Impedance matching of a micromesh bolometer placed in a silicon parallel plate waveguide spectrometer

Abstract

Micromesh bolometers have been used in several instruments working in the submillimeter band. The bolometers are usually placed inside a waveguide and consist of a mesh absorber, at a certain distance from a back short. The mesh is made of SiNi whose thermal conductivity is tuned to the incident power and operating temperature. The impedance of the mesh is initially matched to the incoming wave impedance, but it is then optimized with full wave simulations. The mesh impedance is the one of the incoming wave since the period is usually much smaller than the wavelength. Future instruments will use higher frequencies, where subwavelength mesh period presents a fabrication challenge. In this case the impedance needs to be properly tuned. In this paper an analytical model based on the calculation of the Greenpsilas function is used to design such bolometer impedance. In particular a parallel plate waveguide (PPW) spectrometer is considered, where the radiation couples from an input horn that excites the TE1 PPW mode. Then the signal is focused with a curved grating into a bolometer mesh through waveguides, similar to the Z-Spec instrument. These waveguides were overmoded and needed to be curved which reduced the overall efficiency. This work studies the possibility of receiving the TE1 mode of a silicon-filled PPW directly by a resistive micromesh bolometer placed at the focal point. In order to thermally isolate the bolometer an air gap is needed. Thanks to the analytical model the impact on the matching can be studied and conclude that limits the input angle at which it can be used. The geometry studied is shown in Fig. 1a; an elliptical reflector excited with a waveguide (black rectangle) focuses the signal to a resistive mesh.