Shon Schmidt

Silicon photonic micro-disk resonators for label-free biosensing.

Silicon photonic biosensors are highly attractive for multiplexed Lab-on-Chip systems. Here, we characterize the sensing performance of 3 µm TE-mode and 10 µm dual TE/TM-mode silicon photonic micro-disk resonators and demonstrate their ability to detect the specific capture of biomolecules. Our experimental results show sensitivities of 26 nm/RIU and 142 nm/RIU, and quality factors of 3.3x10(4) and 1.6x10(4) for the TE and TM modes, respectively. Additionally, we show that the large disks contain both TE and TM modes with differing sensing characteristics. Finally, by serializing multiple disks on a single waveguide bus in a CMOS compatible process, we demonstrate a biosensor capable of multiplexed interrogation of biological samples.

Silicon photonic resonator sensors and devices

Silicon photonic resonators, implemented using silicon-on-insulator substrates, are promising for numerous applications. The most commonly studied resonators are ring/racetrack resonators. We have fabricated these and other resonators including disk resonators, waveguide-grating resonators, ring resonator reflectors, contra-directional grating-coupler ring resonators, and racetrack-based multiplexer/demultiplexers. While numerous resonators have been demonstrated for sensing purposes, it remains unclear as to which structures provide the highest sensitivity and best limit of detection; for example, disc resonators and slot-waveguide-based ring resonators have been conjectured to provide an improved limit of detection. Here, we compare various resonators in terms of sensor metrics for label-free bio-sensing in a micro-fluidic environment. We have integrated resonator arrays with PDMS micro-fluidics for real-time detection of biomolecules in experiments such as antigen-antibody binding reaction experiments using Human Factor IX proteins. Numerous resonators are fabricated on the same wafer and experimentally compared. We identify that, while evanescent-field sensors all operate on the principle that the analytes refractive index shifts the resonant frequency, there are important differences between implementations that lie in the relationship between the optical field overlap with the analyte and the relative contributions of the various loss mechanisms. The chips were fabricated in the context of the CMC-UBC Silicon Nanophotonics Fabrication course and workshop. This yearlong, design-based, graduate training program is offered to students from across Canada and, over the last four years, has attracted participants from nearly every Canadian university involved in photonics research. The course takes students through a full design cycle of a photonic circuit, including theory, modelling, design, and experimentation.

Sub-wavelength grating for enhanced ring resonator biosensor

While silicon photonic resonant cavities have been widely investigated for biosensing applications, enhancing their sensitivity and detection limit continues to be an area of active research. Here, we describe how to engineer the effective refractive index and mode profile of a silicon-on-insulator (SOI) waveguide using sub-wavelength gratings (SWG) and report on its observed performance as a biosensor. We designed a 30 μm diameter SWG ring resonator and fabricated it using Ebeam lithography. Its characterization resulted in a quality factor, Q, of 7 · 103, bulk sensitivity Sb = 490 nm/RIU, and system limit of detection sLoD = 2 · 10-6 RIU. Finally we employ a model biological sandwich assay to demonstrate its utility for biosensing applications.

A silicon photonic biosensor using phase-shifted Bragg gratings in slot waveguide

We present a novel silicon photonic biosensor using phase-shifted Bragg gratings in a slot waveguide. The optical field is concentrated inside the slot region, leading to efficient light-matter interaction. The Bragg gratings are formed with sidewall corrugations on the outside of the waveguide, and a phase shift is introduced to create a sharp resonant peak within the stop band. We experimentally demonstrate a high sensitivity of 340 nm/RIU measured in salt solutions and a high quality factor of 1.5 × 10⁴, enabling a low intrinsic limit of detection of 3 × 10⁻⁴ RIU. Furthermore, the silicon device was fabricated by a CMOS foundry, facilitating high-volume and low-cost production. Finally, we demonstrate the devices ability to interrogate specific biomolecular interactions, resulting in the first of its kind label-free biosensor.

Performance of ultra-thin SOI-based resonators for sensing applications

This work presents simulation and experimental results of ultra-thin optical ring resonators, having larger Evanescent Field (EF) penetration depths, and therefore larger sensitivities, as compared to conventional Silicon-on-Insulator (SOI)-based resonator sensors. Having higher sensitivities to the changes in the refractive indices of the cladding media is desirable for sensing applications, as the interactions of interest take place in this region. Using ultra-thin waveguides (<100 nm thick) shows promise to enhance sensitivity for both bulk and surface sensing, due to increased penetration of the EF into the cladding. In this work, the designs and characterization of ultra-thin resonator sensors, within the constraints of a multi-project wafer service that offers three waveguide thicknesses (90 nm, 150 nm, and 220 nm), are presented. These services typically allow efficient integration of biosensors with on-chip detectors, moving towards the implementation of lab-on-chip (LoC) systems. Also, higher temperature stability of ultra-thin resonator sensors were characterized and, in the presence of intentional environmental (temperature) fluctuations, were compared to standard transverse electric SOI-based resonator sensors.

