Large-Area MEMS Tunable Fabry–Perot Filters for Multi/Hyperspectral Infrared Imaging

Ge/BaF2 thin-films for surface micromachined mid-wave and long-wave infrared reflectors

“…Spectrometers [1][2][3][4][5][6][7] in the mid-wave infrared (MWIR: 3 to 5 μm) and long-wave infrared (LWIR: 8 to 12 μm) wavelength ranges are of interest for infrared (IR) spectroscopy, [8][9][10][11] multi/hyper-spectral imaging, [12][13][14][15][16] and compositional analysis 11,17 due to their inherent ability to identify the unique absorption and/or reflected spectra of elements and compounds. Multi/hyper-spectral imaging is commonly differentiated based on the number of spectral channels, where hyperspectral imaging systems generally collect more than 100 adjoining spectral bands and multispectral imaging systems typically acquire <20 non-adjoining spectral bands, 12,16,18,19 even though there is no agreement on precise values. *Address all correspondence to Gurpreet S. Gill, gurpreet.gill@research.uwa.edu.au Fabry-Pérot interferometers (FPIs) provide a spectrometer architecture that is compatible with thin-film surface-micromachined microelectromechanical systems (MEMS).…”

“…25 In this configuration, λ∕4-thick optical films are deposited, where the thickness of each optical layer is equivalent to one quarter of the wavelength for which the DBR is designed. 12,26,27 High reflectivity of the DBRs is assured if there is a high contrast between the refractive indices of the high-and low index materials, which provides a pathway toward the realization of narrowband FPIs. In this paper, germanium (Ge) and barium fluoride ðBaF 2 Þ∕air have been considered as the high and low refractive index materials/media, respectively, for the fabrication of DBRs.…”

“…The λ∕4-thickness 28 and optical constants [29][30][31][32] for the thin-films required for applications in the technologically important MWIR and LWIR bands are listed in Table 1. Ge thin-films have been previously explored by several research groups 2,12,21,27,[33][34][35][36] to fabricate DBRs in the IR region. In particular, our group at The University of Western Australia 12,21,27,33,34 and InfraTec GmbH 2,36 have used Ge thin-films as Table 1 Material parameters for the design of MWIR and LWIR DBRs at center wavelengths of 4 and 10 μm, respectively.…”

“…Ge thin-films have been previously explored by several research groups 2,12,21,27,[33][34][35][36] to fabricate DBRs in the IR region. In particular, our group at The University of Western Australia 12,21,27,33,34 and InfraTec GmbH 2,36 have used Ge thin-films as Table 1 Material parameters for the design of MWIR and LWIR DBRs at center wavelengths of 4 and 10 μm, respectively. This table shows the calculated λ∕4-thicknesses for materials, targeted and achieved mirror flatness, and simulated and measured mirror reflectance.…”

“…On the other hand, ZnS is non-absorbing in the LWIR region but its refractive index of 2.2 as a low refractive index material combined with Ge is not sufficient to provide a high refractive index contrast (a key component to developing narrowband FPIs). Thus, it is impractical for ZnS-based FPIs to achieve a narrow linewidth (typically ≤1% of the design wavelength as inferred from the following studies 7,12,16,[37][38][39] ) in the LWIR region for hyperspectral sensing and imaging, although these FPIs would be suitable for multi-spectral sensing and imaging. To fill this gap, we propose BaF 2 as a low refractive index material due to its low refractive index in the LWIR spectral band, 31,32 combined with Ge for the development of narrow-linewidth FPIs in the LWIR spectral band for hyperspectral sensing and imaging applications.…”