Infrared evanescent field sensing with quantum cascade lasers and planar silver halide waveguides

We demonstrate the first midinfrared evanescent field absorption measurements with an InGaAs/AlInAs/InP distributed feedback (DFB) quantum cascade laser (QCL) light source operated at room temperature coupled to a free-standing, thin-film, planar, silver halide waveguide. Two different analytes, each matched to the emission frequency of a QCL, were investigated to verify the potential of this technique. The emission of a 1650 cm^-1 QCL overlaps with the amide absorption band of urea, which was deposited from methanol solution, forming urea crystals at the waveguide surface after solvent evaporation. Solid urea was detected down to 80.7 μg of precipitate at the waveguide surface. The emission frequency of a 974 cm^-1 QCL overlaps with the CH₃-C absorption feature of acetic anhydride. Solutions of acetic anhydride in acetonitrile have been detected down to a volume of 0.01 μL (10.8 μg) of acetic anhydride solution after deposition at the planar waveguide (PWG) surface. Free-standing, thin-film, planar, silver halide waveguides were produced by press-tapering heated, cylindrical, silver halide fiber segments to create waveguides with a thickness of 300-190 μm, a width of 3 mm, and a length of 35 mm. In addition, Fourier transform infrared (FT-IR) evanescent field absorption measurements with planar silver halide waveguides and transmission absorption QCL measurements verify the obtained results.