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Fabry-Perot tunable infrared filter based on structured reflectors

: Kurth, S.; Hiller, K.; Neumann, N.; Seifert, M.; Ebermann, M.; Specht, H.; Meinig, M.; Gessner, T.


Thienpont, H. (Hrsg.) ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Micro-Optics 2012 : 16-19 April 2012, Brussels, Belgium
Bellingham, WA: SPIE, 2012 (Proceedings of SPIE 8428)
ISBN: 978-0-8194-9120-6
Paper 84281O
Conference "Micro-Optics" <2012, Brussels>
Fraunhofer ENAS ()

This paper reports on the application of sub-wavelength structured single layer reflectors in a Fabry-Perot-Interferometer (FPI) that are used in order to replace distributed Bragg reflectors (DBR). A pair of two-dimensional arrays of ring resonators was analyzed. A 100 nm thin Al layer is regularly patterned to form a meta-surface structure. It shows high reflectance in a sufficiently wide wavelength range. This design approach has the advantage that an optimization can be done by varying geometry parameters of lateral structures only. Moreover, the material is highly compatible to standard MEMS processes. The structures used here are rings that are arranged in a two-dimensional array. Thus, parameters to be varied are the inner and the outer ring diameters and the array pitch. The optimum dimensions of the metal rings have been found iteratively. Samples were fabricated by structuring of two silicon wafers and subsequent wafer bonding. Deep dry etching of the reflecto r carriers from the back side in the areas of the resonator arrays results in free standing silicon nitride membranes that carry the resonators. The carrier membranes elastically suspend the reflecting ring resonators for variation of the cavity width. Finally, the substrates are assembled by a wafer bonding technique utilizing a SU-8 polymer layer with a very definite thickness. A peak transmittance of 55%, a bandwidth FWHM = 100 nm and a modulation contrast of M = 50:1 were achieved. The optical performance was measured by fourier transform infrared spectrometer and compared to the simulation results. It shows a widely good agreement of calculation and measurement.