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 subwavelength patterned structures.

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.

A comprehensive investigation of real-time temperature-induced resonance shift cancellation for silicon wire based biosensor arrays is reported for the first time. A reference resonator, protected by either a SU8 or SiO(2) cladding layer, is used to track temperature changes. The temperature dependence of resonators in aqueous solutions, pertinent to biosensing applications, is measured under steady-state conditions and the operating parameters influencing these properties are discussed. Real-time measurements show that the reference resonator resonances reflect the temperature changes without noticeable time delay, enabling effective cancellation of temperature-induced shifts. Binding between complementary IgG protein pairs is monitored over 4 orders of magnitude dynamic range down to a concentration of 20 pM, demonstrating a resolvable mass of 40 attograms. Reactions are measured over time periods as long as 3 hours with high stability, showing a scatter corresponding to a fluid refractive index fluctuation of ± 4 × 10(-6) in the baseline data. Sensor arrays with a SU8 protective cladding are easy to fabricate, while oxide cladding is found to provide superior stability for measurements involving long time scales.

We demonstrate densely folded silicon wire waveguide ring resonator sensors for label-free biosensing. A spiral ring resonator sensor of 30 μm × 30 μm area has a quality factor of over 20,000, and a cavity length equivalent to a ring of a diameter of 130 μm. We show that extended cavity length increases the quality factor and improves the fabrication robustness of resonators. Real time monitoring of molecular binding resolves ~ 0.2% of a protein monolayer.

In this invited paper we introduce several types of waveguide spectrometers. We present a high-resolution 50-channel AWG spectrometer, including its use to interrogate a fibre Bragg grating sensor with a high wavelength accuracy of 1 pm. A 40-channel AWG operating in a broad wavelength range of 1250–1550 nm is introduced. We present a new 15-channel CWDM demultiplexer using a single curved Si-wire waveguide with an etched sidewall grating and chip size of only 90 µm × 140 µm. Finally, the first planar waveguide Fourier-transform spectrometer is presented, with a greatly increased light gathering capability and a static design.

A method is developed for extracting the coupling and loss coefficients of ring resonators from the peak widths, depths, and spacings of the resonances of a single resonator. Although the formulas used do not distinguish which coefficient is coupling and which is loss, it is shown how these coefficients can be disentangled based on how they vary with wavelength or device parameters.

A new interferometer-based optical sensing platform with nanostructured thin films of ZrO2 or TiO2 as sensing environment has been developed. With the application of an IC compatible Si(3)N(4) waveguide technology, Mach-Zehnder interferometer devices have been fabricated. The application of the glancing angle deposition technique allowed fabrication of nanostructured thin films as the optical sensing environment. Sensing ability of fabricated devices has been demonstrated through the refractive index measurement of a known gas. The transmission spectra and time response measurements have demonstrated a maximum phase shift of Delta phi=pi/10 and a |Delta P(out)|=0.65 dBm. Devices with TiO2 film on the sensing region performed much better than devices with ZrO2, with sensitivity twice as high.

We demonstrate silicon photonic wire evanescent field sensors formed by folding long photonic wire waveguides into dense spiral paths that occupy spot sizes less than 150 mum in diameter, and are hence suitable for biochip array formats. These sensors are incorporated into both ring resonator and Mach-Zehnder interferometer (MZI) interrogation circuits, and tested by monitoring streptavidin protein adsorption. The level of detection is less than 1% of a monolayer in either configuration. Using a balanced MZI design, we show that the sensors can be insensitive to temperature and wavelength, while retaining a very high sensitivity to molecular adsorption.

First implementations of subwavelength gratings in silicon-on-insulator waveguides are discussed and demonstrated by experiment and simulations. The subwavelength effect is exploited for making antireflective and highly reflective waveguide facets as well as efficient fiber-chip coupling structures for photonic wire waveguides.

We demonstrate folded waveguide ring resonators for biomolecular sensing. We show that extending the ring cavity length increases the resonator quality factor, and thereby enhances the sensor resolution and minimum level of detection, while at the same time relaxing the tolerance on the coupling conditions to provide stable and large resonance contrast. The folded spiral path geometry allows a 1.2 mm long ring waveguide to be enclosed in a 150 microm diameter sensor area. The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of approximately 3 pg/mm(2), or a total mass of approximately 5 fg. The real time measurements are used to analyze the kinetics of biotin-streptavidin binding.

Various microphotonic silicon-on-insulator (SOI) waveguide devices developed at the National Research Council of Canada are reviewed. These include a high-resolution arrayed waveguide grating (AWG) spectrometer, a Fourier-transform interleaved AWG spectrometer with a large light gathering capability, and an AWG with the dispersion enhancement using the photonic bandgap effect. Sub-wavelength grating (SWG) structures are introduced, including input couplers and antireflective waveguide facets.

