Lukas Chrostowski


Research Interests

Optics and Photonics
Micro and Nanoelectronics
Manufacturing Processes
quantum computing
semiconductor lasers
silicon photonics

Relevant Thesis-Based Degree Programs


Research Methodology

electron beam lithography


Master's students
Doctoral students
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silicon photonic device and circuit design, nanofabrication, quantum computing, single photon devices

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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Improving the performance of silicon photonic optical resonator-based sensors for biomedical applications (2020)

Silicon photonic biosensors show great potential for applications in medical diagnostics and healthcare services. Near-infrared transparency and high refractive index of silicon allow us to build compact and efficient circuits leveraging CMOS foundries, which provide low-cost mass production and enable the integration of the optoelectronic components on the same chip. Although silicon photonic biosensors have proven performances close to today's gold standard diagnostics, many applications still require higher multiplexing, as well as more sensitive, reliable and quantitative measurements. This dissertation is based on theoretical and experimental studies of silicon photonic sensing architectures in terms of sensor performance improvement and unit-cost reduction.Specifically, two novel sub-wavelength grating-based (SWG) waveguide configurations are presented to improve the sensitivity. Leveraging the advantage of SWG metamaterials, the substrate-overetch (SOE) and multi-box SWG devices present a largely extended modal size and surface contact area, which gives 10-time enhanced sensitivity compared to the conventional devices. In addition, by employing the Bragg grating as the sensing architecture, the multi-box SWG-based grating configuration achieves a lower detection limit compared to the microring resonator (MRR) counterpart and demonstrates the capability for monitoring small molecule interactions.Replacing the laser with a broadband source can provide a lower-cost solution for the optical system. Therefore, two cost-effective broadband light source-based sensing implementations are proposed and demonstrated with acceptable sensitivities. The first implementation uses cascaded MRRs for index monitoring, where the analyte variation is converted to the photocurrent change as the readout. The second implementation uses a phase-shifted Bragg grating-based symmetrical Mach-Zehnder interferometer, where the analyte variation maps the intensity change at the resonant wavelength. Furthermore, a system-level integration of active silicon photonic sensors using Fan-Out Wafer-Level-Packaging (FOWLP) is proposed in the dissertation, which can reduce the die size down to 1 mm² while simplifying the microfluidic and optical integration. Leveraging the CMOS foundries and the proposed FOWLP technique, the unit cost of each packaged sensing die can be reduced to several dollars.

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Reconfigurable silicon photonic integrated circuits (2020)

Integrating photonics with state-of-the-art nanoelectronics in Silicon (Si) is key to enabling new computing paradigms and sensing applications, as it leverages the well-established complementary metal-oxide-semiconductor (CMOS) foundries used to manufacture the electronics chips at a large-scale with low-cost. Towards this goal, great efforts have been made to integrate all the fundamental photonic building blocks on Si. However, due to a number of challenges, there has been no demonstration of a complete fully-integrated silicon photonic (SiP) chip. This dissertation addresses some of the challenges that hold back the deployment of complete fully-integrated Si chips.Due to Si’s temperature dependency, the performance of ring-based filters, switches, and modulators degrade when the surrounding temperature fluctuates. Second, fabrication imperfections lead to a discrepancy between the designed and measured ring-based filters’, switches’, and modulators’ spectral responses. Third, because of Si’s reciprocal lattice, Si cannot be used to realize optical isolators, which are required to integrate lasers on Si as they block back-reflections from flowing back to the laser and destabilizing its operation.This dissertation addresses the aforementioned challenges as follows. By slightly doping the Si waveguides, defect states are introduced which enable sensing and manipulating light in Si waveguides while absorbing minimal optical power. These doped waveguides are introduced into ring-based filters and switches to correct for fabrication errors and demonstrate the tuning of the largest yet most compact ring-based 16×16 optical switch matrix and 14-ring coupled-resonator optical waveguide (CROW) filter. Second, a new design of a microring modulator (MRM) is demonstrated that allows correcting the spectral features (wavelength, bandwidth (BW) and/or extinction ratio (ER)) of fabricated MRMs and maintain the MRM’s free-spectral range (FSR). Third, a new method for measuring propagation losses in optical waveguides is demonstrated. Finally, a stable quantum well (QW) distributed feedback (DFB) laser without an isolator is demonstrated for the first time. Instead of depositing Si-incompatible magneto-optic (MO) materials, a reflection-cancellation circuit (RCC) is proposed and used to demonstrate laser stability against varying levels of back-reflections in real-time. The same circuit was used to further reduce the linewidth of the DFB laser down to 3 kHz.

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Spectral tailoring of silicon integrated Bragg gratings (2020)

Integrated Bragg gratings (IBGs) developed on the silicon-on-insulator (SOI) platform, owning to their high spectral flexibility, have become key components in photonic integrated circuits. Despite the rapid development of silicon IBG devices, there still lacks a comprehensive design methodology to achieve arbitrary, sophisticated, complex (amplitude and phase) spectral responses on IBGs. In addition, various problems also exist in practical designs and implementations of IBGs. The objectives of this thesis are to address these issues and, thus, to facilitate and improve the spectral tailoring of silicon IBG devices.A comprehensive and sophisticated design methodology of IBGs to achieve arbitrary spectral responses has been developed, and each individual step of the design and implementation process has been elaborated in detail. Furthermore, to address the IBG modeling and apodization issues existing in the design process, we have proposed (1) a highly efficient and reliable IBG modeling method by directly synthesizing the physical structure of the gratings; and (2) a high-performance apodization technique for IBGs based on periodic phase modulation.Multichannel photonic Hilbert transformers (MPHTs) based on complex synthesized IBGs have been designed, fabricated, and experimentally characterized. The realizations of these MPHTs are based on using the comprehensive IBG design methodology developed in this thesis. MPHTs with a total wavelength channel number of up to 9 and a single channel bandwidth of up to 625 GHz have been successfully achieved. The impacts of apodization phase errors (APE) on the spectral responses of apodized silicon IBGs have been characterized. The characterization results show that APE can largely distort the spectral responses of apodized IBGs from the designed ones. Then, to address this issue, a methodology to compensate and thus to eliminate APE of an apodized IBG to correct the distorted response has been developed and experimentally validated. A novel apodization profile [k(z)] amplification technique for IBGs has been proposed. Using this k(z) amplification technique for designing IBGs can bring about significant improvements in the apodization performence for the given fabrication constraints. Therefore, this technique can largely overcome the current apodization limitations of silicon IBGs due to fabrication constraints, thus facilitating their spectral tailoring applications.

