Demonstrating low Raman background in UV-written SiO2 waveguides.

Abstract

Raman spectroscopy can give a chemical 'fingerprint' from both inorganic and organic samples, and has become a viable method of measuring the chemical composition of single biological particles. In parallel, integration of waveguides and microfluidics allows for the creation of miniaturized optical sensors in lab-on-a-chip devices. The prospect of combining integrated optics and Raman spectroscopy for Raman-on-chip offers new opportunities for optical sensing. A major limitation for this is the Raman background of the waveguide. This background is very low for optical fibers but remains a challenge for planar waveguides. In this work, we demonstrate that UV-written SiO2 waveguides, designed to mimic the performance of optical fibers, offer a significantly lower background than competing waveguide materials such as Si3N4. The Raman scattering in the waveguides is measured in absolute units and compared to that of optical fibers and Si3N4 waveguides. A limited study of the sensitivity of the Raman scattering to changes in pump wavelength and in waveguide design is also conducted. It is revealed that UV-written SiO2 waveguides offer a Raman background lower than -107.4 dB relative to a 785 nm pump and -106.5 dB relative to a 660 nm pump. Furthermore, the UV-written SiO2 waveguide demonstrates a 15 dB lower Raman background than a Si3N4 waveguide and is only 8.7 - 10.3 dB higher than optical fibers. Comparison with a polystyrene bead (in free space, diameter 7 µm) reveal an achievable peak SNR of 10.4 dB, showing the potential of UV-SiO2 as a platform for a Raman-on-chip device capable of measuring single particles.