Optical Sensitivity of Waveguides Inscribed in Nanoporous Silicate Framework

nanomaterials

Article Optical Sensitivity of Waveguides Inscribed in Nanoporous Silicate Framework

Zhong Lijing 1,2, Roman A. Zakoldaev 1,* , Maksim M. Sergeev 1, Andrey B. Petrov 1, Vadim P. Veiko 1 and Alexander P. Alodjants 1

1 Faculty of Laser Photonics and Optoelectronics, ITMO University, 197101 Saint Petersburg, Russia; [email protected] (Z.L.); [email protected] (M.M.S.); [email protected] (A.B.P.); [email protected] (V.P.V.); [email protected] (A.P.A.) 2 School of Optical and Electronic Information, Huazhong University of Science & Technology, Luoyu Road 1037, Wuhan 430074, China * Correspondence: [email protected]; Tel.: +7-911-144-52-56

Abstract: Laser direct writing technique in glass is a powerful tool for various waveguides’ fabrication that highly develop the element base for designing photonic devices. We apply this technique to fabricate waveguides in porous glass (PG). Nanoporous optical materials for the inscription can elevate the sensing ability of such waveguides to higher standards. The waveguides were fabricated by a single-scan approach with femtosecond laser pulses in the densification mode, which resulted in the formation of a core and cladding. Experimental studies revealed three types of waveguides and quantified the refractive index contrast (up to ∆n = 1.2·10−2) accompanied with ~1.2 dB/cm insertion losses. The waveguides demonstrated the sensitivity to small objects captured by the nanoporous framework. We noticed that the deposited ethanol molecules (3 µL) on the PG surface influence the waveguide optical properties indicating the penetration of the molecule to its cladding. Continuous monitoring of the output near field intensity distribution allowed us to determine the response time (6 s) of the waveguide buried at 400 µm below the glass surface. We found that the minimum distinguishable change of the refractive index contrast is 2 × 10−4. The results obtained pave the Citation: Lijing, Z.; Zakoldaev, R.A.; way to consider the waveguides inscribed into PG as primary transducers for sensor applications. Sergeev, M.M.; Petrov, A.B.; Veiko, V.P.; Alodjants, A.P. Optical Keywords: laser direct writing; porous glass; waveguides; photonic circuits; optofluidics; ethanol; Sensitivity of Waveguides Inscribed small molecules in Nanoporous Silicate Framework. Nanomaterials 2021, 11, 123. https:// doi.org/10.3390/nano11010123 1. Introduction Received: 30 November 2020 A nanoporous silicate framework of porous glass (PG) with multiple buried hollow Accepted: 28 December 2020 channels and pores with a well-controlled size in the range of 2–20 nm [1,2] represents Published: 7 January 2021 a promising matrix, which captures, stores, and transports molecules absorbed from the

Publisher’s Note: MDPI stays neu- environment [3–7]. Recently, portable devices with such a PG loaded with organic indica- tral with regard to jurisdictional clai- tors were demonstrated for monitoring the level of formaldehyde [3], nitrogen dioxide [4], ms in published maps and institutio- and ozone [5,6] in the environment. The interaction of the indicator and the harmful gases nal afﬁliations. changes the color of the entire glass plate. Laser-induced integration of several activated sectors in the single PG aimed to expand the sensor functionality [8]. However, information processing was performed by the spectral analysis of each sector of the glass plate. For a more accurate analysis of chemical reactions in such a nanoporous medium, it is necessary Copyright: © 2021 by the authors. Li- to inscribe a chip-scale optical channel, namely, a bulk waveguide, which typically consists censee MDPI, Basel, Switzerland. of a core and cladding to obtain internal reﬂection of coupled light resulting in a good This article is an open access article balance between light localization and optical losses [9]. The optical signal transmitted distributed under the terms and con- through the waveguide can trace the media state in real-time, for example, the appearance ditions of the Creative Commons At- of new molecules, their chemical interaction, or vice versa evaporation. The combination of tribution (CC BY) license (https:// a nanoporous silicate matrix and bulk waveguide may provide unique sensing abilities for creativecommons.org/licenses/by/ a nanopore sensor, which operates with single molecules [10,11]. The range of applications 4.0/).

Nanomaterials 2021, 11, 123. https://doi.org/10.3390/nano11010123 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 123 2 of 14

depends on the captured molecules and may be demanded in the field of biochemistry [12] or drug investigation [13,14]. Nowadays, femtosecond laser direct writing (LDW) is an advanced technology to fabricate waveguides inside glass materials [15–17]. Such structures represent an integral element for photonics circuits [18,19], lab on a chip [20] and sensorics [21]. In particular, various types of optical waveguides have been proposed—gradient [22], core-cladding [23], plasmonic [24,25], and integrated in a glass chip functioning as thermal [26], fluid [21], or light [25] sensors. Most of these applications are based on an evanescent wave interaction with target molecules deposited on a glass surface [21,25,26]. Such a detecting mechanism supposes a waveguide to be incorporated in the glass pre-surface layer to interact with a target fluid deposited on the glass surface. However, that limits 3D integration of waveguides in a photonic chip. The ability to inscribe waveguides in glass materials enables the development and realization of Mach–Zehnder (MZ) interferometers and quantum photonic circuits possessing low (up to 0.1 dB/cm) dissipation [27,28]. In combination with entangled photon sources of light and efficient single photon detectors, such circuits may be used for linear optics quantum computing [29] and quantum sensorics [30]. In particular, as it is shown in [30], the optofluidic device that contains MZ interferometer and photonic N00N state (with N = 2) at the input, provides the measurement of the concentration of bovine serum albumin (BSA) in aqueous buffer solutions with 10−3 accuracy. Considering all mentioned above, the connection of a waveguide with the sensing medium seems to be relevant. Thus, the concept of waveguides inscription in optically transparent PG can make a significant contribution to the design of optofluidic sensors. Herein, we suggest a concept of the waveguide in PG to demonstrate a sensing platform for the detection of target molecules. In the first step, we hold the research of waveguides inscription in PG by LDW technique. The investigation revealed three types of waveguides possessing the core-cladding structure. For the sensing application, we utilized the waveguide with the highest refractive index contrast (1.2·10−2) and the lowest available propagation losses (~1.2 dB/cm). As a model target molecules, we chose ethanol [31]. Then, we demonstrated that the transmitted laser radiation through the waveguide has a response to ethanol molecules deposited on the PG surface. In particular, the molecules captured by the nanoporous framework transferred to the waveguide cladding and changed the refractive index contrast. This determined and justified the sensitivity of the waveguides to the flow of small objects inside the nanoporous framework. The minimum detectable change in the value of the contrast of the refractive index is 2 × 10−4. This indicates the high sensitivity of such a sensor for determining small volumes of molecules. Besides, we showed that the heat treatment of the waveguides did not affect their structural and optical properties, which confirms the multiple uses of the sensor.