We demonstrate a method of canceling the temperature dependence of ring resonator sensors using a double ring configuration. Temperature calibration curves are provided for SOI photonic wire ring sensors with air and SU8 polymer cladding.

The local oxidation of silicon process is shown to be an effective means of defining optical rib waveguides in silicon on insulator while maintaining the surface planarity required for CMOS integration. Experimental waveguides have shown singlemode operation and losses of less than 1 dB/cm in the telecommunications band around 1550 nm.

We demonstrate a 50-channel high-resolution arrayed waveguide grating microspectrometer with a 0.2 nm channel spacing on a silicon-on-insulator (SOI) platform. The chip size is 8 mm x 8 mm. High channel density and spectral resolution are achieved using high aspect ratio 0.6 mum x 1.5 mum waveguide apertures to inject the light into the input combiner and to intercept different spectral channels at the output combiner focal region. The measured crosstalk is <-10 dB, the 3 dB channel bandwidth is 0.15 nm, and the insertion loss is -17 dB near the central wavelength of lambda = 1.545 mum.

Photonics is playing an increasingly important role in the field of medical diagnostics, genomics and drug discovery. With the advances in molecular biology which gives insight to the processes underlying diseases and drug response, there is a need for new biological sensors to rapidly detect and quantify these processes. Sensing of biological events using planar waveguide evanescent field (EVF) detection offers label-free, high sensitivity, and real time monitoring [1-4]. This is a competing technology with the commercially pervasive SPR (surface plasmon resonance) method, but with the advantage of portability and low cost.

We describe a novel non-destructive technique to measure the sidewall roughness induced scattering loss of SOI ridge waveguides using an integrated 5x17 star coupler. The accuracy of our technique is independent of the coupling efficiency. In our technique, we capture the near field images of the full output waveguides array with varying width ranging from 0.2 to 2.0 mu m and use the intensity maps of these images to produce normalized intensity profiles, from which the relative scattering losses of output waveguides are extracted. Using our technique, we have studied and compared the scattering and polarization dependent losses of three different sets of SOI waveguide samples fabricated by different processes. We have determined the root-mean-square roughness and autocorrelation length of these samples using scanning electron microscopy (SEM). Relating the loss and roughness analysis, we have showed that the process utilizing negative e-beam photoresist and Cr-hardmask with inductively coupled plasma (ICP) etching produced the smoothest waveguide sidewalls and lowest scattering losses. We have also successfully modeled the measured ridge waveguide losses as a function of waveguide width and demonstrated that the theoretical sidewall roughness is in reasonable agreement with the measured roughness from SEM. Our technique is capable of studying roughness induced scattering loss and thus provides an efficient way of optimizing and monitoring process parameters that affect sidewall roughness.

We exploit the unique properties of the silicon-on-insulator material platform to demonstrate a new series of planar waveguide evanescent field sensors for biological / chemical sensing. These sensors, combined with state-of-the-art surface functionalization chemistries, offer a sensitive, label-free means for the specific detection of biomolecules, without the need for fluorescent tags employed in conventional fluorescence-based biochips. The use of silicon photonic wire waveguide technology allows sensors with extremely small footprint and small radius of curvature to be fabricated, facilitating the development of densely packed sensor arrays for multi-parameter analysis, particularly attractive for drug discovery, pathogen detection, genomics and disease diagnostics.We show that high index contrast silicon photonic wire waveguides not only provide the above stated advantages but also offer increased sensitivity over that of evanescent field sensors constructed on other common waveguide material platforms. This results from the unique properties of the optical modes of silicon photonic wire waveguides, which exhibit very large surface electric field magnitude and strong localization near the waveguide surface. We discuss the design and fabrication of silicon-on-insulator-based Mach-Zehnder interferometer sensors and experimentally demonstrate their performance to detect bulk solution refractive index change and to monitor the specific adsorption of streptavidin to biotinylated waveguides.

The design, simulation, and experimental performance of mode converters for coupling from single-mode silicon-on-insulator ridge waveguides to high aspect ratio channel waveguides are described. The converters consist of a two-level adiabatic taper structure. The final channel waveguide is 1.5 mu m high by 0.8 mu m wide. Simulations predict that for total coupler lengths longer than 20 mu m, the coupling loss from the fundamental ridge mode to the slit mode is better than -0.2 dB. The couplers and waveguides were fabricated using a two-step self-aligned process. The measured coupling loss for fabricated mode converters is -0.4 dB.