C. K. Manka

Identification of explosives with two-dimensional ultraviolet resonance Raman spectroscopy.

The first two-dimensional (2D) resonance Raman spectra of TNT, RDX, HMX, and PETN are measured with an instrument that sequentially and rapidly switches between laser wavelengths, illuminating these explosives with forty wavelengths between 210 nm and 280 nm. Two-dimensional spectra reflect variations in resonance Raman scatter with illumination wavelength, adding information not available from single or few one-dimensional spectra, thereby increasing the number of variables available for use in identification, which is especially useful in environments with contaminants and interferents. We have recently shown that 2D resonance Raman spectra can identify bacteria. Thus, a single device that identifies the presence of explosives, bacteria, and other chemicals in complex backgrounds may be feasible.

Streamerless guided electric discharges triggered by femtosecond laser filaments

The time evolution of electrical discharges induced and guided between the cathode of a Van de Graf generator and a ground sphere was studied using a 100 fs Ti:Sapphire laser. Nonlinear focusing and ionization effects produce optical and plasma filaments in the discharge region. Streak camera images often exhibit streamers that propagate towards the cathode, but sudden discharge triggering is frequently observed with no streamer precursors. The typical discharge triggering delay time was measured to be 150 ns. Similar time delays were obtained from an air chemistry code used to model the long time behavior of the plasma induced by the short laser pulse. The model shows that ohmic heating of the filament plasma persists over long time scales and inhibits the decay of electron density due to recombination and attachment processes. Eventually the rise in electron temperature causes the avalanche rate to increase to the point where breakdown occurs. The hydrodynamic density reduction process reported by Tzortzakis et al. [Phys. Rev. E 64, 057401 (2001)] is also taken into consideration. Its main effect is found to be a hastening of the breakdown process.

Far-infrared spectroscopic, magnetotransport, and x-ray study of athermal annealing in neutron-transmutation-doped silicon

We present evidence that the energy introduced by a short laser pulse focused to high intensity on a small spot on the surface of neutron-transmutation-doped silicon electrically activates impurities far away from the focal spot. The activation of the impurities is measured by far-infrared spectroscopy of shallow donor levels and by magnetotransport characterization. Electrical activity is comparable to that obtained with conventional thermal annealing. X-ray rocking curve measurements show strain in the area of the focal spot, but none at large distances from the focal spot.

Production of cumulative jets by ablatively-driven implosion of hollow cones and wedges

Cumulative plasma jets formed by hollow cones imploded via laser ablation of their outer surfaces were observed. The velocity, shape, and density of the jets are measured with monochromatic 0.65keV x-ray imaging. Depending on cone geometry, cumulative jets with ion density ∼2×1020cm−3 and propagation velocities >10km∕s are formed. Similar results are observed when jets are formed by imploding wedges. Such jets can be used to simulate hydrodynamics of astrophysical jets interacting with stellar or interstellar matter.

Athermal annealing of phosphorus-ion-implanted silicon

A 1 cm2 area in phosphorus-implanted silicon samples is annealed by irradiation of a much smaller 0.002 cm2 area with a single laser pulse. Resistivity of the annealed region is uniform and similar to that measured after thermal annealing. Electrically activated donors did not diffuse into the sample and only slightly towards the sample surface. The process is 100% reproducible. We present evidence that the annealing is not caused by heat.

Tunable-multi-wavelength resonance-Raman detection of bacteria and chemicals in complex environments

We are developing tunable-multi-wavelength resonance-Raman spectroscopy and algorithms to enable rapid detection and identification of bacteria and chemicals in complex environments. The system, dubbed SWOrRD, is capable of illuminating a sample containing many chemicals or biological agents with a sequence of laser wavelengths between 210nm and 2000nm; a range which encompasses the resonant frequencies of cells, microorganisms, cellular metabolites, and many chemicals; and measures the resonance-Raman spectra of light scattered from the sample at each laser wavelength. These multiple spectra, which contain much more information about the bond structure that is contained in a single spectrum, are analyzed by a linear-mixture algorithm, based on NRLs ORASIS, to determine the chemical and bacteriological constituents of the sample. The current status of the research will be described.

Specular integrating tube for scattered-light spectroscopy.

A cylindrical sample cell is adapted to the problem of increasing the scattered-light signal from an optically thin liquid sample. The ends of the cylinder are coated with specularly reflecting aluminum to increase the signal by reflecting the stimulating light beam through the medium multiple times. The circumference of the cylinder is similarly coated to increase the fraction of the emitted light that is collected and sent into the slit of a spectrometer. Such a cell can greatly increase the signal measured by an analysis system without any modifications to the system.

Modified Šolc notch filter for deep ultraviolet applications

We present results of the design and testing of a modified optical Šolc notch filter for use in the deep ultraviolet (DUV, 190-300 nm) spectral range. The filter was designed to block a specific wavelength in this region. In addition, a sequence of blocked wavelengths occurs at wavelengths both shorter and longer than the specified wavelength. For Raman applications utilizing tunable lasers, the provision of multiple blocked wavelengths by a single filter may be especially useful. The filter design presented here produces extinction ratios >240 with transmission minima ~1 nm full width at half-maximum. Specific results are shown for the Raman spectra of Teflon excited at 248.4 nm.

Laser-generated shocks and bubbles as laboratory-scale models of underwater explosions

Underwater shocks and bubbles were generated using a high energy pulsed laser system. The advantages of this experimental approach are: (1) precisely controlled and measured experimental conditions; (2) improved diagnostics, including extensive imaging capabilities; (3) unique experiments, including a simultaneously detonated line charge; and (4) the ability to provide validation quality data for hydrodynamic simulation codes. Bubble sensitivity to variation of several experimental parameters was examined. Numerical simulations were performed corresponding to the experimental shots, showing that empirical bubble theory, experimental bubble data, and simulations were all in good agreement.

Athermal annealing of ion-implanted silicon

Phosphorus, arsenic, and boron dopants in neutron-transmutation-doped and ion-implanted silicon were electrically activated by a technique that does not involve the direct application of heat as in conventional thermal annealing or pulsed laser annealing. Dopants activated by this method exhibit much less diffusion than donors activated by standard thermal methods.