Radiation Scattering under Laser Irradiation of a Target Blown by a Two-Phase Stream

We have investigated the spectral characteristics of scattered radiation ( = 0.69 m and = 1.06 m) under probing laser action (q = 10³ W/cm²) on a textolite target of streamline shape blown by a cold two-phase stream (with talc and black particles). By the measured transverse scattering indicatrices the dependences of the scattered radiation intensity on the probing angles and the sort of particles and their number density in the stream have been established. The relative contributions of particles of the stream and the target surface to the total scattering by the target–two-phase-stream system have been determined.     

Second-harmonic conversion of neodymium glass laser radiation with controlled spatial coherence

The experimental results on the second optical harmonic conversion of neodymium glass laser radiation with controlled spatial coherence in a nonlinear KDP crystal are presented. The conversion efficiency is studied depending on the radiation power density on the crystal and the crystal rotation angle in the principal optical plane with respect to the radiation propagation direction. The experiments were performed in the radiation power density range I = 0.2–3 GW/cm² for the number of cavity transverse modes N = 100–1000, radiation divergence 2ч = 0.5–3.5 mrad, spectrum width of 26 Å, and pulse duration of 2.5 ns.     

Laser driven shock wave studies in gold coated Plexiglas targets

Abstract

Laser-driven shock wave propagation in a transparent material such as Plexiglas coated with a thin overlayer of gold is studied using the technique of high speed optical shadowgraphy. A Nd: glass laser was focussed to produce intensities in the range of 10′²-10′⁴W/cm² on the target, within an irradiation spot diameter of 160 pm, optical shadowgrams were recorded bya second harmonic (0.53 pm wavelength) pulse. Shock pressures and scaling of pressure with laser intensity was studied. Shock pressures in gold-coated Plexiglas target was observed to be considerably higher compared to those in uncoated targets. This enhancement of shock pressure has been explained on the basis of contribution of an X-ray driven ablative heat wave in the gold plasma. Shock pressure values show a close agreement with those obtained from a one-dimensional Langrangian hydrodynamic simulation. Shadowgrams of shock fronts produced by non-uniform spatial laser beam irradiation profiles have shown complete smoothing when a gold layer is used on a Plexiglas target.     

Dynamics of erosion plasma formation and formation of a plasma flame with a condensed disperse phase when quasi-continuous laser radiation acts on bismuth

The dynamics of near-surface plasma formation and the formation of an erosion plasma flame was investigated for quasi-continuous laser radiation (λ=1.06 μm, q=0.1–10 MW/cm², τ∼1.5 msec) acting on bismuth targets in air. The absence of low-threshold plasma formation at q<2 MW/cm² was established and explained. Instabilities in the plasma formation were revealed and the range of laser radiation power densities (2≤q≤4 MW/cm²) at which these instabilities take place was determined. At q≥5.4 MW/cm², brightening of the flame was noted. The dependence of the luminescence temperature of the flame on the laser radiation power density q was determined. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 126–133, January–February, 1998.     

Dynamics of the Emission of Light by C2 and C3 Molecules in a Laser Plasma Produced by Two-Pulse Irradiation of the Target

The laser plasma produced by irradiation of a graphite target simultaneously by the first and second harmonics of two Nd³⁺:YAG lasers has been studied by emission spectroscopy methods. The delay between radiation pulses () varied from 0 to 700 sec. It is established that in the absence of a delay between pulses ( = 0) the increase in the intensity of plasma emission at the wavelengths corresponding to the radiative transitions of the C₂ and C₃ molecules is of nonradiative character. The plasma produced by laser radiation at the wavelength = 1064 nm exerts its influence on the radiation spectrum of the plasma produced by laser radiation at the wavelength = 532 nm, if the magnitude of the delay between laser pulses does not exceed 30 sec. The most probable reason for this character of influence of the prepulse on the laser plasma radiation spectrum is sublimation of soot particles caused by laser radiation at = 532 nm.     

