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Embedding strong photonic stopbands into traditional optical fibre that can directly access and sense the outside environment is challenging, relying on tedious nano-processing steps that result in fragile thinned fibre. Ultrashort-pulsed laser filaments have recently provided a non-contact means of opening high-aspect ratio nano-holes inside of bulk transparent glasses. This method has been extended here to optical fibre, resulting in high density arrays of laser filamented holes penetrating transversely through the silica cladding and guiding core to provide high refractive index contrast Bragg gratings in the telecommunication band. The point‐by‐point fabrication was combined with post-chemical etching to engineer strong photonic stopbands directly inside of the compact and flexible fibre. Fibre Bragg gratings with sharply resolved π-shifts are presented for high resolution refractive index sensing from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${n}_{{{{{{\rm{H}}}}}}}$$\end{document}nH = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding., Engineered stop bands to sense an ambient environment can enable many applications. Here, the authors demonstrate well-controlled processes to open high-aspect ratio nanoholes through optical fibre for Bragg gratings in the telecomm spectrum and to enable high-resolution refractive index sensing

Bioelastomers are extensively used in biomedical applications due to their desirable mechanical strength, tunable properties, and chemical versatility; however, three-dimensional (3D) printing bioelastomers into microscale structures has proven elusive. Herein, a high throughput omnidirectional printing approach via coaxial extrusion is described that fabricates perfusable elastomeric microtubes of unprecedently small inner diameter (350-550 µm) and wall thickness (40-60 µm). The versatility of this approach is shown through the printing of two different polymeric elastomers, followed by photocrosslinking and removal of the fugitive inner phase. Designed experiments are used to tune the microtube dimensions and stiffness to match that of native ex vivo rat vasculature. This approach affords the fabrication of multiple biomimetic shapes resembling cochlea and kidney glomerulus and affords facile, high-throughput generation of perfusable structures that can be seeded with endothelial cells for biomedical applications. Post-printing laser micromachining is performed to generate micro-sized holes (520 µm) in the tube wall to tune microstructure permeability. Importantly, for organ-on-a-chip applications, the described approach takes only 3.6 min to print microtubes (without microholes) over an entire 96-well plate device, in contrast to comparable hole-free structures that take between 1.5 and 6.5 days to fabricate using a manual 3D stamping approach.

The formation of volumetric nanogratings in fused silica by femtosecond laser pulses are shown to afford new opportunities for manipulating the physical shape and tailoring the optical properties of the modification zone by harnessing unconventional beam shapes. The nanograting assembly was observed to rigorously follow the beam elongation effects induced with conical-shaped phase fronts, permitting a scaling up of the writing volume. Detailed optical characterization of birefringence, dichroism, and scattering loss pointed to flexible new ways to tune the macroscopic optical properties, with advantages in decoupling the induced phase retardation from the modification thickness by controlling the conical phase front angle. Further insights into an unexpected asymmetric response from Gaussian beams modified with concave and convex phase fronts have been provided by nonlinear propagation simulations of the shaped-laser light.

Using photonic devices, we developed a new approach to traditional spectroscopy where the spectral cross-correlation with a template spectrum can be done entirely on-device. By creating photonic devices with a carefully designed, modulated transmission spectrum, the cross-correlation can be carried out optically without requiring any dispersion, vastly simplifying the instrument and reducing its cost. The measured correlation lag can be used for detecting atomic/molecular species within and determining the radial velocity of a particular astrophysical object. We present an overview of two design approaches that are currently being developed that use different photonic platforms: silicon and fibre-based photonics. The silicon photonic approach utilizes ring resonators that can be thermo-optically modulated to carry out the cross-correlation. The fibre approach uses customized fibre Bragg gratings (FBGs) with transmission spectra that can be strain-modulated. Both approaches have been able to detect molecular gas in a lab setting, and we are now in the process of on-sky testing. Lastly, we discuss the future for these types of devices as their simplicity opens up the possibility of developing low-cost, purpose-built multi-object or integral field spectroscopic instruments that could make significant contributions to scientific programs requiring stellar RV measurements and exoplanet detections., Comment: SPIE Proceedings, Astronomical Telescopes and Instrumentation, July 2022, 10 pages, 7 figures