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Wavelength and polarization control for silicon photonic receiver applications (2020)

No abstract available.

Enabling practical deployment of silicon ring resonator-based systems (2018)

Microring resonators (MRRs) on silicon photonic platforms allow for low-power, dense, and large-scale manipulation of optical signals on-chip. MRR-based modulators, switches, and filters have become key building blocks in integrated optical circuits for applications in future data communications, high-performance computing, and sensing. This thesis presents solutions for overcoming several challenges towards practical deployment of MRR systems. The performance of MRR is highly susceptible to temperature and fabrication variations, which cause significant shifts in the MRR's spectral responses. In-resonator photoconductive heaters (IRPHs), formed by doping MRRs’ waveguides show high responsivities. As IRPHs do not require additional material depositions, photodetectors, or power taps and use the same contact pads for both sense and tune operations, they can be used to automatically tune and temperature stabilize MRRs without compromising the cost or area of the devices. Automatic tuning and stabilization of one- and two-ring filters are demonstrated.Multi-ring filters offer attractive spectral features such as wide pass-bands, steep roll-offs, and large extinction ratios. Using IRPHS, automatic tuning of a four-ring Vernier ring filter across a record 37.6 nm wavelength and wavelength locking to account for a record 65 degrees temperature variation is demonstrated. A tuning algorithm in which the number of iterations scales linearly with the number of coupled rings in the system is presented. As this method typically does not rely on the output spectral shape of the filter, it is applicable to a wider range of coupled resonator systems. Application of this tuning method is then demonstrated for various multi-ring filters by both simulation and experiment. Crosstalk can be a major source of signal degradation in large-scale MRR systems. Interchannel and intrachannel crosstalk of one- and two-ring MRR filters are experimentally investigated. The power penalties due to interchannel crosstalk are presented as functions of channel spacing and adjacent channel isolation. Intrachannel crosstalk of one-ring, cascaded, and series-coupled add-drop filters are compared and spectral conditions that will ensure low intrachannel crosstalk is presented. MRR filters with extremely small radii of 2.75 um, large free spectral ranges of 34.3 nm, and high thermal tuning efficiencies of 2.78 nm/mW are presented.

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Silicon photonic switches for optical communication applications (2017)

Optical switches are used for signal switching in optical communication networks. Silicon photonics is a low-cost and mature technology to develop high-performance optical switches. This thesis is a theoretical and experimental study on silicon photonic switches, featuring broadband, low-power, high-speed, and low-crosstalk performance.Broadband 3-dB couplers are fundamental building blocks for broadband switches based on Mach-Zehnder interferometer (MZI) structures. A broadband 3-dB coupler, which has a 100 nm operation bandwidth with coupling imbalance being much less than its competitors, i.e., adiabatic couplers and multimode interference couplers, has been theoretically designed and experimentally demonstrated.Switches using thermo-optic phase tuning typically have high power consumption. In this thesis, two methods to improve the tuning efficiency of thermo-optic phase shifters have been investigated and employed: 1) using thermal isolation structures and 2) using folded waveguides structures. Accordingly, thermo-optic switches with state-of-the-art, ultra-low power consumption of down to 50μW/π have been demonstrated.MZI switches using carrier injection phase tuning have high-speed performance but with a large switching crosstalk, due to the imbalanced tuning loss in the MZI structure. A novel carrier injection switch based on a balanced nested Mach-Zehnder interferometer (BNMZI) structure has been theoretically proposed. The BNMZI switch has balanced tuning schemes and therefore can be both high-speed and crosstalk-free. Besides, the switch has three switching states: cross, bar, and blocking.Polarization control is necessary for single-mode switches. A high-performance polarization beamsplitter (PBS), which has a 120 nm operation bandwidth with modal isolations of more than 20 dB, has been designed and demonstrated, and it can be used for polarization control for single-mode switches.Characterizing fabrication variability and performing yield prediction for photonic integrated circuits (PICs) are both challenging for photonics designers. We have developed an accurate and cost-efficient characterization method for fabrication variations, which extracts waveguide dimension variations from the spectral response of a single racetrack resonator. In addition, we have proposed a novel yield prediction method for PICs, which, for the first time in silicon photonics, is able to model the impacts of layout-dependent correlated manufacturing variations and take them into account in circuit simulations.

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Sub-wavelength Grating Components for Silicon-on-Insulator Platform (2017)

This dissertation is a theoretical and experimental study of sub-wavelength grating (SWG) based photonic devices for the silicon-on-insulator (SOI) platform, including high-efficiency sub-wavelength grating couplers (SWGCs), broadband SWGCs, broadband SWG directional couplers, and an SWG polarization splitter-rotator. High-efficiency SWGCs with improved operating bandwidths and sup- pressed back reflections have been demonstrated to couple light into and out of SOI based photonic integrated circuits (PICs). One-dimensional SWGs have been proposed and experimentally demonstrated for the first time to make fully-etched grating couplers, which have performances comparable to the state-of-the-art fully-etched grating couplers, but with better fabrication tolerance, reduced fabrication complexity, and less cost. A theoretical study of the operating bandwidths for grating couplers has been presented and a design methodology has been demonstrated for designing SWGCs with design-intent operating bandwidths. SWGCs with 1-dB bandwidths up to 90 nm have been demonstrated, which have improved the operating bandwidth of fully-etched grating couplers by a factor of 3. Such broadband SWGCs are essential components for applications such as wavelength-division multiplexing (WDM) PICs and bio-sensing. Compact directional couplers, with dimensions about 10 times smaller than its alternatives, i.e., adiabatic couplers and multimode interference couplers have been demonstrated for various power splitting ratios. The operating bandwidths of our directional couplers have been improved by a factor of 2 as compared to conventional directional couplers. The dispersion properties of SWGs have been explored and applied to engineer the wave- length dependancy of conventional directional couplers for broad operating bandwidths, which is the first experimental demonstration of such devices. Polarization splitter and polarization rotators are essential components to address the polarization diversity of PICs. An ultra-compact mode- coupling based polarization splitter-rotator (PSR), which combines functionalities of a polarization splitter and a polarization rotator, with dimensions 15-20 times smaller than its alternative, i.e., mode-evolution based PSRs, has been experimentally demonstrated for the first time, where an asymmetric waveguide system consisting of a strip waveguide and an SWG waveguide were used to improve the fabrication tolerance of such devices. A measured peak polarization conversion efficiency of −0.3 dB with crosstalks below −10 dB over the C-band has been achieved.