Interaction of tea CO2 laser light with plasma

High power double discharge type TEA CO₂ laser was used to study laser plasma interaction on carbon target plasma. The maximum output power was 0.3 GW and full width at half maximum intensity was 100 ns. We measured the reflectivity of the laser light at 10.6 μm in wavelength from the carbon plasma. The reflectivity showed a maximum (≈56%) at the laser power intensity of about 1.1 x 10¹⁰ W/cm², and at above this value, the reflectivity decreased. The absorption was introduced effectively above this intensity. The electron density and temperature were also measured.     

Generation of THz Radiation from Laser Beam Filamentation in a Magnetized Plasma

The terahertz (THz) frequency radiation production as a result of nonlinear interaction of high intense laser beam with low density ripple in a magnetized plasma has been studied. If the appropriate phase matching conditions are satisfied and the frequency of the ripple is appropriate then this difference frequency can be brought in the THz range. Self focusing (filamentation) of a circularly polarized beam propagating along the direction of static magnetic field in plasma is first investigated within extended‐paraxial ray approximation. The beam gets focused when the initial power of the laser beam is greater than its critical power. Resulting localized beam couples with the pre‐existing density ripple to produce a nonlinear current driving the THz radiation. By changing the strength of the magnetic field, one can enhance or suppress the THz emission. The expressions for the laser beam width parameter, the electric field vector of the THz wave have been obtained. For typical laser beam and plasma parameters with the incident laser intensity ≈ 10¹⁴ W/cm², laser beam radius (r₀) = 50 μm, laser frequency (ω₀) = 1.8848 × 10¹⁴rad/s, electron plasma (low density rippled) wave frequency (ω₀) = 1.2848 × 10¹⁴ rad/s, plasma density (n₀) = 5.025 × 10¹⁷cm^–3, normalized ripple density amplitude (μ)=0.1, the produced THz emission can be at the level of Giga watt (GW) in power (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

激光驱动飞片过程中激光烧蚀深度的计算

利用激光体烧蚀模型,数值模拟了激光驱动飞片的加速过程,包括激光的吸收和飞片的速度历史等。在光强为GW/cm2量级的激光作用下,激光烧蚀产生的等离子体的流体力学运动可用改编的1维La-grange流体力学计算程序SSS来描述。通过计算得到不同激光能量下的飞片密度剖面,由此给出金属薄膜的烧蚀深度与实验测量值进行比对,二者符合得较好。     

Second-harmonic conversion of partially coherent radiation of neodymium glass laser

The experimental results on the conversion of multimode radiation of a neodymium glass laser with controlled spatial and temporal coherence to the second optical harmonic in nonlinear KDP crystal during the oee interaction implementation are presented. The dependence of the efficiency of the conversion to the second harmonic on the fundamental radiation power density on the crystal in the range I = 0.2 ? 5 GW/cm² is studied at the number of transverse modes in the cavity N = 1000; the radiation divergence φ = 3.5, 4.7, and 5 mrad; the emission spectral width δλ = 5and 42 Å; and the pulse duration of 2.5 ns. The breakdown power density of the KDP crystal is determined, the interference properties of the converted radiation of the second harmonic are studied.     

A Compact Sol-Gel Laser with Dynamic Distributed Feedback

Lasing in dynamic-distributed-feedback (DDFB) laser on the basis of a modified sol-gel matrix activated with Rhodamine 4C is reported. The lasing is achieved in the range of 590 nm with a wavelength tuning of about 20 nm. The slope efficiency of the sol-gel DDFB laser excited by the second harmonic radiation of a YAG:Nd³⁺ laser with a pulse duration of 40 ns is 16.5% at a lasing wavelength of 593 nm. The DDFB laser is also characterized by a threshold pumping energy of 90 J and a pumping energy density up to 40 mJ/cm². The typical spectral width of the laser line is 0.025 nm.     

