All-chalcogenide middle infrared dielectric reflector and filter

We have fabricated a dielectric reflector and a passband filter, both with first order photonic bandgaps in the middle-infrared region around λ = 4 μm. The devices were made from alternating amorphous Ge25S75 and Ge15Te85 chalcogenide films with high transparency in the middle infrared region stacked in multilayers. Due to high thickness accuracy and periodicity of prepared multilayers we also observed second order photonic bandgaps at λ ~ 1.4 μm. The experimental data were in good agreement with theoretical predictions. The work focused on investigation of compositional homogeneity, surface roughness, thermal and optical properties of individual amorphous Ge25S75 and Ge15Te85 films. We confirmed chalcogenide materials as being of suitable choice for designing middle-infrared quarter wave stack devices. FT-IR reflectance spectra confirmed occurrence of 99.4% stopband near λ = 4 μm for fabricated reflector and narrow ~ 50% passband of prepared filter near λ = 3.934 μm.

Research Highlights
► Quarter wave stack (QWS) devices are one-dimensional photonic crystals usually multilayers applied as highly efficient dielectric reflectors and filters.
► Chalcogenides glasses and films exhibited high infrared transparency suitable for designing infrared QWS optical elements.
► Chalcogenide dielectric reflector (R > 99%) and passband filter (T > 50%) with first order photonic bandgaps near λ = 4 μm were prepared by stacking thermally and flash evaporated GeS and GeTe films.
► Photonic bandgaps of GeS/GeTe multilayers can be tuned at any wavelength in the 1–12 μm region simply by changing film thicknesses.