Silicon (Si) photonics is forming a fabless ecosystem, which is enabling low-cost and densely integrated components for optical communications and quantum information. We present a Si optical transmitter for polarization-encoded quantum key distribution (QKD). The chip was fabricated in a standard Si photonic foundry process and integrated together a pulse generator, intensity modulator, variable optical attenuator, and polarization modulator in a $1.3\mathrm{mm}\times 3\mathrm{mm}$ die area. The devices in the photonic circuit meet the requirements for QKD. The transmitter was used in a proof-of-concept demonstration of the BB84 QKD protocol over a 5 km long fiber link. This work shows the potential of using foundry Si photonics for low-cost, wafer-scale fabricated components for quantum information.

Visible and near-infrared spectrum photonic integrated circuits are quickly becoming a key technology to address the scaling challenges in quantum information and biosensing. Thus far, integrated photonic platforms in this spectral range have lacked integrated photodetectors. Here, we report silicon nitride-on-silicon waveguide photodetectors that are monolithically integrated in a visible light photonic platform on silicon. Owing to a leaky-wave silicon nitride-on-silicon design, the devices achieved a high external quantum efficiency of >60% across a record wavelength span from λ ~ 400 nm to ~640 nm, an opto-electronic bandwidth up to 9 GHz, and an avalanche gain-bandwidth product up to 173 ± 30 GHz. As an example, a photodetector was integrated with a wavelength-tunable microring in a single chip for on-chip power monitoring.

Ultra-compact waveguide electroabsorption optical switches and photodetectors with micron- and sub-micron lengths and compatible with silicon (Si) waveguides are demonstrated using the insulator-metal phase transition of vanadium dioxide (VO₂). A 1 μm long hybrid Si-VO₂ device is shown to achieve a high extinction ratio of 12 dB and a competitive insertion loss of 5 dB over a broad bandwidth of 100 nm near λ = 1550 nm. The device, operated as a photodetector, can measure optical powers less than 1 μW with a responsivity in excess of 10 A/W. With volumes that are about 100 to 1000 times smaller than today’s active Si photonic components, the hybrid Si-VO₂ devices show the feasibility of integrating transition metal oxides on Si photonic platforms for nanoscale electro-optic elements.

Emerging applications in quantum information, microscopy, biosensing, depth sensing, and augmented reality demand miniaturized components in the visible (VIS) and near-infrared (NIR) spectrum with wavelengths between 380 and 1100 nm. Foundry silicon photonics, which has been optimized for telecommunication wavelengths, can be adapted to this wavelength range. In this article, we review recent developments in silicon photonics for VIS and NIR wavelengths, with a focus on platforms, devices, and photonic circuits fabricated in foundries. Foundries enable the creation of complex circuitry at a wafer scale. Platforms based on silicon nitride and aluminum oxide wave-guides compatible with complementary metal–oxide–semiconductor (CMOS) foundries are becoming available. As a result, highly functional photonic circuits are becoming possible. The key challenges are low-loss waveguides, efficient input/output coupling, sensitive detectors, and heterogeneous integration of lasers and modulators, particularly those using lithium niobate and other electro-optic materials. These elements, already developed for telecommunications, require further development forλ < 1100 nm. As short-wavelength silicon photonics technology advances, photonic integrated circuits can address a broader scope of applications beyond O- and C-band communication.

We demonstrate novel polarization management devices in a custom-designed silicon nitride (Si₃N₄) on silicon-on-insulator (SOI) integrated photonics platform. In the platform, Si₃N₄ waveguides are defined atop silicon waveguides. A broadband polarization rotator-splitter using a TM0-TE1 mode converter in a composite Si₃N₄-silicon waveguide is demonstrated. The polarization crosstalk, insertion loss, and polarization dependent loss are less than −19 dB, 1.5 dB, and 1.0 dB, respectively, over a bandwidth of 80 nm. A polarization controller composed of polarization rotator-splitters, multimode interference couplers, and thin film heaters is also demonstrated.

We demonstrate optical modulation rates exceeding the conventional cavity linewidth limit using a silicon coupling modulated microring. Small-signal measurements show coupling modulation was free of the parasitic cavity linewidth limitations at rates at least 6× the cavity linewidth. Eye diagram measurements show coupling modulation achieved data rates > 2× the rate attainable by conventional intracavity phase modulation. We propose to use DC-balanced encoding to mitigate the inter-symbol interference in coupling modulation. Analysis shows that coupling modulation can be more efficient than intracavity modulation for large output swings and high-Q resonators. Coupling modulation enables very high-Q resonant modulators to be simultaneously low-power and high-speed, features which are mutually incompatible in typical resonant modulators studied to date.