Kinetics and mechanisms of unimolecular gas-phase reactions of 2-methylpropane radical-ions at times as short as 20 picoseconds and at longer times
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Derrick, P. J.↗
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Superbroadening in H2O and D2O by self-focused picosecond pulses from a YAlG:Nd laser
The dielectric-breakdown intensity threshold, the critical power for self-focusing, and the power threshold for the production of spectral superbroadening have been measured in H2O and D2O. For bandwidth-limited pulses of 30 psec duration at 1.06 microns and of 21 psec duration at 0.53 micron, the superbroadening in water always required power levels sufficient for catastrophic self-focusing and intensities equal to the electric breakdown threshold.
Smith, W. L.↗
A picosecond resolution Time Digitizer for laser ranging
The Time Digitizer capable of covering a range of 0.34 sec in 9.76 psec increments is described. The time interval between a pair of start-stop pulses is digitized coarsely in 20 nsec periods by a very accurate 50 MHz reference clock. The residual fractions of a clock period at the start and the stop end of the measured interval are stretched in two interpolators and digitized in 9.76 psec increments. An equivalent digitizing frequency of 102.4 GHz is thus achieved. The digitizer is built in a minicrate and communicates via a standard crate controller. It is intended for use in the laser ranging between ground stations and the Laser Geodetic Satellite (LAGEOS). It is shown that the distribution in any two adjacent 9.76 psec channels of a small number of identical test time intervals is essentially binomial. The performance of the digitizer and test results are given.
Turko, B.↗
Picosecond pulse generation from a synchronously pumped mode-locked semiconductor laser diode
A semiconductor laser diode was mode locked in an external cavity when synchronously pumped with 90-ps current pulses. Transform-limited optical pulses with a 10-ps pulse width and a peak power of 160 mW were produced. Operating characteristics of such a system are described.
Auyeung, J. C.↗
Time transfer between the Goddard Optical Research Facility and the U.S. Naval Observatory using 100 picosecond laser pulses
A horizontal two-way time comparison link in air between the University of Maryland laser ranging and time transfer equipment at the Goddard Optical Research Facility (GORF) 1.2 m telescope and the Time Services Division of the U.S. Naval Observatory (USNO) was established. Flat mirrors of 25 cm and 30 cm diameter respectively were placed on top of the Washington Cathedral and on a water tower at the Beltsville Agricultural Research Center. Two optical corner reflectors at the USNO reflect the laser pulses back to the GORF. Light pulses of 100 ps duration and an energy of several hundred microjoules are sent at the rate of 10 pulses per second. The detection at the USNO is by means of an RCA C30902E avalanche photodiode and the timing is accomplished by an HP 5370A computing counter and an HP 1000 computer with respect to a 10 pps pulse train from the Master Clock.
Alley, C. O.↗
Picosecond kinetics of excited state decay processes in internally hydrogen-bonded polymer photostabilizers
A construction of economically practical solar energy conversion devices could be based on the employment of inexpensive, visible-radiation transparent, durable, light-fast materials. Difficulties arise in connection with the availability of such materials. Plastics which are not particularly susceptible to solar ultraviolet degradation are, in general, expensive, while less expensive optically transparent films are susceptible to deterioration when exposed to solar radiation. However, techniques are known for protecting polymers from photochemical degradation. According to these techniques, the composition or structure of polymeric material is modified by incorporation of photostabilizers. Two classes of ultraviolet absorbers are used as commercial photostabilizers. These classes include 2-hydroxybenzophenone (HB) and 2-(2-prime -hydroxyphenyl)benzotriazole (HPB). The present investigation is concerned with the results of spectroscopic and kinetics measurements related to the study of the mechanism of excited-state relaxation of polymer photostabilizers in the class HB and HPB.
Huston, A. L.↗
Direct picosecond time resolution of unimolecular reactions initiated by local mode excitation
Attention is given to the first results of direct, picosec measurements of the Delta-nu(OH) 5 local mode transition of H2O2. These time-resolved studies yield a direct measure of the unimolecular dissociation rate, and furnish a lower limit for the rate of energy redistribution from the OH stretch to the O-O reaction coordinate. The data thus determined may be used to ascertain the domain of validity for statistical unimolecular reaction rate theories.
Scherer, N. F.↗
Picosecond coherent anti-Stokes Raman scattering (CARS) study of vibrational dephasing of carbon disulfide and benzene in solution
The vibrational dephasing of the 656/cm mode (nu1, a1g) of CS2 and the 991/cm mode (nu2, a1g) of benzene have been studied as a function of concentration in mixtures with a number of solvents using a ps time-resolved CARS technique. This technique employs two tunable synchronously-pumped mode-locked dye lasers in a stimulated Raman pump, coherent anti-Stokes Raman probe time-resolved experiment. Results are obtained for CS2 in carbon tetrachloride, benzene, nitrobenzene, and ethanol and for benzene nu2 in CS2. The dephasing rates of CS2 nu1 increase on dilution with the polar solvents and decrease or remain constant on dilution with the nonpolar solvents. The CS2/benzene solutions show a contrasting behavior, with the CS2 nu1 dephasing rate being nearly independent of concentration whereas the benzene nu2 dephasing rate decreases on dilution. These results are compared to theoretical models for vibrational dephasing of polyatomic molecules in solution.