The demands for faster networks and growing requirements in data centers have spurred the development of photonic integrated circuits [1] , particularly in designing a universal platform for optical/electrical interconnects. Some headway towards an optical PCB platform has been made, for instance, chip-to-chip polymer waveguide interconnects [2] , ion-exchange waveguide flip-chip packaging [3] , and laser-written 3D circuits in glass [4] . However, the design of an all-optical platform for chip-to-chip or chip-to-network (i.e. chip-to-fiber) communications still faces challenges, namely densification of I/O channels and bending radius constraints for optical waveguide routing. The latter can be addressed by using total internal reflection (TIR) surfaces to bend light at sharp angles, such as for compact fiber coupling to Silicon Photonics (SiP) gratings with a polished glass wedge or angled SMF array [5] . In this regard, we propose a platform which utilizes femtosecond laser assisted chemical etching to embed TIR micro-mirrors and facilitate sharp bending of optical waveguide circuits inside of fused silica glass. Full 3D optical routing is proposed with multi-level waveguides and high channel-density.

The laser inscription of the volume gratings in transparent materials by inducing localized refractive index modification [1] is promising for generating internal colouring effects in flexible designs for wide ranging applications. Nano-explosion of open cavity voids such as provided by elongated laser filaments in glasses [2] are further appealing for enabling high diffraction efficiency due to the large refractive index contrast. While Bessel beams [3] have been widely employed to induce high aspect ratio laser modifications, our group has studied the interplay between Kerr and plasma focusing and surface aberration in forming long filaments in glasses [4] . Spatial light modulators (SLM) present additional opportunities in beam shaping and splitting [5] to enable high resolution multi-positioned interaction volumes of 3D periodic nanostructure. The extension of such techniques to transparent polymers have not be widely explored. Moreover, the point-by-point fabrication technique is slow, time consuming, and prone to positioning errors in 3D space. This paper presents new prospects for high-speed structuring of 3D photonic gratings in polymers.

Femtosecond laser light has been shaped with a spatial light modulator (SLM) to generate optically thin, aberration-free sheets of uniform intensity for inscription of fiber Bragg gratings (FBGs) inside the core of SMF-28 telecommunication fiber. The combination of flexible beam shaping and sheet-by-sheet writing offers facile means in controlling the coupling to cladding or radiation modes while facilitating spectral tuning flexibility that is not available with interference-based techniques. Spectral responses of uniform first order FBGs fabricated with single-pulse exposures are presented.

Femtosecond laser welding was extended to optical silica fiber (SMF-28) by focusing through fused silica substrates and ferrules to form all-glass weld seams. Laser radiation was focused into the fiber cladding to create a welding zone, which drove molten glass to fill as much as a 3 𝜇𝑚 gap around a contact line to form a crack free pseudo-continuous welding seams along the contact line. The strong weld seams up to 30 𝜇𝑚 wide were generated in fiber-to-plate and fiber-to-ferrule geometries without inducing photochemical or thermal degradation of a fiber Bragg grating (FBG) positioned only 62.5 𝜇m from the weld zone. Welding was optimized by real-time monitoring of the FBG thermo-optical shift during laser scanning. Four-point bending tests confirmed a high mechanical strength while thermal annealing showed stable mechanical and FBG responses up to 1000 ˚C. Femtosecond laser writing and welding thus demonstrated a flexible means for photonics fabrication and packaging of FBGs, enabling reliable, high frequency vibration sensing suited for high temperature and strain environments.

A two-dimensional (2D), point-by-point writing technique of forming filament arrays with femtosecond laser pulses was applied inside of single-mode optical fiber to open new opportunities for 2D photonic bandgap engineering and highresolution spectroscopy. A small grating period of ~300 nm provided first-order diffraction externally to the fiber cladding, with spectral, blazing, and self-focusing properties tailored by varying the 1D and 2D grating design. The spectral properties of the lens-less, all-fiber spectrometer have been tuned with varying grating dimension, chirping rate, and blazing design that can meet wide ranging criteria for design of compact grating spectrometers in narrow to broad spectral ranges of the visible and telecommunication bands.

he gegravity Python package is a collection of tools for analyzing general equilibrium, structural gravity models of international trade. It provides a framework for estimating structural gravity models, simulating counterfactual trade experiments, and conducting Monte Carlo simulations to derive measures of statistical precision for model estimates. The package is based on prominent models used in the literature and aims to make structural gravity modeling more readily accessible to researchers and policy analysts.

