Short, broadband, and polarization-insensitive adiabatic Y-junction power splitters.

Adiabatic Y-junction power splitters have low loss, large bandwidth, high polarization insensitivity, and high tolerance to fabrication errors. However, the adiabatic transition lengths required are generally much longer than other power splitters. Using a nonlinear taper profile can considerably shorten the device length. Here, we introduce a taper profile optimization algorithm based on polynomial functions, which significantly reduces the lengths of the adiabatic power splitters without increasing losses. We experimentally demonstrate the performance of the adiabatic power splitters for minimum feature sizes of 80 nm, 120 nm, and 160 nm on the 220 nm silicon-on-insulator (SOI) platform. Our best device has a minimum feature size of 120 nm and a length of 14 µm, with measured losses of 0.25 dB and 0.23 dB for the transverse electric (TE) and transverse magnetic (TM) modes, respectively, in the 1500-1600 nm region. This device has an average transmission of -3 ± 0.5 dB in the 1500-1600 nm region, indicating highly balanced splitting over a large spectral range.

Vertically integrated spot-size converter in AlGaAs-GaAs.

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850 nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or -0.4 dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850 nm.

Correlated photon pair generation in AlGaAs nanowaveguides via spontaneous four-wave mixing.

We demonstrate a source of correlated photon pairs which will have applications in future integrated quantum photonic circuits. The source utilizes spontaneous four-wave mixing (SFWM) in a dispersion-engineered nanowaveguide made of AlGaAs, which has merits of negligible two-photon absorption and low spontaneous Raman scattering (SpRS). We observe a coincidence-to-accidental (CAR) ratio up to 177, mainly limited by propagation losses. Experimental results agree well with theoretical predictions of the SFWM photon pair generation and the SpRS noise photon generation. We also study the effects from the SpRS, propagation losses, and waveguide lengths on the quality of our source.

Low-power continuous-wave four-wave mixing wavelength conversion in AlGaAs-nanowaveguide microresonators.

We experimentally demonstrate enhanced wavelength conversion in a Q∼7500 deeply etched AlGaAs-nanowaveguide microresonator via degenerate continuous-wave four-wave mixing with a pump power of 24 mW. The maximum conversion efficiency is -43 dB and accounts for 12 dB enhancement compared to that of a straight nanowaveguide. The experimental results and theoretical predictions agree very well and show optimized conversion efficiency of -15 dB. This work represents a step toward realizing a fully integrated optical devices for generating new optical frequencies.

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.

Design of algal film photobioreactors: material surface energy effects on algal film productivity, colonization and lipid content.

A parallel plate air lift reactor was used to examine the growth kinetics of mixed culture algal biofilms grown on various materials (acrylic, glass, polycarbonate, polystyrene and cellulose acetate). The growth kinetics of the algal biofilms were non-linear overall and their overall productivities ranged from 1.10-2.08g/m(2)day, with those grown on cellulose acetate having the highest productivity. Overall algal biofilm productivity was largely explained by differences in the colonization time which in turn was strongly correlated to the polar surface energy of the material, but weakly correlated to water-material contact angle. When colonization time was taken into account, the productivity for all materials except acrylic was not significantly different at approximately 2g/m(2)/day. Lipid content of the algal biofilms ranged from 6% to 8% (w/w) and was not correlated to water-material contact angle or polar surface energy. The results have potential application for selecting appropriate materials for algal film photobioreactors.