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A Microfluidic Platform to Study Real-Time Tumour Cell and Tumour Spheroid Response to Chronic and Transient Hypoxia (2016)

Cell-based screening of cancer treatments is used early in the drug development process to test the efficacy and toxicity of treatment candidates prior to animal and human testing. Current cell-based screening methods offer limited predictive capacity, contributing to the high percentage of drugs that fail during the clinical trial stage (80-95% for cancer treatments). One shortcoming of traditional cell-based screening platforms is their inability to recreate many aspects of the natural environment of tumour cells, which can affect treatment response. One important aspect of the microenvironment that can affect cell behaviour and treatment response is oxygenation. Irregular blood vessel formation can cause tumour oxygenation to be much lower than that of surrounding tissue, and spatial and temporal variations in oxygen can be present. Temporal variations can occur at timescales up to several cycles/hour: changes that are too fast to recreate using standard technologies like well plates due to their long diffusion distances. This thesis presents a novel microfluidic platform to expose cells to both chronic and time-varying oxygen profiles and study their response. Microfluidics technology is combined with 3-D cell culture in tumour spheroids, which can better recreate other aspects of the tumour microenvironment (such as cell-cell and cell-matrix interactions) than traditional 2-D culture. The functionality of the oxygen control device is verified using both finite-element modelling and integrated optical oxygen sensors. Two novel methods for oxygen sensor microfabrication are presented, and the functionality of sensors during long-term experiments is studied. Precise oxygen control is demonstrated using the microfluidic system, with oxygen switching times of
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High Performance Silicon Photonic Filters for Dense Wavelength-Division Multiplexing Applications (2016)

This dissertation presents theoretical and experimental results for silicon optical ring resonator filters that meet many of the typical commercial specifications for dense wavelength-division multiplexing (DWDM) filters. First, we theoretically demonstrate a silicon quadruple Vernier racetrack resonator that meets 4-port filter commercial specifications for a clear window of 0.08 nm and a channel spacing of 0.8 nm while being tolerant to typical fabrication variations. Next, we experimentally demonstrate a silicon quadruple Vernier racetrack resonator that meets many 3-port filter commercial specifications for a clear window of 0.048 nm and a channel spacing of 0.8 nm. Then, enhanced resonant tuning range using the Vernier effect is theoretically and experimentally demonstrated using a thermally tunable silicon quadruple Vernier racetrack resonator. Also, we sent 12.5 Gbps data through a thermally tunable silicon quadruple Vernier racetrack resonator and show open eye diagrams in both the drop port and through port of the filter, even within one of the minor through port notches. We then present theoretical and experimental results on a high performance silicon double microring resonator filter using Mach-Zhender interferometer-based coupling that meets numerous 3-port filter commercial specifications for a clear window of 8 GHz and a channel spacing of 200 GHz as well as having an FSR larger than the span of the C-band and low through port passband dispersion. Next, we present a FSR-eliminated silicon Vernier racetrack resonator filter. We demonstrate the performance of this filter both theoretically and experimentally. The FSR of this filter is eliminated by using contra-directional grating couplers (contra-DCs) to suppress all but one of the notches and peaks of the filter's spectra. Lastly, a process calibration procedure is demonstrated that accurately determines the coupling coefficients of fabricated contra-DCs and is used to design a FSR-eliminated silicon Vernier racetrack resonator filter that meets 3-port filter commercial specifications for a clear window of 13 GHz and a channel spacing of 200 GHz. This filter also has low drop port dispersion and low dispersion within the passbands of the through port.

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Feasibility of Optical Gyroscopic Sensors in Silicon-On-Insulator Technology (2015)

In the last decade, silicon photonics has become a strategic technology for the development of telecommunications and sensors. Due to its compatibility with well-developed complementary metal oxide semiconductor (CMOS) fabrication processes, silicon on insulator (SOI) wafers can be processed to create thousands of devices per die in a fast and inexpensive way. Being solid state devices with no movable parts, optical gyroscopes have longer life expectancies and shock resistance compared to micro-electro-mechanical gyroscopes. Thus, the implementation of SOI-based gyroscopes is desirable for large-scale, low-cost production. This thesis presents a study of the feasibility of implementing optical gyroscopes in SOI technology. A comprehensive theoretical study has been carried out to develop a device-level optimization and robustness analysis, showing that the most crucial resonator parameter is the propagation loss, followed by length and coupling. For a given propagation loss, there is an optimal resonator size, beyond which the angular speed resolution is severely degraded. On the system level, the impact of signal-to-noise ratio and insertion loss on the resolution are described.Given that the propagation loss is the most important parameter, strategies were proposed to reduce it as much as possible while still using CMOS-compatible processes. The quality factor, Q was chosen as the figure of merit to be maximized during the design iterations. As a result, the largest Q factors reported to date on SOI, using standard CMOS-compatible processes, were achieved. These Q factors are comparable to, or exceed, those of optical resonators intended for gyroscopic applications that are fabricated in materials such as indium phosphide (InP). Innovative approaches to compensate for fabrication variations are proposed, such as thermally-tuneable coupling and reference rings for differential measurements. Complex mechano-opto-electrical measurement setups were designed and implemented to characterize SOI gyroscopes, both at rest and under rotation. As a result, the Microsystem Integration Platform for Silicon-Photonics (Si-P MIP) was created. This characterization platform is now being commercialized by CMC Microsystems for academic and industrial applications. The main practical and theoretical challenges regarding the implementation of optical ring gyroscopes on SOI have been identified. Schemes to address them and suggestions for future work are proposed.