2. Experiments 2.1. LDW of Waveguides in PG In this work, the femtosecond laser-induced densification approach [32,33] was applied for the inscription of waveguides in PG with an average pore size ~10 nm, and total porosity 26%. Glass composition consists of high silica proportion, SiO2 > 96% (mass fraction, %) [34]. These PG plates possess high transparency (~90%) in the visible spectra range, with an average refractive index n ≈ 1.34. The samples were impregnated with water before laser irradiation to expand the ranges of laser processing parameters. Water presence in PG glass provides a more optically homogeneous medium. In [33] we showed that the water impregnation step increases the range of acceptable values of power densities by 15%. The impregnation stage was conducted by immersing the PG sample in distilled water for 96 h at room temperature and atmospheric pressure. LDW was performed with the linear polarized Gaussian laser beam by Yb-doped fiber laser (Avesta TETA-20, Russia) operating at 1035 nm wavelength with a pulse duration 220 fs, and a fixed repetition rate of 1 MHz. The laser beam was focused by an objective Nanomaterials 2021, 11, 123 3 of 14

lens (LOMO, 20×, NA = 0.4). The objective lens formed the beam waist with a diameter of 2.5 µm and the Rayleigh length z0 = 5.5 µm. The PG samples were mounted on an XYZ translation stage based on a stepper motor with the step equalling 1 µm and being controlled by the driver (SMC-AD3). The waveguides writing was performed by translating the sample perpendicular to the laser beam axis at speed 0.0125–3.75 mm/s and incident pulse energy Ep = 0.6, 0.8, and 1.6 µJ. The position of the laser beam focus is 400 µm beneath the upper surface of the glass sample. The length of the writing track was 10 mm. After the laser writing step, the samples were polished at both facets and heated in a furnace for 2 h at 500 ◦C, and investigated by optical microscopy (Carl Zeiss, Axio Imager) in the transmission mode and cross-polarized light to show the absence of cracks, stresses, and to study the birefringence of waveguides structure.

2.2. Waveguides Testing Since the fabricated tracks possess light-guiding properties, they are important to study in terms of the refractive index contrast according to methodology demonstrated by us formerly [35]. For this purpose, a setup equipped with a He-Ne laser and a pair of objectives can register the near-field intensity distribution of radiation transmitted through the waveguide (Figure1a). In more detail, laser radiation is focused by an objective (60×, 0.85 NA) to couple light into the waveguide. An out-coupled beam is collected by another objective (40×, 0.65 NA). CCD camera (C1) controls the position of the input beam to ensure which part of the waveguide is irradiated. CCD camera (C2) aligns the focus plane of the objective with the out-coupled plane of the waveguide. After that, the near field distribution of the out-coupled beam is captured by CMOS beam profiler (Gentec-EO, Beamage 3.0, QC, Canada). Based on the captured near-field distribution, the refractive index profile of the waveguide is estimated numerically by solving the Helmholtz equation [36] λ2 ∇2E(x, y) n2(x, y) = [n + ∆n(x, y)]2 = n2 − (1) b e f f 4π2 E(x, y)

where, nb is the refractive index of initial glass, neff is the effective refractive index of the propagating mode. Although neff is an unknown constant, the refractive index change profile ∆n(x,y) is unaffected by the magnitude of neff. With the approximation neff = nb, 2 2 ∆n(x,y) can be numerically evaluated using n (x,y) ≈ nb + 2nb ∆n(x,y). The normalized E-field distribution E(x,y) across the mode is inferred by the measured near-field intensity of the mode I(x,y). In the calculation, a third-order Butterworth filter with optimized cutoff frequency is employed to remove impulsive and high-frequency noise in the power intensity measured without accuracy loss [37]. The same setup is also utilized in this work for near-field intensity monitoring during ethanol liquid deposition on PG surface (Section 3.4). The optical losses are also integral parameter, which characterizes the waveguides. The losses measurement are performed with a fiber-coupling setup equipped with a laser module operated at 975 nm, fiber coupling part and power meter (Figure1b). The input fiber is fixed on high-resolution multi-axis positioning coordinate table with sub-micron accuracy. The Fresnel reflection losses for fiber-to-waveguide connections are considered negligible. The waveguide insertion losses are obtained from the power transmitted through the waveguide and detected with an InGaAs photodiode power sensor (deviation is ±0.5%). The registered signal is normalized to the power propagated through free space with the same distance. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 14

