From ray optics to modal analysis: using photonics web simulations to build intuition and address common misconceptions.

The emerging field of silicon photonics has created a large need for Ph.D. photonic integrated circuit design engineers. Developing intuition for electromagnetic waves at the micron scale is a major challenge facing undergraduate and graduate students in photonics. Students often misapply lessons learned from macroscale ray optics to submicron waveguide modes in dielectric structures. In this work, key student misconceptions were identified and addressed in a research study using photonics training simulations. A learning module with interactive 3D vector field visualizations was deployed in a massive open online course to train the next generation of photonics design engineers.

The photonics and optics workforce: unleashing the potential for greater industry growth-introduction to the feature issue.

This feature issue highlights specific photonics and optics workforce challenges, opportunities for industry support, and state-of-the-art-training methods.

Virtual reality enabled asynchronous learning modules for fiber optic preform manufacturing education.

The demand for skilled workers and novel manufacturing training solutions has increased with the growing demand for fiber optic cables. Web-based simulations can be used for training, and this paper presents an approach for developing a fiber preform manufacturing browser-based VR simulation. Subsequently, a study was conducted to evaluate the effectiveness of the simulation based on learning gains and learner perception of ease of use, usefulness, intention of use, learning outcomes, and workload. A mixed-methods between-subjects study with 63 participants found that the combination of lecture and simulation was significantly better for perceived usefulness and learning outcomes compared to lecture-only or lecture-and-video conditions.

Adaptable middle-skilled labor: a neglected roadblock to photonics industry growth.

A roadblock to long-term growth of the photonics industry is the availability of well-trained, adaptable middle-skilled workers. This research characterizes the middle-skilled workforce gap, including the quantity required and skills needed. We estimate that 42,000 new technical middle-skilled workers are needed by 2030, requiring another 100 technician programs nationwide. Training skills along the supply chain are critical; programs must emphasize testing, troubleshooting, and process design. Middle-skilled workers trained in critical thinking will enable an adaptable workforce capable of handling technology evolution. Finally, recommendations for the academia, industry, and middle-skilled training ecosystem are included to ensure that the latter evolves with technology development.

High spectro-temporal compression on a nonlinear CMOS-chip.

Optical pulses are fundamentally defined by their temporal and spectral properties. The ability to control pulse properties allows practitioners to efficiently leverage them for advanced metrology, high speed optical communications and attosecond science. Here, we report 11× temporal compression of 5.8 ps pulses to 0.55 ps using a low power of 13.3 W. The result is accompanied by a significant increase in the pulse peak power by 9.4×. These results represent the strongest temporal compression demonstrated to date on a complementary metal-oxide-semiconductor (CMOS) chip. In addition, we report the first demonstration of on-chip spectral compression, 3.0× spectral compression of 480 fs pulses, importantly while preserving the pulse energy. The strong compression achieved at low powers harnesses advanced on-chip device design, and the strong nonlinear properties of backend-CMOS compatible ultra-silicon-rich nitride, which possesses absence of two-photon absorption and 500× larger nonlinear parameter than in stoichiometric silicon nitride waveguides. The demonstrated work introduces an important new paradigm for spectro-temporal compression of optical pulses toward turn-key, on-chip integrated systems for all-optical pulse control.

Dynamic Complex Emulsions as Amplifiers for On-Chip Photonic Cavity-Enhanced Resonators.

Despite the recent emergence of microcavity resonators as label-free biological and chemical sensors, practical applications still require simple and robust methods to impart chemical selectivity and reduce the cost of fabrication. We introduce the use of hydrocarbon-in-fluorocarbon-in-water (HC/FC/W) double emulsions as a liquid top cladding that expands the versatility of optical resonators as chemical sensors. The all-liquid complex emulsions are tunable droplets that undergo dynamic and reversible morphological transformations in response to a change in the chemical environment (e.g., exposure to targeted analytes). This chemical-morphological coupling drastically modifies the effective refractive index, allowing the complex emulsions to act as a chemical transducer and signal amplifier. We detect this large change in the refractive index by tracking the shift of the enveloped resonant spectrum of a silicon nitride (Si3N4) racetrack resonator-based sensor, which correlates well with a change in the morphology of the complex droplets. This combination of soft materials (dynamic complex emulsions) and hard materials (on-chip resonators) provides a unique platform for liquid-phase, real-time, and continuous detection of chemicals and biomolecules for miniaturized and remote, environmental, medical, and wearable sensing applications.

CLEAR: A Holistic Figure-of-Merit for Post- and Predicting Electronic and Photonic-based Compute-system Evolution.

