shown in Fig. 6-2 (b)–(d). To observe the elemental distribution changes perpendicular to the laser propagation direction, the glass plate was translated parallel to the beam axis at 2 mm/s to write a modification line. Fig. 6-3(a) and (b) show the microscope images of the modifications inside an alumino-borosilicate glass during and after the exposure of fs laser pulses. In Fig. 6-3(a), only 250 kHz laser pulses were focused at a single spot inside the glass. During laser exposure, strong emission was observed around the photoexcited region and the modifications of two clear boundaries were generated after the laser exposure. In Fig. 6-3(b), 250 kHz laser pulses and 1 kHz laser pulses were focused inside the glass at single spot and four spots, respectively. Bright emission occurred around every photoexcited region, and the shape of the modification had changed. The inner modification became square-shaped and the outer boundary remained circular. The resulting elemental distributions are shown in Fig. 6-3(c) and (d). The shapes of the distributions were also different between single spot irradiation and multiple spot irradiation. For example, the high concentration region of Ca was ring-shaped in the case of a single spot irradiation, but square-shaped in the case of multiple spot irradiation. Although the shapes of the elemental distributions were different, the general tendency of the ion migration was similar. The elemental distributions are illustrated schematically in Fig. 6-3(e). In both cases, Ca was concentrated at the periphery of the molten region, in which the flow of glass melt had been observed during laser irradiation, Al was concentrated just inside the Ca-rich region, and Si was concentrated in the central region. The tendency of the ion migration is follows: the glass network former moves to the higher temperature region, while the modifier moves to the lower temperature region. This trend can be explained by the Soret effect, in which the flow of elements is determined by the temperature gradient and the bond strength of an ion with oxygen [144,145]. In the case of alumino-silicate glass, the glass network former is SiO2, the modifier is CaO, and the intermediate species is Al2O3. Therefore, the distributions of these elements are SiO2,Al2O3 and CaO from the center to the outer region.

Fig. 6-2. (a) Optical setup of parallel irradiation by focused 250 kHz and 1 kHz laser pulses at multiple spots inside glass. (b) Schematic illustration of focusing of 250 kHz and 1 kHz laser pulses inside the sample. 250 kHz laser pulses were focused at a single spot, while 1 kHz laser pulses were focused at four spots. (c) A phase hologram used in the study and (d) the intensity distribution generated by the hologram on the focus plane. 106 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Fig. 6-3. (a), (b) Optical microscope images of the modiﬁed region inside an aluminosilicate glass during (left) and after (right) laser irradiation. (c), (d) EPMA mappings of elements after fs laser irradiation. (e) Schematic illustration of elemental distribution changes. (f) Thermal modiﬁcations by parallel laser irradiation with different distributions of 1 kHz photoexcited regions.

By changing the number of the focused spots and their positions, it is possible to tailor the shape of thermal modiﬁcations. Some examples are shown in Fig. 6-3(f). The triangle-shaped molten region can be generated by 250 kHz laser irradiation at the center and 1 kHz laser irradiation at three spots, and in a similar manner, a hexagonal molten region also can be generated. Therefore, multiple spots irradiated at different repetition rates is a powerful technique for the ﬂexible control of elemental distributions in glass.

6.4. Modiﬁcation of ion migration parallel to the laser propagation

The modification of ion migration parallel to the laser propagation is more complex due to the convex flow of glass melt affecting the migration along that direction. For the investigation of ion migration parallel to the laser propagation, a modification line was written perpendicular to incident laser direction [106]. The positions of photoexcited regions by 250 kHz and 1 kHz laser pulses are illustrated in Fig. 6-4(a). The modifications of elemental distributions have been examined inside a sodalime glass (Schott AG, B 270 Superwite), the main elements of which are Si, Ca, Na and O, by changing the difference between the vertical focus positions of 250 kHz and 1 kHz laser pulses, Dd. Fig. 6-4(b) shows the distributions of Si and Ca inside sodalime glass in the cross section of the modification line written by focused 250 kHz fs laser pulses. The concentration of Si was higher in the lower part of the modified region, while the concentration of Ca was higher at the upper rim of the modified region. However, when irradiation was performed with parallel 250 kHz and 1 kHz pulses at multiple spots, the distributions were significantly different (Fig. 6-4(c)). The distributions depended on the difference between the focal depths of 250 kHz and 1 kHz laser pulses (Dd). When the 1 kHz laser pulses were focused at a deeper position than the 250 kHz laser pulses (Dd 0 mm), the Si-rich region was located in the lower part

