Power-efficient silicon nitride thermo-optic phase shifters for visible light.

We demonstrate power-efficient, thermo-optic, silicon nitride waveguide phase shifters for blue, green, and yellow wavelengths. The phase shifters operated with low power consumption due to a suspended structure and multi-pass waveguide design. The devices were fabricated on 200-mm silicon wafers using deep ultraviolet lithography as part of an active visible-light integrated photonics platform. The measured power consumption to achieve a π phase shift (averaged over multiple devices) was 0.78, 0.93, 1.09, and 1.20 mW at wavelengths of 445, 488, 532, and 561 nm, respectively. The phase shifters were integrated into Mach-Zehnder interferometer switches, and 10 - 90% rise(fall) times of about 570(590) µs were measured.

Multicore fibers with 10 and 16 single-mode cores for the visible spectrum.

We report multicore fibers (MCFs) with 10 and 16 linearly distributed cores with single-mode operation in the visible spectrum. The average propagation loss of the cores is 0.06 dB/m at λ = 445 nm and < 0.03 dB/m at wavelengths longer than 488 nm. The low inter-core crosstalk and nearly identical performance of the cores make these MCFs suitable for spatial division multiplexing in the visible spectrum. As a proof-of-concept application, one of the MCFs was coupled to an implantable neural probe to spatially address light-emitting gratings on the probe.

Optical phased array neural probes for beam-steering in brain tissue.

Implantable silicon neural probes with integrated nanophotonic waveguides can deliver patterned dynamic illumination into brain tissue at depth. Here, we introduce neural probes with integrated optical phased arrays and demonstrate optical beam steering in vitro. Beam formation in brain tissue is simulated and characterized. The probes are used for optogenetic stimulation and calcium imaging.

Low-loss broadband bi-layer edge couplers for visible light.

Low-loss broadband fiber-to-chip coupling is currently challenging for visible-light photonic-integrated circuits (PICs) that need both high confinement waveguides for high-density integration and a minimum feature size above foundry lithographical limit. Here, we demonstrate bi-layer silicon nitride (SiN) edge couplers that have ≤ 4 dB/facet coupling loss with the Nufern S405-XP fiber over a broad optical wavelength range from 445 to 640 nm. The design uses a thin layer of SiN to expand the mode at the facet and adiabatically transfers the input light into a high-confinement single-mode waveguide (150-nm thick) for routing, while keeping the minimum nominal lithographic feature size at 150 nm. The achieved fiber-to-chip coupling loss is about 3 to 5 dB lower than that of single-layer designs with the same waveguide confinement and minimum feature size limitation.

Back-side-on-BOX heterogeneously integrated III-V-on-silicon O-band discrete-mode lasers.

We demonstrate foundry-fabricated O-band III-V-on-silicon discrete-mode lasers. The laser fabrication follows the back-side-on-buried-oxide laser integration process and is compatible with complex, multilayer, silicon-on-insulator based platforms. A series of devices were characterized, with the best devices producing on-chip powers of nearly 20 mW with Lorentzian linewidths below 20 kHz and a side mode suppression ratio of at least 60 dB.

30 GHz heterogeneously integrated capacitive InP-on-Si Mach-Zehnder modulators.

We demonstrate high-bandwidth O-band Mach-Zehnder modulators with indium phosphide-on-silicon (InP-on-Si) capacitive phase shifters that are compatible with heterogeneous laser fabrication processes. An electro-optic conversion efficiency of 1.3 V⋅cm and a 3 dB bandwidth of up to 30 GHz was observed for a phase modulator length of 250 µm at a 0 V bias. Open eye patterns were observed at up to 25 Gb/s.

Visible-light silicon nitride waveguide devices and implantable neurophotonic probes on thinned 200 mm silicon wafers.

We present passive, visible light silicon nitride waveguides fabricated on ≈ 100 µm thick 200 mm silicon wafers using deep ultraviolet lithography. The best-case propagation losses of single-mode waveguides were ≤ 2.8 dB/cm and ≤ 1.9 dB/cm over continuous wavelength ranges of 466-550 nm and 552-648 nm, respectively. In-plane waveguide crossings and multimode interference power splitters are also demonstrated. Using this platform, we realize a proof-of-concept implantable neurophotonic probe for optogenetic stimulation of rodent brains. The probe has grating coupler emitters defined on a 4 mm long, 92 µm thick shank and operates over a wide wavelength range of 430-645 nm covering the excitation spectra of multiple opsins and fluorophores used for brain stimulation and imaging.