Continuing demands for increased computing efficiency and communication bandwidth have pushed the current semiconductor technology to its limit. This led to novel technologies with the potential to outperform conventional electronic solutions such as photonic pre-processors or accelerators, electronic-photonic hybrid circuits, and neural networks. However, the efforts made to describe and predict the performance evolution of compute-performance fall short to accurately predict and thereby explain the actually observed development pace with time; that is all proposed metrics eventually deviate from their development trajectory after several years from when they were originally proposed. This discrepancy demands a figure-of-merit that includes a holistic set of driving forces of the compute-system evolution. Here we introduce the Capability-to-Latency-Energy-Amount-Resistance (CLEAR) metric encompassing synchronizing speed, energy efficiency, physical machine size scaling, and economic cost. We show that CLEAR is the only metric to accurately describe the historical compute-system development. We find that even across different technology options CLEAR matches the observed (post-diction) constant rate-of-growth, and also fits proposed future compute-system (prediction). Therefore, we propose CLEAR to serve as a guide to quantitatively predict required compute-system demands at a given time in the future.

Loss reduction of silicon-on-insulator waveguides for deep mid-infrared applications.

We report that propagation loss of optical waveguides based on a silicon-on-insulator (SOI) material platform can be greatly reduced. Our simulations show that the loss, including SiO2 absorption and substrate leakage, but no scattering loss, is 0.024 and 0.53 dB/cm in the deep mid-infrared at 4.8 and 7.1 µm wavelengths, where the material absorption in SiO2 is 100 and 1000 dB/cm, respectively. The loss becomes negligible, compared to scattering loss in Si waveguides. This is enabled by using the TE10 mode in a pedestal waveguide. We also show that the TE10 mode can be excited in the proposed waveguide by the fundamental mode with a coupling efficiency of >94%. Low propagation loss, high coupling efficiency, and fabrication-friendly design would make it promising for practical use of SOI devices in the deep mid-infrared.

Mid-IR supercontinuum generated in low-dispersion Ge-on-Si waveguides pumped by sub-ps pulses.

Ge-on-Si is an attractive material platform for mid-IR broadband sources on a chip because of its wide transparency window, high Kerr nonlinearity and CMOS compatibility. We present a low-loss Ge-on-Si waveguide with flat and low dispersion from 3 to 11 µm, which enables a coherent supercontinuum from 2 to 12 µm, generated using a sub-ps pulsed pump. We show that 700-fs pump pulses with a low peak power of 400 W are needed to generate such a wide supercontinuum, and the waveguide length is around 5.35 mm.

Nonlinear characterization of GeSbS chalcogenide glass waveguides.

GeSbS ridge waveguides have recently been demonstrated as a promising mid - infrared platform for integrated waveguide - based chemical sensing and photodetection. To date, their nonlinear optical properties remain relatively unexplored. In this paper, we characterize the nonlinear optical properties of GeSbS glasses, and show negligible nonlinear losses at 1.55 µm. Using self - phase modulation experiments, we characterize a waveguide nonlinear parameter of 7 W-1/m and nonlinear refractive index of 3.71 × 10-18 m2/W. GeSbS waveguides are used to generate supercontinuum from 1280 nm to 2120 nm at the -30 dB level. The spectrum expands along the red shifted side of the spectrum faster than on the blue shifted side, facilitated by cascaded stimulated Raman scattering arising from the large Raman gain of chalcogenides. Fourier transform infrared spectroscopic measurements show that these glasses are optically transparent up to 25 µm, making them useful for short - wave to long - wave infrared applications in both linear and nonlinear optics.

Bilayer dispersion-flattened waveguides with four zero-dispersion wavelengths.

We propose a new type of bilayer dispersion-flattened waveguides that have four zero-dispersion wavelengths. Low and flat dispersion can be achieved by using two different material combinations, with a much smaller index contrast as compared to the previously proposed slot-assisted dispersion-flattened waveguides. Without using a nano-slot, dispersion becomes less sensitive to waveguide dimensions, which is highly desirable for high-yield device fabrication. Ultra-low dispersion, high nonlinearity, and fabrication-friendly design would make it promising for practical implementation of nonlinear photonic functions. The proposed waveguide configuration deepens our understanding of the dispersion flattening principle.

Effect of a breather soliton in Kerr frequency combs on optical communication systems.

In this study, we numerically investigate the effect of Kerr-comb-generated breather soliton pulses on optical communication systems. The breather soliton pulse amplitude and spectrum envelope oscillate periodically in time. Simulations show that the spectrum of each comb line in the breather soliton state has multiple sub-teeth due to the periodic oscillation of the comb spectrum. In the simulation, the comb output is modulated with different formats. We find that the sub-teeth distort quadrature phase-shift-keyed signals but have less of an effect on on-off-keyed signals.

Soliton breathing induced by stimulated Raman scattering and self-steepening in octave-spanning Kerr frequency comb generation.

