UC San Diego

Multiphoton laser wave-mixing spectroscopy is presented as a sensitive and specific optical detection technique for liquid and gas phase analytes. Compared to other detection methods, wave-mixing offers numerous advantages including small laser probes, high spatial resolution, excellent sensitivity, and specificity that allows for the identification of analytes. The signal generated is a coherent laser-like beam, thus it can be spatially and optically modulated to enhance the signal-to-noise ratio of the system. The laser wave-mixing signal exhibits a quadratic dependence on analyte absorptivity, thus small changes in concentration result in large changes in signal intensity. Furthermore, laser wave-mixing has a cubic dependence on laser power, allowing for the use of low power laser systems. Laser wave-mixing is shown to be a sensitive detection technique that is adapted for detection beyond normal lab scale distances. The simple two-beam forward scattering arrangement is used to detect molecular bromine gas in a sealed 10 cm gas cell. Bromine is detected at distances from 0.25 to 6 meters while maintaining sensitivity in the parts per billion range. This proof of concept work has applications in environmental and medical applications that require real time detection of gaseous analytes. The detection of explosives in the visible range is an important technique for forensic, defense, and security applications. This work shows that laser wave-mixing can be applied to the colorimetric detection of both trinitrotoluene and triacetone triperoxide. Trinitrotoluene is detected with parts per trillion sensitivity using a 514 nm Argon ion laser and triacetone triperoxide is detected in the parts per billion range using a 405 nm solid state diode laser system. Laser wave-mixing is then adapted to use mid- infrared quantum cascade lasers for the detection of explosives and explosives precursors. Quantum cascade lasers are powerful, compact, lasers that feature wavelength and power tunability. This laser system offers sensitivity and specificity in detection of dinitrotoluene, triacetone triperoxide, and acetone as native species in the gas and liquid phase. The wavelength tunability of quantum cascade lasers allows for the identification of these molecules in the fingerprint mid-IR region with parts per billion detection sensitivities