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Figure 1. Schemes of the experimental setups for the caption of near-ﬁeld intensity distribution Figure(a) and for measuring 1. Schemes losses of waveguides of the (b ).experimental setups for the caption of near-field intensity distribution (a)

and3. Results for and measuring Discussion losses of waveguides (b). 3.1. Waveguide Fabrication The pulse energy and scanning speed are two critical parameters determining future waveguideThe morphology optical and geometry losses during are the LDWalso step. integral The waveguides parameter, observed by which characterizes the waveguides. optical microscopy possess the shape of an elongated ellipse in the cross-section, which is Thethe direct losses evidence ofmeasurement a filament structure appearance are performed [38]. These filaments with are character- a fiber-coupling setup equipped with a la- ized by height in the range of 50–400 µm (Figure2a) and width 4–7 µm. Such an elliptical sershape affectsmodule the form ofoperated the mode distribution. at 975 Another nm, feature fiber is a shift ofcoupling the waveguide part and power meter (Figure 1b). The position along the optical axis, depending on the selected pulse energy and the number of inputlaser pulses. fiber The increase is offixed energy andon number high-resolution of laser pulses shifts the waveguidemulti-axis in the positioning coordinate table with sub- positive Z direction, while the decrease causes it to move in the negative Z-direction, as micronschematically shownaccuracy. in Figure2b. Moreover,The Fresnel with the high pulsesreflection number that isloss 1.0 ×es105 , for fiber-to-waveguide connections are we observed a drop-like structure formed of multiple layers in the cross-section (Figure2c). consideredThe enlarged image shows negligible. that the structure The of the waveguide waveguide consists of a brighterinsertion central losses are obtained from the power transmittedellipsoid surrounded bythrough a darker elliptical the ring andwaveguide followed by another and slightly detected brighter with an InGaAs photodiode power ring (Figure2d). Similar structures were reported in borosilicate glass by Nolte et al. [ 39] sensorand in Corning (deviation Gorilla Glass by Boisvertis ±0.5%). et al. [40]. The The multilayer registered structure of si thisgnal kind is normalized to the power propagated is interpreted as an occurrence of the densified region and rarefaction region of the glass throughframework. More free details space about the waveguidewith the structure same are presented distance. in Section 3.3.

3. Results and Discussion 3.1. Waveguide Fabrication The pulse energy and scanning speed are two critical parameters determining future waveguide morphology and geometry during the LDW step. The waveguides observed by optical microscopy possess the shape of an elongated ellipse in the cross- section, which is the direct evidence of a filament structure appearance [38]. These filaments are characterized by height in the range of 50–400 µm (Figure 2a) and width 4–7 µm. Such an elliptical shape affects the form of the mode distribution. Another feature is a shift of the waveguide position along the optical axis, depending on the selected pulse energy and the number of laser pulses. The increase of energy and number of laser pulses shifts the waveguide in the positive Z direction, while the decrease causes it to move in the negative Z-direction, as schematically shown in Figure 2b. Moreover, with the high pulses number that is 1.0 × 105, we observed a drop-like structure formed of multiple layers in the cross-section (Figure 2c). The enlarged image shows that the structure of the waveguide consists of a brighter central ellipsoid surrounded by a darker elliptical ring and followed by another slightly brighter ring (Figure 2d). Similar structures were reported in borosilicate glass by Nolte et al. [39] and in Corning Gorilla Glass by Boisvert et al. [40]. The multilayer structure of this kind is interpreted as an occurrence of the densified region and rarefaction region of the glass framework. More details about the waveguide structure are presented in Section 3.3. Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 14 Nanomaterials 2021, 11, 123 5 of 14

