Nanophotonics 2015; 4:332–352 Review Article Open Access S. Gross* and M. J. Withford Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications DOI 10.1515/nanoph-2015-0020 ing methods used to create photonic chips are relatively Received July 7, 2015; accepted July 30, 2015 immature, and the common approach is to adapt the pla- nar (2D) lithography methods originally developed for sil- Abstract: Since the discovery that tightly focused fem- icon microelectronics. Unfortunately, this is akin to push- tosecond laser pulses can induce a highly localised and ing a square peg into a round hole because photons, the permanent refractive index modification in a large num- elementary particle of light, have many degrees of free- ber of transparent dielectrics, the technique of ultrafast dom, in contrast to electrons which have few. For exam- laser inscription has received great attention from a wide ple, a stream of electrons can be characterised in terms range of applications. In particular, the capability to create of current and voltage. A stream of photons, on the other three-dimensional optical waveguide circuits has opened hand, can exhibit different traits based on velocity and up new opportunities for integrated photonics that would brightness, the optical equivalent to current and voltage as not have been possible with traditional planar fabrication well as wavelength, polarisation, spatial mode and orbital techniques because it enables full access to the many de- angular momentum. These additional features reflect the grees of freedom in a photon. This paper reviews the basic three dimensionality of light, features that cannot be fully techniques and technological challenges of 3D integrated exploited with planar circuitry. International concerns re- photonics fabricated using ultrafast laser inscription as garding the so-called ‘data crunch’, the point where we well as reviews the most recent progress in the fields of will run out of data bandwidth [1], is one of many drivers astrophotonics, optical communication, quantum photon- stimulating interest in 3D photonic solutions. ics, emulation of quantum systems, optofluidics and sens- The field of ultrafast-laser-inscribed microphotonics ing. originated with the pioneering work of Davis et al. [2] and Glezer et al. [3]. Those seminal studies were quick to recog- nise the opportunities of this direct-write technique for 3D 1 Introduction photonics. In brief, refractive index changes ranging from 10−4 to 10−2 and varying in size from a few micrometres to Integrated computer chips are ubiquitous, enabling all our tens of micrometres, can be induced inside different types consumer electronics and more. It is less well known that of glasses when irradiated with the tightly focused output integrated photonic chips also represent a vital part of of a femtosecond laser. The magnitude and dimensions of modern society. Indeed, the Internet is enabled by pho- the laser-induced index change are comparable to those tonic chips such as arrayed waveguide gratings, optical of conventional optical fibres. In contrast to optical fibres, splitters and Mach–Zehnder modulators that route, split this method allows optical waveguides to be written in 3D. and multiplex optical signals as well as perform the con- version from electrical to optical signals in order to inter- This field has grown significantly in the past 10 years face with computers. However, whereas the root materials with more than 50 research groups and several commer- and manufacturing technology for computer chips is ma- cial enterprises that are currently active in this pursuit. ture and well established, the materials and manufactur- This community has improved the performance character- istics of this fabrication platform to the point that mod- est propagation losses, typically ~0.1 dB/cm, wavelength *Corresponding Author: S. Gross: Centre for Ultrahigh bandwidth versatility [visible to mid-infrared (IR)] and complex ar- Devices for Optical Systems (CUDOS), MQ Photonics Research Cen- chitectures are now possible. This capability has trig- tre, Department of Physics and Astronomy, Macquarie University, gered a diverse range of applications in classical and non- Sydney, Australia; Email: [email protected] M. J. Withford: Centre for Ultrahigh bandwidth Devices for Optical classical optics, waveguide and fibre lasers, telecommu- Systems (CUDOS), MQ Photonics Research Centre, Department of nications, astronomy, bio-photonics and sensing; appli- Physics and Astronomy, Macquarie University, Sydney, Australia © 2015 S. Gross and M. J. Withford, published De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Unauthenticated Download Date | 12/27/15 12:04 PM Challenges and