AlGaAs-On-Insulator Nonlinear Photonics

The combination of nonlinear and integrated photonics has recently seen a surge with Kerr frequency comb generation in micro-resonators as the most significant achievement. Efficient nonlinear photonic chips have myriad applications including high speed optical signal processing, on-chip multi-wavelength lasers, metrology, molecular spectroscopy, and quantum information science. Aluminium gallium arsenide (AlGaAs) exhibits very high material nonlinearity and low nonlinear loss when operated below half its bandgap energy. However, difficulties in device processing and low device effective nonlinearity made Kerr frequency comb generation elusive. Here, we demonstrate AlGaAs-on-insulator as a nonlinear platform at telecom wavelengths. Using newly developed fabrication processes, we show high-quality-factor (Q>100,000) micro-resonators with integrated bus waveguides in a planar circuit where optical parametric oscillation is achieved with a record low threshold power of 3 mW and a frequency comb spanning 350 nm is obtained. Our demonstration shows the huge potential of the AlGaAs-on-insulator platform in integrated nonlinear photonics.Comment: 21 pages, 12 figures, 1 table, 41 reference

Comparison of processing-induced deformations of InP bonded to Si determined by e-beam metrology: direct vs. adhesive bonding

In this paper, we employ an electron beam writer as metrology tool to investigate distortion of an exposed pattern of alignment marks in heterogeneously bonded InP on silicon. After experimental study of three different bonding and processing configurations which represent typical on-chip photonic device fabrication conditions, the smallest degree of linearly-corrected distortion errors is obtained for the directly bonded wafer, with the alignment marks both formed and measured on the same InP layer side after bonding (equivalent to single-sided processing of the bonded layer). Under these conditions, multilayer exposure alignment accuracy is limited by the InP layer deformation after the initial pattern exposure mainly due to the mechanical wafer clamping in the e-beam cassette. Bonding-induced InP layer deformations dominate in cases of direct and BCB bonding when the alignment marks are formed on one InP wafer side, and measured after bonding and substrate removal from another (equivalent to double-sided processing of the bonded layer). The findings of this paper provide valuable insight into the origin of the multilayer exposure misalignment errors for the bonded III-V on Si wafers, and identify important measures that need to be taken to optimize the fabrication procedures for demonstration of efficient and high-performance on-chip photonic integrated devices.Comment: 7 pages, 6 figure

Synthesis and systematic optical investigation of selective area droplet epitaxy of InAs/InP quantum dots assisted by block copolymer lithography

We report on the systematic investigation of the optical properties of a selectively grown quantum dot gain material assisted by block-copolymer lithography for potential applications in active optical devices operating in the wavelength range around 1.55 um and above. We investigated a new type of diblock copolymer PS-b-PDMS (polystyrene-block-polydimethylsiloxane) for the fabrication of silicon oxycarbide hard mask for selective area epitaxy of InAs/InP quantum dots. An array of InAs/InP quantum dots was selectively grown via droplet epitaxy. Our detailed investigation of the quantum dot carrier dynamics in the 10-300 K temperature range indicates the presence of a density of states located within the InP bandgap in the vicinity of quantum dots. Those defects have a substantial impact on the optical properties of quantum dots.Comment: 11 pages, 5 figures, 1 tabl

PriorCVAE: scalable MCMC parameter inference with Bayesian deep generative modelling

In applied fields where the speed of inference and model flexibility are crucial, the use of Bayesian inference for models with a stochastic process as their prior, e.g. Gaussian processes (GPs) is ubiquitous. Recent literature has demonstrated that the computational bottleneck caused by GP priors or their finite realizations can be encoded using deep generative models such as variational autoencoders (VAEs), and the learned generators can then be used instead of the original priors during Markov chain Monte Carlo (MCMC) inference in a drop-in manner. While this approach enables fast and highly efficient inference, it loses information about the stochastic process hyperparameters, and, as a consequence, makes inference over hyperparameters impossible and the learned priors indistinct. We propose to resolve the aforementioned issue and disentangle the learned priors by conditioning the VAE on stochastic process hyperparameters. This way, the hyperparameters are encoded alongside GP realisations and can be explicitly estimated at the inference stage. We believe that the new method, termed PriorCVAE, will be a useful tool among approximate inference approaches and has the potential to have a large impact on spatial and spatiotemporal inference in crucial real-life applications. Code showcasing the PriorCVAE technique can be accessed via the following link: https://github.com/elizavetasemenova/PriorCVA

Threshold Characteristics of Slow-Light Photonic Crystal Lasers

The threshold properties of photonic crystal quantum dot lasers operating in the slow-light regime are investigated experimentally and theoretically. Measurements show that, in contrast to conventional lasers, the threshold gain attains a minimum value for a specific cavity length. The experimental results are explained by an analytical theory for the laser threshold that takes into account the effects of slow-light and random disorder due to unavoidable fabrication imperfections. Longer lasers are found to operate deeper into the slow-light region, leading to a trade-off between slow-light induced reduction of the mirror loss and slow-light enhancement of disorder-induced losses.Comment: 5 pages, 7 figure

Tunable MEMS VCSEL on Silicon substrate

We present design, fabrication and characterization of a MEMS VCSEL which utilizes a silicon-on-insulator wafer for the microelectromechanical system and encapsulates the MEMS by direct InP wafer bonding, which improves the protection and control of the tuning element. This procedure enables a more robust fabrication, a larger free spectral range and facilitates bidirectional tuning of the MEMS element. The MEMS VCSEL device uses a high contrast grating mirror on a MEMS stage as the bottom mirror, a wafer bonded InP with quantum wells for amplification and a deposited dielectric DBR as the top mirror. A 40 nm tuning range and a mechanical resonance frequency in excess of 2 MHz are demonstrated