179 research outputs found
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
Enhancing Optical Forces in InP-Based Waveguides
AbstractCantilever sensors are among the most important microelectromechanical systems (MEMS), which are usually actuated by electrostatic forces or piezoelectric elements. Although well-developed microfabrication technology has made silicon the prevailing material for MEMS, unique properties of other materials are overlooked in this context. Here we investigate optically induced forces exerted upon a semi-insulating InP waveguide suspended above a highly doped InP:Si substrate, in three different regimes: the epsilon-near-zero (ENZ), with excitation of surface plasmon polaritons (SPPs) and phonons excitation. An order of magnitude amplification of the force is observed when light is coupled to SPPs, and three orders of magnitude amplification is achieved in the phonon excitation regime. In the ENZ regime, the force is found to be repulsive and higher than that in a waveguide suspended above a dielectric substrate. Low losses in InP:Si result in a big propagation length. The induced deflection can be detected by measuring the phase change of the light when passing through the waveguide, which enables all-optical functioning, and paves the way towards integration and miniaturization of micro-cantilevers. In addition, tunability of the ENZ and the SPP excitation wavelength ranges, via adjusting the carrier concentration, provides an extra degree of freedom for designing MEMS devices.</jats:p
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
- …