FigureFigure 2. Measured2. Measured height of height waveguides of atwaveguides different pulse energyat different and number pulse (a). Sketch energy map ofand number (a). Sketch map of laser inscribing and waveguide shift (b). The color legend of circles in (a) represents the number laserof laser inscribing pulses per region, and wherewaveguide a smaller diametershift (b means). The the color maximum legend positive of shiftcircles of the in (a) represents the number of laserwaveguide, pulses which per is schematically region, where shown in a (b smaller). Transmission diameter microscope means photos ofthe the maximum waveguide positive shift of the wave- 5 guide,fabricated which by Ep = 0.8 isµ schematicallyJ and 10 pulses (c) and shown its enlarged in ( viewb). Transmission (d). The scale bars are microscope 30 µm (c) and photos of the waveguide 5 µm (d). fabricated by Ep = 0.8 µJ and 105 pulses (c) and its enlarged view (d). The scale bars are 30 µm (c) and3.2. Thermal 5 µm Resistance(d). of Waveguides PG plate is possible to reuse after capturing analyte and/or reagent by heat treatment in the furnace (temperature up to 500 ◦C). This reusability is important for the future 3.2.application Thermal of waveguides Resistance as a sensing of Waveguides element. However, such a temperature may release the residue stress field of the waveguides or change their properties as a previous study showedPG for waveguidesplate is inpossible solid glass to [23 ].reuse As part ofafter this work,capturing we confirmed analyte the thermal and/or reagent by heat treat- mentstability in of waveguidesthe furnace when (temperature PG with inscribed waveguidesup to 500 is heated°C). inThis the furnace. reusability is important for the fu- Thus, we have tested the thermal stability of waveguides while heating in a furnace at turea temperature application of 500 ◦C of for 2waveguides h. After the waveguide’s as a sensin heat treatment,g element. the study However, was held such a temperature may releaseby transmission the microscopyresidue andstress crossed field polarizers of the to examine waveguides birefringent or with change a constant their properties as a previ- ousexposure study time showed (Figure3). The for microscopy waveguides in transmission in solid light glass shows [23]. bright As elongated part of this work, we confirmed regions with the related increased refractive index appeared for peak fluence (Fp) up to the45 J/cm thermal2 and the stability different number of waveguides of pulses (Figure3 whena). The observed PG with top viewinscribed in crossed waveguides is heated in the polarizers shows two cases (Figure3b): bright central part for waveguides fabricated furnace.with an increased number of laser pulses (N > 5600) and smooth waveguides (17 J/cm2, N < 3700Thus, pulses we) whose have core tested does not the have thermal a bright color stability when the of sample waveguides is rotated while heating in a furnace relative to crossed polarizers. The bright light is usually associated with birefringence atphenomena. a temperature However, in of comparison 500 °C with for toughened 2 h. After glasses th [41e], waveguide’s such a glow was found heat treatment, the study was heldboth in by the coretransmission and in the cladding microscopy of the fabricated and waveguide. crossed In ourpolarizers study, there to is no examine birefringent with a 2 constantbright light surroundingexposure the time waveguides (Figure fabricated 3). withThe 17.0 micr < Fp oscopy< 22.7 J/cm inand transmission pulse light shows bright number in the range of 3700–5600. That indicates the absence of lateral residual stresses elongatedaround the waveguides. regions Thewith surrounded the related stresses increased occur for waveguides refractive fabricated index by appeared for peak fluence increasing the number of pulses over 104. Even in this case the surrounded stresses relaxed (Fp) up to 45 J/cm2 and the different number of pulses (Figure 3a). The observed top view after additional thermal treatment (Figure3c). The photos of the waveguides’ central part inin crossedcrossed polarizers polarizers after heat treatmentshows hadtwo no cases changes (Figur (Figure3c).e This3b): suggests bright that central part for waveguides fabricatedthe internal structure with of an the increased waveguides remained number the sameof la provingser pulses their thermal (N stability.> 5600) and smooth waveguides (17 J/cm2, N < 3700 pulses) whose core does not have a bright color when the sample is rotated relative to crossed polarizers. The bright light is usually associated with birefringence phenomena. However, in comparison with toughened glasses [41], such a glow was found both in the core and in the cladding of the fabricated waveguide. In our study, there is no bright light surrounding the waveguides fabricated with 17.0 < Fp < 22.7 J/cm2 and pulse number in the range of 3700–5600. That indicates the absence of lateral residual stresses around the waveguides. The surrounded stresses occur for waveguides fabricated by increasing the number of pulses over 104. Even in this case the surrounded stresses relaxed after additional thermal treatment (Figure 3c). The photos of the waveguides’ central part in crossed polarizers after heat treatment had no changes (Figure 3c). This suggests that the internal structure of the waveguides remained the same proving their thermal stability. Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 15 Nanomaterials 2021, 11, 123 6 of 14

Figure 3. Images of waveguidesFigure 3. Images in of waveguidestransmission in transmission light of light microscopy of microscopy ((aa).). Polarized Polarized microscopy microscopy photos ◦ photos of waveguidesof waveguidesin top view in top before view before (b) (andb) and after after ((cc) heat) heat treatment treatment in a furnace in a (T furnace = 500 C, 2(T h). = The 500 °C, 2 waveguides were fabricated at the peak fluence and number of laser pulses per region, which are h). The waveguides wereindicated fabricated in the figure. at the peak fluence and number of laser pulses per region, which are indicated in the figure. 3.3. Waveguide Properties: Types, Refractive Index Profile, and Losses The refractive index profile reveals three types of sandwich-like structure of fabricated 3.3. Waveguide Properties:waveguides. Types, The difference Refractive between Index them Profile, is in the and reordering Losses of densified and rarefaction layers. The uniform waveguide is written by using relatively low pulse energy 0.6 µJ at The refractivea index high translating profile speed reveals of 3.75 three mm/s (Ntypes ~ 400), of with sandwich-like the peak fluence 17.0structure J/cm2 and of fabricated waveguides.net The fluence difference 6.8·103 J/cm between2. The cross-sectional them is view in ofth*e the waveguidereordering shows of a comet-likedensified and rarefaction layers. shapeThe uniform with the coupling waveguide beam position is written on the tip by (highlighted using relatively by a dot-dash low outline pulse in ener- Figure4a ). The near-field distribution of propagating laser light through the waveguide is gy 0.6 µJ at a highalso translating captured to observe speed a guidingof 3.75 mode mm/s and cross-sectional (N ~ 400), features with (Figurethe peak4b). The fluence near- 17.0 J/cm2 and net fluencefield distribution6.8·103 J/cm exhibits2. The a satisfactory cross-sectional modal profile view confirmed of bythe the waveguide well-confinement shows a of the guiding light. The refractive index profile evaluated from Equation (1) demonstrates −4 −4 comet-like shape withthe core the (∆n couplingcore = 6.5·10 beam) and cladding position (∆nclad on= −the10.5 tip·10 (highlighted) (Figure4c). The refractiveby a dot-dash outline in Figure 4a).index The contrast near-field between the distribution core and cladding of is 1.7propagating·10−3. The shape laser of the light distribution through of the the refractive index also tends to the shape of a ‘comet’, but the structure remains within waveguide is also thecaptured framework to of observe the classical a conceptguiding of amode waveguide. and Thiscross-sectional LDW regime is relativelyfeatures (Fig- ure 4b). The near-fieldfast and distribution allows us to fabricate exhibits the typea satisfactory of waveguides, modal which weprofile have called confirmed a ‘comet- by the well-confinement ofshaped the waveguide’.guiding light. The refractive index profile evaluated from Equa- tion (1) demonstrates the core (Δncore = 6.5·10−4) and cladding (Δnclad = −10.5·10−4) (Figure 4c). The refractive index contrast between the core and cladding is 1.7·10−3. The shape of the distribution of the refractive index also tends to the shape of a ‘comet’, but the structure remains within the framework of the classical concept of a waveguide. This LDW regime is relatively fast and allows us to fabricate the type of waveguides, which we have called a ‘comet-shaped waveguide’. NanomaterialsNanomaterials 2021,2021 11, ,x11 FOR, 123 PEER REVIEW 7 of 14 7 of 14

