Dual angular tunability of 2D infrared notch filters: Analysis, experiments, physics

TL;DR

This paper demonstrates dual angular tunability of 2D infrared notch filters using resonant gratings, combining theoretical analysis and experiments to achieve wide spectral tuning by controlling the plane of incidence and polarization.

Contribution

It introduces a detailed physical analysis and experimental validation of dual angular tunability in 2D resonant gratings for infrared notch filters, utilizing second-order effective-medium theory.

Findings

TM mode tuning exceeds 0.5 μm with angle variation

TE mode remains at constant wavelength during TM tuning

Effective-medium theory accurately models strongly modulated lattices

Abstract

Two-dimensional (2D) resonant gratings enable dual angular tunability by controlling the plane of incidence (POI) under linear polarization. If the POI is set to be perpendicular to the electric field vector (s-polarization or transverse electric (TE) polarization), an excited TE mode provides spectral tuning. The orthogonally propagating TM mode is robust in angle. Conversely, if the POI is set for p-polarization, an ex-cited TM mode provides the tuning. Detailed explanations of the underlying physical processes are set forth by decomposing the 2D lattice into equivalent 1D gratings using second-order effective-medium theory (EMT). This is shown to work extremely well even for a strongly modulated lattice with refractive-index contrast of 3. With proper design and corresponding experiments, a widely tunable notch filter covering longwave infrared bands is demonstrated. Experimentally varying the incidence angle up to 16 degree, a notch channel in TM polarization tunes across a band exceeding 0.5 um with the TE channel remaining at a constant wavelength. The interesting appearance of a resonance channel originating in diagonally propagating leaky modes is briefly examined. The analysis and experiments presented will be useful for realizing diverse 2D tunable filters while furnishing methodology for detailed understanding of the attendant near fields and mode structure.