Design, Fabrication and Characterization of Mid-Infrared Strip Waveguide for Laser Spectroscopy in Liquid Environments

Optical sensors based on the interaction between molecular vibrations and mid infrared radiations attracted high attentions in the last decades. In this study, we present the development of a waveguide evanescent field sensor based on the absorption spectroscopy in liquid environments. The device consists of a single mode strip waveguide integrated with a microfluidic system. With a quantum cascade laser at the wavelength of 5.8 μm, we demonstrate the detection of cocaine dissolved in tetrachloroethylene (PCE). Mid infrared germanium waveguides on silicon have been proposed but not experimentally demonstrated. Here we realize this new type of waveguide with a monocrystalline germanium layer on a silicon substrate (Ge-on-Si) with standard photolithography and reactive ion etching. It is designed to be singlemode in transverse magnetic polarization at 5.8 μm. We used a method based on the Fabry-Perot resonance to measure the propagation loss, and the achieved lowest loss is 2.4 dB/cm. The bending loss was measured to be 0.12 dB per 90∘bend with the radius of 115 μm. A microfluidic chip made from a UV-curable adhesive is integrated to the Ge-on- Si waveguide. This is the first integration of this waveguide with a microfluidic system. Solutions of cocaine in PCE flow in the microfluidic channel and absorb the light in the evanescent field. A reference waveguide and an integrated Y- junction eliminate the noise from coupling fluctuations, and the limit of detection is calculated to be 5 μg/ml. Mid infrared photonics can be further developed based on the platform of Ge-on- Si waveguides. We demonstrate a detection of refractive index changes with a Mach-Zehnder interferometer integrated with a microfluidic system. Subwavelength structures for antireflection of the waveguide facets are also experimentally tested. We foresee an increasing development and applications based on this waveguide in the mid infrared.