Development of polarizing spectral bandpass filter using dual subwavelength metallic gratings

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Montana State University - Bozeman, College of Engineering


Recent work in climate science has shown the need for a polarizing spectral bandpass filter in order to passively and remotely determine cloud water phase. Presented here is a novel design of such a filter based on two subwavelength gold gratings separated by a dielectric surface relief grating. It is shown that a Fabry-Perot resonance occurs in the cavity between the gold gratings while surface plasmon polariton resonances occur at the two metal/dielectric interfaces. It is further shown that these two resonance effects couple together to create a spectral bandpass that is high and narrow enough for the remote sensing application. These resonance effects can be changed in shape and position by adjusting the width, height, period, and separation between the gold gratings. It is further observed that the surface plasmon polariton resonances also have the effect of suppressing light transmission at certain wavelengths, allowing the spectral bandpass shape to be tuned, reducing out-of-band transmission. The polarizing effect of these gratings results from the rejection of light polarized along the grating rulings, a phenomenon that is well documented to occur in subwavelength wire grids. Experimental data on prototype gratings show good agreement with predicted performance calculated using numerical rigorous-coupled wave analysis once we account for uncertainties in the material properties and device geometry due to fabrication and processing variabilities. This numerical method is used in conjunction with analytical approximations like zero-order effective medium theory to develop a design process that can be extended to any wavelength in the shortwave infrared region. All devices undergo a final global parameter optimization procedure to account for any subtle near-field effects. Finally, I present designs of three devices optimized for operation at wavelengths of 1550 nm, 1640 nm, and 1700 nm. These devices share characteristics that make them able to be simultaneously fabricated on the same substrate, a crucial step if they are to be built into an array. The final devices all have a peak transmission of greater than 80%, and spectral widths of less than 40 nm.




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