Modeling of an optical sensor for detection of nanoparticles at 1550 nm

We study an optical sensor in the infrared region of the spectrum based on a Mach-Zehnder interferometer assembled with two single-mode subwavelength diameter silica wires immersed in acetonitrile. We propose to use acetonitrile as the detecting solution, because it has negligible losses in the infrared wavelengths of 1300 nm and 1550 nm in contrast to water which has very high losses in the infrared. By bio-modifying the surface of the sensing wire, we can use the optical sensor to selectively detect nanoparticles. For nanoparticles with size of 12 nm and index of refraction of 1.4 forming a coating around the sensing wire, the propagation constant difference at the output of the optical sensor becomes a maximum for a wire diameter of 1.23 μm. By using a wire length of 955 μm, a maximum phase shift of π radians is obtained at the output of the Mach-Zehnder interferometer. An optical sensor using water as the detecting solution at an operating wavelength of 325 nm will require nanowires with diameter of 240 nm and length of 68 μm, which is much more difficult to implement. Effects of wavelength, wire diameter, and specimen thickness and index on the optical sensor are also studied.