Surface-enhanced Raman scattering provides a promising technology for sensitive and selective detection of protease activity by monitoring peptide cleavage. Not only are peptides and plasmonic hotspots similarly sized, Raman fingerprints also hold large potential for spectral multiplexing. Here, we use a gold-nanodome platform for real-time detection of trypsin activity on a CALNNYGGGGVRGNF substrate peptide. First, we investigate the spectral changes upon cleavage through the SERS signal of liquid-chromatography separated products. Next, we show that similar patterns are detected upon digesting surface-bound peptides. We demonstrate that the relative intensity of the fingerprints from aromatic amino acids before and after the cleavage site provides a robust figure of merit for the turnover rate. The presented method offers a generic approach for measuring protease activity, which is illustrated by developing an analogous substrate for endoproteinase Glu-C.

Surface-Enhanced Raman Spectroscopy (SERS) allows for the highly specific detection of molecules by enhancing the inherently weak Raman signals near the surface of plasmonic nanostructures. A variety of plasmonic nanostructures have been developed for SERS signal excitation and collection in a conventional free-space microscope, among which the gold nanodomes offer one of the highest SERS enhancements. Nanophotonic waveguides have recently emerged as an alternative to the conventional Raman microscope as they can be used to efficiently excite and collect Raman signals. Integration of plasmonic structures on nanophotonic waveguides enables reproducible waveguide-based excitation and collection of SERS spectra, such as in nanoplasmonic slot waveguides. In this paper, we compare the SERS performance of gold nanodomes, in which the signal is excited and collected in free space, and waveguide-based nanoplasmonic slot waveguide. We evaluate the SERS signal enhancement and the SERS background of the different SERS platforms using a monolayer of nitrothiophenol. We show that the nanoplasmonic slot waveguide approaches the gold nanodomes in terms of the signal-to-background ratio. We additionally demonstrate the first-time detection of a peptide monolayer on a waveguide-based SERS platform, paving the way towards the SERS monitoring of biologically relevant molecules on an integrated lab-on-a-chip platform.

We present an on-chip filter that is based on the gratingassisted contra-directional coupler (GACDC) implemented on a silicon nitride rib waveguide platform. This filter enjoys the benefit of an unlimited free spectral range (FSR) on the red side of the stop/pass band. Not like a Bragg reflector, the GACDC filter has the advantage of coupling the rejected light into a bus waveguide, instead of reflecting it into the input. This property makes it an add/drop filter suitable for pump rejection and allows effective cascading between multiple stages to provide even higher rejection ratio compared to the single stage version. In this work, we experimentally show that a 16-stage cascaded GACDC filter possesses a stop band with a bandwidth smaller than 3 nm and rejection ratio as high as 68.5 dB.Recently, silicon nitride (SiN) has become one of the most prominent platforms for the miniaturization of photonic circuits in the visible or near infrared range (< 1 µm), where the silicon is no longer transparent [1]. Like the silicon-on-insulator (SOI) technology, SiN nanophotonics is also a CMOS-compatible technology that allows large-scale, cost-effective fabrication of photonic integrated circuits together with the possibility to cointegrate the electronics.A wide range of high-performance components, including grating couplers [2], polarization splitters [3], and wavelength selective filters [4], have been developed on the silicon nitride platform to open the road to further integration of complicated on-chip systems [5][6][7]. As an essential component, the on-chip wavelength filter has attracted a lot of attention from researchers interested in on-chip lasers [8], optical sensors [9] as well as wavelength-division multiplexers [10]. Filters based on the Bragg gratings, ring resonators, and Mach-Zehnder cascades have been studied and demonstrated to meet the requirement