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ABSTRACT: Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.
Applied Optics 09/2011; 50(26):5158-62. · 1.41 Impact Factor
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ABSTRACT: A microwave-scattering-based resonance-enhanced multi-photon ionization technique is used to detect molecular species such as NO, CO, Xe, and Ar in pure form, and for standoff detection of trace species in atmospheric pressure air. In this paper,the spectra, dynamics, and the detection limits of trace species in air are studied. We demonstrate 10 m scale standoff detection of NO, and show that the system has a linear response down to the parts in 10(9) NO levels in ambient air.
Applied Optics 02/2011; 50(4):A68-73. · 1.41 Impact Factor
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ABSTRACT: In an earlier publication we demonstrated that by using pairs of pulses of different colors (e.g., red and blue) it is possible to excite a dilute ensemble of molecules such that lasing and/or gain-swept superradiance is realized in a direction toward the observer. This approach is a conceptual step toward spectroscopic probing at a distance, also known as standoff spectroscopy. In the present paper, we propose a related but simpler approach on the basis of the backward-directed lasing in optically excited dominant constituents of plain air, N(2) and O(2). This technique relies on the remote generation of a weakly ionized plasma channel through filamentation of an ultraintense femtosecond laser pulse. Subsequent application of an energetic nanosecond pulse or series of pulses boosts the plasma density in the seed channel via avalanche ionization. Depending on the spectral and temporal content of the driving pulses, a transient population inversion is established in either nitrogen- or oxygen-ionized molecules, thus enabling a transient gain for an optical field propagating toward the observer. This technique results in the generation of a strong, coherent, counterpropagating optical probe pulse. Such a probe, combined with a wavelength-tunable laser signal(s) propagating in the forward direction, provides a tool for various remote-sensing applications. The proposed technique can be enhanced by combining it with the gain-swept excitation approach as well as with beam shaping and adaptive optics techniques.
Proceedings of the National Academy of Sciences 02/2011; 108(8):3130-4. · 9.68 Impact Factor
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ABSTRACT: The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.
Science 01/2011; 331(6016):442-5. · 31.20 Impact Factor
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ABSTRACT: The need for molecular standoff detection has motivated the development of a remotely pumped, high gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. High gain is achieved in the near infrared following pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved using prior laser spark dissociation. Comment: 4 pages, 4 figures
11/2010;
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ABSTRACT: Resonant enhanced multiphoton ionization (REMPI) and electron avalanche ionization (EAI) are measured simultaneously in Ar:Xe mixtures at different partial pressures of mixture components. A simple theory for combined REMPI + EAI in gas mixture is developed. It is shown that the REMPI electrons seed the avalanche process, and thus the avalanche process amplifies the REMPI signal. Possible applications are discussed.
Journal of Applied Physics 08/2008; · 2.17 Impact Factor
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ABSTRACT: We present a detailed model for the evolution of resonance enhanced multiphoton ionization (REMPI) produced plasma during and after the ionizing laser pulse in inert gas (argon, as an example) at arbitrary pressures. Our theory includes the complete process of the REMPI plasma generation and losses, together with the changing gas thermodynamic parameters. The model shows that the plasma expansion follows a classical ambipolar diffusion and that gas heating results in a weak shock or acoustic wave. The gas becomes involved in the motion not only from the pressure gradient due to the heating, but also from the momentum transfer from the charged particles to gas atoms. The time dependence of the total number of electrons computed in theory matches closely with the results of coherent microwave scattering experiments.
Journal of Applied Physics 12/2007; 102(12):123103-123103-7. · 2.17 Impact Factor
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ABSTRACT: Circular arrays of plasma filaments induced by femtosecond laser pulses in atmospheric air are shown to support guided modes of electromagnetic radiation in the centimeter and millimeter wavelength range. With the refractive index of laser-induced filaments being lower than the refractive index of nonionized air, arrays of such filaments can serve as a structured waveguide cladding, providing an index guiding of radar signals in a nonionized gas region. In spite of attenuation of radar radiation induced by plasma absorption, filament-array waveguides are shown to enhance radar signal transmission relative to freely propagating radar beams.
