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

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.