Transition metal oxide-based nanophotonic sensors for efficient on-chip Raman spectroscopy
Waveguide-based evanescent Raman sensors have attracted much attention in recent years due to their compact format, increased efficiency over micro-Raman spectrometers, and nanoscale surface sensitivity. Our approach is to utilize transition metal oxides (e.g., titanium dioxide, tantalum pentoxide) as novel material platforms for this application. These transition metal oxides have several advantages, including high index, wide transparency window and negligible fluorescence, which all contribute to stronger light-chemistry interaction. We first focus on titanium dioxide (TiO2) as an example. We develop a bi-layer lift-off fabrication approach to create low-loss amorphous TiO2 waveguides and resonators for visible and near-infrared applications. This approach achieves single-mode waveguide losses as low as 7.5 dB/cm around 633 nm, 4× improvement over previous reports, without the need to optimize etching conditions. We utilize TiO2 integrated optical devices to enable on-chip Raman sensing and compare them with the state-of-the-art silicon nitride devices. Our visibly pumped TiO2 sensors display >50× more Stokes signal per input pump power over that of silicon nitride devices. Also, our TiO2 ring resonators enable an increased on-resonance Stokes emission with peak rates >30× higher than an equivalent length straight waveguide. Furthermore, to overcome the interference from the inevitable Raman background of the dielectric waveguide material, we design a TiO2 slot waveguide, where the slot mode reduces the internal Raman background. Our design allows the theoretical conversion efficiency to be >2× higher than previous strip waveguides. We develop a fabrication process using electron-beam lithography with a two-step chromium etching and the resulting device has a minimal sidewall tapering and low loss (11.4 dB/cm at 780 nm wavelength). Lastly, we extend our on-chip Raman detection from liquid-phase analytes to gaseous molecules by a hollow-core anti-resonant reflecting optical waveguide (ARROW). The geometry of ARROWs confines the guided light in the air mode, where the optical field can efficiently overlap with the gaseous molecules. We fabricate hollow-core ARROWs with alternating layers of tantalum oxide and silicon dioxide and our devices show a propagation loss of ~19 dB/cm at 780 nm wavelength. Future work is needed to address the gas diffusion problem within our device for gas detection.
Raman Spectroscopy; Sensor; Fabrication; photonics; Optics; integrated optics materials; waveguide
Craighead, Harold G.
Wang, Michelle D.; Suntivich, Jin
Ph.D., Applied Physics
Doctor of Philosophy
dissertation or thesis