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Optical Resonator Sensors and Systems (2015)

Silicon-on-insulator (SOI)-based sensors are attractive for sensing applications in environmental safety, oil and gas, medical research, and clinical applications. Since these devices are typically developed using Complementary metal-oxide-semiconductor (CMOS)-compatible multi-project-wafer (MPW) shuttles, they bring the potential for having sensing systems on chips (SSOCs), and for mass fabrication and low cost production. The objective of this thesis is to improve the sensitivity, accuracy, and repeatability of sensors fabricated on the SOI platform. Such sensors have the potential to be the key components of an SSOC.One can increase the sensitivity of a resonator sensor by increasing the interaction between the evanescent field of the guided mode and the analyte. In this thesis, two methods for increasing this interaction in micro-ring resonator-based sensors are investigated: 1) using the transverse electric (TE) guided mode in ultra-thin strip waveguides and 2) using the quasi-transverse magnetic (TM) guided mode in thin strip waveguides. Using analyses and simulations, micro-ring sensors were designed to be fabricated within the constraints of a MPW CMOS-compatible process. Using the TE sensors, the temperature-induced errors were reduced by a factor of three; and the TM sensors exhibited twice the sensitivity of the best SOI micro-ring resonator-based sensors reported to date.Moving towards the actual implementation of an SSOC, a system of sensors was design to correct for unwanted variations in the measurements. This system drew on multivariate techniques to achieve improvements that resulted in measurements that were more repeatable and more accurate in the presence of environmental variations. The capability of this system is investigated by designing a cascade of previously developed micro-ring sensors with various waveguide thicknesses. With this system of sensors, we achieved an R2 value of predictions over 0.996 in the presence of a 2 K temperature drift. This approach significantly improved the repeatability and reliability of the measurements in the presence of undesirable variations and drifts. In another move towards achieving an SSOC, integrating photodetectors in resonator sensors was investigated. To accomplish this, ion-implantation on micro-ring sensors was used. Such integrated photodetector-sensors were designed, fabricated, and tested. Their measured sensitivities were within 90% of the expected values.

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Silicon photonic waveguide Bragg gratings (2014)

Silicon is the most ubiquitous material in the electronics industry, and is now expected to revolutionize photonics. In just over ten years, silicon photonics has become a key technology for photonic integrated circuits. By taking advantage of silicon-on-insulator (SOI) wafers and the existing complementary metal-oxide semiconductor (CMOS) fabrication infrastructure, silicon photonic chips are now being delivered with low cost and rapidly increasing functionality.This thesis presents the integration of a fundamental optical device - Bragg grating - into SOI waveguides. Various types of waveguides and grating structures have been investigated. All designs are fabricated using CMOS foundry services. We have also explored various applications using the fabricated devices.From the beginning, we focused on strip waveguide uniform gratings, as these are the most simple to design and fabricate. We have studied many design variations, supported by experimental results. In parallel, we have provided insight into practical issues and challenges involved with the design, fabrication, and measurement, such as the lithography effects, thermal sensitivity, and wafer-scale nonuniformity. We then introduce phase-shifted gratings that can achieve very high quality factors and be employed in various applications. We have also demonstrated sampled gratings and the Vernier effect in strip waveguides.To obtain narrow-band gratings, we propose the use of a rib waveguide. We also propose a multi-period grating concept by taking advantage of the multiple sidewalls of the rib waveguide, to increases the design flexibility for custom optical filters. The wafer-scale data shows that rib waveguide gratings have better performance uniformity than strip waveguide gratings, and that the wafer thick- ness variation is critical. Additionally, we have demonstrated very compact Bragg gratings using a spiral rib waveguide.Finally, we demonstrate slot waveguide Bragg gratings and resonators, which has great potential for sensing, modulation, and nonlinear optics. We have also developed a novel biosensor using a slot waveguide phase-shifted grating that has a high sensitivity, a high quality factor, a low limit of detection, and can interrogate specific biomolecular interactions.

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Silicon Photonic Filters for Wavelength-Division Multiplexing and Sensing Applications (2012)

This thesis is a theoretical and experimental study of novel silicon photonic filters, such as traveling-wave resonators (TWRs) and grating-assisted, contra-directional couplers (contra-DCs), for on-chip wavelength-division-multiplexing (WDM) systems and sensing applications. To measure optical losses of photonic components such as Y-branch splitters and waveguide crossings, we have developed a ring-resonator based technique which is accurate, simple, and space-efficient. A number of novel devices have been demonstrated using commercial CMOS-photonics fabrication foundries, with the aim of developing large-scale photonic integrated circuits using the standard process development tools. Two types of wavelength-selective, TWR-based reflective filters have been demonstrated for applications such as remote sensing and tunable lasers. Ultra-compact, high-Q microdisk resonators have been demonstrated, with radii of down to 1.5 µm, free spectral ranges (FSRs) of up to 71 nm, loaded Q's of up to 88,000, and unloaded Q's of over 100,000.Contra-DCs have been studied using coupled-mode theory. An add-drop filter designed using contra-DCs in slab-modulated rib waveguides has been proposed and demonstrated, which shows a flat-top response and a narrow bandwidth of 50--100 GHz, promising for dense-WDM applications. Also, we proposed an out-of-phase grating design to suppress the intra-waveguide reflection in contra-DCs. Using this novel anti-reflection (AR) design, we have demonstrated an add-drop filter with a single-band, flat-top response and a wide channel bandwidth of 6.5 nm, which enables athermal operation in a large temperature span of > 70 K. This AR contra-DC can be used to build an on-chip coarse-WDM system for power-efficient, ultra-high-speed optical interconnects. Furthermore, we have proposed and demonstrated an electrically tunable phase-shifted contra-DC.In order to overcome the challenges facing microring resonators, such as limited FSRs and difficulty in controlling the bus-resonator coupling, we have proposed to integrate contra-DCs with microring resonators for selective bus-resonator coupling. Using this method, we have demonstrated a single dominant resonant mode in a microring resonator that originally has a small FSR of 1.3 nm. This grating-coupled microring resonator is promising for applications that need a huge free spectral range, such as cascaded resonator sensor arrays and ultra-high-bandwidth WDM systems.