Nematic liquid crystal was introduced into a nano-hole array grating formed by femtosecond laser filaments in a telecommunication fiber. Capillary alignment has resulted in a strong polarization extinction ratio within a broad Bragg resonance.

Various beam shaping approaches were examined to counter the negative influence of surface aberration arising when inscribing optical waveguides deeply inside of glass with a femtosecond laser. Aberration correction was found unable to completely recover the low-loss waveguide properties, prompting a comprehensive examination of waveguides formed with focused Gaussian–Bessel beams. Diverging conical phase fronts are presented as a hybrid means of partial aberration correction to improve insertion loss and a new, to the best of our knowledge, means of asymmetric beam shaping. In this way, low-loss waveguides are presented over shallow to deep writing depth (2.8 mm) where morphological and modal properties could be further tuned with conical phase front.

Beam shaping of femtosecond lasers was applied in the Kerr-lensing and aberration regime to enable high-aspect-ratio filament tracks to form uniformly through the silica cladding and core waveguide of single-mode fiber (SMF28/450). One- and two-dimensional filament arrays were embedded along the waveguide to form weak to strong photonic stopbands. The filament shape enhanced transverse light scattering into narrow azimuthal radiation zones. Tailoring of chirp and 2D patterns further facilitated high-resolution (~350 pm) spectral focusing onto a CCD camera, defining a compact “Spectrometer-in-fibre” over the visible spectrum. At higher exposure, the filaments opened into narrow nano-channels (200-400 nm diameter) presenting a novel Bragg grating for refractive index sensing of the ambient environment. This lab-in-fiber technology presents a robust, flexible, and ubiquitous communication platform for nano-scale sensing across expansive networks or into tightly confined, sinuous spaces.

A chirped-array of filament tracks, formed in single-mode fiber with femtosecond laser pulses, provided radiative emission and lens less focusing to define an all-fiber spectrometer with 0.3 nm resolution in the visible spectrum.

Femtosecond laser pulses were shaped into uniform intensity sheets for point-by-point writing of micro-volume gratings inside of single-mode optical fiber core, opening flexible means for tailoring spectral properties of fiber Bragg grating devices.

Filament arrays were inscribed off-axis in the core of standard single-mode telecommunication fiber, using femtosecond laser pulses. The flexible line-by-line writing formed uniform, parallel filaments, permitting Bragg grating sensing of the photoelastic response from inside of the narrow grating plane. Active monitoring of the Bragg resonance wavelength while driving a lateral fiber tip displacement directly informed on the fiber mechanics when coupled with opto-mechanical modelling. Overlaying of parallel and orthogonal gratings further provided a strongly contrasting azimuthal sensitivity, which paves the way for multi-dimensional displacement sensing with improved precision.

The countering and enhancing influence of surface aberration in combination with conical phase front beam shaping was examined over shallow to deep focusing inside of fused silica to facilitate the geometric control of chemical etching tracks formed by femtosecond laser inscription. The tuning of the collective shaping effects is shown to compensate for a wide focusing range to favorably manipulate the chemical etching rate, the resolution, the surface roughness, and the cross-section profile of the resulting micro-channels. Using only a single laser scanning track, micro-channels have been tailored from symmetrical to highly asymmetric cross-section of up to 1:13 aspect ratio, permitting distortion-free processing to depths of 1.5 mm. The combined shaping effects thus expand the utility of laser chemical etching for micromachining of transparent substrates.

Femtosecond laser irradiation followed by chemical etching (FLICE) was applied to standard telecommunication fiber, embedding an open-filament, grating array through the silica cladding and guiding core cross-section. Second and third order Bragg air-gratings are presented for the first time, providing nano-fluidic channels accessible to the outer cladding surface. Pishifted fiber Bragg grating responses to chemical etching were used to optimize a novel refractive index sensor for fluids.

