Silicon Nitride Background in Nanophotonic Waveguide Enhanced Raman Spectroscopy

Area of Science:

• Nanophotonics
• Spectroscopy
• Materials Science

Background:

• Evanescent Raman spectroscopy on silicon nitride (SiN) nanophotonic waveguides offers superior signal enhancement over free-space systems.
• Signal-to-noise ratio limitations at low analyte concentrations are primarily due to shot-noise from waveguide background luminescence (WGBL).

Purpose of the Study:

• To identify the dominant components and spectral properties of WGBL in SiN nanophotonic waveguides.
• To quantify the Raman scattering efficiency of WGBL.
• To explore mitigation strategies for reducing WGBL.

Main Methods:

• Characterization of WGBL spectrum in SiN nanophotonic waveguides.
• Determination of Raman scattering efficiency at room temperature for 785 nm excitation.
• Demonstration of mitigation techniques including slotted waveguides and quasi-transverse magnetic polarization.

Main Results:

• WGBL is dominated by broad Raman scattering from amorphous materials (momentum selection-rule breaking) and embedded molecular peaks.
• Maximum WGBL Raman scattering efficiency is 4.5 ± 1 × 10^-9 cm^-1·sr^-1 at a 200 cm^-1 Stokes shift, decreasing monotonically at higher shifts.
• Slotted waveguides and quasi-transverse magnetic polarization show promise as WGBL mitigation strategies.

Conclusions:

• Understanding WGBL's origin and spectral characteristics is crucial for optimizing SiN-based evanescent Raman spectroscopy.
• Quantifying WGBL efficiency provides a benchmark for future improvements and device design.
• Proposed mitigation strategies offer pathways to enhance signal-to-noise ratios in waveguide-enhanced Raman spectroscopy.