The small size, high power, promise of access to any wavelength between 3.5 and 16 microns, substantial tuning range about a chosen center wavelength, and general robustness of quantum cascade (QC) lasers provide opportunities for new approaches to ultra-sensitive chemical detection and other applications in the mid-wave infrared. PNNL is developing novel remote and sampling chemical sensing systems based on QC lasers, using QC lasers loaned by Lucent Technologies. In recent months laboratory cavity-enhanced sensing experiments have achieved absorption sensitivities of 8.5 x 10-11 cm-1 Hz-1/2, and the PNNL team has begun monostatic and bi-static frequency modulated, differential absorption lidar (FM DIAL) experiments at ranges of up to 2.5 kilometers. In related work, PNNL and UCLA are developing miniature QC laser transmitters with the multiplexed tunable wavelengths, frequency and amplitude stability, modulation characteristics, and power levels needed for chemical sensing and other applications. Current miniaturization concepts envision coupling QC oscillators, QC amplifiers, frequency references, and detectors with miniature waveguides and waveguide-based modulators, isolators, and other devices formed from chalcogenide or other types of glass. Significant progress has been made on QC laser stabilization and amplification, and on development and characterization of high-purity chalcogenide glasses, waveguide writing techniques, and waveguide metrology.
Electromagnetic (EM) radiation propagating inside a dielectric particle will develop standing waves if certain boundary conditions are satisfied. The boundary conditions depend on, among other things, the shape of the particle. The standing-wave solutions are widely referred to as morphology-dependent resonances (MDRs). If MDR conditions are satisfied then the intensity of the electric field can be calculated everywhere inside the particle. It has been found that for certain MDR modes in micron-sized spheres the EM field strength is localized close to the particle surface. Energy can be transferred into these MDRs through Gaussian laser beam scattering from the sphere, and the energy becomes trapped in a semi-bound state. The localization and increased lifetimes of MDRs allows them to be used in stimulated Raman scattering experiments. Calculations have shown that the maximum energy is transferred into the MDR when the laser is focused outside the sphere surface. This paper analyzes the internal and external radial wavefunctions involved in the scattering to determine the optimum position of the Gaussian beam.
Recent experiments done in our laboratory provided a preliminary indication that morphology- dependent stimulated Raman scattering could be used to spatially image molecular structure and composition changes in regions located close to the surface of axisymmetric particles. We have just completed a new series of theoretical and experimental studies which confirm our initial results. The results of these studies are described here.
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