Pump Laser Intensity Research Articles

Nearly degenerate four-wave mixing with bichromatic laser fields in a Rb atomic system

We present an experimental study of cw nearly degenerate four-wave-mixing emission in a Doppler broadened Rb atomic system. With the fixed detuning of an intense pump laser for the Rb ${\mathit{D}}_{2}$ line, the nearly degenerate four-wave-mixing emission exhibits three peaks as a probe laser is scanned across the same Rb ${\mathit{D}}_{2}$ line. The observed three-peaked spectrum can be interpreted as the resonances in the energy structure of the dressed atomic states.

A real-time, nonintrusive trace gas detector based on laser photothermal deflection

We present the layout and technical details of a trace gas monitor based on photothermal deflection. The operating principle of this instrument, i.e., the deflection of a (weak) probe laser beam by the thermal refractive index gradient induced by trace gas absorption of an intense pump laser beam, allows nonintrusive measurements with good space and time resolution. An intra-cavity CO2 laser is used as the pump beam and a red HeNe laser as the probe. The latter runs perpendicular to the pump beam to optimize spatial resolution. To increase sensitivity, the probe laser is incorporated in a multipass setup. The instrument is demonstrated by the localization of ethylene emission sites on a cherry tomato and by monitoring ammonia production due to nitrogen fixation by cyanobacteria. Both C2H4 and NH3 can be detected at the 1–3 ppb level, at a spatial resolution of 2 mm (along the pump laser)×0.6 mm (perpendicular to it), and a response time of 0.1 s (without background correction) or 15 s (including background correction). Sensitivity can be increased at the expense of spatial resolution, and vice versa. In principle, this instrument is applicable to all those gases possessing a characteristic (‘‘fingerprint’’) spectrum in the CO2 laser range. The great advantage of the photothermal deflection technique with respect to other trace gas detection schemes lies in the nonintrusive character of the measurements. There is no need to enclose the sample in a vessel or to suck large volumes of air into the detector; measurements can be performed in open air and in real time. This should prove especially useful where sticky (polar) gases, like H2O, NH3, CH3OH, etc., are to be detected quantitatively. Main applications include air quality monitoring, especially concerning dry deposition rate measurements using the eddy correlation technique, and the study of volatile metabolite emission of biological samples.

Generation of coherent hard-x-ray radiation in crystalline solids by high-intensity femtosecond laser pulses.

A process for coherent hard-x-ray generation is suggested, which is based on the idea that high-intensity, ultrashort laser pulses can be coupled into a crystal before the lattice structure is destroyed. In the presence of the lattice periodicity, additional momentum can be transferred to the electrons that are moving in the strong light field. This additional momentum supplied by the crystal results in a substantial decrease of the threshold pump laser intensity required for hard-x-ray generation as compared to free electrons. The lattice is also utilized to select the frequency of the emitted x rays via Bragg coupling. A numerical estimation for the yield of the proposed mechanism gives an output power of some 10 W in the range of 1--10 keV of photon energies by using a state-of-the-art optical pulse with a peak intensity of about ${10}^{17}$ W/${\mathrm{cm}}^{2}$ and a pulse duration of 30 fs.

Investigation of the temperature dependent recombination processes in periodic four-narrow-asymmetric-coupled-quantum-well structures

Continuous-wave intensity-dependent photoluminescence spectra of periodic four-narrow-asymmetric-coupled-quantum-well structures were measured at different temperatures. Above 215 K, the photoluminescence depends linearly on high pump laser intensities and quadratically on low pump laser intensities. This is due to the competition between radiative recombination, and nonradiative recombination of carriers at excitation intensity saturable deep traps. Below 215 K the trapping becomes negligible and the photoluminescence is linearly proportional to all laser intensity levels; radiative recombination dominates. Based on this transitional (from quadratic to linear) behavior of the photoluminescence and our simple theory, the transition intensity, the effective nonradiative decay time of the carriers, and the photoluminescence quantum efficiency have been determined as a function of temperature.

