Three-dimensional characterisation of a two-dimensional photonic bandgap reflector at midinfrared wavelengths

A combined experimental and theoretical study is presented of the reflection properties of a two-dimensional photonic crystal in a three-dimensional optical environment. The crystal is a triangular lattice of cylindrical holes in bulk silicon. The reflection spectra are measured over a wide range of midinfrared wavelengths by using a Fourier-transform spectrometer with a convergent incident beam. Very high reflection coefficients are demonstrated for the first-order forbidden bands, reaching 98%, for the first band (λ≈6-8 μm) in TE polarisation following Γ-M direction. This result is of great promise for future applications of photonic bandgap reflectors in the midinfrared. From the comparison between the results of characterisation and those of numerical simulations, the contributions of the different effects that degrade the reflector performances are separated. The incident-beam divergence is shown to modify the shapes, widths and positions of the higher-order forbidden bands. Diffraction losses at the interface are found to be strongly dependent on the crystal termination and orientation, and can reach 60%, but only for the smallest wavelengths investigated. In turn, the fabrication inhomogeneities such as the small roughness of the interface or the hole-radius dispersion are shown to be the prime cause of degradation as long as diffraction effects remain weak