Narrow Bandwidth Metal-Insulator-Metal-Insulator-Metal Filters for Hyperspectral Imaging Applications

Dagny Fleischman Materials Science, California Institute of Technology, Pasadena, California, USA Luke Sweatlock Materials Science, California Institute of Technology, Pasadena, California, USA N/A, Northrop Grumman, Redondo Beach, California, USA Hirotaka Murakami Materials Science, California Institute of Technology, Pasadena, California, USA N/A, Sony Corporation, Tokyo, Japan Sozo Yokogawa Materials Science, California Institute of Technology, Pasadena, California, USA N/A, Sony Corporation, Tokyo, Japan Harry A. Atwater Materials Science, California Institute of Technology, Pasadena, California, USA

Plasmonic structures have shown promise for applications as color filters in the visible and near IR parts of the spectrum. Due to their subwavelength mode volumes, plasmonic filters are well matched to the small size of state-of-the-art active pixels for dense integration in CMOS image sensor arrays. However, typical plasmonic filters possess a tradeoff between relatively broad transmission bandwidths and multiple resonances. If it were possible to dramatically reduce the FWHM of the transmission spectra while retaining a single peak, CMOS hyperspectral imaging arrays could be realized. We have designed and optimized metal-insulator-metal-insulator-metal (MIMIM) structures that possess a single transmission peak with FWHM as small as 17 nm. Using finite difference time domain calculations and a boundary value solution method for multilayer plasmonic structures, we have calculated MIMIM dispersion relations to identify the nanophotonic parameters responsible for narrowband filter transmittance.

The transmission element of MIMIM filters is an array of parallel sub-wavelength slits perforating the MIMIM stack, which acts as a narrowband transmission resonator. The in-plane periodicity of the slit array dictates the filter transmission wavelength, so the same MIMIM architecture can be densely integrated in adjacent filter elements for multiple wavelength intervals by simply adjusting the spacing between the slits. Additionally, the MIMIM structure itself is highly tunable. Along with changing the periodicity of the slits in the filter, the active plasmon mode can be manipulated by controlling the thickness of the insulating layers, by changing the indices of the constituent materials or by adjusting the thickness of the middle metal layer that couples together the two individual MIM structures of the device.

We have developed an amorphous silicon facilitated lift-off process to fabricate the MIMIM filter structures, because transmission behavior is highly dependent on the straightness of the sidewalls of the slits that perforate the stacked films. The results of the transmission measurements of these filters and prospective ways for integration with CMOS image sensors will also be discussed.

dagny.fleischman@caltech.edu