High-contrast, GeTe₄ waveguides for mid-infrared biomedical sensing applications

Abstract

The ideal molecular "fingerprint" region for biochemical analysis is dominated by the Mid-IR spectral region from 2µm to 13µm. Realisation of single mode waveguides optimised for evanescent field based sensing is essential to detect analytes of very low concentration using their fingerprint molecular vibrations. Since commonly used silicate glasses do not transmit at wavelengths beyond 3µm and some Mid-IR transmitting glasses such as the fluorides degrade when in contact with water, novel materials are required to cover this range. Germanium telluride is a promising material which has a broad IR transparency in the region of 2µm - 20µm. We demonstrate a "high contrast" (Δn ~ 1), GeTe₄ single mode rib waveguide on ZnSe. GeTe₄ thin films were deposited on ZnSe substrates by RF sputtering at room temperature. Deposition parameters such as sputtering pressure, power, and argon flow rate were varied to optimise the deposition conditions and to study their effect on the resulting films. The deposited films were amorphous as confirmed by XRD and the average roughness of the films was between 4 and 11nm as measured by AFM. The films transmitted over the full spectral range from 2µm - 20µm as measured by FTIR. Photolithography followed by reactive-ion etching was carried out to etch the film to produce a rib waveguide structure with low surface roughness and vertical sidewalls, using an optimised mixture of CHF₃ and O₂. Further optical characterization of these waveguides including propagation loss and mode profile will be presented.

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Identifiers

ORCID for Vinita Mittal: orcid.org/0000-0003-4836-5327

ORCID for J.S. Wilkinson: orcid.org/0000-0003-4712-1697

ORCID for G.S. Murugan: orcid.org/0000-0002-2733-3273

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