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A paper-based biosensor integrating a functionalized porous silicon (PSi) membrane as the active sensing element for quantifiable protein detection has been developed. For similar short-time exposures to an analyte, improved molecular transport in PSi membranes when on paper leads to larger signal changes compared to traditional PSi films that remain on a silicon substrate. In this work, we discuss controlling the incubation time of the analyte and the overall testing time of the sensor by incorporating different combinations of wicking and absorbent paper beneath the PSi membrane. With this control, the PSi-on-paper sensor platform has the potential to serve as an effective low-cost rapid diagnostic test with highly sensitive, quantitative readout for a wide range of analytes. 

We experimentally demonstrated slow wave enhanced phase and spectral sensitivity in asymmetric Michelson interferometer sensors with a phase sensitivity 277,750 rad/RIU-cm and theoretical phase sensitivity as high as 461,810 rad/RIU-cm. In the context of low-cost chip integrated photonic packaged sensors, in this paper we will experimentally demonstrate a method for active tuning of interferometer fringes using phase change materials that will potentially overcome fabrication induced variation of interference fringe wavelengths, thus allowing sensor chip packaging with a fixed wavelength laser and available integrated photodetectors. 

Changes in the real and imaginary parts of the waveguide effective index in the presence of analytes have been used in various microcavity and slow light devices for on-chip sensing and absorption spectroscopy respectively in diverse applications. Periodically patterned waveguide sensors in interferometer configurations can lead to small interferometer sizes comparable in dimensions to microcavity resonator sensors, and/or significantly higher sensitivities compared to resonator type sensors. We show our work with compact silicon photonic interferometer devices for on-chip biosensing and absorbance sensing, overcoming fabrication tolerances with post-fabrication phase trimming. 

Evanescent field silicon photonics in a silicon-on-insulator or silicon-nitride-on-insulator platforms have been effectively utilized to demonstrate chemical and biosensors over the past decade with applications in the detection of nucleic acids and protein biomarkers for cancers, viruses and infectious diseases, and environmental toxins. By balancing the requirements for efficient low-loss transmission through the waveguide and enhancing light-matter interaction such as with molecules binding on the high index material surfaces in resonant microcavities, slow light and interferometer geometries, various high sensitivity biosensors have been experimentally demonstrated down to few femtograms/ml. various slotted microcavities and waveguides have been experimentally demonstrated. In recent years, subwavelength waveguides have demonstrated high bulk spectral sensitivities approaching ~500nm/RIU (RIU=refractive index unit) in periodic structures with lattice constant (Λ) <<(λ/2n eff ) where n eff is the effective index at wavelength λ. While most experimental demonstrations have been in subwavelength ring resonator geometries, in this research, in addition to experimental demonstration of bulk spectral sensitivity ~775nm/RIU in subwavelength waveguides in interferometer configurations, we investigate optimized geometries that can reach sensitivities ~70,000nm/RIU in compact dimensions. In contrast to Mach-Zehnder interferometer (MZI) sensors of the same geometric interferometer arm lengths, the reflected path in Michelson interferometers (MI) doubles the optical path length, and thus effectively doubles the phase shift in the presence of an analyte. The interference fringe linewidths are narrowed compared to the equivalent MZI and would thus enable smaller changes in analyte concentration to be discerned from the fringe spectra.