Mid-infrared liquid spectroscopy using chalcogenide optical waveguide

Abstract

Biochemical analysis for medical diagnostics is normally carried out by trained operators in sophisticated laboratories using large instruments. These techniques are out of reach to many people living in remote areas or are too slow for critically ill patients who need urgent care. A rapid, automated, compact, low cost and disposable integrated lab on a chip device which can be operated by a minimally trained person is required not only for the detection and diagnosis of the diseases, but also for water, food and environmental safety. Optical waveguide devices allow integration of various optoelectronic and microfluidic components to help in realizing sensing of liquid analytes and require 3-6 orders less sample volume compared with many conventional medical tests. The mid-infrared (mid-IR) spectral window covering wavelengths from 2 to 14 µm is ideal for biochemical analysis due to the fundamental fingerprint molecular absorption, and waveguide evanescent field based spectroscopy can potentially offer rapid high sensitivity, real-time measurement of complex mixtures to identify and quantify species in clinical samples for point of care diagnosis without any biomarkers. To realize such devices, mid-IR transparent chalcogenide materials are used to fabricate optical waveguides. In our previous work, we have demonstrated waveguiding in the mid-IR region using germanium telluride (GeTe 4) waveguide cores on zinc selenide (ZnSe) substrates [1]. In this work, ZnSe rib waveguides were fabricated on oxidized silicon with the dimensions shown in Fig 1(a). ZnSe films were deposited by thermal evaporation and the ribs were fabricated using photolithography and Ar + ion-beam etching. Light from a tunable optical parametric oscillator (M-squared) operating between 2.5-3.7 µm wavelengths was butt-coupled into the waveguide using a single mode ZrF 4 fibre (core diameter ~ 9 µm) and the waveguide output was collected using a multimode InF 3 fibre (core diameter ~100 µm), directed onto a TE cooled Mercury Cadmium Telluride detector (MCT, Vigo System) and recorded on a computer. After optimizing coupling into the waveguide by imaging the waveguide output using a mid-IR camera from above, as shown in Fig 1(b), two spectral scans were recorded, one of the output power from the waveguide alone and the other with a10 µL DI water droplet on the waveguide surface of length ~0.6 cm shown in Fig 1(c) inset. The ratio, representing the waveguide absorption spectrum of water, is plotted in Fig 1(c) (black points) and compared with a numerical model of the water-coated waveguide (blue line) using the published complex refractive index of water [2], for a waveguide length of 0.6 cm. These preliminary experimental results are in good agreement with the theoretical results. The elevated transmission in the absorption band is due to substrate radiation and the transmission above unity is due to reduction in waveguide scattering loss with the water cladding. Future work will study the absorption spectra of clinically relevant analytes. Fig. 1 (a) Theoretical mode profile of ZnSe rib waveguide, (b) Infrared camera image of the guiding rib waveguide sample with a water drop on its surface and (c) Experimental and theoretical IR-spectra of water on the surface of a ZnSe waveguide (inset: photograph of waveguide sample while taking measurements).

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Identifiers

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

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

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

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