Frequency conversion in nano-waveguides using bound-state-in-continuum.

Optical frequency conversion in semiconductor nanophotonic devices usually imposes stringent requirements on fabrication accuracy and etch surface roughness. Here, we adopt the concept of bound-state-in-continuum (BIC) for waveguide frequency converter design, which obviates the limitations in nonlinear material nano-fabrication and requires to pattern only a low-refractive-index strip on the nonlinear slab. Taking gallium phosphide (GaP) as an example, we study second-harmonic generation using horizontally polarized pump light at 1.55 µm phase matching to vertically polarized BIC modes. A theoretical normalized frequency conversion efficiency of 1.1×104 % W -1 c m -2 is obtained using the fundamental BIC mode, which is comparable to that of conventional GaP waveguides.

2D Monte Carlo simulation of a silicon waveguide-based single-photon avalanche diode for visible wavelengths.

Integrated photonics platforms are crucial to the development and implementation of scalable quantum information and networking schemes, but many such devices still rely on external bulk photodetectors. We report the design and simulation of a waveguide-based single-photon avalanche diode (SPAD) for visible wavelengths. The SPAD consists of a p-n junction implemented in a doped silicon waveguide, which is end-fire coupled to an input silicon nitride waveguide. We developed a 2D Monte Carlo model to simulate the avalanche multiplication process of charge carriers following the absorption of an input photon, and calculated the photon detection efficiency (PDE) and timing jitter of the SPAD. We investigated the SPAD performance at a wavelength of 640 nm and temperature of 243K for different device dimensions and device doping configurations. For our simulated parameters, we obtained a maximum PDE of 0.45 at a reverse bias voltage of ~20 V, and full-width-half-max (FWHM) timing jitter values <8 ps.

Freestanding dielectric nanohole array metasurface for mid-infrared wavelength applications.

We designed and simulated freestanding dielectric optical metasurfaces based on arrays of etched nanoholes in a silicon membrane. We showed 2π phase control and high forward transmission at mid-infrared (mid-IR) wavelengths around 4.2 µm by tuning the dimensions of the holes. We also identified the mechanisms responsible for high forward scattering efficiency and showed that these conditions are connected with the well-known Kerker conditions already proposed for isolated scatterers. A beam deflector was designed and optimized through sequential particle swarm and gradient descent optimization to maximize transmission efficiency and reduce unwanted grating orders. Such freestanding silicon nanohole array metasurfaces are promising for the realization of silicon-based mid-IR optical elements.

Broadband silicon polarization beam splitter with a high extinction ratio using a triple-bent-waveguide directional coupler.

We report on the design and experimental demonstration of a broadband silicon polarization beam splitter (PBS) with a high extinction ratio (ER)≥30 dB. This was achieved using triple-bent-waveguide directional coupling in a single PBS, and cascaded PBS topology. For the single PBS, the bandwidths for an ER≥30 dB are 20 nm for the quasi-TE mode, and 70 nm for the quasi-TM mode when a broadband light source (1520-1610 nm) was employed. The insertion loss (IL) varies from 0.2 to 1 dB for the quasi-TE mode and 0.2-2 dB for the quasi-TM mode. The cascaded PBS improved the bandwidth of the quasi-TE mode for an ER≥30 dB to 90 nm, with a low IL of 0.2-2 dB. To the best of our knowledge, our PBS system is one of the best broadband PBSs with an ER as high as ∼42 dB and a low IL below 1 dB around the central wavelength, and experimentally demonstrated using edge-coupling.

Optimal geometry of nonlinear silicon slot waveguides accounting for the effect of waveguide losses.

The optimal geometry of silicon-organic hybrid slot waveguides is investigated in the context of the efficiency of four-wave mixing (FWM), a χ(3) nonlinear optical process. We study the effect of slot and waveguide widths, as well as waveguide asymmetry on the two-photon absorption (TPA) figure of merit and the roughness scattering loss. The optimal waveguide core width is shown to be 220nm (symmetric) with a slot width of 120nm, at a fixed waveguide height of 220nm. We also show that state-of-the-art slot waveguides can outperform rib waveguides, especially at high powers, due to the high TPA figure-of-merit.

Entanglement measurement of a coupled silicon microring photon pair source.

Using two-photon (Franson) interferometry, we measure the entanglement of photon pairs generated from an optically-pumped silicon photonic device consisting of a few coupled microring resonators. The pair-source chip operates at room temperature, and the InGaAs single-photon avalanche detectors (SPADs) are thermo-electrically cooled to 234K. Such a device can be integrated with other components for practical entangled photon-pair generation at telecommunications wavelengths.

Silicon microring-based wavelength converter with integrated pump and signal suppression.

We fabricate a two-stage wavelength converter in silicon by cascading a microring wavelength mixer with a five-ring coupled-resonator filter. A p-i-n diode is incorporated into the microring for electronic carrier sweep-out, and microheaters are incorporated into the filter for tunability. The generated idler wavelength is effectively separated from the input pump and signal, with nearly 50 dB of suppression.

Quantum light generation on a silicon chip using waveguides and resonators.

Integrated optical devices may replace bulk crystal or fiber based assemblies with a more compact and controllable photon pair and heralded single photon source and generate quantum light at telecommunications wavelengths. Here, we propose that a periodic waveguide consisting of a sequence of optical resonators can outperform conventional waveguides or single resonators and generate more than 1 Giga-pairs per second from a sub-millimeter-long room-temperature silicon device, pumped with only about 10 milliwatts of optical power. Furthermore, the spectral properties of such devices provide novel opportunities for chip-scale quantum light sources.

