Jeff Young


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
A silicon photonic circuit for optical trapping and characterization of single nanoparticle (2017)

In this thesis, two slightly different silicon-on-insulator (Silicon-on-Insulator (SOI)) planar photonic integrated circuits for optically trapping and characterizing single nanoparticles are designed, fabricated, and fully characterized. These symmetric (input/output) structures are formed by etching two dimensional patterns through a 220 nm thick silicon slab atop a micrometer thick layer of silicon dioxide, and are operated in a fluidic cell at wavelengths of ≈ 1.55 μm. Each consist of two grating couplers, two parabolic tapered waveguides, two single mode ridge waveguides, two photonic crystal waveguides and a single photonic crystal slot (PCS) microcavity, designed using a Finite Difference Time Domain (FDTD) electromagnetic simulation tool. The circuits are designed to concentrate continuous wave laser light incident on the input grating coupler to a small volume within the fluidic channel of the microcavity in order to achieve a high electric field intensity gradient capable of attracting and trapping nanoparticles from the solution via optical gradient forces.The fabricated PCS cavities exhibit Q factors > 7500 and resonant transmissions as high as T = 6%, when operated in hexane and without undercutting the cavities. Due to fabrication imperfections, the cavity Q and peak transmission values were not as high as simulation predicted, nevertheless, these robust, devices were successfully used to optically trap single sub-50 nm Au nanospheres and nanorods with
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Nonlinear optical response of triple-mode silicon photonic crystal microcavities coupled to single channel input and output waveguides (2017)

Optical and opto-electronic components play important roles in both classical and quantum information processing technologies. Despite fundamental differences in these technologies, both stand to benefit greatly from moving away from bulky, individually packaged components, toward a scalable platform that supports dense integration of low power consumption devices. Planar photonic circuits, composed of devices etched in a thin slab of high refractive index material, are considered an excellent candidate, and have been used to realize many key components, including low-loss waveguides, light sources, detectors, modulators, and spectral filters. In this dissertation, a novel triple-microcavity structure was designed, externally fabricated, and its linear and nonlinear optical properties were thoroughly characterized. The best of the structures exhibited both high four-wave mixing conversion efficiencies and low threshold optical bistability, which are relevant to frequency conversion and all-optical switching applications. The device consisted of three coupled photonic crystal (PC) microcavities with three nearly equally spaced resonant frequencies near telecommunication wavelengths (λ ~ 1.5 μm), with high quality factors (~ 10⁵, 10⁴ and 10³). The microcavity system was coupled to independent input and output PC waveguides, and the cavity-waveguide coupling strengths were engineered to maximize the coupling of the input waveguide to the central mode, and the output waveguide to the two modes on either side. A novel and sophisticated measurement and analysis protocol was developed to characterize the devices. This involved measuring and modelling the linear and nonlinear transmission characteristics of each of the modes separately with a single tunable laser, as well as the frequency conversion efficiency (via stimulated four-wave mixing) when two tunable lasers pumped two of the modes, and the power generated in the third mode was monitored. Comparisons of the entire set of model and experimental results led to the conclusion that this structure can be used to achieve both low-power-threshold optical switching and high efficiency four-wave-mixing-based frequency conversion. The advantages of this structure over others in the literature are its small footprint, multi-mode functionality and independent input and output channels. The main disadvantage that requires further refinement, has to do with its sensitivity to fabrication imperfections.

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Luminescent properties of PB-based (PbX) colloidal quantum dots (CQDs) in vacuum, on silicon and integrated with a silicon-on-insulator (SOI) photonic integrated circuit (PIC) (2016)

