Boosting Detection Sensitivity by Using a Surface-Wave-Enabled Darkfield Aperture (SWEDA)

Abstract

The on-chip detection of a weak optical signal in biological experiments can easily be complicated by the presence of an overwhelming background signal, and as such, pre-detection background suppression is substantively important for weak signal detection. In this paper, we report a structure that can be directly incorporated onto optical sensors to accomplish background suppression prior to detection. This structure, termed surface-wave-enabled darkfield aperture (SWEDA), consists of a central sub-wavelength hole surrounded by concentric grooves that are milled onto a gold layer. Incoming light can be collected and converted into surface waves (SW) by the concentric grooves and then be recoupled into propagating light through the central hole. We show that the SW-assisted optical component and the direct transmission component of the central hole can cancel each other, resulting in near-zero transmission under uniform illumination (observed suppression factor of 1230). This structure can therefore be used to suppress a light field's bright background and allow sensitive detection of localized light field non-uniformity (observed image contrast enhancement of 27dB). We also show that under a coherent background illumination, a CMOS pixel patterned with the proposed structure achieves better SNR performance than an un-patterned single pixel.