Design and applications of single-photon devices based on waveguides coupled to quantum emitters

Engineering photon-photon interactions is fundamentally challenging because photons in vacuum do not interact with each other. While their interactions can be mediated using optical nonlinearities, these effects are negligible for individual photons. This thesis explores two topics related to optical nonlinearities in waveguides. In the first part, we perform a numerical simulation study of hollow core antiresonant reflecting optical waveguides (ARROWs) fabricated using standard lithographic techniques in the context of their suitability as a platform for on-chip photonic quantum information processing. We investigate the effects of the core size, the number of pairs of antiresonant layers surrounding the hollow core, and the refractive index contrast between cladding materials on propagation losses in the waveguide. Additionally, we explore the feasibility of integrating these waveguides with Bragg gratings and dielectric metasurface mirrors to form on-chip cavities, that when loaded with atomic ensembles could act as nonlinear optical devices controllable with single photons. The second part of this thesis studies the application of a 3 level quantum emitter coupled to a directional optical waveguide to deterministically subtract a single photon from a propagating optical pulse. Subtracting a single photon from a light state is one of the most fundamental operations with important applications in quantum information processing. However, current methods to subtract a photon such as using a low reflectivity beam splitter suffer from inherently low success probabilities as well as a strong dependence on the number of photons in the input. We explore implementing this single photon subtraction operation in our proposed system when the optical input is a continuous wave coherent state, coherent pulsed state containing a finite number of photons, or a Fock state

Broadband repeatable <0.025 dB average loss rapid adiabatic based 3-dB coupler in a 45 nm SOI CMOS process

We demonstrate a 75 µm-long rapid adiabatic coupler (RAC) with an average insertion loss <0.025 dB/coupler and an average power splitting ratio of 50±1.09% over 40 nm bandwidth and 68 reticles across a 300 mm 45 nm SOI CMOS wafer.Accepted manuscrip

Carrier Dynamics and Lasing Applications of Colloidal Quantum Dots

Colloidal quantum dots have seen progress over the last three decades as an active material for solution processed optoelectronics. Quantum dots offer a tunable optical bandgap from the UV to the mid-IR via control over size and chemical composition. Their optical and electronic properties can be further manipulated through surface engineering and heterostructuring. These materials are processed from solution, enabling low-cost fabrication; and are compatible with a wide range of substrates.In this thesis, I investigate properties of colloidal quantum dots for lasing applications. My findings illuminate fundamental processes that determine their performance in lasing; and point to strategies to overcome present-day limitations. First, I investigate the effects of temperatures reached during continuous-wave excitation on the charge carrier dynamics in CdSe/CdS core/shell QDs, and their effect on the lasing threshold. Modelling and experimental characterization reveal a temperature-activated sub- picosecond electron trapping process that depletes the population of excited QDs. Accordingly, a small decrease in the athermal lasing threshold can yield a large decrease in the continuous- wave lasing threshold due to reduced heat generation. In CdSe/CdS QDs, built-in biaxial strain reduces the valence band-edge degeneracy, lowering the athermal and CW lasing threshold by 30% and 70% respectively. Next I investigate graded CdSe/CdS shells on infrared InAs QDs to suppress non-radiative biexciton Auger recombination. Infrared InAs QDs are promising materials for infrared light emitting devices, but their Auger lifetime is much shorter than those found in more widely explored cadmium and lead chalcogenide materials. The graded CdSe/CdS shells on InAs which I develop herein result in a 2x increase in the Auger lifetime relative to the best value reported in prior InAs QD literature. Finally, I propose a method to achieve nanosecond deep-blue lasing using CsPbCl3 QDs. These perovskite quantum dots suffer from fast biexciton Auger lifetimes, and are consequently able to sustain lasing only under femtosecond pulsed photoexcitation. Forming a superlattice of QDs with aligned dipoles, and coupling them to a high Q-factor distributed feedback grating, is a step toward quasi-CW lasing in this materials system. I design the grating for single mode operation within the gain spectrum of the CsPbCl3 QDs.Ph.D

Giant alloyed hot injection shells enable ultralow optical gain threshold in colloidal quantum wells

