Finite size narrow-band transmission filters for real-time short wave IR spectroscopy and imaging

This contribution presents the design and simulation of finite size narrow band transmission filters for applications in realtime and remote short wave infrared (SWIR) spectroscopy and imaging. The results can ultimately be extended to hyperspectral imaging that has gained interest in many fields ranging from agriculture to surveillance. As chemical compounds possess unique set of sharp spectral signatures, they can be differentiated using wideband spectral scanning. However, such standard approaches require bulky instruments and long acquisition times together with post-processing of acquired data. These technological bottlenecks result in a diminished usefulness of such systems in scenarios demanding short response times, such as law enforcement, homeland security, and military applications. To overcome these limitations, narrow-band and pixel-size filters can be incorporated to focal plane arrays for real-time spectral imaging systems. In this study, we explored several designs of narrow-band, finite-sized (<10λ) SWIR transmission filters based on one dimensional and two dimensional guided mode resonance (GMR) gratings. The filter designs utilize dielectrics such as SiO₂ and TiO₂ and are optimized for high throughput for a narrow transmission band (~ 10 nm) within a large stop band (~ 500 nm). The electric field, fringing fields, and transmission response are investigated utilizing HFSS, CST, and Matlab software for normal incidence (dielectric losses accounted for in simulations). The fringing fields at the edges are also explored for various thicknesses of conductive reflectors fencing the filter edges. Tunability of the filters are also presented as a function of their design features, including dielectric thicknesses, grating periodicity, and grating duty cycle.