Supercontinuum in integrated photonics: generation, applications, challenges, and perspectives.

Frequency conversion in nonlinear materials is an extremely useful solution to the generation of new optical frequencies. Often, it is the only viable solution to realize light sources highly relevant for applications in science and industry. In particular, supercontinuum generation in waveguides, defined as the extreme spectral broadening of an input pulsed laser light, is a powerful technique to bridge distant spectral regions based on single-pass geometry, without requiring additional seed lasers or temporal synchronization. Owing to the influence of dispersion on the nonlinear broadening physics, supercontinuum generation had its breakthrough with the advent of photonic crystal fibers, which permitted an advanced control of light confinement, thereby greatly improving our understanding of the underlying phenomena responsible for supercontinuum generation. More recently, maturing in fabrication of photonic integrated waveguides has resulted in access to supercontinuum generation platforms benefiting from precise lithographic control of dispersion, high yield, compact footprint, and improved power consumption. This Review aims to present a comprehensive overview of supercontinuum generation in chip-based platforms, from underlying physics mechanisms up to the most recent and significant demonstrations. The diversity of integrated material platforms, as well as specific features of waveguides, is opening new opportunities, as will be discussed here.

Photo-Thermal Tuning of Graphene Oxide Coated Integrated Optical Waveguides.

We experimentally investigate power-sensitive photo-thermal tuning (PTT) of two-dimensional (2D) graphene oxide (GO) films coated on integrated optical waveguides. We measure the light power thresholds for reversible and permanent GO reduction in silicon nitride (SiN) waveguides integrated with one and two layers of GO. For the device with one layer of GO, the power threshold for reversible and permanent GO reduction are ~20 and ~22 dBm, respectively. For the device with two layers of GO, the corresponding results are ~13 and ~18 dBm, respectively. Raman spectra at different positions of a hybrid waveguide with permanently reduced GO are characterized, verifying the inhomogeneous GO reduction along the direction of light propagation through the waveguide. The differences between the PTT induced by a continuous-wave laser and a pulsed laser are also compared, confirming that the PTT mainly depend on the average input power. These results reveal interesting features for 2D GO films coated on integrated optical waveguides, which are of fundamental importance for the control and engineering of GO's properties in hybrid integrated photonic devices.

Mid-infrared supercontinuum generation in silicon-germanium all-normal dispersion waveguides.

We demonstrate coherent supercontinuum generation spanning over an octave from a silicon germanium-on-silicon waveguide using ∼200fs pulses at a wavelength of 4 µm. The waveguide is engineered to provide low all-normal dispersion in the TM polarization. We validate the coherence of the generated supercontinuum via simulations, with a high degree of coherence across the entire spectrum. Such a generated supercontinuum could lend itself to pulse compression down to 22 fs.

Positive and negative phototunability of chalcogenide (AMTIR-1) microdisk resonator.

We demonstrate externally photo-induced partially-reversible tuning of the resonance of a microdisk made of AMTIR-1 (Ge(33)As(12)Se(55)). We have achieved for the first time, to the best of our knowledge, both positive and negative shift in a microresonator with external tuning. A positive resonance shift of 1 nm and a negative resonance shift of 0.5 nm on a single microdisk has been measured. We have found that this phenomenon is due to initial photo-expansion of the microdisk followed by the photo-bleaching of the AMTIR-1. The observed shifts and the underlying phenomenon is controllable by varying the illumination power (i.e. the low power illumination suppresses the photobleaching process). We measure a loaded quality factor of 1.2x10(5) at 1550nm (limited by the measuring instrument). This holds promise for non-contact low power reversible-tunning of photonic circuit elements.

Integrated optical auto-correlator based on third-harmonic generation in a silicon photonic crystal waveguide.

The ability to use coherent light for material science and applications is linked to our ability to measure short optical pulses. While free-space optical methods are well established, achieving this on a chip would offer the greatest benefit in footprint, performance and cost, and allow the integration with complementary signal-processing devices. A key goal is to achieve operation at sub-watt peak power levels and on sub-picosecond timescales. Previous integrated demonstrations require either a temporally synchronized reference pulse, an off-chip spectrometer or long tunable delay lines. Here we report a device capable of achieving single-shot time-domain measurements of near-infrared picosecond pulses based on an ultra-compact integrated CMOS-compatible device, which could operate without any external instrumentation. It relies on optical third-harmonic generation in a slow-light silicon waveguide. Our method can also serve as an in situ diagnostic tool to map, at visible wavelengths, the propagation dynamics of near-infrared pulses in photonic crystals.