As an attractive materials system for high-performance optoelectronics, colloidal nanoplatelets (NPLs) benefit from atomic-level precision in thickness, minimizing emission inhomogeneous broadening. Much progress has been made to enhance their photoluminescence quantum yield (PLQY) and photostability. However, to date, layer-by-layer growth of shells at room temperature has resulted in defects that limit PLQY and thus curtail the performance of NPLs as an optical gain medium. Here, we introduce a hot-injection method growing giant alloyed shells using an approach that reduces core/shell lattice mismatch and suppresses Auger recombination. Near-unity PLQY is achieved with a narrow full-width-at-half-maximum (20 nm), accompanied by emission tunability (from 610 to 650 nm). The biexciton lifetime exceeds 1 ns, an order of magnitude longer than in conventional colloidal quantum dots (CQDs). Reduced Auger recombination enables record-low amplified spontaneous emission threshold of 2.4 μJ cm-2 under one-photon pumping. This is lower by a factor of 2.5 than the best previously reported value in nanocrystals (6 μJ cm-2 for CdSe/CdS NPLs). Here, we also report single-mode lasing operation with a 0.55 mJ cm-2 threshold under two-photoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm-2 from CdSe/CdS CQDs in the Fabry-Pérot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultrahigh performance NPLs.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio

Contactless measurements of photocarrier transport properties in perovskite single crystals

The remarkable properties of metal halide perovskites arising from their impressive charge carrier diffusion lengths have led to rapid advances in solution-processed optoelectronics. Unfortunately, diffusion lengths reported in perovskite single crystals have ranged widely - from 3 μm to 3 mm - for ostensibly similar materials. Here we report a contactless method to measure the carrier mobility and further extract the diffusion length: our approach avoids both the effects of contact resistance and those of high electric field. We vary the density of quenchers - epitaxially included within perovskite single crystals - and report the dependence of excited state lifetime in the perovskite on inter-quencher spacing. Our results are repeatable and self-consistent (i.e. they agree on diffusion length for many different quencher concentrations) to within ± 6%. Using this method, we obtain a diffusion length in metal-halide perovskites of 2.6 μm ± 0.1 μm.This publication is based in part on work supported by the US Department of the Navy, Office of Naval Research (Grant Award No.: N00014-17-1-2524), by the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engi- neering Research Council (NSERC) of Canada

Suppression of Auger Recombination by Gradient Alloying in InAs/CdSe/CdS QDs

Colloidal quantum dots are promising for low-cost optoelectronic devices such as solar cells, light-emitting diodes (LEDs), lasers, and photodetectors. InAs-based quantum dots (QDs) are well suited for near-infrared (NIR) applications; however, to date, the highest-QY InAs QDs have exhibited short biexciton Auger lifetimes of similar to&lt;50 ps. Here, we report a band engineering strategy that doubles the Auger lifetime in InAs CQDs. By developing a continuously graded thick CdSexS1-x shell, we synthesize InAs/CdSexS1-x/CdS CQDs that enable a smooth progression from the core to the outer shell, slowing the Auger process. We report a biexciton Auger lifetime of similar to 10(5) ps compared to 17 ps for control InAs/CdSe/CdS CQDs. This represents a 2x increase of the Auger lifetime relative to the best value reported for InAs CQDs in prior literature. Copyright © 2020 American Chemical Society.1

Deep-Blue Perovskite Single-Mode Lasing through Efficient Vapor-Assisted Chlorination

This is the peer-reviewed version of the following article: Pina, J. M., Parmar, D. H., Bappi, G., Zhou, C., Choubisa, H., Vafaie, M., Najarian, A. M., Bertens, K., Sagar, L. K., Dong, Y., Gao, Y., Hoogland, S., Saidaminov, M. I., Sargent, E. H., Deep‐Blue Perovskite Single‐Mode Lasing through Efficient Vapor‐Assisted Chlorination. Adv. Mater. 2021, 33, 2006697, which has been published in final form at https://doi.org/10.1002/adma.202006697. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Metal halide perovskites have emerged as promising candidates for solution-processed laser gain materials, with impressive performance in the green and red spectral regions. Despite exciting progress, deep-blue-an important wavelength for laser applications-remains underexplored; indeed, cavity integration and single-mode lasing from large-bandgap perovskites have yet to be achieved. Here, a vapor-assisted chlorination strategy that enables synthesis of low-dimensional CsPbCl3  thin films exhibiting deep-blue emission is reported. Using this approach,  high-quality perovskite thin films having a low surface roughness (RMS ≈ 1.3 nm) and efficient charge transfer properties are achieved. These enable us to document low-threshold amplified spontaneous emission. Levering the high quality of the gain medium,  vertical-cavity surface-emitting lasers with a low lasing threshold of 6.5 µJ cm-2  are fabricated. This report of deep-blue perovskite single-mode lasing showcases the prospect of increasing the range of deep-blue laser sources.This work was supported financially by the R&D Center U.S. San Jose Laboratory, A Division of Sony Corporation of America (2018 Sony Research Award Program Ref. No. 2019‐0669)