Optimization of passively Q-switched lasers

In optimizing passively Q-switched lasers, there is a unique choice of output coupler and unsaturated absorber transmission which maximizes the laser output energy and efficiency for each three-way combination of laser gain medium, absorber medium, and pump intensity (i.e, inversion density). In the present paper, we generalize and solve the three coupled differential equations which describe the passively Q-switched laser to obtain closed form solutions for key laser parameters such as the output energy and pulsewidth. We then apply the Lagrange multiplier technique to determine the optimum mirror reflectivities and unsaturated absorber transmissions as a function of two dimensionless variables. The first variable, z, corresponds to the ratio of the logarithmic round-trip small signal gain to the round-trip dissipative (nonuseful) optical loss and is identical to that which was used in previous treatments to optimize the rapidly Q-switched laser. The second variable, /spl alpha/, is unique to the passively Q-switched laser and is equal to the saturation energy density of the amplifying medium divided by the saturation energy density of the absorber. It is largely determined by the ration of the absorber to stimulated emission cross sections, but also depends on the speed of relaxation mechanisms in the amplifying and absorbing media relative to the resonator photon decay time. Several design curves, valid for all four level amplifying and absorbing media, are then generated. These permit the design of an optimum passively Q-switched laser and an estimate of its key performance parameters to be obtained quickly with the aid of a simple hand calculator. In the limit of large /spl alpha/ (>10), the design curves are virtually indistinguishable from the rapidly Q-switched case. The curves can also be used to perform rapid tradeoff studies of different absorbing materials. The theory can also be applied to CW-pumped, repetitively Q-switched systems through a simple multiplicative factor for the laser gain. The theory is applied to the analysis of a passively Q-switched Nd:YAG laser previously reported in the literature and shown to give excellent agreement with the experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Output characteristics of a KrF-laser-pumped Fe vapor laser

The output characteristics of a KrF-laser-pumped Fe vapor laser have been investigated experimentally. A YAG laser of 1.5 J/pulse is used for producing Fe vapor and a KrF laser of about 34 mJ/pulse is employed for optically pumping the Fe vapor. The Fe laser output is measured as a function of He buffer gas pressure, delay time, vaporizing laser intensity and pumping laser intensity. A maximum laser energy of 1.08 μJ and a maximum optical conversion efficiency ([Fe laser energy]/[KrF laser energy]) of 6.5 × 10 −3(%) are obtained. The optical gains at 299.95 and 304.04 nm are 1.53 and 1.77/cm, respectively.

Coherent stimulated Raman emission from a microcavity-excited optical resonator

We report the observation of coherent emission from a planar Fabry-Perot resonator driven by a single, coherently emitting microcavity. The microcavity, either a liquid microdroplet or microcylinder, emits coherent stimulated Raman scattering when irradiated by an intense pump laser; the Raman light, in turn, couples to modes of the resonator. The spatiotemporal characteristics of resonator emission are strongly influenced by the microcavity source within; notably, a significant excess time delay in the emission of stimulated Raman scattering (i.e., compared with that of the microcavity in free space) accompanies the appearance of a sharp resonator spatial interference pattern. This and other noteworthy features are described, and a qualitative discussion of their significance is given.

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N2 and CO2 as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/I sat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and I sat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/I sat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/I sat≈1.

Saturation effects in gas-phase degenerate four-wave mixing spectroscopy: nonperturbative calculations

Saturation effects in gas-phase degenerate four-wave mixing (DFWM) are investigated for conditions of interest for diagnostic applications in flames and plasmas. In particular, the case in which Doppler and collisional broadening are comparable, as is often the case for flame species such as NO and OH, is investigated. DFWM line shapes and signal intensities are calculated nonperturbatively and compared with high-resolution laser measurements. In the nonperturbative calculations the time-dependent density-matrix equations for a two-level system interacting with three laser fields are integrated directly on a grid of spatial locations along the phase-matching axis. The electric-field amplitude for the DFWM signal is determined by multiplying the time-varying, laser-induced polarization at each grid point by the appropriate phase factor and then by summing over all grid points. The calculations are in excellent agreement with measurements of DFWM line shapes and signal intensities for NO in a buffer gas of He over a wide range of He pressure. Under saturation conditions the pressure dependence of the DFWM signal is reduced greatly compared with the unsaturated case. The signal level is still dependent on the ratio of pure dephasing to quenching collisions, even for saturation conditions. From the standpoint of minimal dependence on collisional processes it appears that operation with pump-laser intensities approximately equal to the saturation intensity is optimal. The dependence of the DFWM line shape and signal intensity on the ratio of probe to pump-laser intensity is also investigated.