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Modeling of the transistor vertical cavity surface emitting laser (2011)

The direct modulation of semiconductor lasers has many applications in data transmission. However, due to the frequency response it has been challenging to use directly modulated lasers for high speed digital transmission at bit-rates above 10 Gbps. With this in mind, designing a laser with a large modulation bandwidth to be used in high data-rate optical links is very important. Transistor lasers (TLs) are a potential candidate for this purpose.Based on these motivations, the main focus of this PhD research is on understanding the physics of the TL and predicting its performance. A detailed model that correctly incorporates the transistor effects on laser dynamics did not exist. The previous models do not differentiate between the bulk carriers and the quantum well (QW) carriers in the rate equations, do not include the effects of the capture and escape lifetimes in the QW, and significantly overestimate the bandwidth.To account for these phenomena, a model has been developed to study the dynamics of the TL. The model is based on the continuity equation in the separate confinement hetero-structure region of the conventional laser and the base region of the TL. It uses the quantum mechanical escape and capture of carriers in the quantum well region and the laser rate equations to model the laser operation. The model has been used to gain insight into the conventional separate confinement hetero-structure lasers, and the results of the model have been compared with the experimental results obtained for 850 nm vertical cavity surface emitting lasers(VCSELs). Analytical expressions have been derived for DC and AC parameters of the TL operating in common-base and common-emitter configurations. It has been shown that the TL operating in the common-emitter configuration has a similar modulation bandwidth as a conventional laser (~ 20 GHz). The common-base configuration, on the other hand, has a very large small-signal modulation bandwidth (> 40 GHz) due to bandwidth equalization in the TL. The large-signal performance of the TL has been studied. Finally, it has been shown that the common-emitter configuration with feedback has improved bandwidth by a factor of 1.5 in high bias currents.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Compact electro/optical packaging enabled by photonics wire bonding (2023)

Three-dimensional freeform optical waveguides, or photonic wired bonding (PWB), is a versatile technology that enables low-loss interconnects between photonics components. With two-photon lithography, PWB can match the mode profile and position mismatch between single-mode fibers, photonics integrated chips (PIC), and semiconductor lasers, rendering the costly active alignment obsolete. Despite the advantages of the PWB technology, compact packaging solutions must be developed to fully cultivate its potential in interconnecting photonics components and work with existing integrated photonics functionalities. We review the basics of photonics packaging. The working principle and technical specifications of PWB are summarized. Laser integration and fiber attachment methods are reviewed.More technical details like the fabrication process and structure of PWB are introduced. Moreover, we proposed a series of simulations to optimize the PWB geometry for efficient coupling. The mode coupler shape and its vertical offset tolerance are simulated by mode overlapping, and the constant cross-section waveguide's single operation condition and bending loss are investigated. The mode size convertor between mode couplers and constant cross-section waveguide is optimized to balance the footprint and good transmission.An intensity-modulated microring modulator (IM-MRR) based optical computing unit is demonstrated with PCB-based co-packaging with PWB for optical interconnection. A chip post-processing recipe is developed and calibrated for metalization rework. This is the first demonstration of using PWB for packaging neuromorphic systems. The potential of PWB to enable high-density optical I/O for the neuromorphic system is discussed.We propose a hybrid on-chip laser integration technology enabled by PWB. The photonics integrated chip (PIC) post-process and packaging process is illustrated. The deep reactive ion etching (DRIE) process is calibrated for etching on-chip recess cavities. Pure indium and Au₈₀S₂₀ preforms are tested for laser die soldering on bare silicon and gold-coated surface. A PWB-compatible compact assembly solution for electrical/optical co-packaging is developed. The assembly solution is implemented for investigating short-range optical feedback effects on distributed feedback lasers. The methods feature the least change on existing nanofabrication techniques while maintaining flexibility for off-the-shelf laser dies integration. This approach has great potential to enable low-cost hybrid integration for low-to-medium volume applications.

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High efficiency optical coupling to and from silicon photonic integrated circuits (2023)

In this thesis, I present two new approaches to achieving high optical coupling efficiency to and from silicon photonic devices. The first approach is a new way of optimizing silicon photonic devices by using machine learning trained with measured data obtained from fabricated devices. I call this new approach, fab-in-the-loop reinforcement learning. I show that by applying fab-in-the-loop reinforcement learning to grating coupler design, I am able to make grating couplers with a lower insertion loss and higher bandwidth then by traditional design methods. The second approach is based on a technology called photonic wire bonding. Photonic wire bonding consists of 3D printing a polymer waveguide between two photonic components using two photon polymerization. I show that it is possible to successfully use photonic wire bonds in cryogenic environments by a careful selection of materials and the appropriate techniques for installation of these bonds.