Applied Optics 09/2007; 46(23):5593-7. · 1.41 Impact Factor
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ABSTRACT: Multiphoton ionization and electron recombination processes are studied in argon using coherent microwave Rayleigh scattering from a localized, resonance-enhanced multiphoton ionization produced plasma. A time dependent one-dimensional plasma dynamic model is developed to predict the time evolution of the microwave scattering from the plasma. Experimental results of the argon ionization spectrum and electron recombination rates are in good agreement with the model predictions.
Physical Review Letters 07/2007; 98(26):265005. · 7.37 Impact Factor
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ABSTRACT: Coherent Rayleigh-Brillouin scattering in gases has been studied experimentally for the first time in the kinetic regime and shown to give line shapes that differ significantly from the spontaneous Rayleigh-Brillouin scattering. A kinetic model was developed to obtain an analytic solution of the line shape for monatomic gases, and good agreement with the experimental data was achieved.
Physical Review Letters 11/2002; 89(18):183001. · 7.37 Impact Factor
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ABSTRACT: We report the first gas temperature measurements in plasmas to our knowledge obtained by filtered Rayleigh scattering (FRS). A narrow-linewidth Ti:sapphire laser is used as the illumination source, and a mercury filter provides strong suppression of elastic background. We perform measurements in weakly ionized glow discharges in pure argon and in an argon-plus-1%-nitrogen mixture. Where possible, we verify the FRS technique by comparing filtered measurements with unfiltered measurements. We present point measurements of axial temperature with uncertainties of less than 5%. We use a planar scheme to obtain radial temperature profiles with uncertainties of 10%.
Applied Optics 07/2002; 41(18):3753-62. · 1.41 Impact Factor
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ABSTRACT: We demonstrate, for the first time to our knowledge, the utility of coherent Rayleigh scattering (CRS) for temperature measurements in low-density gases and weakly ionized plasmas by measuring the translational temperature of neutral argon in a glow discharge. By analysis of the near-Gaussian spectral profile of the CRS signal, we determine temperatures with an uncertainty of </=3%. We also investigate the intensity range over which this simple Gaussian analysis can be used for temperature measurements and discuss its potential for gas diagnostics.
Optics Letters 02/2002; 27(3):161-3. · 3.40 Impact Factor
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ABSTRACT: We report single-pulsed, spatially-resolved temperature measurements in unseeded low-speed and supersonic air using the acoustic decay of laser-created thermal gratings. A low energy 193 nm excitation scheme was employed to create thermal gratings by predissociation of O2 in the flow. The acoustic decay was monitored over a few hundred ns by a continuous-wave argon-ion laser. This technique is insensitive to laser energy fluctuations as it is a frequency rather than an intensity measurement. Single-pulse measurements in Mach 3.9 and Mach 2.0 nozzle flows were analyzed to a temperature uncertainty of better than 7% by use of power spectra of the time domain signals. Higher accuracy is expected with improved calibration measurements.
Optics Communications.
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ABSTRACT: This work presents a simplified model of microwave scattering during the avalanche ionization stage of laser breakdown and corresponding experimental results of microwave scattering from laser breakdown in room air. The model assumes and measurements confirm that the breakdown regime can be viewed as a point dipole scatterer of the microwave radiation and thus directly related to the time evolving number of electrons. The delay between the laser pulse and the rise of the microwave scattering signal is a direct measure of the avalanche ionization process. © 2006 American Institute of Physics.
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ABSTRACT: Coherent Rayleigh scattering (CRS) is studied in the high intensity regime, where the optical lattice potential ap-proaches the thermal energy of the gas particles. We describe this optical-kinetic process using a 1-D Boltzmann equa-tion with an arbitrary strong optical force and show that in this regime a line shape narrowing phenomenon is predicted and that the signal intensity saturates when complete trapping of particles is reached. We discuss the impact of these processes on the application of coherent Rayleigh scattering as gas phase diagnostic tool.
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ABSTRACT: Phase-matched four-wave mixing is demonstrated for millijoule nanosecond pulses guided by photonic band-gaps of hollow fibers with a two-dimensionally periodic cladding and a core diameter of 50 m. Raman reso-nances related to the stretching vibrations of water molecules inside the hollow fiber core are detected in the spectrum of the four-wave mixing signal, suggesting phase-matched coherent anti-Stokes Raman scattering in hollow photonic-crystal fibers as a convenient sensing technique for condensed-phase species adsorbed on the inner fiber walls and trace gas detection. © 2005 Optical Society of America OCIS codes: 190.4370, 320.7140.