Theory of strong photothermal nonlinearity from sub-surface non-stationary (?breathing?) cracks in solids

A model for the description of the strong thermal-wave nonlinearity exhibited by a non-stationary (“breathing”) crack (delamination) in a solid is proposed. The description of the nonlinear thermal-wave frequency spectrum both in the regime of the laser-induced thermal tansparency and of the laser-induced thermal darkening of the crack is presented. The conditions under which the amplitude of the thermal-wave second harmonic becomes on the order of the fundamental thermal-wave amplitude are derived. It is further demonstrated that the dependence of the photothermal response on the pump-laser intensity provides mechanical information on the crack (delamination). This type of information is different from, and additional to, that provided by the traditional measurements of the photothermal-response dependence on laser-beam intensity modulation frequency.

Energy transfer in mercury vapour following laser excitation of the 71S0 state by two-photon absorption

After excitation of the 71S0 mercury state by two photon absorption of a laser radiation at 313 nm the time integrated intensities of the 407.8 (I0) and 404.7 nm (I1) lines emitted from the 71S0 and 73S1 states were determined as functions of the pump laser intensity, I, and mercury pressure. At high pump laser intensities I0 and I1 do not follow anymore a quadratic law in I, I0 weakens to In<2 while, in contrast, I1 increases to In>2. The time evolutions of the populations of these 71S0, 73S1 states and of the 61P1, 63P0,1,2 states were determined. The set of experimental results is well reproduced by introducing collisions of two Hg(6P) atoms as well as two Hg(71S0) collisions. Effects of these collisions are discussed.

Femtosecond optical response of Y-Ba-Cu-O thin films: The dependence on optical frequency, excitation intensity, and electric current.

We have performed a series of femtosecond reflectivity experiments on various Y-Ba-Cu-O thin films at temperatures ranging from 12 to 300 K. In particular, the dependence of the optical response on probing laser frequency, pumping laser intensity, and bias electric current has been measured. Results obtained at room temperature provide quantitative information on the position of the Fermi level in films with different oxygen content. Systematic analysis of the measurements performed in the superconducting state indicates that the optical response associated with nonequilibrium properties of Y-Ba-Cu-O depends strongly on excitation intensity, sample thickness, and bias current. The results cannot be satisfactorily interpreted as the relaxation dynamics of quasiparticles, and a simple two-fluid model is shown to fail to explain data obtained under low laser excitation. Several tentative explanations are proposed, which provide a more comprehensive understanding of the transient optical response of Y-Ba-Cu-O.

Molecular vibration dynamics in molecule-surface interactions: subpicosecond transient infrared spectroscopy

Theoretical studies of molecular vibration dynamics at metal surfaces are made in connection with subpicosecond transient pump-probe spectroscopy. The rate equation is tentatively used to describe the transient behaviour of molecular vibration. The observed absorption spectra are reproduced qualitatively as a function of the pump laser intensity and also the time delay between pump and probe lasers. The rate constant for the decay process involved in the rate equation is estimated within the Markov approximation on the basis of the electronic damping theory of molecular vibration at metal surfaces.

Microsphere-based short-wavelength recombination x-ray laser.

We describe a scheme for obtaining very short wavelengths (\ensuremath{\lambda}\ensuremath{\sim}10 \AA{}) in recombination lasers. The rapid cooling rates necessary to achieve population inversion during recombination are attained by adiabatic expansion of submicrometer spheres. The lasing region is made up of many such spheres. The spheres are heated impulsively by a powerful picosecond laser. First, they ionize, then as they expand, they cool and recombine. We have calculated the optimum sphere size and initial temperature for maximum gain in the n=3 to 2 transition of hydrogenlike ions of elements with atomic numbers, Z, between 10 and 30. Gain of about 250 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is calculated in aluminum at 38.8 \AA{}. Gain rapidly decreases with Z so that gain in titanium at 13.6 \AA{} is about 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. We have calculated the required pump-laser intensity and found it to be attainable with current lasers. The propagation of the pump through the ``gas'' of spheres is considered and the problems arising from pump scattering by the spheres are discussed.