Low propagation loss silicon-on-sapphire waveguides for the mid-infrared.

We report record low loss silicon-on-sapphire nanowires for applications to mid infrared optics. We achieve propagation losses as low as 0.8 dB/cm at λ = 1550 nm, ~1.1 to 1.4 dB/cm at λ = 2080 nm and < 2dB/cm at λ = 5.18 µm.

Slow-light enhanced correlated photon pair generation in a silicon photonic crystal waveguide.

We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 µm. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip.

Third-harmonic generation in slow-light chalcogenide glass photonic crystal waveguides.

We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (∼2), we obtain a relatively large conversion efficiency (1.4×10(-8)/W(2)), which is 30× higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode.

Measuring the dispersive properties of liquids using a microinterferometer.

Using a single-beam, compact interferometer, we measure the refractive index of liquids in the near IR. This highly compact device relies on a silica capillary with a 50 µm inner diameter: it uses a minimal volume of test liquid, isolates the liquid from the humid atmosphere, has broadband operation, and is inherently mechanically stable. These characteristics, in combination with straightforward data acquisition, make it particularly well-suited for measuring the optical properties in the near IR of a wide range of liquids. Using this refractometer, we measure the refractive index of high-index liquids that are expected to be hydroscopic. The accuracy of the refractometer (±0.1%) is demonstrated through measuring the indices of air and pure water. We show that the hydroscopic behavior of the probed liquids has little influence on their optical properties in the near IR.

Photosensitive and thermal nonlinear effects in chalcogenide photonic crystal cavities.

We investigate the photosensitive and thermo-optic nonlinear properties of chalcogenide glass photonic crystal (PhC) cavities at telecommunications wavelengths. We observe a photosensitive refractive index change in AMTIR-1 (Ge(33)As(12)Se(55)) material in the near-infrared, which is enhanced by light localization in the PhC cavity and manifests in a permanent blue-shift of the nanocavity resonance. Thermo-optic non-linear properties are thoroughly investigated by i) carrying out thermal bistable switching experiments, from which we determined thermal switching times of 63 µs and 93 µs for switch on and switch off respectively and ii) by studying heating of the cavity with a high peak power pulsed laser input, which shows that two-photon absorption is the dominant heating mechanism. Our measurements and analysis highlight the detrimental impact of near-infrared photosensitivity and two-photon absorption on cavity based nonlinear optical switching schemes. We conclude that glass compositions with lower two-photon absorption and more stable properties (reduced photosensitivity) are therefore required for nonlinear applications in chalcogenide photonic crystal cavities.

Investigation of phase matching for third-harmonic generation in silicon slow light photonic crystal waveguides using Fourier optics.

Using Fourier optics, we retrieve the wavevector dependence of the third-harmonic (green) light generated in a slow light silicon photonic crystal waveguide. We show that quasi-phase matching between the third-harmonic signal and the fundamental mode is provided in this geometry by coupling to the continuum of radiation modes above the light line. This process sustains third-harmonic generation with a relatively high efficiency and a substantial bandwidth limited only by the slow light window of the fundamental mode. The results give us insights into the physics of this nonlinear process in the presence of strong absorption and dispersion at visible wavelengths where bandstructure calculations are problematic. Since the characteristics (e.g. angular pattern) of the third-harmonic light primarily depend on the fundamental mode dispersion, they could be readily engineered.

Photowritten high-Q cavities in two-dimensional chalcogenide glass photonic crystals.

We demonstrate a high-Q(approximately 125,000) photonic crystal (PhC) cavity formed using a postprocessing optical exposure technique where the refractive index of a photosensitive chalcogenide PhC is modified locally. The evolution of the cavity resonances was monitored in situ during writing using a tapered fiber evanescent coupling system, and the Q of 125,000 represents 1 order of magnitude increase over previously reported cavities in two-dimensional chalcogenide glass PhC.

Slow light enhancement of nonlinear effects in silicon engineered photonic crystal waveguides.