Silicon nitride-on-silicon bi-layer grating couplers designed by a global optimization method.

Silicon nitride-on-silicon bi-layer grating couplers were designed for the O-band using an optimization-based procedure that accounted for design rules and fabricated on a 200 mm wafer. The designs were sufficiently robust to fabrication variations to function well across the wafer. A peak fiber-to-chip coupling efficiency to standard single mode fiber of -2.2 dB and a 1-dB bandwidth of 72.9 nm was achieved in the representative device. Over several chips across the wafer, we measured a median peak coupling efficiency of -2.1 dB and median 1-dB bandwidth of 70.8 nm. The measurements had good correspondence with simulation.

Multi-layer silicon nitride-on-silicon polarization-independent grating couplers.

A polarization-independent grating coupler is proposed and demonstrated in a 3-layer silicon nitride-on-silicon photonic platform. Polarization independent coupling was made possible by the supermodes and added degrees of geometric freedom unique to the 3-layer photonic platform. The grating was designed via optimization algorithms, and the simulated peak coupling efficiency was -2.1 dB with a 1 dB polarization dependent loss (PDL) bandwidth of 69 nm. The fabricated grating couplers had a peak coupling efficiency of -4.8 dB with 1 dB PDL bandwidth of over 100 nm.

U-shaped PN junctions for efficient silicon Mach-Zehnder and microring modulators in the O-band: erratum.

We correct two minor errors in the manuscript. The effective diameter of the ring modulator should be 62.5 µm rather than 65 µm. The factor, g, in the FOM for comparing between the O- and C-band results should be 0.83 instead of 0.7.

Integration of an O-band VCSEL on silicon photonics with polarization maintenance and waveguide coupling.

We demonstrate the hybrid integration of an O-band vertical-cavity surface-emitting laser (VCSEL) onto a silicon photonic chip using a grating coupler that is optimized to simultaneously provide feedback to maintain the single emission polarization and efficient in-plane coupling. The grating coupler was fabricated on silicon-on-insulator using a standard silicon photonics foundry process, and integrated with a commercially available VCSEL. A transparent VCSEL submount was fabricated with femtosecond laser templating and chemical etching to simplify the passive and active alignment steps. A record-high VCSEL-to-chip coupling efficiency of -5 dB was obtained at a bias current of 2.5 mA. The slope efficiency and output power are competitive with microcavity hybrid silicon lasers. The results show the feasibility of VCSELs as low threshold current on-chip sources for silicon photonics.

Flip-chip integrated silicon Mach-Zehnder modulator with a 28nm fully depleted silicon-on-insulator CMOS driver.

We present a silicon electro-optic transmitter consisting of a 28nm ultra-thin body and buried oxide fully depleted silicon-on-insulator (UTBB FD-SOI) CMOS driver flip-chip integrated onto a Mach-Zehnder modulator. The Mach-Zehnder silicon optical modulator was optimized to have a 3dB bandwidth of around 25 GHz at -1V bias and a 50 Ω impedance. The UTBB FD-SOI CMOS driver provided a large output voltage swing around 5 Vpp to enable a high dynamic extinction ratio and a low device insertion loss. At 44 Gbps, the transmitter achieved a high extinction ratio of 6.4 dB at the modulator quadrature operation point. This result shows open eye diagrams at the highest bit rates and with the largest extinction ratios for silicon electro-optic transmitter using a CMOS driver.

Tri-layer silicon nitride-on-silicon photonic platform for ultra-low-loss crossings and interlayer transitions.

We present a three-layer silicon nitride on silicon platform for constructing very large photonic integrated circuits. Efficient interlayer transitions are enabled by the close spacing between adjacent layers, while ultra-low-loss crossings are enabled by the large spacing between the topmost and bottommost layers. We demonstrate interlayer taper transitions with losses < 0.15 dB for wavelengths spanning from 1480 nm to 1620 nm. Our overpass waveguide crossings exhibit insertion loss < 2.1 mdB and crosstalk below -56 dB in the wavelength range between 1480 nm and 1620 nm with losses as low as 0.28 mdB. Our platform architecture is suited to meet the demands of large-scale photonic circuits which contain hundreds of crossings.