Modeling of terahertz radiation emission from a free‐electron laser

In this article, we report the generation of terahertz (THz) radiation using the interaction of a laser‐modulated relativistic electron beam (REB) with a surface plasma wave. Two laser beams propagating through the modulator interact with the REB, leading to velocity modulation of the beam. This results in pre‐bunching of the REB. The pre‐bunched beam travels through the drift space, where the velocity modulation translates into density modulation. The density‐modulated beam, on interacting with the surface plasma pump wave, acquires an oscillatory velocity that couples with the modulated beam density to give rise to a nonlinear current density which acts as an antenna to give THz radiation. By optimizing the parameters of the beam and the wiggler, we obtain power of the order of 10⁻⁴ using the current scheme.     

Optimum regime of passive mode locking in the yttrium aluminate laser

In order to obtain pulses of maximum energy and minimum time, the passive mode-locking regime was optimized for the yttrium aluminate-neodymium laser on the basis of a range of polymethine dyes with different nonlinear parameters (bleaching intensity and relaxation time). Ultrashort pulses with a minimum time of 5 ps and power 0.4 GW were generated. The radiant strength of yttrium aluminate was estimated and found to be 1.3 GWcm^–2 for ultrashort pulses (USP). Dyes were identified which provided for the USP minimum time and maximum energy. A pulse time of 5 ps was found to be limiting for the given radiant strength of the crystal.     

Resotron: A Novel Tunable Millimeter and Submillimeter Wave Source of Radiation

The need for tunable radiation sources in the millimeter and submillimeter range for spectroscopic purposes is still a research area of great interest. The tunable radiation source, proposed in this paper, is a special free electron laser device with the prerequisite of low electron energy. The output power density in the millimeter (GHz)-range is of the order of MW/cm² and in the submillimeter (THz)-range of the order of kW/cm². The device consists of an electron source with electron optics, wiggler/microwiggler, and a longitudinal magnetic field. The wiggler for the THz-regime has a very short wiggler period of approximately 400 m and could be manufactured with laser micromachining techniques. The free electron laser operates in magnetoresonance and shows surprisingly stable electron orbits and therefore narrow output frequencies. Computational results of the temporal behavior of the output power done with a multi-frequency code are reported.     

Investigation on the hydrodynamics of slab x-ray laser plasma by nonuiform line focused laser irradiation

Based on the two-dimensional model,this paper compares the hydrodynamics of slab x-ray laser plasma produced by different nonuniform line focused irradiations.It finds that the average intensity and the duration of laser pulse and the overall shape of the intensity distribution in the focal line have different influences on the plasma.Calculations show that the evolution of temperature variation is more sensitive to the pulse duration and the electron density variation is more sensitive to the pulse intensity.Pulses with duration of 200 ps to 500 ps and with intensity of 0.2 TW/cm 2 to 1.0 TW/cm 2 are proved acceptable in slab x-ray lasers.     

Dynamics of high-temperature plasma formation during laser irradiation of three-dimensionally structured,low-density matter

Results are presented from an experimental investigation of the properties of the plasma produced by the action of a radiation pulse at the second harmonic of a Nd laser, with average intensity ~5·10¹⁴ W/cm² in the focal spot, on flat targets consisting of porous polypropylene (CH)_x with an average density of 0.02 g/cm³ (close to the critical plasma density) and with ~50 μm pores. The properties of the laser plasma obtained with porous and continuous targets are substantially different. The main differences are volume absorption of the laser radiation in the porous material and much larger spatial scales of energy transfer. The experimentally measured longitudinal ablation velocity in the porous material was equal to (1.5–3)·10⁷ cm/s, which corresponds to a mass velocity of (3–6)·10⁵ g/cm²· s, and the transverse (with respect to the direction of the laser beam) propagation velocity of the thermal wave was equal to ~(1–2) ·10⁷ cm/s. The spatial dimensions of the plasma plume were ~20–30μm. The plasma was localized in a 200–400μm region inside the target. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 462–467 (10 October 1996)     

Study of Direct Amplification of Ultrashort Light Pulses in a Laser Amplifier with a View to Obtaining High Radiation Contrast

An amplification scheme for ultrashort laser pulses of high radiation contrast was used to perform experiments on ablation pressure symmetrization using a prepulse upon acceleration of thin foils. It is shown that the spontaneous radiation of the regenerative amplifier restricts the energy contrast in the amplification of chirped pulses at a level of 10^-4–10^-3. The possibility of direct amplification of a short pulse with a view to increasing the energy contrast ratio was considered. Experiments were performed on the PICO laser facility to demonstrate that a 10-ps pulse amplification achieved an intensity 100 GW/cm², a gain factor of 1.2, and an inversion dumping factor >30%.     

