Inverse-designed waveguide-based biosensor for high-sensitivity, single-frequency detection of biomolecules.

Integrated silicon photonic waveguide biosensors have shown great potential for detecting bio-molecules because they enable efficient device functionalization via a well-developed surface chemistry, as well as simple scalable manufacturing, which makes them particularly suitable for low-cost point-of-care diagnostic. The on-chip integrated biosensors can be broadly classified into two types: (i) high-quality factor resonator sensors and (ii) interferometric sensors relying on non-resonant optical elements such as e.g. integrated waveguides. The former type usually requires a broadband or a tunable light source as well as complicated signal post-processing to measure a shift of the resonance frequency, while the latter exhibits a relatively low sensitivity due to the lack of efficient light recycling and phase accumulation mechanism in low quality factor elements. Additionally, high quality factor resonant photonic structures can be very sensitive to the presence of other non-target molecules in the water solution, causing sensor vulnerability to any noise. In this work, we combine a computational "inverse design" technique and a recently introduced high-contrast probe cleavage detection (HCCD) technique to design and optimize waveguide-based biosensors that demonstrate high sensitivity to the target molecule while being less sensitive to noise. The proposed biosensors only require a single frequency (or narrow-band) source and an intensity detector, which greatly simplifies the detection system, making it suitable for point-of-care applications. The optimal integrated sensor design that we demonstrate shows 98.3% transmission for the positive (target detected, probes cleaved) state and 4.9% transmission for the negative (probes are still attached) state at 1550 nm wavelength. The signal intensity contrast (20.06-fold transmission increase) shown in this work is much greater than the shift of the resonance frequency (less than 1% wavelength shift) observed in conventional ring-resonator-based biosensors. The new design may pave the way for realizing a single-frequency highly sensitive and selective optical biosensor system with a small physical footprint and a simple optical readout on a silicon chip.