Design and fabrication of amorphous germanium thin film-based single-material distributed Bragg reflectors operating near 2.2 μm for long wavelength applications

We report the design and fabrication of single-material distributed Bragg reflectors (DBRs) composed of amorphous germanium (a-Ge) thin films by a glancing angle deposition and their optical reflectance characteristics in the long wavelength region of 1.7-3.1 μm, together with the aid of theoretical analysis using a rigorous coupled-wave analysis method. The refractive index of the a-Ge films is estimated from the measured transmittance spectra. The high and low refractive indices a-Ge films (i.e., Δn ~ 2.18), which compose the alternative layers of DBRs, were fabricated at two incident vapor flux angles (θ_α) of 0° and 80°, respectively, by determining the quarter wavelength thicknesses of 135 nm at θ_α = 0° and 291 nm at θ_α = 80° for a center wavelength (λ_c) of ~2.2 μm. For the fabricated a-Ge/a-Ge DBR with only 5 pairs, the normalized stop bandwidth (Δλ/λ _c) of ~22.3% was measured while maintaining high reflectance values of >99% over a wide wavelength range of 2.06-2.55 μm, which indicates a reasonable consistency with the calculated reflectance results.