Kashif M. Awan

Tuneable four-wave mixing in AlGaAs nanowires.

We have experimentally demonstrated broadband tuneable four-wave mixing in AlGaAs nanowires with the widths ranging between 400 and 650 nm and lengths from 0 to 2 mm. We performed a detailed experimental study of the parameters influencing the FWM performance in these devices (experimental conditions and nanowire dimensions). The maximum signal-to-idler conversion range was 100 nm, limited by the tuning range of the pump source. The maximum conversion efficiency, defined as the ratio of the output idler power to the output signal power, was -38 dB. In support of our explanation of the experimentally observed trends, we present modal analysis and group velocity dispersion numerical analysis. This study is what we believe to be a step forward towards realization of all-optical signal processing devices.

Enhanced spectral sensitivity of a chip-scale photonic-crystal slow-light interferometer

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

Post-process wavelength tuning of silicon photonic crystal slow-light waveguides

Silicon photonic crystal waveguides have enabled a range of technologies, yet their fabrication continues to present challenges. Here, we report on a post-processing method that allows us to tune the operational wavelength of slow-light photonic crystal waveguides in concert with optical characterization, offsetting the effects of hole-radii and slab thickness variations. Our method consist of wet chemical surface oxidation, followed by oxide stripping. Theoretical modelling shows that the changes in optical behavior were predictable, and hence controlled tuning can be achieved by changing the number of processing cycles, where each cycle removes approximately 0.25 nm from all exposed surfaces, producing a blueshift of 1.6±0.1  nm in operating wavelength.

Fabrication and optical characterization of GaN waveguides on (−201)-oriented β-Ga 2 O 3

Gallium nitride (GaN), a wide-bandgap III–V semiconductor material with a bandgap wavelength λg = 366 nm (for Wurtzite GaN) and transparency window covering the visible spectrum, has a large number of applications for photonics and optoelectronics. However, the optical quality of this material suffers from growth imperfections due to the lack of a suitable substrate. Recent studies have shown that GaN grown on (−201) β – Ga2O3 (gallium oxide) has better lattice matching and hence superior optical quality as compared to GaN grown traditionally on Al2O3 (sapphire). In this work, we report on the fabrication of GaN waveguides on Ga2O3 substrate, followed by a wet-etch process aimed at the reduction of waveguide surface roughness and improvement of side-wall verticality in these waveguides. The propagation loss in the resulting waveguides has been experimentally determined to be 7.5 dB/cm.

Nonlinear photonics on-a-chip in III-V semiconductors: quest for promising material candidates

We propose several designs of nonlinear optical waveguides based on quaternary III-V semiconductors AlGaAsSb and InGaAsP. These semiconductor materials have been widely used for laser sources. Their nonlinear optical properties, however, yet remain unexplored, while the materials definitely hold promise for nonlinear photonics on-a-chip. The latter argument is based on the fact that III-V compounds tend to exhibit high values of the nonlinear optical susceptibilities, while the nonlinear absorption in these materials can be minimized in the wavelength range of interest through a proper selection of the material composition. We present the modal analysis for the designed waveguide structures and show that the effective mode area much less than 1  μm2 can be achieved through a design optimization in each of the two compounds. We also present specific waveguide designs that demonstrate zero dispersion at the wavelengths of interest. The designed AlGaAsSb and InGaAsP waveguides are thus expected to demonstrate high values of the nonlinear coefficient and efficient nonlinear optical interactions.

Demonstration of optical nonlinearity in InGaAsP/InP passive waveguides

Abstract We report on the study of the third-order nonlinear optical interactions in InxGa1-xAsyP1-y/InP strip-loaded waveguides. The material composition and waveguide structures were optimized for enhanced nonlinear optical interactions. We performed self-phase modulation, four-wave mixing and nonlinear absorption measurements at the pump wavelength 1568 nm in our waveguides. The nonlinear phase shift of up to 2.5 π has been observed in self-phase modulation experiments. The measured value of the two-photon absorption coefficient α 2 was 19 cm/GW. The four-wave mixing conversion range, representing the wavelength difference between maximally separated signal and idler spectral components, was observed to be 45 nm. Our results indicate that InGaAsP has a high potential as a material platform for nonlinear photonic devices, provided that the operation wavelength range outside the two-photon absorption window is selected.

Gallium nitride on gallium oxide substrate for integrated nonlinear optics

Gallium Nitride (GaN), being a direct bandgap semiconductor with a wide bandgap and high thermal stability, is attractive for optoelectronic and electronic applications. Furthermore, due to its high optical nonlinearity — the characteristic of all 111-V semiconductors — GaN is also expected to be a suitable candidate for integrated nonlinear photonic circuits for a plethora of apphcations, ranging from on-chip wavelength conversion to quantum computing. Although GaN devices are in commercial production, it still suffers from lack of a suitable substrate material to reduce structural defects like high densities of threading dislocations (TDs), stacking faults, and grain boundaries. These defects significandy deteriorate the optical quality of the epi-grown GaN layer, since they act as non-radiative recombination centers. Recent studies have shown that GaN grown on (−201) β-Gallium Oxide (Ga2O3) has superior optical quality due to a better lattice matching as compared to GaN grown on Sapphire (Al2O3) [1-3]. In this work, we report on the fabrication of GaN waveguides on GaiOj substrate and their optical characterization to assess their feasibihty for efficient four-wave mixing (FWM).

Epitaxially-grown Gallium Nitride on Gallium Oxide substrate for photon pair generation in visible and telecomm wavelengths

Gallium Nitride (GaN), along with other III-Nitrides, is attractive for optoelectronic and electronic applications due to its wide direct energy bandgap, as well as high thermal stability. GaN is transparent over a wide wavelength range from infra-red to the visible band, which makes it suitable for lasers and LEDs. It is also expected to be a suitable candidate for integrated nonlinear photonic circuits for a wide range of applications from all-optical signal processing to quantum computing and on-chip wavelength conversion. Despite its abundant use in commercial devices, there is still need for suitable substrate materials to reduce high densities of threading dislocations (TDs) and other structural defects like stacking faults, and grain boundaries. All these defects degrade the optical quality of the epi-grown GaN layer as they act as non-radiative recombination centers. In this work, we propose GaN epitaxially grown on (-201) β-Ga2O3 as a suitable candidate for correlated photon pair generation, leading to on-chip quantum sources for both telecomm and visible spectrum. We also present designs for GaN waveguides to achieve efficient four-wave mixing (FWM) based on the experimental absorption and dispersion data of epitaxially grown GaN on Ga2O3.

Aluminium gallium arsenide waveguide designs for efficient four-wave mixing

Explosive growth in internet traffic has prompted huge interest in the area of all-optical signal processing. It can provide much higher data rates at much lower heat dissipation as compared to electronics. Wavelength conversion is an important aspect of all-optical signal processing, since it enables adding, dropping, multiplexing and de-multiplexing of WDM channels. All-optical wavelength conversion can readily be achieved by employing the third-order optical nonlinearity that leads to four-wave mixing (FWM).

Post-Process Tuning of Slow Light Photonic Crystal Waveguides

We present a wet chemical method for tuning the slow light operating wavelength of silicon photonic crystal waveguides. This procedure compensates for the effects of slab thickness - and systematic hole radii - variations.