Figure 4. 4.Cross-sectional Cross-sectional view ofview a comet-shaped of a comet-shaped waveguide (0.6waveguideµJ pulse energy, (0.6 µJ ~400 puls pulses)e energy, ~400 pulses) (a), µ 4 (rectangular-sectioneda), rectangular-sectioned waveguide waveguide (0.6 J(0.6 pulse µJ energy, pulse ~10 energy,pulses) ~10 (d) and4 pulses) cylindrical-shaped (d) and cylindrical-shaped waveguide (1.6 µJ pulse energy, ~600 pulses) (g) with corresponding measured near-field distribution (waveguideb,e,h) and estimated (1.6 µJ refractive pulse indexenergy, profile ~600 (c,f ,ipulses)). Dash white (g) with squares corresponding indicate the laser radiationmeasured near-field distribu- tioncoupling (b,e position.,h) and Scaling estimated bar equals refractive 10 µm. index profile (c,f,i). Dash white squares indicate the laser radiation coupling position. Scaling bar equals 10 µm. With the same pulse energy 0.6 µJ but at the lowest translating speed 0.125 mm/s (N ~ 104) with the same peak fluence but with larger net fluence ~2 × 105 J/cm2, an elongatedWith densified the same region pulse is observed energy in the 0.6 cross-sectional µJ but at the view lowest (Figure4 d).translating That oc- speed 0.125 mm/s (Ncurs ~ due 10 to4) thewith self-focusing the same of peak the propagating fluence wavefront,but with i.e.,larger the pulse net collapsesfluence into~2 × 105 J/cm2, an elon- gateda filament densified near the focus region [42]. Theis observed captured near-field in the image cross-sectional demonstrates aview rectangular (Figure 4d). That occurs shape distribution with its maximum in the central part (Figure4e). The central mode profiledue to is the elliptical. self-focusing The refractive of the index propagating profile shows wavefront, an opposing result,i.e. the where pulse the collapses into a fila- −4 positivement near rectangular the corefocus (∆ n[42].core = 5The·10 )captured is surrounded near-field by symmetrically image located demonstrates nega- a rectangular −3 shapetive cladding distribution(∆nclad = − with2.7·10 its) (Figure maximum4f). The refractivein the central index contrast part between(Figure the 4e). The central mode core and cladding is 3.2·10−3. The second type of waveguides are called ‘rectangular- profilesectioned is waveguides’. elliptical. The refractive index profile shows an opposing result, where the pos- itiveThe rectangular third type of waveguidescore (Δncore are written= 5·10 with−4) is a relativelysurrounded high pulse by energysymmetr of 1.6icallyµJ located negative at a translating speed of 2.5 mm/s (N ~ 600) with a larger peak fluence of 45.3 J/cm2 and cladding (Δnclad = −2.7·10−3) (Figure 4f). The refractive index contrast between the core net fluence 2.7·104 J/cm2. Here, we see a 2-micron-sized cylindrical-shaped core located −3 andbetween cladding two rarefaction is 3.2·10 layers. (FigureThe second4g). Basically, type such of waveguides a structure occurs are as called a result of‘rectangular-sectioned waveguides’.self-focusing and defocusing by self-generated plasma [42]. The coupling of He-Ne laser radiationThe into third the core type shows of waveguides a nearly single ellipticalare written mode distributionwith a relatively and is captured high pulse energy of 1.6 in Figure4h. Again, the refractive index profile demonstrates a positive elliptic core with µJ at a translating speed of 2.5 mm/s (N ~ 600) with a larger peak fluence of 45.3 J/cm2 and net fluence 2.7·104 J/cm2. Here, we see a 2-micron-sized cylindrical-shaped core located between two rarefaction layers (Figure 4g). Basically, such a structure occurs as a result of self-focusing and defocusing by self-generated plasma [42]. The coupling of He- Ne laser radiation into the core shows a nearly single elliptical mode distribution and is captured in Figure 4h. Again, the refractive index profile demonstrates a positive elliptic core with maximum Δncore = 6.0·10−4 surrounded with a highly depressed cladding with maximum Δnclad = −1.2·10−2. The contrast between the core and cladding is 1.2·10−2 (Fig- ure 4i). The third type of waveguides are called ‘cylindrical-shaped waveguides’. This type possesses the highest contrast, the absence of residual stresses in the cladding, and is chosen for the following losses and sensing investigations. Insertion losses (α) are measured for the waveguides, which refers to the second type waveguide (Figure 4d–f) and third type waveguide (Figure 4g–i). This was accom- Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 14

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plished by coupling laser module radiation into the core of the waveguide on the fiber- maximum ∆n = 6.0·10−4 surrounded with a highly depressed cladding with maximum core in coupling setup−2 (Figure 1b). The input power (P −)2 was equal to 1.4, 3.9, and 6.9 mW. The ∆nclad = −1.2·10 . The contrast between the core and cladding is 1.2·10 (Figure4i). The thirdoutput type ofpower waveguides (Pout are) calledof the ‘cylindrical-shaped light after passing waveguides’. through This type the possesses waveguide was also registered the(Figure highest contrast,5). The the insertion absence of residuallosses stresses are estimated in the cladding, as and is chosen for the following losses and sensing investigations. Insertion losses (α) are measured for the waveguides,α =⋅ which refers() to the second type waveguide (Figure4d–f) and third type waveguide (Figure104g–i). lg ThisPP wasin accomplished out . (2) by coupling laser module radiation into the core of the waveguide on the ﬁber-coupling setup (FigureThus,1b). the The second input power type ( Pin )waveguide was equal to 1.4, possesses 3.9, and 6.9 mW. a Theslightly output higher inversion loss com- power (Pout) of the light after passing through the waveguide was also registered (Figure5) . Thepared insertion with losses the are estimatedthird type as waveguide, while the average insertion loss of these waveguides is ~1.2 dB/cm. α = 10·lg(Pin/Pout). (2)