We report nonlinear measurements on 80microm silicon photonic crystal waveguides that are designed to support dispersionless slow light with group velocities between c/20 and c/50. By launching picoseconds pulses into the waveguides and comparing their output spectral signatures, we show how self phase modulation induced spectral broadening is enhanced due to slow light. Comparison of the measurements and numerical simulations of the pulse propagation elucidates the contribution of the various effects that determine the output pulse shape and the waveguide transfer function. In particular, both experimental and simulated results highlight the significant role of two photon absorption and free carriers in the silicon waveguides and their reinforcement in the slow light regime.

Reconfigurable microfluidic photonic crystal slab cavities.

We demonstrate the spectral and spatial reconfigurability of photonic crystal double-heterostructure cavities in silicon by microfluidic infiltration of selected air holes. The lengths of the microfluidic cavities are changed by adjusting the region of infiltrated holes in steps of several microns. We systematically investigate the spectral signature of these cavities, showing high Q-factor resonances for a broad range of cavity lengths. The fluid can be removed by immersing the device in toluene, offering complete reconfigurability. Our cavity writing technique allows for tolerances in the infiltration process and provides flexibility as it can be employed at any time after photonic crystal fabrication.

High-Q microfluidic cavities in silicon-based two-dimensional photonic crystal structures.

We demonstrate postprocessed microfluidic double-heterostructure cavities in silicon-based photonic crystal slab waveguides. The cavity structure is realized by selective fluid infiltration of air holes using a glass microtip, resulting in a local change of the average refractive index of the photonic crystal. The microcavities are probed by evanescent coupling from a silica nanowire. An intrinsic quality factor of 57,000 has been derived from our measurements, representing what we believe to be the largest value observed in microfluidic photonic crystal cavities to date.

Characterizing photonic crystal waveguides with an expanded k-space evanescent coupling technique.

We demonstrate a direct, single measurement technique for characterizing the dispersion of a photonic crystal waveguide (PCWG) using a tapered fiber evanescent coupling method. A highly curved fiber taper is used to probe the Fabry-Pérot spectrum of a closed PCWG over a broad k-space range, and from this measurement the dispersive properties of the waveguide can be found. Waveguide propagation losses can also be estimated from measurements of closed waveguides with different lengths. The validity of this method is demonstrated by comparing the results obtained on a 'W1' PCWG in chalcogenide glass with numerical simulation.

Nanowire coupling to photonic crystal nanocavities for single photon sources.

We demonstrate highly efficient evanescent coupling via a silica loop-nanowire, to ultra-small (0.5 (lambda/n)(3) ), InAs/InP quantum dot photonic crystal cavities, specifically designed for single photon source applications. This coupling technique enables the tuning of both the Q-factor and the wavelength of the cavity mode independently, which is highly relevant for single photon source applications. First, this allows for the optimization of the extraction efficiency while maintaining a high Purcell factor. Second, the cavity mode can be matched with a spectrally misaligned quantum dot without changing the structure or degrading the Q-factor: a 3 nm resonance shift is reported.

Photosensitive post tuning of chalcogenide photonic crystal waveguides.

We present experimental results on post-tuning the dispersion of a two-dimensional photonic crystal waveguide made from Ge(33)As(12)Se(55) chalcogenide glass by exploiting the material photosensitivity to near-bandgap light. The change in the refractive index and volume of the material in response to exposure to 633nm light resulted in a shift of more than 5nm in the resonant coupling wavelength between a tapered optical fiber and the modes of a W1 waveguide. This represents a first proof of principle demonstration of the photosensitive post-tuning of a planar photonic crystal device.

Characterization and modeling of Fano resonances in chalcogenide photonic crystal membranes.

We demonstrate resonant guiding in a chalcogenide glass photonic crystal membrane. We observe strong resonances in the optical transmission spectra at normal incidence, associated with Fano coupling between free space and guided modes. We obtain good agreement with modeling results based on three-dimensional finite-difference time-domain simulations, and identify the guided modes near the centre of the first Brillouin zone responsible for the main spectral features.

Efficient coupling to chalcogenide glass photonic crystal waveguides via silica optical fiber nanowires.

We demonstrate highly efficient evanescent coupling between a highly nonlinear chalcogenide glass two dimensional photonic crystal waveguide and a silica fiber nanowire. We achieve 98% insertion efficiency to the fundamental photonic crystal waveguide mode with a 3dB coupling bandwidth of 12nm, in good agreement with theory. This scheme provides a promising platform to realize low power nanocavity based all-optical switching and logic functions.