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Investigation of photonic crystal cavities for a silicon photonic quantum computing with spin qubit architecture (2023)

Quantum computing promises to fundamentally change our current computational capabilities, offering exponential speedup in many key areas. While many different quantum computing architectures exist, a clear way to achieve scalable, fault-tolerant quantum computing remains elusive. Photonic quantum computing with spin qubits is a promising novel quantum computing architecture. It aims to solve many of the present-day issues; however, not all necessary photonic components are available as of now. In particular, a photonic cavity for the spin qubit-photon interface is missing.In this work, we give the key characteristics and requirements for such a photonic cavity and experimentally demonstrate a cavity system with potential to meet them. To this end, we design, simulate, and experimentally characterise a photonic crystal slot-bridge nanobeam cavity fulfilling the first key characteristic: an optimised quality factor to mode volume ratio. We investigate the sensitivity of the cavity for different design parameters and achieve good agreement with simulations. This cavity demonstration also serves as a guideline for future design optimisation aiming to achieve better performance values.We further demonstrate a permanent post-fabrication trimming method based on laser-assisted local surface oxidation on the slot-bridge nanobeam cavities. This fulfills the second key characteristic, namely that of trimming capability of the cavity resonance.Finally, we give an outlook on possible future work to improve the trimming method and cavity optimisation.

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Ring resonator/modulator simulation automation and substrate scattering analysis for applications in integrated silicon photonic circuits (2023)

A micro-ring resonator/modulator simulation automation system is presented centered on Lumerical’s application programming interface (API) system called LumAPI.A clean and user-friendly graphical user interface (GUI) is integrated with the system to increase accessibility while designing the passive ring structure and the P-Njunction used for modulation. Furthermore, a database system is implemented torecord data for every component simulation that comprises the complete ring. Thisremoves the need for duplicate simulations from ever being executed in the future,thus, reducing the overall simulation time required when designing micro-rings.This process allows the user to model the spectral behaviors of micro-ring resonators with ease and provides the capability to investigate high-speed modulationof micro-ring modulators.The use of micro-ring resonators as a source of photon-pair generation viaspontaneous four-wave mixing is then discussed, along with the application of cascaded contra-directional couplers as pump rejection filters. An analysis of potentialnoise sources is presented along with a novel way to improve the suppression ratioof these filters.

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Diagnosing electron beam lithography quality using optical ring resonators (2022)

Electron beam lithography is a useful fabrication method in the field of silicon photonics, enabling rapid one-off prototyping and offering small feature sizes relative to optical lithography alternatives. For applications in quantum information technology where every photon counts, it is critical to thoroughly gauge the optical performance of fabricated devices and optimize the process to ensure proper device performance. This thesis begins by motivating the need for standardized test structures through write condition variations, with the intention of minimizing propagation loss and feature size errors. Throughout this testing, the need arose for a more accurate method of measuring propagation loss than commonly used spiral cutback structures, which are susceptible to large uncertainties due to both fabrication and measurement errors. It also became apparent that density-related sizing errors are prevalent on several different fabrication processes and a convenient method for quantifying and negating them is needed. Therefore, we present optical ring resonator-based methods for determining both propagation losses and feature size variations with greater accuracy than conventional methods.

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Photoconductive heater-detectors for photonic integrated circuits (2021)

Electro-optic feedback control is an active research area within integrated photonics, where detection and tuning elements are used to dynamically control devices and circuits. However, the need for discrete control elements increases the number of electrical connections to a photonic chip, and can require large area on-chip to integrate. Therefore, a single element that can perform both detection and tuning would provide great benefit over their disjoint counterparts as photonic circuit density increases. Photoconductive heater-detectors (PCHD) have proven viable as a hybrid control and detection element, but the lack of models available make it unlikely for circuit designers to adopt them in their designs.We propose an empirical compact model for PCHDs based on measured results. Core electro-optic relationships are pulled from literature and empirically modeled. A compact model for the general structure of a PCHD is implemented in Lumerical INTERCONNECT using standard library elements populated with parameters specific to the n-doped PCHDs that were measured. The compact model is used in a variety of simulations and compared against measured results.We also demonstrate the design of a widely tunable ring-based silicon photonic notch filter. We present measured results demonstrating the device capability of tuning the filtering frequency, the free spectral range (two states), the optical bandwidth from 5 to 34 GHz, and the extinction ratio in excess of 30 dB, all independently of each other. We also provide circuit simulations using the PCHD model to demonstrate feedback loops used to automatically reconfigure the circuit based on specific spectral property optimizations.Lastly, we propose an advanced silicon photonic biosensor architecture for the detection of COVID-19 and other pathogens, enabled by PCHDs. By integrating the detector and tuner as a single element within the resonant cavity and operating in the O-band rather than the C-band, cheap single wavelength lasers can be used as an optical source rather than the standard sweepable lasers required to operate photonic biosensors. Simulated results of the sensor highlight the trade-off between environmental sensitivity and measured signal strength as the size of the sensing region increases.

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Silicon photonic sensors based on low-cost optical sources (2021)

Silicon photonic (SiPh) sensors hold tremendous potential for the advancement of global healthcare. Leveraging mature complementary metal-oxide semiconductor (CMOS) foundry processes, hundreds of SiPh sensors can be integrated into tiny devices, enabling the detection of multiple pathogens, and eliminating the need for expensive in-lab chemical processing. While the performance of SiPh sensors is similar to clinical standards, the implementation costs remain quite high. To realize the potential that SiPh sensor systems hold for global healthcare, their overall cost must be reduced. SiPh sensors typically rely on high-resolution tunable lasers, which remain an expensive off-chip component. This thesis first summarizes alternative optical sources that are used for SiPh sensors. Fixed-wavelength lasers are a low-cost alternative, and benefit from relative ease of coupling to chip. Unfortunately, the corresponding sensor designs are very sensitive to noise. Broadband optical sources are another lower-cost alternative source; however, their use often requires expensive detection equipment. Despite their drawbacks, implementing these alternative optical sources could significantly reduce the overall cost of a SiPh system.Three low-cost SiPh architectures are presented in this thesis: two that use a broadband source, and one that uses a fixed-wavelength laser. The broadband SiPh architectures use a sensor-tracker system, where one component, a microring resonator (MRR) or a Mach-Zehnder interferometer (MZI), acts as a sensor and a second component acts as a tracker (by electrically tracking wavelength shifts). Since wavelength shifts from the sensor can be read as electrical power shifts in the tracker, this system eliminates the need for expensive detection equipment. Sensitivity values of 78.9 nm/RIU (refractive index unit) and 218.5 nm/RIU were obtained, with system limits of detection of 3.4x10^⁻⁴ RIU and 7.7 x10^⁻⁴ RIU for the MRR and MZI designs, respectively.In the fixed-wavelength system, a heater-detector tuning element is placed in the MRR loop. This similarly enables electrical tracking of wavelength shifts, thus reducing the noise sensitivity commonly found in fixed-wavelength systems. Simulation results report sensitivities up to 76 nm/RIU, with calculated intrinsic limits of detection down to 3.8 x10^⁻⁴ RIU. The results obtained demonstrates that high-resolution tunable lasers are not required to achieve high sensor performance.