FigureFigure 5. 5.Insertion Insertion losses losses measurement measurement of the second of type the waveguide second (0.6typeµJ pulsewaveguide energy, (0.6 µJ pulse energy, ~104 4 µ ~10pulses),pulses), and and the the third third type waveguidetype waveguide (1.6 J pulse energy,(1.6 µJ ~600 pulse pulses). energy, ~600 pulses). Thus, the second type waveguide possesses a slightly higher inversion loss compared with the third type waveguide, while the average insertion loss of these waveguides is ~1.2 dB/cm.Notably, the insertion losses account for both coupling losses and propagation losses. Notably,The cross-sectional the insertion losses accountmorphology for both coupling determine losses ands propagationthe spatial losses. distribution of the mode sup- The cross-sectional morphology determines the spatial distribution of the mode supported byported the waveguide. by the Anywaveguide. mismatch between Any themismatch mode ﬁelds be oftween the waveguide the mode and the fields of the waveguide and inputthe beaminput will beam result inwill increased result insertion in increased losses. Meanwhile, insert asion in silicate losses. glass Meanwhile, [43], as in silicate glass the insertion losses originated from the non-symmetrical morphology of the waveguide can[43], be dramaticallythe insertion reduced losses to below originated 1 dB/cm at 1550 from nm, bythe optimizing non-symmetrical the fabrication morphology of the wave- process.guide Moreover,can be ifdramatically we compare our resultreduced to a waveguide to below in porous 1 dB/cm silicon [ 44at], which1550 nm, by optimizing the fab- isrication applied for process. sensing application, Moreover, our waveguides if we compare possess an order our of result magnitude to lowera waveguide in porous silicon losses. Therefore, we can assume that, for a primary transducer with a length of several centimeters,[44], which the obtained is applied loss value for is sensing suitable. application, our waveguides possess an order of mag- nitudeWhen lower the waveguide losses. is used Therefore, as the analyzer we of liquid,can assume the liquid penetratesthat, for to thea primary transducer with a cladding through the porous framework, mitigates the refractive index contrast between the corelength and cladding, of several and deteriorates centimeters, the waveguiding the obtained property. Notably,loss value waveguides is suitable. with When the waveguide is used as the analyzer of liquid, the liquid penetrates to the cladding through the porous framework, mitigates the refractive index contrast between the core and cladding, and deteriorates the waveguiding property. Notably, waveguides with a higher refractive index contrast are preferable for liquid sensor applications. Therefore, the third type of waveguides was selected for the sensing demonstration.

3.4. Opto-Fluidic Waveguide for Ethanol Molecule Sensing The occurrence of the optical micro-channel in PG is a remarkable feature, as the fabricated waveguide exhibits sensitivity to the changes happening in the nanoporous framework. Such sensitivity is associated with the waveguide’s cladding filling with molecules deposited on the glass surface. First, these changes can be noticed when the refractive index contrast changes, then there will be a distortion of the intensity distribution at the waveguide output. The key sensor parameter of such a waveguide is a response time. Here, we demonstrate an approach to detect molecules captured by PG during the registration of time-dependent changes in the near-field mode distribution of the wave- Nanomaterials 2021, 11, 123 9 of 14

a higher refractive index contrast are preferable for liquid sensor applications. Therefore, the third type of waveguides was selected for the sensing demonstration. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 14 3.4. Opto-Fluidic Waveguide for Ethanol Molecule Sensing The occurrence of the optical micro-channel in PG is a remarkable feature, as the fabricated waveguide exhibits sensitivity to the changes happening in the nanoporous framework. Such sensitivity is associated with the waveguide’s cladding filling with moleculesguide output. deposited onIn thethe glass experiment surface. First, we these use changes the can‘cylindrical-shaped be noticed when the waveguide’ due to the refractivefollowing index contrastreasons: changes, (i) uniform then there will elliptical be a distortion mode of the of intensity distribution; distribution (ii) lowest losses; (iii) the at the waveguide output. The key sensor parameter of such a waveguide is a response time. presenceHere, we demonstrateof space anfor approach liquid to detectprecipitation molecules captured in the by PGform during of the rarefaction reg- layers around the istrationwaveguide of time-dependent core. To changes conduct in the this near-field study, mode widely distribution available of the waveguide ethanol, a so-called “small mol- output.ecule” In the[45], experiment was chosen we use the as ‘cylindrical-shaped the liquid, since waveguide’ the duemolecules to the following easily penetrate through PG reasons: (i) uniform elliptical mode of distribution; (ii) lowest losses; (iii) the presence ofnanoporous space for liquid framework precipitation in [46]. the form of rarefaction layers around the waveguide core. ToThe conduct experimental this study, widely scheme available for ethanol, waveguide a so-called “smalltesting molecule” is given [45], in Figure 6. He-Ne laser was chosen as the liquid, since the molecules easily penetrate through PG nanoporous frameworkradiation [46 ].is coupled into the waveguide. An objective-objective connection is applied to laserThe experimentalradiation schemecoupling for waveguide and registering testing is given the in Figuretransmitted6. He-Ne laser laser radiation through the radiation is coupled into the waveguide. An objective-objective connection is applied towaveguide laser radiation (Figure coupling and6a). registering The objectives the transmitted are fixed laser radiation on the through multiple-axis the translation stages to waveguideprovide (Figure accurate6a). The positioning objectives are of fixed both on the the multiple-axis in-coupling translation beam stages and out-coupling beam (Fig- toure provide 6b). accurate A CMOS positioning camera of both located the in-coupling behind beamobjective and out-coupling 2 captures beam the near-field intensity dis- (Figure6b). A CMOS camera located behind objective 2 captures the near-field intensity distribution,tribution, as as schematically schematica shownlly in shown Figure1a. in Figure 1a.

