Abstract
We theoretically demonstrate a mid-infrared polarization-independent guided-mode-resonance transmission filter. We designed a structure based on a deeply-etched 2D grating above a thin slab of the same material respectively supporting transverse magnetic and transverse electric fundamental modes with identical effective index, which propagate along orthogonal directions. This device relates to multi-resonant guided-mode-resonance filters, and more particularly to the concept of zero-contrast gratings (ZCG), which can operate either as wideband reflectors [R. Magnusson, Optics Letters 39, 4337 (2014)] or bandpass filters [M. Niraula, J. W. Yoon, and R. Magnusson, Optics Letters 40, 5062 (2015)]. However, contrary to the latter, this new generation of filters is not bound by stringent material requirements inherent to conventional ZCGs. In particular, ZCGs are demonstrated with high to low refractive index ratio below 2, using germanium as high-index material over a low-index zinc sulfide substrate. These filters exhibit a transmission peak with a full-width at half-maximum of about 30 pm, and a maximum transmission close to 100 % lying in a 46-nm-wide rejection window.
Highlights
Our goal is to design polarization-insensitive transmission filters in the mid-IR range, between 3 and 4 μm, whose central wavelength could be varied for adjacent filters in a matrix
In a previous study,[11] we demonstrated 1D narrowband filters in the mid-IR based on pure transverse electric (TE) zero-contrast gratings (ZCG)
We use a deeply-etched 2D grating above a thin slab of the same material, which is designed such that the TE mode is mostly guided in the slab, while the transverse magnetic (TM) one lies within the ridges of the grating
Summary
Our goal is to design polarization-insensitive transmission filters in the mid-IR range, between 3 and 4 μm, whose central wavelength could be varied for adjacent filters in a matrix. Zerocontrast gratings (ZCGs) have proven to be interesting solutions for the design of such filters: tunability of the resonance, few layers, planar structuration approach Their principle, as described in Ref. 1, is to excite two distinct guided modes through two different grating orders in a single high-index partiallyetched layer. We use a deeply-etched 2D grating above a thin slab of the same material, which is designed such that the TE mode is mostly guided in the slab, while the TM one lies within the ridges of the grating (see Fig. 1) Because these modes are confined within separate parts of the high-index structure, their effective index can be tuned almost independently.
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