Highly-Nonlinear Chalcogenide Glass-in-Silica Waveguides

Chalcogenide glasses (ChGs) draw much attention in photonics applications, due to their high refractive indices, broad transparency windows, pronounced nonlinearities, and photo-sensitivity effects. ChGs were successfully employed in demonstrations of strong nonlinear-optical and opto-mechanical interactions on a chip. Most waveguides used in previous demonstrations were defined using dry etching. The process is challenging to implement due to the relative instability of ChGs, and requires specific modifications and calibrations for each new glass composition. In addition, the process is not fully compatible with CMOS electronics fabrication.

In this work we propose and demonstrate an alternative approach to the fabrication of ChG waveguides, with small cross-sectional areas, for nonlinear-optical and opto-mechanical applications. The proposed waveguides consist of a ChG core that is surrounded by silica cladding from three sides. Devices are fabricated in silica-on-silicon wafers. Patterns are first defined as isolated, narrow and comparatively deep trenches in the silica layer. A layer of As₂S₃ ChG composition is deposited onto the sample by thermal evaporation, as a final process step. Deposition partially fills the etched silica trenches with a ChG core region. The proposed fabrication protocol circumvents the etching (or any other processing) of the ChG layer, and supports the deposition of any ChG composition with no process modifications. It relies on well-established practices for the etching of silica, which are fully compatible with CMOS electronics. Last but not least, the ChG core region supports guided modes of light and sound waves, a pre-requisite for efficient stimulated Brillouin scattering. Waveguides with 2 µm-wide and 1 µm-thick core regions were examined in simulations and experiments. The linear losses of fabricated devices were comparatively high: 1.25 dB/mm. Losses are primarily due to roughness in the etched silica sidewalls. Despite these losses, four-wave mixing was successfully demonstrated in a 4.5 mm-long waveguide. The nonlinear coefficient γ of the waveguide was measured as 6.5 ± 1 [W×m]^-1, in good agreement with predictions. The nonlinear coefficient is 5,000 times larger than that of standard single-mode fiber, and comparable to those of previous As₂S₃ waveguides fabricated using dry etching. The results provide a proof-of-concept for the proposed approach to the fabrication of highly nonlinear ChG waveguides. 

* M.Sc. research supervised by: Prof. Avi Zadok