Spectra of secondary emission during generation of optical harmonics in globular photonic crystals

Secondary emission spectra of globular silica photonic crystals when their surfaces were exposed to laser pulses 250 fs long at a power density to 1 TW/cm² have been studied. Optical harmonics and plasma emission were detected in this case. For the opal matrix containing pores filled with air, in the reflection mode, the third optical harmonic with a conversion efficiency of ～10% arises. The highest conversion efficiency for exciting radiation with wavelengths of 1026 or 513 nm is implemented when the frequencies of the exciting radiation or the second harmonic are near the stop band edge. In globular photonic crystals filled with sodium nitrite or barium titanate ferroelectrics, the second optical harmonic is observed. The exciting radiation conversion efficiency to the second optical harmonic was a few percent and depended on the frequency of exciting radiation and photonic crystal globule diameters. It is found that the plasma emission intensity increases with the exciting radiation power density. The dependences of the intensity of the second and third optical harmonics on the pump intensity are constructed for various photonic crystal globule diameters.     

纳秒激光诱导空气等离子体射频辐射特性研究

开展了波长为532 nm、脉宽为8 ns的纳秒激光诱导空气等离子体射频电磁辐射特性实验研究,基于锥形天线探测空气等离子体在30-800 MHz频谱范围有较强的射频电磁辐射,是等离子体内电偶极子振荡变速运动造成的.实验结果表明:随激光能量增加,30-200 MHz范围内射频辐射强度逐渐变强,但360-600 MHz频率范围射频辐射强度逐渐变弱.等离子体射频辐射的空间分布依赖于入射激光的偏振方向,当激光偏振方向与天线放置方向一致时,该方向上空气等离子体的射频辐射强度高,谱线较丰富.射频辐射总功率随激光能量先增加后降低,采用等离子体电子密度变化对等离子体频率及等离子体衰减系数影响(制约)关系,对射频辐射总功率随激光能量的变化规律进行了解释.     

Laser Annealing of SiO2 Glass with Copper Nanoparticles

Composite layers formed in SiO₂ by implantation of 50keV Cu⁺ ions with a dose of 8·10¹⁶ cm^–2 at an ion current density of 10 A/cm² have been investigated. It is shown that ion implantation carried out under the chosen conditions allows one to synthesize copper nanoparticles in the surface region of a dielectric. The exposure of composites to highpower pulses of an excimer krypton laser at a wavelength in the SiO₂ transmission region has been investigated. In the absence of effective optical absorption by a glass substrate, the dynamics of the change in the structure of the layer with metallic nanoparticles is determined by the number of laser pulses. It has been established that at the initial stage of pulsed irradiation fragmentation of the largest nanoparticles occurs followed by the inverse process of their agglomeration as a result of slight heating of the glass matrix; further exposure to the laser irradiation leads to an effective accumulation of energy in the particles and, as a consequence, to their melting and dissociation into small clusters and individual atoms.     

Influence of infrared radiation on the excitonic absorption edge of CuCl

We report changes in the excitonic absorption edge of CuCl caused by intense CO₂ laser radiation at 10.6 m, a wavelength which lies in the infrared (ir) transparency region of CuCl. With an ir intensity of 0.4 GW/cm² we observe a 100% absorption increase for the Kr⁺ laser probe wavelength of 406.7nm. The effect scales linearly with ir intensity but does not depend on relative polarization. We explain the effect by laser field induced electroabsorption of the exciton. The magnitude of the effect is closely related to electroabsorption induced by static external fields and by internal electric fields from optical phonons.