U-shaped PN junctions for efficient silicon Mach-Zehnder and microring modulators in the O-band.

We demonstrate U-shaped silicon PN junctions for energy efficient Mach-Zehnder modulators and ring modulators in the O-band. This type of junction has an improved modulation efficiency compared to existing PN junction geometries, has low losses, and supports high-speed operation. The U-shaped junctions were fabricated in an 8" silicon photonics platform, and they were incorporated in travelling-wave Mach-Zehnder modulators and microring modulators. For the high-bandwidth Mach-Zehnder modulator, the DC VπL at -0.5 V bias was 4.6 V·mm. It exhibited a 3dB bandwidth of 13 GHz, and eye patterns at up to 24 Gb/s were observed. A VπL as low as ~2.6 V·mm at a -0.5 V bias was measured in another device. The ring modulator tuning efficiency was 40 pm·V-1 between 0 V and -0.5 V bias. It had a 3-dB bandwidth of 13.5 GHz and open eye patterns at up to 13 Gb/s were measured. This type of PN junctions can be easily fabricated without extra masks and can be incorporated into generic silicon photonics platforms.

Binary particle swarm optimized 2 × 2 power splitters in a standard foundry silicon photonic platform.

Compact power splitters designed ab initio using binary particle swarm optimization in a 2D mesh for a standard foundry silicon photonic platform are studied. Designs with a 4.8 µm×4.8 µm footprint composed of 200 nm×200 nm and 100 nm×100 nm cells are demonstrated. Despite not respecting design rules, the design with the smaller cells had lower insertion losses and broader bandwidth and showed consistent behavior across the wafer. Deviations between design and experiments point to the need for further investigations of the minimum feature dimensions.

Wavelength-size hybrid Si-VO(2) waveguide electroabsorption optical switches and photodetectors.

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 (VO(2)). A 1 µm long hybrid Si-VO(2) 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-VO(2) devices show the feasibility of integrating transition metal oxides on Si photonic platforms for nanoscale electro-optic elements.

Unidirectional hybrid silicon ring laser with an intracavity S-bend.

We demonstrate a hybrid silicon ring laser with an internal amplifying S-bend that couples a fraction of the counter-clockwise circulating light into the the clockwise direction. The device supported single-mode, unidirectional laser oscillation at certain bias conditions. A spatial field distribution model is derived to describe the unidirectional operation. A unidirectional clockwise laser output with a suppression ratio up to 18.6 dB over the counter-clockwise mode was achieved.

Electrically controllable extraordinary optical transmission in gold gratings on vanadium dioxide.

Highly tunable optical transmission through one-dimensional gold gratings patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. Dense electrical integration is enabled by grating features that also function as electrical contacts to the VO2. Extraordinary optical transmission is observed in the VO2 insulator phase, and the optical transmission is extinguished by up to about 6 dB in a 170 nm thick VO2 film. Measurements of gratings with varying duty cycles demonstrate the dependence of the optical transmission and tuning on the device geometry.

Tunable single-mode coupled-cavity laser in a standard InP photonics platform.

We present a wavelength tunable, coupled-cavity laser in a standard indium phosphide multiproject wafer shuttle which did not support distributed feedback gratings. The single-mode operation was enabled by reflections from slots in the laser cavity. The wavelength of the laser emission was tunable over 20 nm near a wavelength of 1560 nm via the currents applied to each section of the laser. A maximum side-mode suppression ratio of 46 dB was observed. The delayed self-heterodyne spectrum of the laser showed a Voigt line shape, corresponding to optical linewidths of 3.7 MHz for the Lorentzian and 88 MHz for the Gaussian contributions.

Dimensional variation tolerant silicon-on-insulator directional couplers.

We design silicon ridge/rib waveguide directional couplers which are simultaneously tolerant to width, height, coupling gap, and etch depth variations. Using wafer-scale measurements of structures fabricated in the IMEC Standard Passives process, we demonstrate the normalized standard deviation in the per-length coupling coefficient (a metric for the splitting ratio variation) of the variation-tolerant directional couplers is up to 4 times smaller than that of strip waveguide designs. The variation-tolerant couplers are also the most broadband and the deviation in the coupling coefficient shows the lowest spectral dependence.