Free-standing tantalum pentoxide waveguides for gas sensing in the mid-infrared

Abstract

Typical applications of integrated photonics in the mid-infrared (MIR) are different from near-infrared (telecom) range and, in many instances, they involve chemical sensing through MIR spectroscopy. Such applications necessitate tailored designs of optical waveguides. Both cross-sectional designs and processing methods of MIR waveguides have been a subject of extensive research, where material transparency and substrate leakage of guided modes have been the most common challenges. Both these challenges can be solved simultaneously with air-suspended waveguides. In this paper, tantalum pentoxide (Ta₂O₅, tantala) thin films deposited on silicon were tested for two different dry under-etching procedures, XeF₂ and SF₆ plasma, with both of them facilitating selective removal of silicon. We analyze the advantages and limitations of these two methods and optimize the processing for fabricating membranes with arbitrary length and cross-sectional aspect ratio over 300. The performance of these high-aspect-ratio membranes as a framework for single-mode waveguides is rigorously analyzed at 2566 nm wavelength. With tantala being transparent up to 10 µm wavelength, such waveguides are particularly well suited for gas sensing in MIR.