Optically clear and elastic conductors are critical for the next generation of fully imperceptible stretchable electronics that are not only optically transparent, but also invisible under typical lighting conditions and reading distances. Such conductors have a central role in a wide range of emerging applications such as wearable computing, soft bioelectronics, and biologically-inspired robotics. Here, we introduce a materials architecture and laser-based microfabrication technique to produce electrically conductive circuitry (sheet resistance = 2.95 Ω/sq; conductivity = 5.65×105 S/m) that are soft, elastic (strain limit > 100%), and optically transparent. The circuitry is composed of a grid-like array of visually imperceptible liquid metal (LM) lines on a clear elastomer. The laser fabrication approach allows for fully imperceptible electronics that have not only high optical transmittance (>85% at 550 nm) but also are invisible under typical lighting conditions and reading distances. This unique combination of properties is enabled by using a direct laser writing technique that results in LM grid patterns with a line width and pitch as small as 4.5 μm and 100 μm, respectively – yielding grid-like wiring on a transparent polydimethylsiloxane (PDMS) elastomer substrate that has adequate conductivity for digital functionality but is also well below the threshold for visual perception. The fabricated LM wiring can be readily interfaced with conventional circuit components (e.g., lead wiring, or LED chips) to enable optically clear digital electronics. The electrical, mechanical, electromechanical, and optomechanical characterization of fabricated LM circuits shows that the high conductivity and transparency are preserved at tensile strains of ~100%.

When studying the formation of trade between two countries, traditional modeling has described it as being primarily dependent on individual and bilateral characteristics of the two trading partners. It is likely, however, that trade is dependent not only on the two countries involved but on the patterns by which all countries trade. Standard efforts to control for these complex network dependencies such as the inclusion of multilateral resistance terms in gravity models provide only a blunt reflection of these dependencies and overlook many of their details. This paper describes the explicit incorporation of complex network patterns in trade models. Two types of models are considered: gravity models that incorporate network covariates and exponential random graph models (ERGMs) that analyze trade from a network perspective. Estimates of both models provide evidence that network dependencies are influential in international trade. Comparisons of both models indicate that each approach outperforms the other at capturing and replicating certain types of network patterns. These results indicate that complex network patterns are an important determinant of trade, that gravity models can capture much of this dependency, and that other network models such as ERGMs can be valuable tools for capturing some types of network dependencies.

A comprehensive design of wavefront-splitting interferometers (WSIs) is introduced for integration into wafer light circuits and optical fiber systems. The WSI detects the coherent interference of two wavefronts after being equally split by a single buried resonator and collected into a single mode waveguide. WSIs present a desirable compact sensor as possible with Fabry Perot interferometers (FPIs), with the additional benefits of strong visibility contrast and high sensitivity to external parameters as expected with Mach–Zehnder interferometers. Theoretical models and finite-difference time-domain simulations show the WSI spectral responses to be 2 $\times$ to 120 $\times$ more sensitive to changes in refractive index, temperature, and strain over comparable Bragg grating waveguides and FPIs. Femtosecond laser irradiation with selective chemical etching provided the flexible means for 3-D geometric structuring and waveguide integration below the surface, delivering precise WSIs with a small $\sim$ 12-nm rms surface roughness. Temperature and vacuum sensing were demonstrated with high sensing resolution (0.8 ${}^{\circ}$ C and 1.8 $\times$ 10 ${}^{-5}$ RIU) and sensitivity (60.6 pm/ ${}^{\circ}$ C and 2800 nm/RIU) to match theoretically anticipated values. Such high finesse optical elements open a new realm of optical sensing and integrated optical circuit concepts without relying on tedious nanoprecision assembly methods or the use of large optical components.

