Efficiency of evanescent excitation and collection of spontaneous Raman scattering near high index contrast channel waveguides.

Ashim Dhakal,Ananth Z Subramanian,Roel Baets,Frédéric Peyskens,Stéphane Clemmen,Ali Raza,Nicolas Le Thomas

doi:10.1364/oe.23.027391

Abstract

We develop and experimentally verify a theoretical model for the total efficiency η0 of evanescent excitation and subsequent collection of spontaneous Raman signals by the fundamental quasi-TE and quasi-TM modes of a generic photonic channel waveguide. Single-mode silicon nitride (Si3N4) slot and strip waveguides of different dimensions are used in the experimental study. Our theoretical model is validated by the correspondence between the experimental and theoretical absolute values within the experimental errors. We extend our theoretical model to silicon-on-insulator (SOI) and titanium dioxide (TiO2) channel waveguides and study η0 as a function of index contrast, polarization of the mode and the geometry of the waveguides. We report nearly 2.5 (4 and 5) times larger η0 for the fundamental quasi-TM mode when compared to η0 for the fundamental quasi-TE mode of a typical Si3N4 (TiO2 and SOI) strip waveguide. η0 for the fundamental quasi-TE mode of a typical Si3N4, (TiO2 and SOI) slot waveguide is about 7 (22 and 90) times larger when compared to η0 for the fundamental quasi-TE mode of a strip waveguide of the similar dimensions. We attribute the observed enhancement to the higher electric field discontinuity present in high index contrast waveguides.

Highlights

Integrated photonics has been established as a very promising platform for lab-on-a-chip sensing applications [1]
We conclude that due to enhanced excitation and emission (Purcell enhancement) near the high index contrast waveguides, the overall efficiency of excitation and collection of a practical single mode waveguide can be higher than the ideal free space excitation and collection
Based on our measurements of evanescently collected Raman signals with Si3N4 slot and strip waveguides, we have been able to validate a theoretical model for the collection efficiency of generic channel waveguides

Summary

Introduction

Integrated photonics has been established as a very promising platform for lab-on-a-chip sensing applications [1]. An emergent type of photonic sensors is based on light-matter interaction with the evanescent wave of nanophotonic waveguide modes This has led to the realization of on-chip absorption spectroscopy [2] and spontaneous Raman spectroscopy [3]. In this paper we consider Raman sensors in which the molecules under study are evanescently excited through a high-index channel waveguide and the corresponding Raman signal is evanescently collected using the same single-mode photonic waveguide. A single-mode waveguide carries the pump beam and collects the Stokes light with the smallest possible étendue It opens up the potential of integration with very compact photonic components, such as an Arrayed Waveguide Grating (AWGs) acting as an on-chip spectrometer [6]. The derivation of the key equation for determining η0 is elaborated in the Appendix

Measurement of η0 for strip and slot Si3N4 waveguides

Findings

Conclusion