emerging applications of 3D integrated photonics Ë 333 particular the fabrication of 3D lightwave circuits. Recent progress and emerging applications of 3D-integrated pho- tonics, with a particular focus on astrophotonics and opti- cal communication, are discussed. Furthermore, the latest developments in quantum photonics, modelling of quan- tum systems using waveguide arrays, and optofluidics as well as sensing are presented. The first section gives a gen- eral overview and background of ultrafast laser inscrip- tion, whilst the second section highlights and discusses the particular challenges faced when fabricating 3D light- wave circuits. The third section introduces the aforemen- tioned applications that benefit from the 3D fabrication ca- Figure 1: Longitudinal (A) and transverse (B) ultrafast laser inscrip- pability inherent to ultrafast laser inscription, followed by tion. The arrow indicates the sample’s translation direction. a conclusion. 2 Background Ultrafast laser inscription relies on a tightly focused fem- tosecond laser beam. The sample is placed on computer- controlled motion control equipment to translate it in three dimensions with respect to the focal spot. The sam- ple can either be translated parallel to the laser beam di- rection, the so-called longitudinal writing, or perpendic- ular to the laser beam, that is, transverse writing as il- lustrated in Figure 1. The latter is more commonly used because the maximum device length is not restricted by Figure 2: Waveguide geometries in glass and crystals. (A) Waveg- the working distance of the focusing objective. However, uide with a positive index contrast based on a smooth Type I mod- achieving circular waveguides in the transverse writing ge- ification. (B) Depressed cladding waveguide created from partially overlapping Type I modifications surrounding an unmodified core. ometry is more challenging as it is discussed in Section 3. (C) Stress-induced waveguide in crystals based on two parallel Type The high peak intensity at the location of the focal II tracks. The guided mode is located in between the two damage spot during ultrafast laser inscription causes nonlinear op- lines. The anisotropic stress-field, indicated by the red-dashed tical breakdown of the material. This results in energy de- lines, typically only guides a single polarization. (D) Depressed position, triggering a highly localised structural modifica- cladding geometry based on Type II modifications in crystals. tion of the substrate. In general, three different types of structural modification can be identified depending onthe cations that capitalise on both 2D (single chip and hy- peak intensity of the femtosecond laser pulse [12]. For low brid) and, in particular, 3D (single chip and hybrid) device intensities, a smooth refractive index change, referred to designs. Demonstrations of 3D photonic circuits include as Type I modification is generated [13]. The sign ofre- three-port couplers [4], 2D waveguide arrays [5], pho- fractive index change can be either positive or negative. tonic lanterns [6], orbital angular momentum state gener- At intermediate intensities, a non-isotropic index change ators [7] and quantum random walks [8]. With respect to has been observed, resulting in birefringence. The bire- a photon’s higher degrees of freedom listed earlier, there fringence is caused by self-aligned nanogratings that are have been reports of polarisation-encoded qubits [9], opti- perpendicular to the electric field vector of the inscription cal remapping and phase control aimed at stellar interfer- laser [14]. This type of index change has been observed in ometry [10] and spatial mode conversion on a chip aimed fused silica [15, 16], multicomponent silicate [17] and fluo- at addressing future telecommunications needs [11], all of roaluminate glass [18] as well as in a few crystals such as which exploit 3D photonic designs. TeO2 [19] and sapphire [20]. These in volume nanogratings This paper reviews the fabrication technology and should not be confused with laser-induced surface nanos- challenges associated with ultrafast laser inscription, in tructures. At high peak intensities, optical damage and Unauthenticated Download Date | 12/27/15 12:04 PM 334 Ë S. Gross and M. J. Withford voids caused by micro-explosions are created [21]. These cient spatial overlap between pulses is required, limiting modifications are referred to as Type II. the translation speed to tens of micrometres per second. Nonlinear optical breakdown can be triggered inside Hence the fabrication of relatively complex devices can virtually any transparent dielectric, making ultrafast laser take from a few hours to days, thereby making the pro- inscription exceptionally