Fig. 6-4. (a) Distributions of photoexcited regions by 1 kHz and 250 kHz laser pulses. 250 kHz pulses were focused at the center, and 1 kHz pulses were focused at four spots. Dd is deﬁned as the difference between the vertical focus positions of 1 kHz and 250 kHz laser pulses. (b) EPMA mapping of Si and Ca in the cross section of the modiﬁcation line written inside a sodalime glass by focused 250 kHz fs laser pulses (c) EPMA mapping of Si and Ca parallel to the laser propagation direction after parallel irradiation with 1 kHz and 250 kHz laser pulses with varied Dd. T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 107

Fig. 6-5. (a), (b) Optical microscope images of the modiﬁed regions inside a sodalime glass during irradiation with 1 kHz and 250 kHz laser pulses. In (a), 1 kHz pulses were focused at the upper regions of the molten region (Dd = +15 lm), while in (b) at the lower regions (Dd = 15 lm). (c) Proposed model of ﬂow of glass melt and Si condensation during laser irradiation in the case of Dd = +15 lm.

Fig. 6-6. Optical microscope images during fs laser irradiation inside sodalime glass. (a) 250 kHz irradiation at a single spot, (b)–(d) 250 kHz irradiation at the center and 1 kHz irradiation at four spots. of the modification. At Dd =15mm, the Si-rich layer of about 10 mm thickness was formed in the upper part of the modified region. The distributions of Ca were always nearly opposite to that of Si. This result means that the position of the Si-rich region can be controlled by tuning the focus position of 1 kHz pulses relative to that of 250 kHz pulses. To discover the origin of the unique distribution of Si by parallel laser irradiation, the flow of glass melt was observed during laser irradiation [106]. The snap shots of the observed flow of glass melt at different Dd are shown in Fig. 6-5 (a) and (b). The vortex flow of glass melt was generated around the photoexcited region by 1 kHz laser pulses. At Dd = 15 mm, the glass melt flowed from the vicinity of the 250 kHz photoexcited region in the downward direction. The direction of the melt flow was opposite at Dd = +15 mm. Based on the observation of the glass melt flow, the authors explained the generation of a Si-rich region as follows: glass melts may be composed of Si-poor (or Ca-rich) melts (or clusters) and Si-rich (or Ca-poor) melts. Normally, the mobility of Si-rich melts is lower than that of Si-poor melts, because silicate glass 108 T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113

Table 7 Literature summary of femtosecond laser induced ion migration in dielectrics.

Composition Laser used (Wavelength, Type of Observed Ref. pulse width, rep. rate) structure migration

32.5Li2O-27.5Nb2O5-40SiO2 800 nm, 130 fs, 200 kHz Static Si, Nb [11,101] microstructure Crown glass S1111 (Matsunami Co. Ltd) 800 nm, 120 fs, 200 kHz Static Si, O, K, Na, [40] microstructure Ca, Zn QxErSpa100 and QXErSpa47 Kigre Phosphate glasses 1030 nm, 400 fs, 500 kHz Waveguide K, Na, P, Si, Zn, [41,97,100] Si, La, Al

33Li2O–33Nb2O5–34SiO2 (mol%) 1030 nm, 300 fs, 300 kHz Scanned Si, Nb [93,94] microstructure

50TeO2-20P2O5-20Na2O-5ZnO-5ZnF2 1040 nm, 400 fs, 1 MHz Waveguide Na, Te [96]

20Na2O–10CaO–70SiO2–4Eu2O3 (mol%) 800 nm, 80 fs, 250 kHz Static Si, O, Ca, Eu [99] microstructure