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Computational lithography for silicon photonics design (2020)

A lithography model is built using physical measurements obtained from a fabricated test pattern. The method is able to accurately predict the proximity and smoothing effects characteristic of a 193~nm deep-ultraviolet (DUV) lithography process.The accuracy of the model is verified by visually inspecting the fabricated test patterns and comparing them to the predictions of the lithography model. Furthermore, using a benchmark device (the contra-directional coupler), the prediction accuracy of the optical response is compared against experimental measurements. The comparisons showed the predictions had good agreement with the fabricated devices.Subsequently, an application of the lithography model is demonstrated. Design correction methods enabled by the lithography model are performed on the contra-directional coupler. The new designs were fabricated using electron-beam lithography and their experimental measurements confirmed an improved optical performance.

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Efficient layout-aware statistical analysis for photonic integrated circuits (2020)

Fabrication variability significantly impacts the performance of photonic integrated circuits (PICS), which makes it crucial to quantify the impact of fabrication variations at the design and simulation stage. The variability analysis enables circuit and system designers to optimize their designs to be more robust and obtain maximum yield when designing for manufacturing. The variability analysis requires a total of six parameters to model spatially correlated manufacturing variations in photonic circuits: mean, standard deviation, and correlation length for both width and thickness variations of photonic components. The correlation lengths are spatial parameters that describe how the width and thickness variations are distributed along a chip’s or a wafer’s surface. The methods that allow for the non-invasive characterization of variations are limited to extracting mean and standard deviations of width and thickness variations. In this thesis, we present a method to extract the physical correlation lengths, which are crucial to model manufacturing variations.In this thesis, we also present the Reduced Spatial Correlation Matrix based Monte Carlo (RSCM-MC), a methodology to study the impact of spatially correlated manufacturing variations on the performance of photonic circuits. The presented methodology is compared with another layout-dependent Monte Carlo (MC) simulation methodology, called Virtual Wafer-based Monte Carlo (VW-MC). First, we describe the process of generating spatially correlated physical variations using the presented methodology and use the generated correlated physical variations to conduct MC simulations. We then use a Mach-Zehnder lattice filter photonic circuit as a benchmark circuit to study the accuracy of the proposed method. We compare the statistical parameters of quantities defining the flatness of the transmission spectra of the filter. We then compare the computation performance of RSCM-MC with VW-MC using a combination of a small-sized circuit (two-stage Mach-Zehnder filter) and a large circuit (a 16x16 ring matrix) with thousands of components. For the best case, i.e. the small-sized circuit, we observe a decrease in computational times by 98.9% and a reduction in memory requirement by 72%. For the worst case, i.e. the 16x16 ring matrix, we observe a decrease in computational times by 99.8% and a reduction in memory requirement by 87%.

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Ring resonator based silicon photonic transmitter and receiver (2019)

Ring resonators in silicon photonics platform hold great potential in various applications due to their compact size and wavelength selectivity, enabling densely integrated optical systems. This thesis focuses particularly on the application of ring resonators in silicon photonic transmitters and receivers. In transmitters, all-pass ring resonators with PN junctions can be driven in depletion mode to provide high-speed binary modulated signals. Pulse-amplitude-modulation-4 (PAM4) schemes can be adopted to achieve higher bit rates by providing four levels of amplitude instead of two. Instead of relying on a power-hungry digital-to-analog converter (DAC) in the driver, the four optical levels can be realized by using two separate non-return-to-zero (NRZ) drivers on either a single ring resonator with segmented PN junctions or a dual cascaded ring resonator. In this thesis, the two DAC-less PAM4 modulation methods in ring resonators are compared using frequency and time domain analytic equations, with a target bit rate of 25Gb/s. Under the same constraints in terms of ring resonator dimensions and electrical signal voltages, the single ring resonator with segmented PN junctions is found to be the superior candidate, due to the smaller number of stabilization circuits required while achieving a larger modulation amplitude.In receivers, add-drop ring resonators can be used as wavelength division multiplexing (WDM) channel filters, but they suffer from high polarization dependence, which motivates the need for a polarization management solution on chip. In this thesis, a 4-channel polarization-insensitive WDM receiver is designed by forming a waveguide loop between the two output ports of a polarization-splitter-rotator. The input signals in the quasi-transverse-electric and the quasi-transverse-magnetic polarization states can be demultiplexed without active polarization tuning or independent processing of the two polarization states. Large signal measurements at 10 Gb/s indicate that the design can tolerate a signal delay of up to 30% of the unit interval (UI) between the two polarization states, which implies that compensating for manufacturing variability with optical delay lines on chip is not necessary for a robust operation. The inter-channel crosstalk is found negligible down to 50 GHz spacing, proving its compatibility with dense WDM systems.

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Highly Efficient Thermo-Optic Switches on Silicon-On-Insulator (2015)

We analyze and demonstrate the performance of dense dissimilar waveguide routing as a method for increasing the efficiency of thermo-optic phase shifters on a silicon-on-insulator platform. Optical, mechanical, and thermal models of the phase shifters are developed and used to propose metrics for evaluating device performance. The lack of cross-coupling between dissimilar waveguides allows highly dense waveguide routing under heating elements and a corresponding increase in efficiency. We demonstrate a device with highly dense routing of 9 waveguides under a 10 μm wide heater and, by thermally isolating the phase shifter by removal of the silicon substrate, achieve a low switching power of 95 μW, extinction ratio greater than 20 dB, and less than 0.1 dB ripple in the through spectrum. The device has a footprint of less than 800 μm x 180 μm. The increase in waveguide density achieved by using dissimilar waveguide routing is found not to negatively impact the switch response time.

