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Polarization-independent, narrowband, near-IR spectral filters via guided mode resonances in ultra-thin a-Si nanopillar arrays Ryan Cecil Ng, Juan C. Garcia, Julia R Greer, and Katherine T. Fountaine ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019
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ACS Photonics
Polarization-independent, narrowband, near-IR spectral filters via guided mode resonances in ultrathin a-Si nanopillar arrays Ryan C. Ng,†,* Juan C. Garcia,‡ Julia R. Greer,§ & Katherine T. Fountaine‡
†Division
of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, United States
‡NG
Next, Northrop Grumman Corporation, One Space Park, Redondo Beach, CA 90278, United States
§Division
of Engineering and Applied Sciences, California Institute of Technology, Pasadena, CA 91125, United States
Abstract
We report the optical properties obtained through experiments, simulation, and theory, of ultra-thin (1 m consisting of many layers that require a complicated and expensive fabrication process.13 To compare the performance of filters that operate in different wavelength regimes to one another, we quantify the filter spectral resolution, i.e. bandwidth, using / which is the full width at half maximum (FWHM) divided by the peak wavelength, as
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bandwidths tend to be narrower at shorter wavelengths. Interference filters currently represent the commercial standard, which can achieve narrow near unity pass/stop bands with / on the order of 10-2 enabled by their large footprint, thickness, and fabrication complexity. Many plasmonic-based designs, such as subwavelength nanorod/hole arrays14–16 or metal-insulator-metal (MIM) arrays,17,18 have been explored for color and other spectral filtering because they have the potential to be small footprint and can exhibit an optical response in the visible and IR ranges, but these plasmonicbased designs are known to exhibit significant loss due to the materials used which leads to low filtering efficiencies.8,9,19 In one plasmonic demonstration, Xu et al. created MIM stack arrays consisting of 100 nm thick ZnSe sandwiched between two 40 nm thick Al layers, which allowed them to obtain / as low as ~ 10-1 and a transmission of ~4060% with a filter area dimension of 10 m x 10 m.17 With the use of nanophotonic design principles, a dielectric-based design offers the same potential for small pixel sizes as metal-based plasmonic designs, while minimizing material losses due to insignificant absorption. Appropriately-designed dielectric subwavelength 1D gratings and 2D pillar arrays exhibit narrow-band near-unity spectral variations at normal light
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incidence, which make them particularly useful in filtering applications.20–24 Niraula et al. reported a near-unity band in a dielectric design, with of 3.7 x 10-4 centered at 1304 nm with a 272 nm thick patterned periodic c-Si layer on a 248 nm thick slab layer of the same material.23 These high-performance filter designs usually require thicknesses on the order of 1 m. In dielectric subwavelength arrays, only specular reflection and transmission are observed
outside
the
grating
without
any
higher
order
diffraction
or
reflection/transmission efficiencies, which gives rise to very narrow, near-unity bandwidths. These particular spectral features are passively-tunable via geometric variation of the array over a large spectral range, which is advantageous for the simultaneous production of many filters which cumulatively form a much larger contiguous spectral band desired for hyperspectral imaging. A number of theoretical explanations for these spectral features have been discussed in literature, including the interference of axially propagating waveguide modes,25–27 interference of periodic scatterers,28 and guided mode resonances.29–31 The first approach approximates the resonance to be created by two interfering axially propagating waveguide modes, and the narrowband resonances arise from their constructive and destructive interference.25–
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Magnusson et al. demonstrated that this approximation breaks down for gratings with
an antireflection layer deposited at the transmission interface or for systems with very thin structures that exhibit resonances beyond the cut-off point of the waveguides.32 This resonance effect has also been described by treating the observed resonance with analytical Mie theory as the result of interfering electric and magnetic dipole modes in an infinite array of weakly-coupled dielectric scatterers.28 In the lattermost formalism, periodic design enables an incident light wave to couple into a guided mode within the array via a grating vector, which also allows re-radiation into the cover or the substrate. This phenomenon is most commonly referred to as a guided mode resonance (GMR).29– 31
Sturmberg, et al. attempted to reconcile some of these different formalisms by
developing an analytical formulation of these spectral features as a Fano resonance, indicated by an asymmetric spectral line-shape, in a 1D slab array that arises from the interference between an axially propagating Fabry-Perot mode and an in-plane propagating grating vector-coupled waveguide mode.33 However, this theory is only valid for arrays with a fill fraction near unity as it approximates the array as a homogeneous slab with an effective index and loses accuracy the further the fill fraction
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of high-index material decreases. The GMR formalism is universally applicable in describing the observed near-unity narrowband reflection/transmission peaks of subwavelength periodic structures without requirements such as high fill fraction or sufficiently thick waveguide layers to be valid and, thus, it is the formalism we use for this study.
We report the optical properties of ultra-thin (