FigureFigure 6. (a 6.) Schematic (a) Schematic view of liquid-immersing view of liquid-immersing procedure for PG with procedure the inscribed for cylindrical- PG with the inscribed cylindrical- shapedshaped waveguide. waveguide. (b) Photo ( ofb) the Photo experimental of the setup experimental for waveguide sensorsetup ability for waveguide testing. sensor ability testing.

In the experiment, a 3 µL-drop of ethanol liquid was deposited on the PG surface, where the monitoring of near-field intensity distribution at the waveguide output occurred during 45 min after ethanol deposition (Figure 7). The exposure time of the CMOS camera was constant (20 ms) to trace the intensity change of the guiding light. The RMS noise of the CMOS camera is 1000:1. In the first minute of ethanol penetration, there was a sharp jump in the value of the transmitted intensity, the same was observed in the value of the contrast (red dots/curve in Figure 7). After 15 min, the mode shape was changed with the corresponding intensity decrease. Meanwhile, the filling of nanopores with ethanol mitigates the refractive index contrast between the core and cladding of the waveguide, and thus deteriorates its light guiding ability. After 17 min, we noticed the maximum decrease in the intensity, the added ethanol diffused to the region adjacent to the waveguide through connected nanopores of PG. 22 min later, the intensity began to recover, reaching its original state after 42 min. The results obtained show the waveguide possesses an ability to detect small molecules such as ethanol captured by PG upon registration of time-dependent changes in the near-field intensity distribution at the waveguide output. Nanomaterials 2021, 11, 123 10 of 14

In the experiment, a 3 µL-drop of ethanol liquid was deposited on the PG surface, where the monitoring of near-field intensity distribution at the waveguide output occurred during 45 min after ethanol deposition (Figure7). The exposure time of the CMOS camera was constant (20 ms) to trace the intensity change of the guiding light. The RMS noise of the CMOS camera is 1000:1. In the first minute of ethanol penetration, there was a sharp jump in the value of the transmitted intensity, the same was observed in the value of the contrast (red dots/curve in Figure7). After 15 min, the mode shape was changed with the corresponding intensity decrease. Meanwhile, the filling of nanopores with ethanol mitigates the refractive index contrast between the core and cladding of the waveguide, and Nanomaterials 2021, 11, x FOR PEER REVIEWthus deteriorates its light guiding ability. After 17 min, we noticed the maximum10 decreaseof 15 in the intensity, the added ethanol diffused to the region adjacent to the waveguide through connected nanopores of PG. 22 min later, the intensity began to recover, reaching its original state after 42 min. The results obtained show the waveguide possesses an ability to detect by PGsmall upon molecules registration such of as time-dependent ethanol captured changes by PG in upon the near-field registration intensity of time-dependent distribution changesat the waveguide in the near-field output. intensity distribution at the waveguide output.

FigureFigure 7. Time 7. Time dependence dependence of the of peak the peakintensity intensity of near-field of near-ﬁeld distribution distribution (black(black triangle) triangle) and the and the changechange of the of refractive the refractive index index contrast contrast between between core coreand andcladding cladding of waveguides of waveguides (red (red triangle) triangle) af- after ter PGPG impregnation impregnation with with 3-µL 3-µL ethanol ethanol liquid. liquid.

To estimateTo estimate the response the response time time of the of wave the waveguide,guide, we captured we captured the evolution the evolution of the of the near-fieldnear-field intensity intensity distribution distribution during during the fi therst firstminute minute after after the ethanol the ethanol deposition deposition with with an intervalan interval of 6 ofs (that 6 s (that corresponds corresponds to the to theminimum minimum measurement measurement interval interval of CMOS of CMOS camera)camera) and anda constant a constant exposure exposure time timeof 20 of ms. 20 The ms. Theresult result is given is given in Figure in Figure 8, where8, where an an approximatelyapproximately linear linear decline decline is revealed is revealed with with a slope a slope of 0.00248 of 0.00248 s−1. sThe−1. Thetime time of ethanol of ethanol liquidliquid deposition deposition on the on PG the surface PG surface was set was as setthe asbeginning the beginning of the recording. of the recording. There is Therea significantis a significant decrease decreasein the intensity in the of intensity about 17% of about within 17% 60 s, within which 60 can s, whichbe easily can distin- be easily guisheddistinguished by most photodiode by most photodiode detectors detectors(for example, (for example,Hamamatsu Hamamatsu Photonics Photonics S5972 PIN S5972 Photodiode).PIN Photodiode). Applying Applying our CMOS our CMOScamera cameraparameters, parameters, we can we establish can establish the response the response timetime is fewer is fewer than than 6 s. 6Meanwhile, s. Meanwhile, our ournumeri numericalcal simulation simulation of the of the refractive refractive index index pro- profile −4 file revealsreveals a a decrease decrease of of refractive refractive index index contrast contrast of ~5 of × ~5 10×−4 with10 awith deviation a deviation error of error ±1 of −4 × 10−±4 within1 × 10 60 withins. Therefore, 60 s. Therefore, the minimum the minimum distinguishable distinguishable change in change the value in the of value the re- of the −4 fractiverefractive index contrast index contrast is ~2 × is10 ~2−4. × 10 . NanomaterialsNanomaterials 2021, 11, x2021 FOR, 11PEER, 123 REVIEW 11 of 15 11 of 14

Figure 8.Figure Evolution 8. Evolution of the peak of intensity the peak intensityof the near-fiel of thed near-field distribution distribution and the change and the of changerefractive of refractive index contrastindex during contrast the during first theminute first after minute ethanol after ethanoldeposition deposition on the PG on thesurface. PG surface. The inserted The inserted im- images ages are theare near-field the near-field distributi distributionsons at 6, at 18, 6, 24, 18, 48, 24, and 48, and60 s. 60 s.

