W. Sinclair

Silicon-on-insulator guided mode resonant grating for evanescent field molecular sensing

We present experimental and theoretical results of label-free molecular sensing using the transverse magnetic mode of a 0.22 mum thick silicon slab waveguide with a surface grating implemented in a guided mode resonance configuration. Due to the strong overlap of the evanescent field of the waveguide mode with a molecular layer attached to the surface, these sensors exhibit high sensitivity, while their fabrication and packaging requirements are modest. Experimentally, we demonstrate a resonance wavelength shift of approximately 1 nm when a monolayer of the protein streptavidin is attached to the surface, in good agreement with calculations based on rigorous coupled wave analysis. In our current optical setup this shift corresponds to an estimated limit of detection of 0.2% of a monolayer of streptavidin.

Photonic wire biosensor microarray chip and instrumentation with application to serotyping of Escherichia coliisolates

A complete photonic wire molecular biosensor microarray chip architecture and supporting instrumentation is described. Chip layouts with 16 and 128 independent sensors have been fabricated and tested, where each sensor can provide an independent molecular binding curve. Each sensor is 50 μm in diameter, and consists of a millimeter long silicon photonic wire waveguide folded into a spiral ring resonator. An array of 128 sensors occupies a 2 × 2 mm2 area on a 6 × 9 mm2 chip. Microfluidic sample delivery channels are fabricated monolithically on the chip. The size and layout of the sensor array is fully compatible with commercial spotting tools designed to independently functionalize fluorescence based biochips. The sensor chips are interrogated using an instrument that delivers sample fluid to the chip and is capable of acquiring up to 128 optical sensor outputs simultaneously and in real time. Coupling light from the sensor chip is accomplished through arrays of sub-wavelength surface grating couplers, and the signals are collected by a fixed two-dimensional detector array. The chip and instrument are designed so that connection of the fluid delivery system and optical alignment are automated, and can be completed in a few seconds with no active user input. This microarray system is used to demonstrate a multiplexed assay for serotyping E. coli bacteria using serospecific polyclonal antibody probe molecules.

Silicon photonic wire evanescent field sensors: From sensor to biochip array

Silicon photonic wire waveguides have a remarkably high response to surface molecular binding. Evanescent field waveguide sensors based on silicon can be interrogated using Mach-Zehnder interferometers, ring resonators, or by probing surface gratings in a reflection geometry. This paper compares these approaches from a theoretical viewpoint and through recent experimental results, with the goal of defining a path from our existing individual sensors to practical biosensor array chips.

A fully integrated silicon photonic wire sensor array chip and reader instrument

A complete instrumentation system for interrogating silicon photonic wire waveguide sensor array chips has been built and demonstrated. The system is designed to read 16 or more photonic wire sensors on a single silicon chip simultaneously and in real time, while delivering sample fluid to the sensors through microfluidic channels fabricated monolithically on the chip. The chip can be inserted into the instrument, automatically aligned with input optics and fluid connection, and be ready for measurement in a few minutes.

Applications of subwavelength grating structures in silicon-on-insulator waveguides

We discuss several applications of both resonant and non-resonant subwavelength gratings (SWGs) for silicon photonics. We present results of evanescent field molecular sensing using the transverse magnetic mode of a 0.22 μm thick silicon slab waveguide with a resonant SWG, which couples a free space laser beam to the silicon waveguide mode. The optical readout of this configuration is almost identical to the established surface plasmon resonance sensing technology. Using calibrated sucrose solutions, we demonstrate a bulk refractive index sensitivity of 111 nm/RIU in good agreement with rigorous coupled wave analysis calculations. The binding of a monolayer of streptavidin protein on the waveguide surface is monitored in real time with a signal-to-noise ratio of ~500. In another application, non-resonant SWGs are used to create effective dielectric media with a refractive index that can be tuned between the values of silicon (3.48) and SU-8 polymer used for the cladding (1.58). For example, we present SWG waveguides with an effective core index of approximately 2.65, which exhibit lower propagation loss than photonic wire waveguides of similar dimensions. We use these SWG waveguides to demonstrate highly efficient fiber-chip couplers.

