Quantum cascade lasers for mid-infrared chemical sensing.

摘要

The mid-infrared (MIR) spectral range (2-20 mum) is particularly useful for chemical sensing due to the excitation of fundamental rotational and vibrational modes. In the 'fingerprint region' (10-20 mum), most organic analytes have unique absorption patterns; absorption measurements in this region provide molecule-specific information with high sensitivity.; Quantum cascade lasers (QCLs) present an ideal light source for (MIR) chemical sensing due to their narrow linewidth, high spectral density, compact size, and ease of fabrication of nearly any MIR wavelength. As the emission wavelength is dependent on layer size within the heterostructure rather than material composition, various wavelengths in the MIR can be achieved through bandstructure engineering.; High sensitivity measurements have been achieved in both gas and liquid phase by developing integrated sensing systems. The laser emission frequency is selected to match a strong absorption feature for the analyte of interest where no other interfering bands are located. A waveguide is then developed to fit the application and wavelength used.; Gas sensing applications incorporate silica hollow waveguides (HWG) and an OmniGuide fiber which consists of a one dimensional photonic crystal wrapped into a cylindrical shape to create a photonic bandgap HWG. Analyte gas is injected into the hollow core allowing the HWG or OmniGuide to serve simultaneously as a waveguide and miniaturized gas cell. Sensitivities of parts per billion (ppb) are achieved with a response time of 8 s and a sample volume of approximately 1 mL.; Liquid sensing is achieved via evanescent wave measurements with novel planar waveguides of silver halide (AgX) and gallium arsenide (GaAs). GaAs waveguides developed in this work are grown via molecular beam epitaxy on a GaAs wafer substrate and have a thickness on the order of the wavelength of light to achieve a single-mode waveguide, providing a significant improvement in evanescent field strength over conventional multimode fibers or attenuated total reflection elements used in infrared spectroscopy. Liquid samples of muL volume at the waveguide surfaces are detected and waveguide mode calculations indicate high evanescent intensity at the waveguide surface, which leads to monolayer sensing applications.; QCLs have begun to be utilized as a light source in the MIR regime over the last decade. The next step in this field is the establishment of useful chemical and biological sensing applications, along with compact and highly integrated device platforms which take full advantage of this technology. The demonstrations of both gas and liquid phase MIR chemical sensing in this work advance the field towards finding key applications in medical, biological, environmental, and atmospheric measurements.