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Silicon Photonic Modulators and Filters for Optical Interconnects (2015)

The goal of this work is to enhance the performance of and demonstrate new applications for silicon photonic modulators and filters. We demonstrate a variety of novel designs and applications of silicon photonic devices for integrated optical interconnects. First, we demonstrate a biasing scheme for travelling-wave Mach-Zehnder modulators in which the bias voltage is applied separately from the data signal. Using this biasing scheme, there is no low frequency cutoff and there is no power consumption in the termination resistor from the bias voltage, which results in an improved modulator having a lower overall power consumption. We experimentally show, as a proof-of-concept, successful high-speed modulation, at a data rate of 28 Gb/s, of a modulator which uses this biasing scheme. Next, we present a novel modulator design in which a microring modulator is placed into each arm of a Mach-Zehnder interferometer. This design uses the sharp phase response of a microring resonator near its resonance so that the light experiences a large phase change when a voltage is applied to the p-n junction phase shifters within the microring. We use temporal coupled mode theory to simulate the time-domain response of this modulator. We then demonstrate a novel modulator design which uses a quarter-wave phase-shifted Bragg grating. The modulator, which was fabricated using 193 nm optical lithography, has open eye diagrams at a data rate of 32 Gb/s. We also show that using a 2³¹-1 pseudorandom binary sequence pattern, the modulator has a bit error rate (BER) less than 10⁻¹² at a data rate of 20 Gb/s and has a BER less than 10⁻¹⁰ at a data rate of 25 Gb/s. Finally, we demonstrate a contra-directional grating coupler-based filter on silicon in an optical add-drop multiplexer configuration and show that it can successfully add and drop a 12.5 Gb/s signal at the same wavelength without substantial signal degradation.

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Sub Wavelength Grating Ring Resonators Based on Silicon-on-Insulator Technology for Sensing Applications (2015)

Standard silicon photonic strip waveguides offer a high intrinsic refractive index contrast; this permits strong light confinement, leading to compact bends, which in turn facilitate the fabrication of devices with a small footprint. Waveguides based on sub-wavelength gratings (SWGs) can allow the fabrication of devices with specific, engineered optical properties. The combination of SWG waveguides with optical resonators can offer the possibility of achieving ring resonators with properties different from the traditional Silicon-On-Insulator (SOI) rings. One important property that SWG rings can offer is decreased light confinement in the waveguide core; this improves the resonator’s sensitivity to changes in the cladding refractive index, making the rings ideal for refractive index sensing applications. This thesis presents the design and experimental characterization of SWG-based rings realized on SOI chips without upper cladding, permitting their use as sensors. The sensitivity achieved was 400 nm/RIU and the limit of detection was 9.9 x 10−⁴ .

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Grating coupler design based on silicon-on-insulator (2013)

Silicon-on-insulator has become a promising platform for high-density integrated photonics circuits. The large refractive index contrast between the functional silicon layer and its cladding raises a coupling issue between an optical fibre and on-chip devices. Grating coupler provides a compact and efficient way to tackle the coupling issue between the optical fibre and silicon waveguide. In this thesis, a universal design methodology, which accommodates various etch depths, silicon thicknesses, and cladding materials has been demonstrated and verified by both FDTD simulation and measurement results. A fully etched grating coupler with a sub-wavelength grating structure has been proposed to reduce the large back reflection of existing fully etched grating couplers. Back reflection of the proposed fully etched grating coupler has been reduced from more than 20% to about 5%. The insertion loss and bandwidth of the proposed structure have also been improved. In addition, a bidirectional grating coupler for vertical coupling has been proposed to improve the insertion loss and bandwidth of the traditional grating coupler. A simulated insertion loss of -1.5dB with a 3dB bandwidth of more 100nm has been achieved with the proposed structure.

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Photonic device design flow: from Masklayout to device measurement (2012)

Mask layout design is an important part in silicon photonic device design flow; the space used and the quality of the mask directly affect the cost of fabrication and quality of the outcome. To effectively minimize time spent on drawing masks, fixing design violations, and reducing unused spacings between each structure, we use effective approaches in the mask design process to ensure the listed criteria are met. Using the PCell and the hierarchy drawing methods, GDS files that contain different device parameters can be generated efficiently. As a result, direct GDS modeling efficiency is improved. An experimental setup that is capable of obtaining high quality measurement data is critical to device measurement. The concept of an automated measurement station can effectively reduce work needed from the experimenter while providing quality results. With the implemented fiber-to-fiber and fiber array automated measurement station, multi-device measurement can be set up to run automatically in minutes whereas traditional manual measurement stations require one's presence and constant attention. In this thesis, we have illustrated several mask drawing approaches and showed the drawing steps of two masks in detail. We have described two automated experimental setups, fiber-to-fiber and fiber array, in detail and included various measurement results to show the capabilities of these two stations.

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Fabrication and characterization of InP semiconductor ring lasers (2010)

This thesis investigates the fabrication of 1550 nm emitting InP semiconductor racetrack resonator lasers (SRLs) via wet etching techniques. The method of choice for SRL fabrication is reported to be via dry etching. Dry etching is a complex, time consuming and expensive process which leaves relatively rougher surfaces and sidewalls compared to wet etching techniques. In this thesis, coupling, racetrack resonators, and edge emitter laser theory were studied for the SRL design. Then, a fabrication process for the SRLs was developed via wet etching techniques. The light emitting diode (LED) characteristics of the fabricated devices were observed and successfully measured. The spectrum of the device was also measured with optical spectrum analyzer (OSA) and resonances were observed.However, lasing was not observed. The cleaving process is a major limiting step in the fabrication and it is being improved. In parallel to the wet etching fabrication at UBC, dry etching (the common method for SRL fabrication) is being performed at the Centre de Recherche en Nanofabrication et Nanocaractérisation (CNR2) at the Université de Sherbrooke.

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