The design of micro-optical resonator arrays are introduced and tailored towards refractive index sensing applications, building on the previously unexplored benefits of open dielectric stacks. The resonant coupling of identical hollow cavities present strong and narrow spectral resonance bands beyond that available with a single Fabry Perot interferometer. Femtosecond laser irradiation with selective chemical etching is applied to precisely fabricate stacked and waveguide-coupled open resonators into fused silica, taking advantage of small 12 nm rms surface roughness made available by the self-alignment of nanograting planes. Refractive index sensing of methanol-water solutions confirm a very attractive sensing resolution of 6.5 × 10−5 RIU. Such high finesse optical elements open a new realm of optofluidic sensing and integrated optical circuit concepts for detecting minute changes in sample properties against a control solution that may find importance in chemical and biological sensors, telecom sensing networks, biomedical probes, and low-cost health care products.

A multi-level nanophotonic structure is a major goal in providing advanced optical functionalities as found in photonic crystals and metamaterials. A three-level nano-grating phase mask has been fabricated in an electron-beam resist (ma-N) to meet the requirement of holographic generation of a diamond-like 3D nanostructure in photoresist by a single exposure step. A 2D mask with 600 nm periodicity is presented for generating first order diffracted beams with a preferred π/2 phase shift on the X- and Y-axes and with sufficient 1(st) order diffraction efficiency of 3.5% at 800 nm wavelength for creating a 3D periodic nanostructure in SU-8 photoresist. The resulting 3D structure is anticipated to provide an 8% complete photonic band gap (PBG) upon silicon inversion. A thin SiO2 layer was used to isolate the grating layers and multiple spin-coating steps served to planarize the final resist layer. A reversible soft coating (aquaSAVE) was introduced to enable SEM inspection and verification of each insulating grating layer. This e-beam lithographic method is extensible to assembling multiple layers of a nanophotonic structure.

Femtosecond laser interactions inside transparent dielectric films of refractive index, n film, with tight focusing presents strong nonlinear interactions that can be preferentially confined at the fringe maxima as formed by Fabry-Perot interference, to generate thin nanoscale plasma disks separated on half-wavelength, λ/2n film. The nano-thin disk explosions can be controlled inside the film to cleave open subwavelength internal cavities at single or multiple periodic depths at low laser exposure, while higher exposure will eject a quantised number of film segments with segment thickness defined by the laser wavelength. This new method enables high-resolution film patterning for ejecting nanodisks at quantised film depth for colouring and three-dimensional (3D) surface structuring, as well as for fabrication of free-standing nanofilms.

A material architecture and laser-based microfabrication technique is introduced to produce electrically conductive films (sheet resistance = 2.95 Ω sq-1 ; resistivity = 1.77 × 10-6 Ω m) that are soft, elastic (strain limit >100%), and optically transparent. The films are composed of a grid-like array of visually imperceptible liquid-metal (LM) lines on a clear elastomer. Unlike previous efforts in transparent LM circuitry, the current approach enables fully imperceptible electronics that have not only high optical transmittance (>85% at 550 nm) but are also invisible under typical lighting conditions and reading distances. This unique combination of properties is enabled with a laser writing technique that results in LM grid patterns with a line width and pitch as small as 4.5 and 100 µm, respectively-yielding grid-like wiring that has adequate conductivity for digital functionality but is also well below the threshold for visual perception. The electrical, mechanical, electromechanical, and optomechanical properties of the films are characterized and it is found that high conductivity and transparency are preserved at tensile strains of ≈100%. To demonstrate their effectiveness for emerging applications in transparent displays and sensing electronics, the material architecture is incorporated into a couple of illustrative use cases related to chemical hazard warning.

Kerr-lens self-channelling of femtosecond laser light offers a novel high-aspect geometry for laser processing inside transparent materials. In glass materials, the laser filaments enable white-light continuum generation, scribing, nanochannel formation, and refractive index modification. In the present work, refractive index matching oils were applied around optical fiber to eliminate astigmatic aberration and thereby form highly symmetric and uniform filaments selectively in the cladding or core waveguide of standard single-mode optical fibre (SMF-28). Under tight focusing, long filaments exceeding 20 um length were formed with single pulses to sub-micron diameter. Arrays of 0.5 um spaced filaments are verified by formation of strong fiber Bragg gratings (FBGs). Flexible positioning of the filament arrays within the fiber core offers wide scope for coupling to cladding and radiation modes and creating new types of in-fibre optical devices.