High Color Purity Lead‐Free Perovskite Light‐Emitting Diodes via Sn Stabilization

Perovskite-based light-emitting diodes (PeLEDs) are now approaching the upper limits of external quantum efficiency (EQE); however, their application is currently limited by reliance on lead and by inadequate color purity. The Rec. 2020 requires Commission Internationale de l’Eclairage coordinates of (0.708, 0.292) for red emitters, but present-day perovskite devices only achieve (0.71, 0.28). Here, lead-free PeLEDs are reported with color coordinates of (0.706, 0.294)—the highest purity reported among red PeLEDs. The variation of the emission spectrum is also evaluated as a function of temperature and applied potential, finding that emission redshifts by <3 nm under low temperature and by <0.3 nm V−1 with operating voltage. The prominent oxidation pathway of Sn is identified and this is suppressed with the aid of H3PO2. This strategy prevents the oxidation of the constituent precursors, through both its moderate reducing properties and through its forming complexes with the perovskite that increase the energetic barrier toward Sn oxidation. The H3PO2 additionally seeds crystal growth during film formation, improving film quality. PeLEDs are reported with an EQE of 0.3% and a brightness of 70 cd m−2; this is the record among reported red-emitting, lead-free PeLEDs.H.Y.L. acknowledges financial support from the National Natural Science Foundation of China (NSFC) (No. 51771132), Recruitment Program of Global Youth Experts of China Talents Plan of China, the Natural Sciences and Engineering Research Council of Canada (NSERC) Postdoctoral Fellowship (PDF). E.H.S. and all co-authors from the Department of Electrical and Computer Engineering at the University of Toronto acknowledge the financial support by the Global Research Outreach program of Samsung Advanced Institute of Technology and the Ontario Research Fund Research Excellence Program and the Natural Sciences and Engineering Research Council of Canada (NSERC)

Quantum Dot Self‐Assembly Enables Low‐Threshold Lasing

Perovskite quantum dots (QDs) are of interest for solution-processed lasers; however, their short Auger lifetime has limited lasing operation principally to the femtosecond temporal regime the photoexcitation levels to achieve optical gain threshold are up to two orders of magnitude higher in the nanosecond regime than in the femtosecond. Here the authors report QD superlattices in which the gain medium facilitates excitonic delocalization to decrease Auger recombination and in which the macroscopic dimensions of the structures provide the optical feedback required for lasing. The authors develope a self-assembly strategy that relies on sodiumd—an assembly director that passivates the surface of the QDs and induces self-assembly to form ordered three-dimensional cubic structures. A density functional theory model that accounts for the attraction forces between QDs allows to explain self-assembly and superlattice formation. Compared to conventional organic-ligand-passivated QDs, sodium enables higher attractive forces, ultimately leading to the formation of micron-length scale structures and the optical faceting required for feedback. Simultaneously, the decreased inter-dot distance enabled by the new ligand enhances exciton delocalization among QDs, as demonstrated by the dynamically red-shifted photoluminescence. These structures function as the lasing cavity and the gain medium, enabling nanosecond-sustained lasing with a threshold of 25 μJ cm–2.C.Z., J.M.P., and T.Z. contributed equally to this work. L.Z. and H.D. acknowledge the National Natural Science Foundation of China (Grant Nos. 61875256, 61675219, and 91950201). This work was partially funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). M.I.S. acknowledges the support of Banting Postdoctoral Fellowship Program, administered by the Government of Canada. WAXS and SAXS measurements were performed in the Canadian Light Source CLS, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), NSERC, the National Research Council (NRC), the Canadian Institutes ofHealth Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan. The authors acknowledge the technical assistance and scientific guidance of C.-Y. Kim and A. Leontowich at the CLS. The authors also acknowledge the help and useful discussions of A. Johnston, S. Teale, and J. Fan regarding the WAXS and SAXS data. They also acknowledge the useful discussions with R. Sabatini regarding transient PL data. H.D. acknowledges the Youth Top-notch Talent Support Program in Shanghai. The authors again acknowledge the University of Chinese Academy of Sciences (UCAS) Joint Ph.D. Training Program