60SiO2-20Al2O3-20CaF2-10YbF3 (mol%) 800 nm, 150 fs, 250 kHz Scanned Si, O, Al, Ca, [102] microstructure Yb, F

60SiO2-20Al2O3-20CaF2-12YbF3 (mol%) 800 nm, 40 fs, 250 kHz Static Si, Al, Ca, Yb [103] microstructure

15Na2O–85GeO2 (mol%) 800 nm, 70 fs, 250 kHz Static Na, O, Ge [104] microstructure Schott AG, B 270 Superwite 800 nm, 80 fs, 250 kHz Static Si, O, Ca, Al [105,106,108] together with 120 fs, 1 Hz microstructure

20K2O-10CaO-70SiO2, 20Cs2O-10CaO-70SiO2 and 10K2O- 800 nm, 80 fs, 250 kHz Static Si, O, Ca, K, Cs [107]

10Cs2O-10CaO-70SiO2 microstructure

50CaO-50SiO2 (mol%) 800 nm, 80 fs, 250 kHz Static Si, Ca, O [144,145] microstructure

33.3SrO-16.7TiO2-50SiO2 1030 nm, 300 fs, 300 kHz Scanned Si, Ti, Sr [146] microstructure

33Li2O-33Nb2O5-34SiO2 1030 nm, 300 fs, 300 kHz Waveguide No observed [80] migration Schott borosilicate glass (OG570) 1030 nm, 750 fs, 473 kHz & 1 Waveguide Si, Na, K, Zn [147–149] MHz

10Bi2O3–90GeO2 and 40Bi2O3-60GeO2 (mol%) 1030 nm, 370 fs, 500 kHz Grating planes Bi, Ge, O [81] Lead chalcogenide glass doped with PbS QD 800 nm, 120 fs, 250 kHz Waveguide Si, Pb, S, Zn, B, [82] K, O

56ZnO-42P2O5-0.7Er2O3-1.3Yb2O3 and 28.0ZnO-28.0MgO- 1030 nm, 750 fs, 1 MHz Waveguide P, Zn, Mg [150]

42.0P2O5-0.7Er2O3-1.3Yb2O3 (mol%)

64SiO2-17Al2O3-5B2O3-15CaO 1030 nm, 370 fs, 500 kHz Static Si, Al, Ca, O [83] microstructure

10Na2O-90TeO2 doped with 2% La2O3 or 1% Al2O3 (mol%) 800 nm, 120 fs, 1 kHz Scanned Te, Na, La, Al [152] microstructure

xNa2O–(100-x)SiO2 x = 21% and 30% (mol%) 800 nm, 80 fs, 250 kHz Static Si, Na [161] microstructure

60SiO2–20Al2O3–20CaF2–3Bi2O3 (mol%) 800 nm, 150 fs, 250 kHz Static B, Ca, Al, O [84] microstructure

Fig. 7. Migrating elements observed in the perpendicular (a) and parallel direction (b) with respect to the incident focused high repetition rate femtosecond laser pulses. In both cases, ion migration only occurs inside the molten zone. In (a) the color-coded concentric circular zones are within the central molten region with the outer zone not shown. In (b), white and black regions are within the molten zone, with the outer zone indicated by grey. with higher Si concentration has higher viscosity. Therefore, when the glass melt flows near the boundary of the molten region, Si-rich melts would be trapped near the boundary because of the higher viscosity while Si-poor melts would keep flowing because of the lower viscosity (Fig. 6-5(c)). Therefore, the Si-poor melts would flow back to the central part of T.T. Fernandez et al. / Progress in Materials Science 94 (2018) 68–113 109 the molten region, while the Si-rich melts with a higher viscosity would be accumulated near the boundary. As a result, the Si-rich region would be generated near the boundary of the molten region and the 1 kHz photoexcited region.