Spatial Light Modulators (SLM) are emerging as a power tool for laser beam shaping whereby digitally addressed phase shifts can impose computer-generated hologram patterns on incoming laser light. SLM provide several additional advantages with ultrashort-pulsed lasers in controlling the shape of both surface and internal interactions with materials. Inside transparent materials, nonlinear optical effects can confine strong absorption only to the focal volume, extend dissipation over long filament tracks, or reach below diffraction-limited spot sizes. Hence, SLM beam shaping has been widely adopted for laser material processing applications that include parallel structuring, filamentation, fiber Bragg grating formation and optical aberration correction. This paper reports on a range of SLM applications we have studied in femtosecond processing of transparent glasses and thin films. Laser phase-fronts were tailored by the SLM to compensate for spherical surface aberration, and to further address the nonlinear interactions that interplay between Kerr-lens self-focusing and plasma defocusing effects over shallow and deep focusing inside the glass. Limits of strong and weak focusing were examined around the respective formation of low-loss optical waveguides and long uniform filament tracks. Further, we have employed the SLM for beam patterning inside thin film, exploring the limits of phase noise, resolution and fringe contrast during interferometric intra-film structuring. Femtosecond laser pulses of 200 fs pulse duration and 515 nm wavelength were shaped by a phase-only LCOS-SLM (Hamamatsu X10468-04). By imposing radial phase profiles, axicon, grating and beam splitting gratings, volume shape control of filament diameter, length, and uniformity as well as simultaneous formation of multiple filaments has been demonstrated. Similarly, competing effects of spherical surface aberration, self-focusing, and plasma de-focusing were studied and delineated to enable formation of low-loss optical waveguides over shallow and deep focusing conditions. Lastly, SLM beam shaping has been successfully extended to interferometric processing inside thin transparent film, enabling the arbitrary formation of uniform or non-uniform, symmetric or asymmetric patterns of flexible shape on nano-scale dimensions without phase-noise degradation by the SLM patterning. We present quantized structuring of thin films by a single laser pulse, demonstrating λ/2nfilm layer ejection control, blister formation, nano-cavities, and film colouring. Closed intra-film nanochannels with high aspect ratio (20:1) have been formed inside 3.5 um thick silica, opening new prospects for sub-cellular studies and lab-in-film concepts that integrate on CMOS silicon technologies.

Multifunctional lab in fiber technology seeks to translate the accomplishments of optofluidic, lab on chip devices into silica fibers. a robust, flexible, and ubiquitous optical communication platform that can underpin the ‘Internet of Things’ with distributed sensors, or enable lab on chip functions deep inside our bodies. Femtosecond lasers have driven significant advances in three-dimensional processing, enabling optical circuits, microfluidics, and micro-mechanical structures to be formed around the core of the fiber. However, such processing typically requires the stripping and recoating of the polymer buffer or jacket, increasing processing time and mechanically weakening the device. This paper reports on a comprehensive assessment of laser damage in urethane-acrylate-coated fiber. The results show a sufficient processing window is available for femtosecond laser processing of the fiber without damaging the polymer jacket. The fiber core, cladding, and buffer could be simultaneously processed without removal of the buffer jacket. Three-dimensional lab in fiber devices were successfully fabricated by distortion-free immersionlens focusing, presenting fiber-cladding optical circuits and progress towards chemically-etched channels, microfluidic cavities, and MEMS structure inside buffer-coated fiber.