Silicon photonic wire biosensors: Sensing, instrumentation, and applications

Photonic wire evanescent field (PWEF) sensor chips have been developed for multiplexed label free molecular detection. The sensors are made using 260 nm × 450 nm cross-section silicon waveguides folded into spirals less than 200 μm in diameter, but with an overall sensor length of more than a millimeter. The long propagation length gives a response to molecular binding much better than currently available tools for label-free molecular sensing. These sensors can be arrayed at densities up to ten or more per square millimeter. This talk reviews our ongoing work on the photonic wire sensor chip design and layout, on-chip integrated fluidics, optical coupling, and chip interrogation using arrays of grating couplers formed using sub-wavelength patterned structures. The goal is to develop a commercially viable sensor platform by addressing cost-of-instrumentation, cost per measurement, ease-of-use, and by increasing the number of sensors that can be simultaneously monitored.

Silicon photonic wire evanescent field sensors: sensor arrays and instrumentation

We are developing a photonic wire evanescent field (PWEF) sensor chip using 260 nm x 450 nm cross-section silicon photonic wire waveguides. The waveguide mode is strongly localized near the silicon surface, so that light interacts strongly with molecules bound to the waveguide surface. The millimeter long sensor waveguides can be folded into tight spiral structures less than 200 micrometers in diameter, which can be arrayed at densities up to ten or more independent sensors per square millimeter. The long propagation length in each sensor element gives a response to molecular binding much better than currently available tools for label-free molecular sensing. Cost of instrumentation, cost per measurement, ease-of-use, and the number of sensors that can be simultaneously monitored on a sensor array chip are equally important in determining whether an instrument is practical for the end user and hence commercially viable. The objective of our recent work on PWEF sensor array chips and the associated instrumentation is to address all of these issues. This conference paper reviews our ongoing work on the photonic wire sensor chip design and layout, on-chip integrated fluidics, optical coupling, and chip interrogation using arrays of grating couplers formed using sub-wavelength patterned structures.

Engineering light at the sub-wavelength scale using silicon photonics

As a result of the evolution semiconductor fabrication tools and methods over several decades, it now possible to routinely design and make optical devices with features comparable to or smaller than the wavelength of the light that propagates through these structures. This paper will review some silicon optical structures with critical features at these extremely short length scales. For example it becomes possible to create segmented waveguide structures with optical properties that can be tuned continuously between those of the cladding and waveguide core, using lithographic patterning rather than varying etch depth. Using thin high index contrast waveguides and the correct polarization, the optical electric field profiles can be shaped to maximize the coupling to molecular monolayers or cladding layers with specific functionality. Examples are given from our recent work on optical biosensors chips which employ grating couplers made by sub-wavelength digital patterning, and use waveguides optimized for coupling to molecular monolayers.

Subwavelength nanophotonics: From a new waveguide principle to practical components at telecom wavelengths

We report on a new waveguide principle using subwavelength gratings. It is known that diffraction effects are suppressed for waves propagating in materials structured at the subwavelength scale. Subwavelength gratings can create artificial media engineered using microscopic inhomogeneities to enact effective macroscopic behaviour. For the high-index-contrast waveguides, a subwavelength grating can be created by the combination of high-refractive-index and low-refractive-index materials, for example single crystal silicon and amorphous silica in silicon-on-insulator platform. These periodic structures frustrate diffraction provided they operate outside the Bragg condition and behave like a homogeneous medium. In a subwavelength grating (SWG) waveguide with a core consisting of a periodic arrangement of segments (Fig. 1), light excites a Bloch mode, which can theoretically propagate through the SWG waveguide with minimal scattering losses. Unlike other periodic waveguides such as line-defects in a 2D photonic crystal lattice, a subwavelength grating waveguide confines the light like a conventional index-guided structure and does not exhibit optically resonant behaviour.

Silicon Photonic Guided Mode Resonance Filter for Evanescent Field Molecular Sensing

We present experimental and theoretical results of label-free molecular sensing with the TM mode of a 0.22 µm thick silicon slab waveguide used in a guided mode resonance configuration. Due to the strong overlap of the evanescent field of the waveguide mode with a molecular layer attached to the surface, these sensors exhibit high sensitivity, while their fabrication and packaging requirements are minimal. Experimentally, we demonstrate a resonance wavelength shift of ~1 nm when a monolayer of streptavidin is attached to the surface, in good agreement with calculations based on rigorous coupled wave analysis.