We present a silicon (Si) based infrared (IR) absorption sensor which is suitable for integration into a
miniaturized sensor system. The sensor is designed to operate in the wavelength region around λ=5 μm. We particularly
discuss the design, the modeling and the optical characterization of the used materials. The sensor operates as a singlemode
Si waveguide (WG) on low refractive index 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 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 device concepts.
The single-mode requirements lead to WG dimensions of 2 μm width x 600 nm height for an operation at λ=5 μm, which
are verified by 3D simulations. For those parameters, the WG will support one transverse electric (TE) mode and one
transverse magnetic (TM) mode. Efficient guidance is only obtained for the fundamental TE and TM modes. As an
example, it is shown that mode TE1 is a non-guided mode. The experimentally obtained WG dimensions are 605 nm
height and 2 μm width. In our paper we discuss issues with the design, the material characterization and first
experimental results obtained with the recently fabricated prototypes.