Silicon photonics in the mid-infrared: Waveguide absorption sensors

We present a silicon (Si) based infrared (IR) absorption sensor for fluids which is suitable for integration into a miniaturized sensor system. The sensor is designed to operate in the mid infrared (MIR) region ranging in the wavelengths λ=5 μm to λ=6 μm. We present the design, the modeling and the optical characterization of the used materials. The sensor is designed as a single-mode Si waveguide (WG) on low refractive index Si₃N₄/SiOx/ Si₃N₄ membrane. The single-mode requirement for the WG is needed to avoid losses due to imperfections on the WG walls causing redistribution of the carried energy among the different modes. The waveguide interacts with the fluid sample by means of the evanescent field which extends into the sample. This sensor configuration is not only compatible to the Si technology, but can also be realized on a single chip. In addition, the principle of operation is not limited to a single wavelength: by changing the waveguide dimensions, it can be applied to a broad spectral range. Thus, by its dimensions, performance and Si-compatibility, the sensor is expected to overcome previously published devices1 [1].