In the rapidly evolving field of experimental quantum information processing, one important sub-field pursues a potentially scalable implementation that transports quantum information encoded in photons throughout “photonic circuits” fabricated in a silicon wafer. A key component is an efficient on-demand source of these single photons, and this dissertation aimed to assess the feasibility of one proposed realization of such a source by integrating few PbSe colloidal quantum dots (CQDs, demonstrated single photon emitters in nanoparticle form) into the mode volume of an optical microcavity designed to efficiently direct quantum dot emission into a silicon photonic circuit. Although no direct evidence of {\it single} photon emission was observed, results prompted a number of follow-up experiments and considerable theoretical modeling to understand this quantum dot, photonic circuit system.The methods of investigation included (1) temporally-, spectrally-, and spatially-resolved photoluminescence (PL) measurements of PbSe CQDs integrated into SOI PICs and relatable environments (solution, thick film, thin film), (2) temperature-dependent, air-exposure studies of PbSe CQD thick film PL, (3) development and application of kinetic and quantum mechanical cavity-coupled modeling that admit complete accounting of the photonic density of states, depolarization effects, and non-radiative decay, and (4) a photon coincidence test of single photon emission.The main findings of this work are: (1) while capture of cavity-enhanced PbSe CQD emission into a silicon photonic circuit was demonstrated, the overall photon generate rate is inadequate for any useful implementation, (2) the measured coupling rate can be modeled and explained in terms of system parameters extracted from auxiliary experimental results obtained with the PbSe CQDs in isolation, or on isolated microcavities, and (3) consistent results could only be obtained after nontrivial depolarization factors and non-radiative decay processes are properly accounted for. From this it is clear that the performance of PbSe CQDs in this configuration of a single photon source in silicon is currently limited by a long-lived trap state with a several microsecond lifetime, and large depolarization effects that inhibit emission. Although plausible future efforts may mitigate these effects substantially, performance may still be hindered by the intrinsic emission strength of PbSe CQDs.

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Molecular beam epitaxy growth technology and properties of GaAsBi alloys (2014)

In this thesis, molecular beam epitaxy (MBE) technology and the MBE growth of GaAsBi are investigated. MBE is a non-equilibrium technique whereby precisely controlled molecular beams are deposited onto a heated substrate at temperatures much lower than for equilibrium growth techniques. A novel closed-cycle cooling setup is implemented to replace liquid nitrogen (LN₂) cooling of the MBE cryo-shroud. The temperature dependence of cryopanel pumping is explored, and GaAs and AlGaAs layers grown using the new cooling setup and with LN₂ cooling of the shroud are characterized. Strong AlGaAs photoluminescence and low impurity concentrations indicate closed-cycle cooling is a promising cost-saving technique for MBE. The relatively unexplored III-V-Bi family of alloys is an exciting frontier of III-V semiconductor alloy exploration. The GaAsBi alloy exhibits many novel properties, including an unparalleled bandgap reduction per change in the size of the crystal lattice, presenting a wide range of potential device applications. A systematic study of the dependence of Bi incorporation on MBE growth conditions is presented. Bi incorporation is found to rapidly increase as the As₂:Ga flux ratio is lowered to 0.5 and saturate for lower flux ratios. This indicates Bi incorporation is sensitive to the surface stoichiometry. A GaAsBi growth model is proposed where Bi from a wetting layer incorporates on surface sites which are terminated by Ga. Low growth temperatures are required as the weak Bi-Ga incorporation bond can be broken thermally, ejecting Bi back to the wetting layer. GaAsBi layers with up to 21.8% Bi, record Bi-content, were grown at temperatures as low as 200C. These layers have up to 2.6% mismatch from the GaAs substrates and show unusually large critical thicknesses for relaxation, a result of the low growth temperature.Optical absorption measurements on pseudomorphic GaAsBi layers with up to 18.7% Bi show the bandgap decreases strongly with increasing Bi-content, reaching 0.5 eV at 18.7% Bi. Si-doped n-GaAsBi layers with up to 4% Bi show the concentration of acceptor states increases rapidly with increasing Bi-content. The acceptor concentration is equal to that of closed Bi3 clusters, suggesting they are the source of deep acceptor states in GaAsBi.