4. Conclusions4. Conclusions In conclusion,In conclusion, we have we applied have appliedLDW in LDWPG to in fabricate PG to fabricate new types new of typeswaveguides. of waveguides. Specifically,Specifically, a gradual a gradualsequence sequence of uniform of uniform ‘comet-shaped’, ‘comet-shaped’, ‘rectangular-sectioned’, ‘rectangular-sectioned’, and and ‘cylindrical-shaped’‘cylindrical-shaped’ waveguides waveguides was achieved was achieved in PG by in varying PG by varying pulse energy pulse energyand/or and/orthe the number numberof accumulated of accumulated laser pulses. laser pulses.The ‘cylindrical-shaped’ The ‘cylindrical-shaped’ waveguide waveguide is written is writtenby by the followingthe following parameters: parameters: 1 MHz, 11.6 MHz, µJ pulse 1.6 µ energy,J pulse energy,and 2.5 andmm/s 2.5 translating mm/s translating speed, speed, −2 which iswhich characterized is characterized by relatively by relatively high ∆n high ~1.2·10∆n− ~1.22. The·10 obtained. The obtained value is valuesuitable is suitablefor for the currentthe tasks current of tasksthe sensor of the element. sensor element. For other For applications other applications this value this can value be increased can be increased by adoptingby adopting the multi-scan the multi-scan technique technique [47]. However, [47]. However, the detailed the detailed investigation investigation of the of the level of nanoporelevel of nanopore collapse collapse in the waveguide in the waveguide cross-section cross-section is required. is required. Waveguide Waveguide optical optical losses representlosses represent one of important one of important issues issuesfor various for various applications applications in photonics in photonics and quan- and quantum tum technologies.technologies. For the For currently the currently available available LDW LDW setup, setup, the insertion the insertion losses losses of fabricat- of fabricated ed ‘cylindrical-shaped‘cylindrical-shaped waveguide’ waveguide’ are equal are equal to 1.2 to dB/cm. 1.2 dB/cm. The obtained The obtained level levelof inser- of insertion tion losseslosses is due is due to the to thetechnical technical imperfecti imperfectionsons of the of experimental the experimental setup. setup. The losses The losses can can be be significantlysignificantly reduced reduced by implementing by implementing with with the the currently currently available available technology technology uti- utilized the lized theposition position accuracy accuracy of of hundreds hundreds of of nanometers nanometers [48 [48].]. In particular, the ‘cylindrical-shaped’ waveguide is used for the detection of ethanol In particular,molecules the deposited ‘cylindrical-shaped’ on PG upon registration waveguide of is time-dependentused for the detection changes of inethanol the near-field moleculesdistribution deposited at on the PG waveguide upon registration output. The of waveguidetime-dependent shows changes a short responsein the near- time of ~6 s field distributionand high sensitivityat the waveguide with a minimum output. The detectable waveguide change shows in the a short value response of the refractive time index of ~6 s andcontrast high ofsensitivity the numerical with a method minimum of ~2 detectable× 10−4. Thechange results in the show value the of optical the refrac- sensitivity of tive indexwaveguides contrast of inscribed the numerical in PG for method the detection of ~2 × of 10 small−4. The molecules results suchshow as the ethanol. optical Then, on sensitivitythe of next waveguides step, the calibration inscribed procedure in PG for of the such detection a sensor of is requiredsmall molecules for every such target as molecule. ethanol.Depending Then, on the on next the propertiesstep, the calibration of the molecules, procedure the of calibration such a sensor can be is accomplishedrequired for by the Nanomaterials 2021, 11, 123 12 of 14

response time, minimum/maximum intensity level shift at the waveguide output, and the recovery time. The size of pores places a restriction on the molecule to be detected— molecules larger than the pore size cannot be detected because they are unable to penetrate the waveguide cladding. It is worth noticing that our recent work enables to use PG waveguides for applications in the quantum domain even in the presence of some moderate level of losses. In particular, further improvement of the considered technology allows to design MZ interferometers as a basis for high precision sensors and photonic information processing circuits [49]. For sensor applications, one can use the waveguide in an interferometer arm to obtain an interference pattern to show the concentration variation of captured molecules. The entangled states and especially, N00N states at the input of the interferometer can improve the visibility for this method [30].

Author Contributions: Conceptualization, R.A.Z. and V.P.V.; Data curation, Z.L., A.B.P. and A.P.A.; Funding acquisition, A.P.A.; Investigation, Z.L., R.A.Z. and M.M.S.; Methodology, Z.L., R.A.Z. and M.M.S.; Project administration, A.P.A.; Resources, R.A.Z. and M.M.S.; Software, Z.L.; Supervision, V.P.V. and A.P.A.; Validation, Z.L.; Visualization, Z.L.; Writing—original draft, Z.L. and R.A.Z.; Writing—review and editing, R.A.Z., V.P.V. and A.P.A. All authors have read and agreed to the published version of the manuscript. Funding: The work on laser direct writing of waveguide and its investigation are funded by Russian Foundation for Basic Research (RFBR) no. 19-52-52012 МНТ_a. The development and testing of the opto-fluid system are funded by the grant of Russian Science Foundation (project no. 20-71-10103). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Acknowledgments: Z.L. acknowledges the support from the China Scholarship Council (201708090140). R.A.Z. thanks Avesta Ltd. (http://avesta.ru/en) for the opportunity to work with the femtosecond laser. A.P.A. is grateful to Alexey Melnikov for fruitful discussions. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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