Far Infrared Resonant Nanocavity Arrays Based on GaAs Surface Phonon Polaritons

Optical nanocavity arrays are constructed with deep sub-wavelength gratings of a metal-insulator-polar dielectric layered structure, which supports hybrid modes of surface plasmon-phonon polaritons as a surface waveguide. We utilized a 300 nm thick silicon dioxide (SiO2) film as a space layer on polar dielectric substrate, gallium arsenide (GaAs) which exhibits Reststrahlen band at far-infrared (33.9 – 37.1 microns, 33.4 – 36.6 meV) region. The 40 nm gold (Au) grating patterned using deep UV photolithography followed by electron beam metal deposition. The analytical solutions are validated through experimental reflectance measurements of the device with a custom-made setup using the back input channel of the Fourier Transform Infrared spectrometer. To measure the s-polarization dependence, we rotate the sample to maintain s-polarized illumination while varying the incidence polarization angle in relation to the grating direction. A strong resonance is observed due to the Fabry-Pérot resonance condition based on the FIR within the Reststrahlen band of GaAs. We studied two devices with different gold grating widths (3 μm and 3.5 μm with gap size of 2.5 μm) and demonstrated the resonance shifts to a longer wavelength with an increase in the cavity width of the devices.