Optimized sensitivity of Silicon-on-Insulator (SOI) strip waveguide resonator sensor

Evanescent field sensors have shown promise for biological sensing applications. In particular, Silicon-on-Insulator (SOI)-nano-photonic based resonator sensors have many advantages for lab-on-chip diagnostics, including high sensitivity for molecular detection and compatibility with CMOS foundries for high volume manufacturing. We have investigated the optimum design parameters within the fabrication constraints of Multi-Project Wafer (MPW) foundries that result in the highest sensitivity for a resonator sensor. We have demonstrated the optimum waveguide thickness needed to achieve the maximum bulk sensitivity with SOI-based resonator sensors to be 165 nm using the quasi-TM guided mode. The closest thickness offered by MPW foundry services is 150 nm. Therefore, resonators with 150 nm thick silicon waveguides were fabricated resulting in sensitivities as high as 270 nm/RIU, whereas a similar resonator sensor with a 220 nm thick waveguide demonstrated sensitivities of approximately 200 nm/RIU.

Improving the performance of silicon photonic rings, disks, and Bragg gratings for use in label-free biosensing

Silicon photonics biosensors continue to be an area of active research, showing the potential to revolutionize Labon- Chip applications ranging from environmental monitoring to medical diagnostics. As near-infrared light propagates through nano-scale silicon wires on an SOI chip, a portion of the light resides outside the waveguide and interacts with biomolecules and the biological matrix on the waveguide’s surface. This capability makes silicon photonics an ideal platform for label-free biosensing. Additionally, the SOI platform is compatible with standard CMOS fabrication processes, facilitating manufacturing at the economies of scale offered by today’s foundries. In this paper, we describe our efforts to improve the performance of SOI-based biosensors—specifically, TE and TM mode microring resonators, thin waveguide resonators, sub-wavelength grating resonators, as well as strip and slot Bragg gratings. We compare device performance in terms of sensitivity, intrinsic limit of detection, and their potential for biosensing applications in Lab-on-Chip systems.

Silicon-on-insulator sensors using integrated resonance-enhanced defect-mediated photodetectors

A resonance-enhanced, defect-mediated, ring resonator photodetector has been implemented as a single unit biosensor on a silicon-on-insulator platform, providing a cost effective means of integrating ring resonator sensors with photodetectors for lab-on-chip applications. This method overcomes the challenge of integrating hybrid photodetectors on the chip. The demonstrated responsivity of the photodetector-sensor was 90 mA/W. Devices were characterized using refractive index modified solutions and showed sensitivities of 30 nm/RIU.

Assessing silicon photonic biosensors for home healthcare

Silicon nanophotonics has the transformative potential to produce highly integrated, low-cost medical biosensors for point-ofcare (POC) clinics and home healthcare diagnostic applications. The recent literature has seen significant advances, reporting silicon photonic biosensors capable of delivering results with clinically relevant sensitivities. Additionally, the repertoire of molecules that bind to specific biomarkers, which signifies the presence of certain pathogens or disease states, is rapidly expanding. Detecting and quantifying the circulating biomarkers is a route to diagnosis, and such analyses can operate in increasingly complex samples such as saliva and human serum, a major component of blood. By leveraging existing CMOS fabrication processes and their economies of scale, silicon photonic sensors offer significant advantages over traditional biosensing platforms, such as thousands of sensors assembled on a single millimeter-scale chip. In the near future, these biosensors could displace traditional clinical assays that rely on multistep liquid handling, trained operators, and benchtop instrumentation. However, the field must overcome numerous challenges to realize a fully integrated silicon photonic biosensing platform. These include determining the optimal sensor characteristics, fluidic and analyte control, and on-chip lasers and detectors. Our work reviews important sensor performance metrics in aqueous environments similar to saliva, serum, or blood, that are essential for medical diagnostic applications.1 Evanescent field sensors, such as surface plasmon resonance (SPR) or planar waveguide-based sensors (including silicon photonics resonators), represent some of the most popular optical techniques for sensitive and label-free detection of biological analytes (see Figure 1). Compared to other sensing methods, these optical sensors have the advantages of high sensitivity, Figure 1. The silicon photonic sensor measurement setup2: the optical input fiber is connected to a tunable laser source with wavelength at around 1550nm, and the output optical fiber is connected to an optical power sensor. Measurements are performed on a temperaturecontrolled stage. Fluidics are connected to a syringe pump for biological fluid delivery.