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.

Design and fabrication of SOI micro-ring resonators based on sub-wavelength grating waveguides.

Standard silicon photonic strip waveguides offer a high intrinsic refractive index contrast; this permits strong light confinement, leading to compact bends, which in turn facilitates the fabrication of devices with small footprints. Sub-wavelength grating (SWG) based waveguides can allow the fabrication of low loss devices with specific, engineered optical properties. The combination of SWG waveguides with optical micro-resonators can offer the possibility of achieving resonators with properties different from the traditional 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. In this paper, we present the design and experimental characterization of SWG based rings realized on SOI chips without upper cladding (permitting their use as sensors). The fabricated rings offer quality factors in the range of ~1k-6k, depending on SWG parameters. Based on the comparison of experimental and simulated data we expect sensitivities exceeding 383 nm/RIU in water and 270 nm/RIU in air, showing excellent potential for use in sensing applications.

Sub-wavelength grating components for integrated optics applications on SOI chips.

In this paper we demonstrate silicon on insulator (SOI) sub-wavelength grating (SWG) optical components for integrated optics and sensing. Light propagation in SWG devices is studied and realized with no cladding on top of the waveguide. In particular, we focused on SWG bends, tapers and directional couplers, all realized with compatible geometries in order to be used as building blocks for more complex integrated optics devices (interferometers, switches, resonators, etc.). Fabricated SWG tapers for TE and TM polarizations are described; they allow for connecting SWG devices to regular strip waveguides with loss lower than 1 dB per taper. Our SWG directional coupler presents a very compact design and a negligible wavelength dependence of its crossover length (and as a consequence of its coupling coefficient, κ), over a 40 nm bandwidth. This wavelength flatten response represents a bandwidth enhancement with respect to standard directional couplers (made using strip or rib waveguides), in particular for the TE mode. SWG bends are demonstrated, their loss dependence on radius is analyzed, and fabricated bends have a loss in the range 0.8-1.6 dB per 90 degrees bend. Simulated and measured results show promise for large-scale fabrication of complex optical devices and high sensitivity sensors based on SWG waveguides with engineered optical properties, tailored to specific applications.

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.

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.

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.

Optical biosensors to analyze novel biomarkers in oncology.

Many cancer types are characterized by poor survival and unpredictable therapy response. Easy-to-perform molecular analyses may help patient stratification and treatment tailoring. Several integrated devices have been proposed to overcome current analysis equipment limitations. They offer improved sensitivity and easy availability of parallel detection. Particularly, unlabelled optical biosensors combine the manifold advantages of integrated sensors (e.g. easy handling, portability and low-volume requirement) with detection of target molecules in their original form. Here, we review integrated optical biosensor current features, and discuss their possible application to the detection of protein variants from body fluids, with particular regard to histone modifications. Indeed, histone post-translational modifications are a set of epigenetic markers frequently deregulated in cancer. Available technology does not allow a comprehensive analysis of all histone modifications in a single patient. Thus, label-free optical biosensors may pave the way to the discovery and detection of a novel class of biomarkers in oncology.