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Master's Student Supervision (2010 - 2020)
A NbTiN superconducting nanowire single photon detector (SNSPD) on a silicon-on-insulator substrate (2017)

Single photon detectors are essential part of most optical quantum information applications. Among all candidates, superconducting nanowire single photon detectors (SNSPD) have the advantages of low jitter, low dark count rates and high maximum count rate. Commercial systems based on these detector elements currently cost on order $50-100K. In this project a circular meander design of a free-space SNSPD is fabricated and tested in house. Although the absolute absorption efficiency of the detector is low, because it was fabricated on a substrate optimized for other applications, the measured and modelled values for both incident polarizations agree within the uncertainties. The bias current dependence is almost constant from 50% to 100% of the breakdown threshold value, which should allow operation at intrinsic dark count rates extrapolated to be
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Characterization of photonic crystal silicon-on-insulator optical circuits fabricated by a CMOS foundry (2011)

Prototype silicon photonic circuits in a silicon-on-insulator (SOI) wafer fabricated using a CMOS-based single layer process, are thoroughly characterized. Thousands of devices were fabricated on a single 200 mm diameter wafer using deep UV lithography at the IMEC silicon photonics foundry. The devices studied integrate three key elements: i) input/output grating couplers, ii) waveguides, and iii) microcavities. The photonic crystal cavities are symmetrically coupled to input and output single mode channel waveguides, which couple light into tapered waveguides that are terminated by two-dimensional photonic crystal gratings couplers.The grating coupler efficiencies and bandwidths are studied independently from the other device components, both experimentally and by simulations, using free-space optics. At a fixed angle of incidence, light over a bandwidth of approximately 14 nm (TE) and 30 nm (TM) is coupled from grating to grating, in both experiments and simulations. The centre frequency of this coupling spectrum is tuned by varying the angle of incidence on the grating, by coating the grating with photoresist, and by varying the size of the holes that form the grating.The maximum net single-grating coupling efficiencies are measured to be 15% (26%) for TE (TM) coupling. Taking into account the limited aperture of the collection optics, and aberrations of the input coupling lenses, these measured net efficiencies are reasonably consistent with simulated true efficiencies of 21%(49%) found using Lumerical FDTD Solutions and MODE Solutions commercial software.Resonant transmission measurements from free space, via an input grating, through the complete integrated photonic circuit, including the photonic crystal microcavity, and off-chip via an output grating are measured for a number of different cavities. The transmission of light from the end of one tapered waveguide, through the cavity, to the end of the other tapered waveguide is found to be ~10%. The maximum microcavity quality factor measured is ~ 5000.This work demonstrates that fully integrated photonic circuits can be successfully fabricated using IMEC's CMOS foundry service. It further shows that useful overall coupling efficiencies can be realized using free space optics, which will be useful for probing such circuits when they are placed inside optical cryostats.

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Optical Trapping Using a Photonic Crystal Cavity: Design and Sample Fabrication (2010)

A photonic crystal (PC) structure for trapping a 50nm radius dielectric particle at a precise location on a silicon surface in an organic solvent environment has been designed and all of its key components have been fabricated. The high gradient of electric field intensity in a PC cavity mode, with wavelength ~ 1.5 microns, exerts a radiation force toward the center of the cavity. The Finite Difference Time Domain (FDTD) modeling method was used to design a symmetric (input/output) structure that consists of two grating couplers, two parabolic tapered waveguides, two single mode ridge waveguides, two photonic crystal waveguides and a single three-missing-hole (L3) PC cavity.The radiation force on the dielectric sphere was exactly calculated using FDTD simulations to evaluate the Maxwell Stress Tensor (MST) in the presence of the particle to be trapped. This result was compared to that obtained using the simpler dipole approximation, and good agreement between them was found. The fabrication of the structure was done by electron beam lithography and chlorine plasma etching. The Q factors for some of the fabricated samples were measured from the cavity enhanced photoluminescence emission of PbSe quantum dots deposited on the sample surface. A vertical Q factor of 3600 (in vacuum environment) was measured for an isolated cavity, which corresponds to a Qv of 3800 ( in solvent environment) in the FDTD simulations. Also, the Q, of the overall structure (cavity and the waveguides) was measure to be 1050 in vacuum, which from simulations is equivalent to a Q of 1800 in a solvent. These Q values and the resonant frequencies of the modes are in close, but not perfect agreement with the simulation results.

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