A STUDY ON "WATER WINDOW" RANGE 2p-3d POPULATION INVERSION OF He-LIKE AI IONS X-RAY LASER

The results of experimental and theoretical study on population inversion in the He-like Al ions are presented for the pumping laser intensity range of ～2×1014W|cm2. The effects of different target structure (foil, plane and half-cylinder) on the 2p-3d population inversion of the He-like Al ions (45nm) in a spot focused laser plasma have been studied experimentally. The plasma parameters such as electron temperature, electron density and the ratio of H-like ion density to He-like ion density, are diagnosed by spectroscopic methods. Results are compared with C-R model calculations.

Energy-transfer-assisted lasing from microdroplets seeded with fluorescent sol

We report observations of lasing emission from liquid microdroplets of Fluorescein 548 dye in ethanol, seeded with submicrometer-sized fluorescent sol. An incident pump laser excites fluorescein dye molecules, which in turn couple energy to sol-dye molecules, generating lasing in the sol. The pump laser can also generate fluorescein lasing in the droplets, which may excite lasing in the sol. Other noteworthy findings include the absence of sol emission for larger sol and the presence of sol emission, even without any observable fluorescein emission, for smaller sol. All emissions are at wavelengths corresponding to morphology-dependent resonances of the droplet. Studies of the dependence of these emissions on pump laser intensity and sol concentration suggest that they are driven either by Förster energy transfer between fluorescein dye molecules and dye within the sol or by enhanced radiative transfer that occurs when fluorescein emission couples to morphology-dependent resonances of the droplet microcavity.

Harmonic generation in laser-pumped FELs and stimulated backscattering from plasmas

Abstract Stimulated backscattered harmonic radiation generated by the interaction of an intense pump laser field with an electron beam or plasma is analyzed using a nonlinear, relativistic, fluid theory valid to all orders in the pump laser amplitude. The backscattered radiation occurs at odd harmonics of the Doppler shifted incident laser frequency. The growth rate and saturation level of the backscattered harmonics are calculated and thermal limitations are discussed. This mechanism may provide a practical method for producing coherent radiation in the XUV regime.

Determination of the gain coefficient of an NO laser at 218 nm

The authors have observed laser oscillation on the B'2 Delta -X2 Pi (3-10) and (3-11) of an F2 laser pumped nitric oxide laser. The maximum total output pulse energy was 280 mu J. They present here the results of an investigation to determine the maximum intra-cavity loss which still allows laser oscillation to occur in this system. This information has been used to find the effective gain coefficient of the NO laser, operating on the B'2 Delta -X2 Pi (3-10) transition at 218 nm by applying the condition for cw laser oscillation. The effective cw gain coefficient was found to be 0.25 cm-1 for pump laser intensities of 20 MW cm-2.

Frequency shifting of pulsed narrow-band laser light in a multipass Raman cell

A multipass cell is described which allows efficient stimulated Raman frequency shifting for low pump laser intensities and low gas pressures. The latter is important for Raman shifting of narrow-band Fourier-transform limited light pulses (Δ v=75 MHz). It is shown that frequency broadening of the Raman shifted light can be largely avoided in the Dicke narrowing regime at low pressures. For 75 MHz pump pulses and an H 2 density of 2.5 amagat we found a negligible broadening to 90 MHz of the stimulated Stokes light. This is far below the value of 250 MHz expected from spontaneous emission. The narrow-band Stokes pulses achieved in CO 2 enabled us to measure the pressure shift coefficient (-0.71×10 -2 cm -1/amagat) of this gas. It is demonstrated, for the example of benzene, that our technique provides a very practical light source for high resolution molecular spectroscopy.

Generation of stimulated backscattered harmonic radiation from intense-laser interactions with beams and plasmas.

A comprehensive analysis is presented that describes the generation of harmonic radiation by the stimulated backscattering of intense laser fields from electron beams and from plasmas. The dynamics of the intense laser-electron interaction are analyzed using a fully nonlinear, relativistic, cold fluid model valid to all orders in the amplitude of the pump laser. In general, the backscattered radiation, from an electron beam or stationary plasma, occurs at odd harmonics of the Doppler-shifted incident laser frequency. The strength of the harmonics is strongly dependent on the incident laser intensity. The growth rate and saturation level of the backscattered harmonics are calculated, and the limitations due to thermal, space-charge, and collisional effects are discussed. Significant radiation generation at high harmonics requires sufficiently intense pump laser fields and sufficiently cold axial electron distributions. This mechanism may provide a practical method for producing coherent radiation in the xuv regime.