Abstract
We review our work developing optical waveguide‐based evanescent field sensors for the label‐free, specific detection of biological molecules. Using high‐index‐contrast silicon photonic wire waveguides of submicrometer dimension, we demonstrate ultracompact and highly sensitive molecular sensors compatible with commercial spotting apparatus and microfluidic‐based analyte delivery systems. We show that silicon photonic wire waveguides support optical modes with strong evanescent field at the waveguide surface, leading to strong interaction with surface bound molecules for sensitive response. Furthermore, we present new sensor geometries benefiting from the very small bend radii achievable with these high‐index‐contrast waveguides to extend the sensing path length, while maintaining compact size. We experimentally demonstrate the sensor performance by monitoring the adsorption of protein molecules on the waveguide surface and by tracking small refractive index changes of bulk solutions.
Highlights
Sensing techniques employing fluorescent labeling of molecules are commonly used for simple binding event detection to determine the presence or absence of molecular targets in solution
In the case of surface plasmon resonance (SPR) sensors, the optical field is the surface plasmon mode which propagates along the metal-analyte interface, whereas for the planar waveguide evanescent field sensor it is the evanescent tail of the optical waveguide mode
We present experimental results demonstrating the performance of these devices using two different waveguide configurations: the Mach-Zehnder interferometer (MZI) and the ring resonator
Summary
Sensing techniques employing fluorescent labeling of molecules are commonly used for simple binding event detection to determine the presence or absence of molecular targets in solution. The SOI material system consists of a top silicon waveguide core layer positioned on a silicon dioxide lower cladding layer grown on a silicon substrate [17] and has the highest refractive index contrast of all commonly available planar waveguide systems In this manuscript, we review our work developing evanescent field sensors in SOI. We show that silicon nanophotonic waveguides with dimensions smaller than the wavelength of light they guide have a strong evanescent field at the waveguide surface for the transverse magnetic (TM) polarized waveguide mode This results in increased interaction with surface bound molecules, which leads to larger response compared to waveguide sensors fabricated on traditional low-index-contrast material platforms [16, 18]. We illustrate the performance of these sensors to track homogeneous refractive index change of bulk solutions and to monitor the formation of thin molecular layers on the waveguide surface
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