The nonlinear interactions of femtosecond lasers are driving multiple new application directions for nanopatterning and structuring of thin transparent dielectric films that serve in range of technological fields. Fresnel reflections generated by film interfaces were recently shown to confine strong nonlinear interactions at the Fabry-Perot fringe maxima to generate thin nanoscale plasma disks of 20 to 40 nm thickness stacked on half wavelength spacing, λ/ 2n film , inside a film (refractive index, n film ). The following phase-explosion and ablation dynamics have resulted in a novel means for intrafilm processing that includes ‘quantized’ half-wavelength machining steps and formation of blisters with embedded nanocavities. This paper presents an extension in the control of interferometric laser processing around our past study of Si 3 N 4 and SiO x thin films at 515 nm, 800 nm, and 1044 nm laser wavelengths. The role of laser polarization and incident angle is explored on fringe visibility and improving interferometric processing inside the film to dominate over interface and / or surface ablation. SiO x thin films of 1 μm thickness on silicon substrates were irradiated with a 515 nm wavelength, 280 fs duration laser pulses at 0° to 65° incident angles. A significant transition in ablation region from complete film removal to structured quantized ejection is reported for p- and s-polarised light that is promising to improve control and expand the versatility of the technique to a wider range of applications and materials. The research is aimed at creating novel bio-engineered surfaces for cell culture, bacterial studies and regenerative medicine, and nanofluidic structures that underpin lab-in-a-film. Similarly, the formation of intrafilm blisters and nanocavities offers new opportunities in structuring existing thin film devices, such as CMOS microelectronics, LED, lab-on-chips, and MEMS.

Second harmonic generation (SHG) is demonstrated in femtosecond laser written waveguides in fused silica through a combination of thermal poling and laser-based quasi-phase-matching (QPM) techniques. Quasi-phase-matching was controlled by the periodic erasure of induced nonlinearity through femtosecond laser erasure. A maximum SHG conversion efficiency of 6.6±0.5×10−5%/W is reported for the fundamental wavelength of 1552.8 nm with a phase-matching bandwidth of 4.4 nm for a 10.0 mm long waveguide. For a shorter sample, an effective second-order nonlinearity of χ(2)=0.012±0.001 pm/V was measured. Chirped QPM structures for wider SHG bandwidths also were demonstrated. Such periodically poled waveguides are promising for introducing nonlinear optical components within the 3D passive optical circuits that can be flexibly formed in fused silica by femtosecond laser writing.

An O-band VCSEL is integrated with a silicon photonic chip using a grating coupler that provides feedback to maintain the emission polarization. The VCSEL-to-chip coupling efficiency is −5 dB at a current of 2.5 mA.

The lab-on-chip (LOC) platform has presented a powerful opportunity to improve functionalization, parallelization, and miniaturization on planar or multilevel geometries that has not been possible with fiber optic technology. A migration of such LOC devices into the optical fiber platform would therefore open the revolutionary prospect of creating novel lab-in-fiber (LIF) systems on the basis of an efficient optical transport highway for multifunctional sensing. For the LIF, the core optical waveguide inherently offers a facile means to interconnect numerous types of sensing elements along the optical fiber, presenting a radical opportunity for optimizing the packaging and densification of diverse components in convenient geometries beyond that available with conventional LOCs. In this paper, three-dimensional patterning inside the optical fiber by femtosecond laser writing, together with selective chemical etching, is presented as a powerful tool to form refractive index structures such as optical waveguides and gratings as well as to open buried microfluidic channels and optical resonators inside the flexible and robust glass fiber. In this approach, optically smooth surfaces (~12 nm rms) are introduced for the first time inside the fiber cladding that precisely conform to planar nanograting structures when formed by aberration-free focusing with an oil-immersion lens across the cylindrical fiber wall. This process has enabled optofluidic components to be precisely embedded within the fiber to be probed by either the single-mode fiber core waveguide or the laser-formed optical circuits. We establish cladding waveguides, X-couplers, fiber Bragg gratings, microholes, mirrors, optofluidic resonators, and microfluidic reservoirs that define the building blocks for facile interconnection of inline core-waveguide devices with cladding optofluidics. With these components, more advanced, integrated, and multiplexed fiber microsystems are presented demonstrating fluorescence detection, Fabry-Perot interferometric refractometry, and simultaneous sensing of refractive index, temperature, and bending strain. The flexible writing technique and multiplexed sensors described here open powerful prospects to migrate the benefits of LOCs into a more flexible and miniature LIF platform for highly functional and distributed sensing capabilities. The waveguide backbone of the LIF inherently provides an efficient exchange of information, combining sensing data that are attractive in telecom networks, smart catheters for medical procedures, compact sensors for security and defense, shape sensors, and low-cost health care products.