6.5. Possible applications of ion migration with multiple spot irradiation

Parallel femtosecond laser irradiation at different repetition rates can be used to suppress the accumulation of microbubbles or nano-particles in the molten region. The generation of micro-bubbles and nanoparticles has been observed inside various glasses at various laser irradiation conditions. Because the accumulation of bubbles and nanoparticles could scatter the laser beam, they often prevent stable laser writing and lead to photonic circuits with increased waveguide loss. Fig. 6-6(a) shows the modification inside a sodalime glass during 250 kHz laser irradiation. At 2 s, stable photoexcitation by 250 kHz laser irradiation was observed. However, 15 s after the laser irradiation, microbubbles began to be generated in the middle. At 90 s after the laser irradiation, larger bubbles were generated by the accumulated microbubbles which scattered the subsequent incident preventing further photoexcitation. The accumulation of microbubbles can be reduced by a parallel laser irradiation, and stable photoexcitation can be achieved over a longer dwell time. Fig. 6-6(b)–(d) show the modification inside sodalime glass during parallel 1 kHz and 250 kHz laser irradiation. At Dd = 15 mm, microbubbles were formed at 5 s after the laser irradiation, but they disappeared quickly by a strong flow of the glass melt around the 1 kHz photoexcited regions. Therefore, microbubbles were not accumulated in the molten region, and photoexcitation could be sustained over several minutes. The dynamics of microbubbles changed drastically when 1 kHz femtosecond laser pulses were focused at Dd = +15 mm (Fig. 6-6(d)), so the accumulation of microbubbles can be modulated by selecting the positions of 1 kHz photoexcitations. For example, when the 1 kHz photoexcited regions are near the upper boundary of the molten region, a short capillary of microbubbles was formed near the lower boundary of the molten region, as shown in the image at 60 s in Fig. 6-6(d). These dynamics of melt flow suggests that parallel photoexcitation by 250 kHz and 1 kHz femtosecond laser pulses can modulate not only the ion migration but also the generation, movement, accumulation and disappearance of microbubbles.

7. Conclusion

In this Review, we have described the important role of ion migration on the refractive index profile produced by focused femtosecond laser pulses in glasses. The behavior of ion migration strongly depends on the laser repetition rate. At high repetition rates (>200 kHz), there is an accumulation of heat between successive pulses which can drive ion migration near the focal volume. Although there is not sufficient thermal accumulation at lower repetition rates (<10 kHz) to drive ion migration, the spatial distributions of heat buildup and ion migration can be modulated with a hybrid approach based on parallel irradiation with high and low repetition rate sources. Other laser writing parameters for controlling the elemental distributions in glass include spherical aberration and pulse front tilt. The behavior of ion migration depends also on the direction of observation. Table 7 gives a literature summary of femtosecond laser induced ion migration in dielectrics. Fig. 7 shows the general trend for elements migrating due to femtosecond laser irradiation: when observed perpendicular to the laser direction (Fig. 7(a)), the glass formers segregate in the epicenter whereas the intermediate and the network modifiers segregate in outer concentric regions with the molten region. This elemental redistribution can be partially explained from the transient temperature distribution and Soret effect but the situation becomes complex when there is a strong flow of glass melt at high repetition rates and net fluences, requiring more sophisticated elemental and molecular dynamic simulations. The trend of elements migrating along the parallel direction during high repetition rate femtosecond laser irradiation (Fig. 7(b)) is for multivalent ions to segregate in the densified zone with the single valent ions migrating to the rarefied zone. For both parallel and perpendicular directions, no evidence of ion migration has been observed in the cladding zone outside the inner molten region. In this Review, we have demonstrated how the properties of optical waveguides, which are the nerves and spine of photonic devices, can be easily tailored using ion migration. One bottleneck of waveguides inscribed by focused ultrashort laser pulses is their maximum achievable refractive index change, which hinders the design of ultra-compact photonic chips. This restriction is lifted by exploiting ion migration, which provides unmatched control over the refractive index profile. Several applications of ion migration during femtosecond laser writing were highlighted including space selective phase separation, laser based ion exchange method, and an optical micropipette. We expect this Review will stimulate new research to provide even more insight and control over ion migration, leading to greatly improved photonic device performance, opening up pre- viously unexplored application areas for femtosecond laser microfabrication.

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