We investigate the impact of stimulated Raman scattering (SRS) and self-steepening (SS) on breather soliton dynamics in octave-spanning Kerr frequency comb generation. SRS and SS can transform chaotic fluctuations in cavity solitons into periodic breathing. Furthermore, with SRS and SS considered, bandwidth of the soliton breathes more than two times stronger. The simultaneous presence of SRS and SS also make the soliton breathe slower and degrades the coherence of the soliton.

Label-free water sensors using hybrid polymer-dielectric mid-infrared optical waveguides.

A chip-scale mid-IR water sensor was developed using silicon nitride (SiN) waveguides coated with poly(glycidyl methacrylate) (PGMA). The label-free detection was conducted at λ=2.6-2.7 µm because this spectral region overlaps with the characteristic O-H stretch absorption while being transparent to PGMA and SiN. Through the design of a hybrid waveguide structure, we were able to tailor the mid-IR evanescent wave into the PGMA layer and the surrounding water and, consequently, to enhance the light-analyte interaction. A 7.6 times enhancement of sensitivity is experimentally demonstrated and explained by material integration engineering as well as waveguide mode analysis. Our sensor platform made by polymer-dielectric hybrids can be applied to other regions of the mid-IR spectrum to probe other analytes and can ultimately achieve a multispectral sensor on-a-chip.

Nonlinear conversion efficiency in Kerr frequency comb generation.

We analytically and numerically investigate the nonlinear conversion efficiency in ring microresonator-based mode-locked frequency combs under different dispersion conditions. Efficiency is defined as the ratio of the average round trip energy values for the generated pulse(s) to the input pump light. We find that the efficiency degrades with growth of the comb spectral width and is inversely proportional to the number of comb lines. It depends on the cold-cavity properties of a microresonator only and can be improved by increasing the coupling coefficient. Also, it can be increased in the multi-soliton state.

A nonreciprocal racetrack resonator based on vacuum-annealed magnetooptical cerium-substituted yttrium iron garnet.

Vacuum annealed polycrystalline cerium substituted yttrium iron garnet (CeYIG) films deposited by radio frequency magnetron sputtering on non-garnet substrates were used in nonreciprocal racetrack resonators. CeYIG annealed at 800°C for 30 min provided a large Faraday rotation angle, close to the single crystal value. Crystallinity, magnetic properties, refractive indices and absorption coefficients were measured. The resonant transmission peak of the racetrack resonator covered with CeYIG was non-reciprocally shifted by applying an in-plane magnetic field.

Mid-infrared spectrometer using opto-nanofluidic slot-waveguide for label-free on-chip chemical sensing.

A mid-infrared (mid-IR) spectrometer for label-free on-chip chemical sensing was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. Utilizing the large refractive index contrast (Δn ∼ 2) between the liquid core of the waveguide and the Si cladding, a broadband mid-IR lightwave can be efficiently guided and confined within a nanofluidic capillary (≤100 nm wide). The optical-field enhancement, together with the direct interaction between the probe light and the analyte, increased the sensitivity for chemical detection by 50 times when compared to evanescent-wave sensing. This spectrometer distinguished several common organic liquids (e.g., n-bromohexane, toluene, isopropanol) accurately and could determine the ratio of chemical species (e.g., acetonitrile and ethanol) at low concentration (<5 µL/mL) in a mixture through spectral scanning over their characteristic absorption peaks in the mid-IR regime. The combination of CMOS-compatible planar mid-IR microphotonics, and a high-throughput nanofluidic sensor system, provides a unique platform for chemical detection.

Mid-infrared materials and devices on a Si platform for optical sensing.

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.

Generation of two-cycle pulses and octave-spanning frequency combs in a dispersion-flattened micro-resonator.

We show that octave-spanning Kerr frequency combs with improved spectral flatness of comb lines can be generated in dispersion-flattened microring resonators. The resonator is formed by a strip/slot hybrid waveguide, exhibiting a flat and low anomalous dispersion between two zero-dispersion wavelengths that are separated by one octave from near-infrared to mid-infrared. Such flattened dispersion profiles allow for the generation of mode-locked frequency combs, using relatively low pump power to obtain two-cycle cavity solitons on a chip, associated with the octave-spanning comb bandwidth. The wavelength dependence of the optical loss and of the coupling coefficient and thus wavelength dependent Q-factor are also considered.

Direct-write thermocapillary dewetting of polymer thin films by a laser-induced thermal gradient.

A positive-tone 2D direct-write technique that can achieve sub-wavelength patterning by non-linear overlap effects in a conventional polymer system is described. The technique involves relatively inexpensive free-space optics, skips the usual development step, and promises the possibility of a lithographic method that is solvent-free.