Abstract

Waveguide enhanced Raman spectroscopy (WERS) is a promising sensing technology requiring accurate waveguide optimization to increase the pump/signal surface intensity. Traditionally, WERS has been focused on single-mode operation, which results in stringent waveguide parameter control and increased propagation losses. In this work, we have studied theoretically and experimentally the impact of planar waveguide thickness on surface scattering losses and the waveguide propagation losses. In our study, we consider radiation from a Raman-emitting dipole on a waveguide can be captured back into the waveguide in polarization and spatial modes different from the pump mode, provided the waveguide can support them. In the case of randomly-distributed dipoles, we consider the Raman gain coefficients corresponding to all possible combinations of pump and signal polarization and spatial modes. We have introduced a new generalized FOM to optimize planar waveguide-based WERS sensors, which is a promising candidate for flexible disposable biomedical sensing, under multimode excitation/collection operation and waveguide-thickness- and mode-dependent propagation losses. The FOM is shown to increase with the mode order as a result of the combination of substantial reduction in propagation loss, increased number of collecting modes, longer optimum sensing lengths, and this occurs despite the concomitant surface intensity reduction. This implies that in the case of randomly distributed dipoles, the single-mode pump excitation is not strictly required, and multimode pump excitation will give superior conversion efficiencies.