Michael G. Wood

12.5 pm/V hybrid silicon and lithium niobate optical microring resonator with integrated electrodes

We present a silicon microring resonator with a lithium niobate top cladding and integrated tuning electrodes. Submicrometer thin films of z-cut lithium niobate are bonded to silicon microring resonators via benzocyclobutene. Integrated electrodes are incorporated to confine voltage controlled electric fields within the lithium niobate thin film. The electrode design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The hybrid material system combines the electro-optic functionality of lithium niobate with the high index contrast of silicon waveguides, enabling compact low tuning voltage microring resonators. Optical characterization of fabricated devices results in a measured loaded quality factor of 11,500 and a free spectral range of 7.15 nm in the infrared. The demonstrated tunability is 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs.

Compact cantilever couplers for low-loss fiber coupling to silicon photonic integrated circuits

We demonstrate coupling from tapered optical fibers to 450 nm by 250 nm silicon strip waveguides using compact cantilever couplers. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers. Finite difference time domain simulations are used to design the length of the silicon inverse width taper to as short as 6.5 μm for a cantilever width of 2 μm. Modeling of various strip waveguide taper profiles shows reduced coupling losses for a quadratic taper profile. Infrared measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are less than 1 dB per connection. The compact, broadband, and low-loss coupling scheme enables direct access to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.

Electrically tunable optical polarization rotation on a silicon chip using Berry’s phase

The continued convergence of electronics and photonics on the chip scale can benefit from the voltage control of optical polarization for applications in communications, signal processing and sensing. It is challenging, however, to electrically manipulate the polarization state of light in planar optical waveguides. Here we introduce out-of-plane optical waveguides, allowing access to Berrys phase, a quantum-mechanical phenomenon of purely topological origin. As a result, electrically tunable optical polarization rotation on the chip scale is achieved. Devices fabricated in the silicon-on-insulator material platform are not limited to a single static polarization state. Rather, they can exhibit dynamic tuning of polarization from the fundamental transverse electric mode to the fundamental transverse magnetic mode. Electrical tuning of optical polarization over a 19 dB range of polarization extinction ratio is demonstrated with less than 1 dB of conversion loss at infrared wavelengths. Compact system architectures involving dynamic control of optical polarization in integrated circuits are envisioned.

Optimization of electron beam patterned hydrogen silsesquioxane mask edge roughness for low-loss silicon waveguides

Abstract. We carried out a multiparameter fabrication study designed to reduce the line edge roughness (LER) of electron beam (e-beam) patterned hydrogen silsesquioxane resist for the purpose of producing low-loss silicon strip waveguides. Reduced mask roughness was achieved for 50°C pre-exposure baking, 5000  μC/cm2 dose with a beam spot size more than twice as large as the electron beam step size, development in 25% tetramethylammonium hydroxide and postdevelopment baking with rapid thermal annealing in an O2 ambient at 1000°C. The LER caused by pattern fracturing and stage stitches was reduced with multipass writing and per-pass linear and rotational offsets. Si strip waveguides patterned with the optimized mask have root-mean-square sidewall roughness of 2.1 nm with a correlation length of 94 nm, as measured by three-dimensional atomic force microscopy. Measured optical propagation losses of these waveguides across the telecommunications C-band were 2.5 and 2.8  dB/cm for the transverse magnetic and transverse electric modes, respectively. These reduced loss waveguides enable the fabrication of advanced planar lightwave circuit topologies.

Degenerate band edge resonances in periodic silicon ridge waveguides

We experimentally demonstrate degenerate band edge resonances in periodic Si ridge waveguides that are compatible with carrier injection modulation for active electro-optical devices. The resonant cavities are designed using a combination of the plane-wave expansion method and the finite difference time domain technique. Measured and simulated quality factors of the first band edge resonances scale to the fifth power of the number of periods. Quality factor scaling is determined to be limited by fabrication imperfections. Compared to resonators based on a regular transmission band edge, degenerate band edge devices can achieve significantly larger quality factors in the same number of periods. Applications include compact electro-optical switches, modulators, and sensors that benefit from high-quality factors and large distributed electric fields.

7 nm/V DC tunability and millivolt scale switching in silicon carrier injection degenerate band edge resonators

We demonstrate electro-optical tuning of degenerate band edge resonances in Si photonic waveguides for applications including tunable filters, low voltage switches, and modulators. Carrier injection modulation is enabled by introducing periodic Si slabs to electrically connect the resonator to P and N dopants. Measured devices yield a large DC tunability of 7.1 nm/V and a peak switching slope of 206 dB/V. Digital data transmission measurements at 100 Mb/s show 3 dB of switching with a swing voltage of 6.8 mV, 91.4 aJ/bit switching energy, and 1.08 pJ/bit holding energy.

Compact silicon strip waveguide cantilever couplers for low-loss and broadband fiber-to-chip coupling

We demonstrate low-loss fiber-to-chip coupling using 7.3 μm long silicon strip waveguide cantilever couplers. Average coupling losses are measured to be less than 0.62 dB per connection throughout the optical telecommunications C band.

Experimental Verification of Degenerate Band Edge Dispersion in Silicon Photonic Integrated Circuits

On resonance, periodic dielectric media of finite length exhibit large and distributed internal electric field distributions. The large and distributed fields have been exploited for applications in light emission, optical switching, and nonlinear optics. Resonance quality factors scale as the number of periods cubed near a regular band edge with quadratic dispersion. For chip-scale integrated photonics, significant footprint reduction is possible by exploiting the quartic dispersion of a degenerate band edge with quality factors that scale to the fifth power of the number of periods. Band diagrams with quartic dispersion are extracted from transmission measurements of fabricated devices realized in silicon photonics. Transmission measurements show Fano resonances with a sharp transmission peak-to-bandgap extinction ratio of 20 dB and quality factors of 27 000.