Bret D. Cannon

Frequency stabilization of quantum-cascade lasers by use of optical cavities

We report a heterodyne beat with a linewidth of 5.6+/-0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 microm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop.

External Cavity Quantum Cascade Laser for Quartz Tuning Fork Photoacoustic Spectroscopy of Broad Absorption Features

We demonstrate mid-infrared spectroscopy of large molecules with broad absorption features using a tunable external cavity quantum cascade laser. Absorption spectra for two different Freons are measured over the range 1130-1185 cm(-1) with 0.2 cm(-1) resolution via laser photoacoustic spectroscopy with quartz tuning forks as acoustic transducers. The measured spectra are in excellent agreement with published reference absorption spectra.

Computer simulation of electron thermalization in CsI and CsI(Tl)

A Monte Carlo (MC) model was developed and implemented to simulate the thermalization of electrons in inorganic scintillator materials. The model incorporates electron scattering with both longitudinal optical and acoustic phonons. In this paper, the MC model was applied to simulate electron thermalization in CsI, both pure and doped with a range of thallium concentrations. The inclusion of internal electric fields was shown to increase the fraction of recombined electron-hole pairs and to broaden the thermalization distance and thermalization time distributions. The MC simulations indicate that electron thermalization, following γ-ray excitation, takes place within approximately 10 ps in CsI and that electrons can travel distances up to several hundreds of nanometers. Electron thermalization was studied for a range of incident γ-ray energies using electron-hole pair spatial distributions generated by the MC code NWEGRIM (NorthWest Electron and Gamma Ray Interaction in Matter). These simulations revealed ...

Computer simulation of the light yield nonlinearity of inorganic scintillators

To probe the nature of the physical processes responsible for the nonlinear scintillation light yield of inorganic scintillators, we have combined an ab initio based Monte Carlo code for calculating the microscopic spatial distributions of electron-hole pairs with an atomistic kinetic Monte Carlo (KMC) model of energy-transfer processes. In the present study, we focus on evaluating the contribution of an annihilation mechanism between self-trapped excitons (STE) to the scintillation response of pure CsI and Ce-doped LaBr3. A KMC model of scintillation mechanisms in pure CsI was developed previously and we introduce in this publication a similar model for Ce-doped LaBr3. We show that the KMC scintillation model is able to reproduce both the kinetics and efficiency of the scintillation process in Ce-doped LaBr3. Relative light output curves were generated at several temperatures for both scintillators from simulations carried out at incident γ-ray energies of 2, 5, 10, 20, 100, and 400 keV. These simulation...

Shock Wave Mediated Plume Chemistry for Molecular Formation in Laser Ablation Plasmas.

Although it is relatively straightforward to measure the ionic, atomic, molecular, and particle emission features from laser ablation plumes, the associated kinetic and thermodynamic development leading to molecular and nanocluster formation remain one of the most important topics of analytical chemistry and material science. Very little is known, for instance, about the evolutionary paths of molecular and nanocluster formation and its relation to laser plume hydrodynamics. This is, to a large extent; due to the complexity of numerous physical processes that coexist in a transient laser-plasma system. Here, we report the formation mechanisms of molecules during complex interactions of a laser-produced plasma plume expanding from a high purity aluminum metal target into ambient air. It is found that the plume hydrodynamics plays a great role in redefining the plasma thermodynamics and molecular formation. Early in the plasma expansion, the generated shock wave at the plume edge acts as a barrier for the combustion process and molecular formation is prevalent after the shock wave collapse. The temporally and spatially resolved contour mapping of atoms and molecules in laser ablation plumes highlight the formation routes and persistence of species in the plasma and their relation to plume hydrodynamics.

Kinetic Monte Carlo Model of Scintillation Mechanisms in CsI and CsI(Tl)

We have developed a computational model of energy transfer processes in scintillators using the kinetic Monte Carlo (KMC) approach. In this publication, we focus on the alkali halide compound CsI both pure and doped with a range of thallium concentrations. The KMC model makes use of an explicit atomistic representation of the crystal lattice, activator sites, defect sites, and individual electron-hole pairs. The probability of individual diffusion, recombination, and scintillation events is calculated from rate equations parameterized with data published in the literature. Scintillation decay curves, relative intensities of emission peaks, and light yields are computed and found to be in good agreement with experimental data for a range of temperatures and thallium concentrations. This demonstrates that the KMC scintillation model is capable of reproducing both the kinetics and the efficiency of the scintillation process in CsI. In addition, novel predictions emerge from our simulations such as the diffusion distance distributions of self-trapped holes and excitons. Finally, the KMC scintillation model provides a framework for probing possible physical processes responsible for the nonlinear relationship between scintillation light yield and incident gamma-ray energy.

Laser-enhanced electron-impact ionization spectroscopy.

Sequential photon-excitation, electron-impact ionization with subsequent mass analysis has been applied to a barium atomic beam. High-resolution, Doppler-free laser excitation produces the 6s6p(1)P(1) excited state, which is then ionized by electron bombardment. The excited state is selectively ionized when bombardment energies are between the excited- and ground-state ionization thresholds. Mass discrimination has permitted the recording of individual optical spectra for all the stable isotopes, including the 0.1% abundant (130)Ba and (132)Ba, in a sample with natural isotopic abundances.

Design and performance of a sensor system for detection of multiple chemicals using an external cavity quantum cascade laser

We describe the performance of a sensor system designed for simultaneous detection of multiple chemicals with both broad and narrow absorption features. The sensor system consists of a broadly tunable external cavity quantum cascade laser (ECQCL), multi-pass Herriott cell, and custom low-noise electronics. The ECQCL features a fast wavelength tuning rate of 2265 cm-1/s (15660 nm/s) over the range of 1150-1270 cm-1 (7.87-8.70 μm), which permits detection of molecules with broad absorption features and dynamic concentrations, while the 0.2 cm-1 spectral resolution of the ECQCL system allows measurement of small molecules with atmospherically broadened absorption lines. High-speed amplitude modulation and low-noise electronics are used to improve the ECQCL performance for direct absorption measurements. We demonstrate simultaneous detection of Freon-134a (1,1,1,2-tetrafluoroethane), ammonia (NH3), and nitrous oxide (N2O) at low-ppb concentrations in field measurements of atmospheric chemical releases from a point source.

A new laser concept for isotopically selective analysis of noble gases

A new laser approach for the isotopically selective analysis of noble gases is presented. This approach uses noble gas atoms prepared in the 1s5 metastable state. Hyperfine levels in the 1s5 and 2p9 states form two-level systems in Ne, Ar, Kr, and Xe which can be excited by a commercially available single-frequency laser system. Absorption of photons from such a laser, and the resulting momentum transfer, can be used to selectively deflect the desired isotope from a supersonic atomic beam into a detection area. Light from the same laser can then be used to selectively count atoms of the desired isotope using the photon-burst technique. Thus, enrichment and selective detection are accomplished with a single laser in a single pass through the apparatus.The problem of analyzing for85Kr in a sample of noble gases extracted from the air is examined in detail. This is a stringent test of the selectivity of this approach because85Kr has the same nuclear spin, and thus similar hyperfine splittings, as naturally occurring83Kr. Calculations indicate that isotopic selectivity of the new approach is easily adequate to resolve85Kr in a 1010 excess of83Kr.

Emission and Propagation Properties of Midinfrared Quantum Cascade Lasers

We report divergence, astigmatism, and beam propagation factor (M2) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M2 yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M2 reported for midinfrared QCLs.