Perry, Joseph W.↗
Spectral and picosecond temporal properties of flared guide Y-coupled phase-locked laser arrays
Spatiospectral and spatiotemporal characteristics of flared waveguide Y-coupled laser arrays are studied for the cases of both CW and pulsed operation. Regular sustained self-pulsations were observed for both operation modes. It is suggested that the pulsations are due to the destabilization of phase locking which is caused by amplitude phase coupling.
Defreez, R. K.↗
Synchronous pumping of picosecond dye laser using high efficiency second harmonic generation from optical fibres
The stable operation of a mode-locked dye laser synchronously pumped by the second harmonic of an Nd:YAG laser produced in an Nd codoped germanosilicate optical fiber is reported. The optical fiber preparation technique, which results in a second harmonic conversion efficiency of 2 percent, is described. This optical fiber SHG conversion efficiency is the highest reported to date using a continuous-wave mode-locked laser.
Lawandy, N. M.↗
Picosecond sources for sub-centimeter laser ranging
Some of the tradeoffs involved in selecting a laser source for space-based laser ranging are outlined, and some of the recent developments in the laser field most relevant to space-based lasers for ranging and altimetry are surveyed. Laser pulse width and laser design are discussed. It is argued that, while doubled/tripled ND-host lasers are currently the best choice for laser ranging in two colors, they have the shortcoming that the atmospheric transmission at 355 nm is significantly poorer than it is at longer wavelengths which still have sufficient dispersion for two-color laser ranging. The life requirement appears to demand that laser diode pumping be used for space applications.
Krebs, Danny J.↗
The solid state detector technology for picosecond laser ranging
We developed an all solid state laser ranging detector technology, which makes the goal of millimeter accuracy achievable. Our design and construction philosophy is to combine the techniques of single photon ranging, ultrashort laser pulses, and fast fixed threshold discrimination while avoiding any analog signal processing within the laser ranging chain. The all solid state laser ranging detector package consists of the START detector and the STOP solid state photon counting module. Both the detectors are working in an optically triggered avalanche switching regime. The optical signal is triggering an avalanche current buildup which results in the generation of a uniform, fast risetime output pulse.
Prochazka, Ivan↗
Picosecond imaging of sprays
Preliminary results from applying a Kerr-Fourier imaging system to a water/air spray produced by a shear coaxial element are presented. The physics behind ultrafast time-gated optical techniques is discussed briefly. A typical setup of a Kerr-Fourier time gating system is presented.
Breisacher, Kevin↗
Determination of Cross-Sectional Area of Focused Picosecond Gaussian Laser Beam
Measurement of the waist diameter of a focused Gaussian-beam at the 1/e(sup 2) intensity, also referred to as spot size, is key to determining the fluence in laser processing experiments. Spot size measurements are also helpful to calculate the threshold energy and threshold fluence of a given material. This work reports an application of a conventional method, by analyzing single laser ablated spots for different laser pulse energies, to determine the cross-sectional area of a focused Gaussian-beam, which has a nominal pulse width of approx. 10 ps. Polished tungsten was used as the target material, due to its low surface roughness and low ablation threshold, to measure the beam waist diameter. From the ablative spot measurements, the ablation threshold fluence of the tungsten substrate was also calculated.
Ledesma, Rodolfo↗
Detection of Trace Silicone Contamination on Carbon Fiber Reinforced Composite (CFRP) Surfaces by Picosecond Laser Induced Breakdown Spectroscopy (LIBS)
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Rodolfo Ledesma↗
Picosecond laser generated plasma as a source of singly charged ions for external injection into an EBIS
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S. Kondrashev↗
Temperature dependence of the transient photoconductive response in stretched trans-polyacetylene films with below-gap excitation
The temperature dependence of the picosecond photocurrent with below-gap excitation (1.06 microns) has been measured for a highly oriented form of transpolyacetylene. The one-dimensional picosecond photocurrent is independent of temperature. The three-dimensional picosecond photocurrent is temperature dependent with an activation energy of 63 meV. These results demonstrate the photoproduction of nonlinear charged carriers (solitons and polarons) at energies that are below the principal interband absorption edge.
Walser, A. D.↗
Laser damage of HR, AR-coatings, monolayers and bare surfaces at 1064 nm
Laser induced damage thresholds and morphologies were investigated in a variety of uncoated and coated surfaces, including monolayers and multi-layers of different chemical compositions. Both antireflective (AR) and highly reflective (HR) were tested. Testing was done at 1064 nm with 25 picosecond and 8 nanosecond YAG/Nd laser single pulses. Spot diameter in the experiments varied from 0.09 to 0.22 mm. The laser damage measurement procedure consisted of 1-on-1 (single laser pulse in the selected site) and N-on-1 experiments including repeated irradiation by pulses of the same fluence and subsequently raised from pulse to pulse fluence until damage occurred. The highest picosecond damage thresholds of commercially available coatings averaged 12 - 14 J/sq cm, 50 percent less than thresholds obtained in bare fused silica. Some coatings and bare surfaces revealed a palpable preconditioning effect (an increase in threshold of 1.2 to 1.8 times). Picosecond and nanosecond data were compared to draw conclusions about pulse width dependence. An attempt was made to classify damage morphologies according to the type of coating, class of irradiating, and damage level.
Garnov, S. V.↗