Today, the ultrafast laser has become a powerful and robust tool for both science and commercial applications, opening new discoveries in high intensity interaction with materials through to enabling new medical procedures or manufacturing of novel product concepts. One of the most rewarding areas has been the novel means for inducing absorption inside of normally transparent materials. In Prof. Herman’s group, these interactions are studied in the context of strong nonlinear optical effects and the unfolding of remarkably energetic processes that begin in the non-equilibrium domain of thermodynamics. By tuning such processes, our group has sought out the technical benefits in three-dimensional (3D) structuring of optical materials to nano-scale dimensions, aiming to invent new forms of 3D additive and subtractive manufacturing. The research often leads to invention of new processes and product concepts touching the wide areas of photonics, optical communications, optical packaging, fibre optics, data storage, security marking, optical sensing, heads-up display, biosensing, lab-in-a-chip, and lab-in-fibre. The present paper builds on a myriad of additive and subtractive laser techniques that have culminated in transparent glasses to address a significant challenge in packaging of photonics devices. On the level of laser processing, femtosecond laser interactions were exploited for full-scale 3D structuring of the internal dimensions of glass wafers to define optical circuits and to guide etching selectively along laser tracks. Following chemical etching, the optical glass circuits become precisely arranged with alignment slots for enabling a facile assembly and packaging with optical fibers or other optical devices, with the focus here on the silicon photonic chip and multi-core optical fiber. The alignment slots provide exception self-guidance in optical-to-optical interconnections to sub-micron dimensions. The optical interposer presented herein demonstrates the potential for scaling up of laser 3D writing to enable high-density optical packaging, specifically addressing the major bottleneck for efficiently connecting optical fibers to silicon photonic processors as required in telecom and data centers. Moreover, such 3D additive and subtractive processing promises higher scale integration and rapid photonic assembly and packaging of micro-optic components for broader-based applications from integrated biophotonic chips to wearable displays.

A systematic study of glass scribing is presented on the benefits of ultrafast laser burst trains in generating filamentation tracks to guide cleaving of glass substrates. The interplay of Kerr self-focusing, plasma defocusing, and burst-train accumulation effects in filament formation was characterized by time-resolved in-situ microscopic imaging. Various filament-track scribing geometries were compared with and without assistance from burst-train pulse delivery or surface V-groove ablation. The cleaving guidance and reproducibility were examined together with the breaking force, facet morphology and flexural strength of cleaved substrates to assess the overall scribing and cleaving quality. The reported results attest to the benefits and flexibility of burst-mode ultrafast laser interactions to assist cleaving of optically transparent materials along well formed filament arrays.

We present multiple methods of high aspect ratio hole drilling in fused silica glass, taking advantage of high power and high repetition rate picosecond lasers and flexible beam delivery methods to excise deep holes with minimal collateral damage. Combinations of static and synchronous scanning of laser focus were explored over a range of laser repetition rates and burst-train profiles that dramatically vary laser plume interaction dynamics, heat-affected zone, and heat accumulation physics. Chemically assisted etching of picosecond laser modification tracks are also presented as an extension from femtosecond laser writing of volume nanograting to form high aspect ratio (77) channels. Processing windows are identified for the various beam delivery methods that optimize the laser exposure over energy, wavelength, and repetition rate to reduce microcracking and deleterious heating effects. The results show the benefits of femtosecond laser interactions in glass extend into the picosecond domain, where the attributes of higher power further yield wide processing windows and significantly faster fabrication speed. High aspect ratio holes of 400 μm depth were formed over widely varying rates of 333 holes per second for mildly cracked holes in static-focal positioning through to one hole per second for low-damage and taper free holes in synchronous scanning.

We study the exposure limitations of femtosecond laser microsurgery and multiphoton imaging in a microfluidic chip environment, assessing damage thresholds at various interfaces as well as interference from bubble formation in the hosting solution. Both heat accumulation and incubation effects from multipulse laser exposures at 1-MHz repetition rate were evaluated. For demonstration, three microsurgery approaches of laser scribing, percussion drilling and trepanning were applied to arterial walls loaded in vitro in a lab-on-a-chip device. We report that deleterious effects from interface damage and microbubble formation can be avoided to offer laser processing windows for damage-free fluorescence imaging and precise microsurgery of live tissue hosted inside small microfluidic chambers.