E. Post

A Silicon-on-Insulator Photonic Wire Based Evanescent Field Sensor

We demonstrate a new, highly sensitive evanescent field sensor using silicon-on-insulator (SOI) photonic wire waveguides. Theoretical analysis shows that thin SOI waveguides can provide higher sensitivity over devices based in all other common planar waveguide material systems for the probing of both thin adsorbed biomolecular layers and bulk homogeneous solutions. A Si photonic wire waveguide was incorporated into a Mach-Zehnder interferometer based sensor, configured to monitor the index change of a homogeneous solution. High effective index change of 0.31 per refractive index unit (RIU) change of the solution was measured, confirming theoretical predictions

Folded cavity SOI microring sensors for high sensitivity and real time measurement of biomolecular binding

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.

A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with sub-micrometer aperture waveguides.

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.

High bandwidth SOI photonic wire ring resonators using MMI couplers

A ring resonator in SOI photonic wire waveguides is demonstrated using a compact MMI coupler with 3mum x 9 mum footprint as the coupling element. We achieved high bandwidth of 0.25 nm, and a quality factor Q of ~ 6000 for rings with a radius of 50 mum. Unlike directional coupler based rings, these resonators have a wavelength independent Q and extinction ratio over more than 30 nm wavelength range, and there is no loss penalty for increasing the bandwidth. Compared to their directional coupler based counterparts, these resonators also have less demanding fabrication requirements and are compatible with high speed signal processing and optical delay lines.

Real-time cancellation of temperature induced resonance shifts in SOI wire waveguide ring resonator label-free biosensor arrays.

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.

Gradient-index antireflective subwavelength structures for planar waveguide facets

We demonstrate the use of subwavelength gratings etched into the facets of silicon-on-insulator ridge waveguides as a means of reducing facet reflectivity by the gradient-index effect. Reflectivities as low as 2.0% and 2.4% for the fundamental TE and TM modes, respectively, are demonstrated experimentally for light of 1.55 microm wavelength, in agreement with both effective medium theory and finite-difference time domain calculations. Simulations show that facet reflectivites can be further reduced to less than 1% by increasing the grating modulation depth.

Extracting coupling and loss coefficients from a ring resonator

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.

Microphotonic Elements for Integration on the Silicon-on-Insulator Waveguide Platform

This paper presents an overview of our recent work on several fundamental optical elements and their integration on the silicon-on-insulator (SOI) waveguide platform. Theory, design and experimental results are presented for monolithically integrated asymmetric graded-index waveguide couplers, as well as output couplers based on total internal reflection mirrors. Design strategies for dispersive elements on SOI, for example, ring resonators and arrayed waveguide gratings, are discussed with special emphasis on methods to eliminate the polarization sensitivity. Finally, the properties and applications of evanescent fields in SOI waveguides are reviewed

Wavelength-Dependent Model of a Ring Resonator Sensor Excited by a Directional Coupler

The spectral characteristics of a ring resonator made of Si photonic wires are modeled using mode expansion of supermodes of the directional coupler. The influence of the coupling coefficient, loss factor and waveguide dispersion on the spectral features are analyzed in detail. The model is then compared with the experimental data of a ring resonator designed for sensing purposes. The model that includes a wavelength dependence on coupling length reproduces the large variations of the envelope of the experimental spectrum, when coupling coefficient cover its full range from 0 to 1. Fitting parameters explain the details of the experimental spectrum and contribute to the sensor optimization, as well as illustrating general guidelines for ring resonator design.

Subwavelength Grating Structures in Silicon-on-Insulator Waveguides

First implementations of subwavelength gratings (SWGs) in silicon-on-insulator (SOI) 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. We demonstrate experimentally that by etching triangular SWGs into SOI waveguide facets, the facet power reflectivity can be reduced from 31% to <2.5%. Similar structures using square gratings can also be used to achieve high facet reflectivity. Finite difference time-domain simulations show that >94% facet reflectivity can be achieved with square SWGs for 5  𝜇 m thick SOI waveguides. Finally, SWG fiber-chip couplers for SOI photonic wire waveguides are introduced, including design, simulation, and first experimental results.