Mid-infrared photonic crystals for gas sensing

Infrared spectroscopy is the key technology used for gas sensing since it facilitates the determination of the concentration of a gas species with high precision, undisturbed by other gases in the environ- ment. In particular, the mid-infrared region of the electromagnetic spectrum between 3 and 8 ➭m hosts many absorption lines of gases relevant for chemical and biological sensing. However, infrared spectrometers often require large interaction volumes, typically 0.3-10 L, to achieve the necessary interaction length for measurable gas absorption, which prevents their miniaturization and on-chip integration. In this thesis a novel gas spectrometer is developed that uses a mid-infrared photonic crystal to enhance the interaction between light and gas molecules and allows for a reduced sensor volume of only ∼10−6 L. In particular, it uses the effect of slow light, i.e. a reduced Propagation group velocity, to enhance the absorption of light by gas molecules in the photonic crystal. For this purpose a 2D mid-infrared photonic crystal consisting of high aspect ratio microtube arrays was developed. This structure was designed and characterized via optical simulation methods, and subsequently fabricated by a process that combines photo-assisted etching of macropouros silicon with chemical vapor deposition, and other wet and dry etching techniques. Due to their high aspect ratios of up to 1:60, these structures are perceived to be semi-infinite in the z-direction normal to the lattice plane. The high geometrical uniformity and mechanical robustness of the microtubes was investigated by optical, electron and atomic force microscopy, as well as mechanical shock tests and FEM simulations. Optical transmission experiments reveal strong transmittance of mid-infrared light through the photonic crystals far above the light line at frequencies that do not support guided modes in conventional 2D photonic crystals. Attenuations as low as 0.27 dB/100 ➭m were recorded, surpassing the limitations for light guiding above the light line in conventional 2D pho- tonic crystals that rely on total internal reflection for confinement in z-direction. Fair qualitative agreement is obtained between experimentally measured transmission spectra, 2D band structure, and 2D FDTD simulations, whereby the dependence of the transmission spectra on the photonic crystal geometry was reproduced in the simulations. Gas absorption in the photonic crystals was experimentally investigated at a propene absorption band between 5300 and 5500 nm. Enhanced gas absorption by factors between 4 and 10 due to low group velocities of 0.08-0.10c in the photonic crystals was found. As further concept for the microtube technology, tunable and reconfigurable photonic crystals were developed by incorporating vanadium dioxide into the microtube core. Here, shifts of the photonic band structure by 70-100 nm due to a thermally induced phase transition of the vanadium dioxide were recorded.