Spatially-resolved measurements of spin valley polarization in MOCVD-grown monolayer WSe2.

Time-resolved Kerr rotation microscopy is used to generate and measure spin valley polarization in MOCVD-grown monolayer tungsten diselenide (WSe2). The Kerr signal reveals bi-exponential decay with time constants of 100 ps and 3 ns. Measurements are performed on several triangular flakes from the same growth cycle and reveal larger spin valley polarization near the edges of the flakes. This spatial dependence is observed across multiple WSe2 flakes in the Kerr rotation measurements but not in the spatially resolved reflectivity or microphotoluminescence data. Time-resolved pump-probe overlap measurements further reveal that the Kerr signal's spatial dependence is not due to spin diffusion on the nanosecond timescale.

Long-Lived Hole Spin/Valley Polarization Probed by Kerr Rotation in Monolayer WSe2.

Time-resolved Kerr rotation and photoluminescence measurements are performed on MOCVD-grown monolayer tungsten diselenide (WSe2). We observe a surprisingly long-lived Kerr rotation signal (∼80 ns) at 10 K, which is attributed to spin/valley polarization of the resident holes. This polarization is robust to transverse magnetic field (up to 0.3 T). Wavelength-dependent measurements reveal that only excitation near the free exciton energy generates this long-lived spin/valley polarization.

The effect of doping on low temperature growth of high quality GaAs nanowires on polycrystalline films.

The increasing demand for miniature autonomous sensors requires low cost integration methods, but to date, material limitations have prevented the direct growth of optically active III-V materials on CMOS devices. We report on the deposition of GaAs nanowires on polycrystalline conductive films to allow for direct integration of optoelectronic devices on dissimilar materials. Undoped, Si-doped, and Be-doped nanowires were grown at Ts  = 400 °C on oxide (indium tin oxide) and metallic (platinum and titanium) films. Be-doping is shown to significantly reduce the nanowire diameter and improve the nanowire aspect ratio to 50:1. Photoluminescence measurements of Be-doped nanowires are 1-2 orders of magnitude stronger than undoped and Si-doped nanowires and have a thermal activation energy of 14 meV, which is comparable to nanowires grown on crystalline substrates. Electrical measurements confirm that the metal-semiconductor junction is Ohmic. These results demonstrate the feasibility of integrating nanowire-based optoelectronic devices directly on CMOS chips.

Amplifying optical rotation using a coupled waveguide and ring resonator.

Measuring optical rotations in materials is a useful tool in many experimental studies. Research may be limited by the ability to measure small rotations due to weak interactions. We propose a novel scheme wherein we use a coupled waveguide and ring resonator to amplify the effects of optical rotation, potentially opening new avenues for investigation. Our proposed device can increase the resulting optical rotation by up to six orders of magnitude.

Optimizing nanophotonic cavity designs with the gravitational search algorithm.

Designing photonic crystal cavities with high quality factors and low mode volumes is of great importance for maximizing interactions of light and matter in metamaterials. Previous work on photonic crystal cavities has revealed dramatic improvements in performance by fine-tuning the device design. In L3 cavities, slight shifts of the holes on the edge of the cavity have been found to greatly increase quality factors without significantly altering the mode volume. Here we demonstrate utilizing a nature inspired search algorithm to efficiently explore a large parameter space. The results converge upon a new cavity model with a high quality factor to mode volume ratio (Q/V = 798,000 (λ/n)(-3)).

Time-resolved two-pulse excitation of quantum dots coupled to a photonic crystal cavity in the Purcell regime.

We investigate the nonlinear emission dynamics of quantum dots coupled to photonic crystal cavities in the Purcell regime using luminescence intensity autocorrelation. Two laser pulses with a controlled time delay sequentially excite the coupled system inducing emission that depends on the delay and laser power. We find distinct contrasts between exciton and biexciton emission as a function of time delay which originate from different nonlinearities. A quantum optical simulation is also performed that accounts for the interaction between the laser pulses, exciton, and cavity mode.

Photoluminescence imaging of focused ion beam induced individual quantum dots.

We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 µm apart, with a minimum single dot emission line width of 160 µeV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV.

Electron spin polarization-based integrated photonic devices.

The lack of optical isolators has limited the serial integration of components in the development of photonic integrated circuits. Isolators are inherently nonreciprocal and, as such, require nonreciprocal optical propagation. We propose a class of integrated photonic devices that make use of electrically-generated electron spin polarization in semiconductors to cause nonreciprocal TE/TM mode conversion. Active control over the non-reciprocal mode coupling rate allows for the design of electrically-controlled isolators, circulators, modulators and switches. We analyze the effects of waveguide birefringence and absorption loss as limiting factors to device performance.

Raman amplification of 40 Gb/s data in low-loss silicon waveguides.

We demonstrate on-chip Raman amplification of an optical data signal at 40 Gb/s in a silicon-on-insulator p-i-n rib waveguide. Using 230 mW of coupled pump power, on/off gain of up to 2.3 dB is observed, while signal integrity is maintained. In addition, the gain is measured as a function of signal wavelength detuning from the Stokes wavelength. The Lorentzian linewidth of the Raman gain profile is determined to be approximately 80 GHz. This provides applicability for the selective amplification of individual DWDM optical channels.

Demonstration of wavelength conversion at 40 Gb/s data rate in silicon waveguides.

We report an efficient wavelength conversion via four-wave-mixing in reverse biased silicon-on-isolator p-i-n rib waveguides and demonstrate, for the first time, the conversion of a high-speed optical pseudo-random bit sequence data at 40 Gb/s. Results give a wavelength conversion efficiency of -8.6dB using a 8cm long waveguide with clear open eye on the wavelength converted signal . Conversion efficiency as functions of pump power and bias voltages has also been investigated. We show a slope efficiency close to 2 as predicted by theory.