Spectrally multiplexed and tunable-wavelength photon pairs at 1.55 µm from a silicon coupled-resonator optical waveguide.

Using a compact optically pumped silicon nanophotonic chip consisting of coupled silicon microrings, we generate photon pairs in multiple pairs of wavelengths around 1.55 µm. The wavelengths are tunable over several nanometers, demonstrating the capability to generate wavelength division multiplexed photon pairs at freely chosen telecommunications-band wavelengths.

Low-power continuous-wave four-wave mixing in silicon coupled-resonator optical waveguides.

We demonstrate four-wave mixing in silicon-on-insulator coupled-resonator optical waveguides consisting of 35 and 65 microring resonators, using a cw pump with coupled power below 20 mW and observed parametric conversion across more than 10 THz. The conversion efficiency is enhanced by +16 dB relative to a silicon straight waveguide of equivalent length, due to the slowing factor of the coupled-resonator structure.

Integrated avalanche photodetectors for visible light.

Integrated photodetectors are essential components of scalable photonics platforms for quantum and classical applications. However, most efforts in the development of such devices to date have been focused on infrared telecommunications wavelengths. Here, we report the first monolithically integrated avalanche photodetector (APD) for visible light. Our devices are based on a doped silicon rib waveguide with a novel end-fire input coupling to a silicon nitride waveguide. We demonstrate a high gain-bandwidth product of 234 ± 25 GHz at 20 V reverse bias measured for 685 nm input light, with a low dark current of 0.12 µA. We also observe open eye diagrams at up to 56 Gbps. This performance is very competitive when benchmarked against other integrated APDs operating in the infrared range. With CMOS-compatible fabrication and integrability with silicon photonic platforms, our devices are attractive for sensing, imaging, communications, and quantum applications at visible wavelengths.

Broadband Silicon-On-Insulator directional couplers using a combination of straight and curved waveguide sections.

Broadband Silicon-On-Insulator (SOI) directional couplers are designed based on a combination of curved and straight coupled waveguide sections. A design methodology based on the transfer matrix method (TMM) is used to determine the required coupler section lengths, radii, and waveguide cross-sections. A 50/50 power splitter with a measured bandwidth of 88 nm is designed and fabricated, with a device footprint of 20 µm × 3 µm. In addition, a balanced Mach-Zehnder interferometer is fabricated showing an extinction ratio of >16 dB over 100 nm of bandwidth.

Polarisation independent silicon-on-insulator slot waveguides.

The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and miniaturising photonic components. In this work, we present a systematic study of the slot waveguide geometries required for having zero birefringence (ZB). We show that the rail widths required for ZB are more strongly dependent on the height of the waveguide than on the slot separation. After which, we demonstrate that the ZB geometry is significantly affected by the slanting of the waveguide walls. This paper proceeds to show that within the range studied, one can fix the height, slot, slant angle, and bend radius, and still achieve ZB by varying the widths of both of the rails. Given a fabrication tolerance of 5 nm, we show that a coherence length on the order of a hundred microns can be achieved. We finish by showing that for straight and bent ZB waveguides, having symmetric rails is preferable due to higher tolerances and lower sensitivity to bending. Since any arbitrarily shaped slot waveguide is a combination of both single mode straight and bent waveguides, we have a toolbox from which we can achieve ZB for any given slot and height.

Controlling the spectrum of photons generated on a silicon nanophotonic chip.

Directly modulated semiconductor lasers are widely used, compact light sources in optical communications. Semiconductors can also be used to generate nonclassical light; in fact, CMOS-compatible silicon chips can be used to generate pairs of single photons at room temperature. Unlike the classical laser, the photon-pair source requires control over a two-dimensional joint spectral intensity (JSI) and it is not possible to process the photons separately, as this could destroy the entanglement. Here we design a photon-pair source, consisting of planar lightwave components fabricated using CMOS-compatible lithography in silicon, which has the capability to vary the JSI. By controlling either the optical pump wavelength, or the temperature of the chip, we demonstrate the ability to select different JSIs, with a large variation in the Schmidt number. Such control can benefit high-dimensional communications where detector-timing constraints can be relaxed by realizing a large Schmidt number in a small frequency range.

Electronic control of optical Anderson localization modes.

Anderson localization of light has been demonstrated in a few different dielectric materials and lithographically fabricated structures. However, such localization is difficult to control, and requires strong magnetic fields or nonlinear optical effects, and electronic control has not been demonstrated. Here, we show control of optical Anderson localization using charge carriers injected into more than 100 submicrometre-scale p-n diodes. The diodes are embedded into the cross-section of the optical waveguide and are fabricated with a technology compatible with the current electronics industry. Large variations in the output signal, exceeding a factor of 100, were measured with 1 V and a control current of 1 mA. The transverse footprint of our device is only 0.125 µm(2), about five orders of magnitude smaller than optical two-dimensional lattices. Whereas all-electronic localization has a narrow usable bandwidth, electronically controlled optical localization can access more than a gigahertz of bandwidth and creates new possibilities for controlling localization at radiofrequencies, which can benefit applications such as random lasers, optical limiters, imagers, quantum optics and measurement devices.

White beam diffraction in a two-dimensional photonic crystal.

We investigate diffraction of a white beam in a two-dimensional photonic crystal. Experimental and computational results for a halogen light source are presented. Applying equal-frequency surface analysis, we reproduce the experimental results by computer simulations. The white beam is modeled computationally as a sum of many circular rings; each ring is made up of many wavelength components. We also present computer simulations of the diffraction pattern for a fluorescent light source.