Absorption of ultrashort laser pulses in strongly overdense targets

Absorption measurements on solid conducting targets have been performed in s and p polarization with ultrashort, high-contrast Ti:sapphire laser pulses at intensities up to 5x10{16}W/cm{2} and pulse duration of 8 fs. The particular relevance of the reported absorption measurements lies in the fact that the extremely short laser pulse interacts with matter close to solid density during the entire pulse duration. A pronounced increase of absorption for p polarization at increasing angles is observed reaching 77% for an incidence angle of 80 degrees . Simulations performed using a 2D particle in cell code show a very good agreement with the experimental data for a plasma profile of L/lambda approximately 0.01.     

X-ray production by irradiation of solid targets with sub-picosecond excimer laser pulses

A table-top excimer laser system generating sub-ps pulses was used to irradiate solid targets at intensities of up to 4×10¹⁵ W/cm². Soft X-ray spectra of various materials were measured. The X-ray conversion efficiencies were between 1–5%. Streak camera measurements showed instrument-limited X-ray pulse duration of a few ps.Partially based on the plenary talk X-Ray Generation by Sub-Picosecond UV Laser by F. P. Schäfer, G. Kühnle, S. Szatmári, and M. Steyer, presented at the XVI Int. Quant. Electron. Conf., Tokio, July 18–21, 1988, Technical Digest (The Japan Society of Applied Physics) p. 2     

Femtosecond X-ray line emission from multilayer targets irradiated by short laser pulses

The invention of high-power, ultra-short-pulse lasers has opened the way to investigations aimed at the creation of a new type of bright X-ray source for various uses including material science applications and time-resolved X-ray diffraction for biology. The efficiency with which laser energy incident on a solid target is converted into an X-ray emission depends on many factors, including the temporal profile of the laser pulse. Here we report the results of our theoretical and experimental investigations of the line X-ray emission from layered solid targets irradiated by ultra-short laser pulses. The laser prepulse parameters and target thickness are optimized to convert the maximum laser energy into an emission in the selected X-ray line. Multilayer foils are proposed to increase the energy of the K-line emission from laser plasma while simultaneously keeping the X-ray pulse duration at a hundred femtoseconds. The emission is studied both experimentally and theoretically by means of an analytical model and numerical simulations. PACS 52.38.Ph; 52.38.Dx; 52.50.Jm     

Generation of harmonics during the interaction of ultrashort superstrong laser pulses with solid targets

Generation of even and odd harmonics in the skin layer formed during the interaction of a short relativistic laser pulse with solid targets is considered. The complex motion of free electrons in the skin layer along the electric field vector and along the direction of propagation of a laser wave is analyzed. The Fourier expansion of the trajectory of this motion is used to obtain the components of the conductivity tensor and of the amplitude of the transverse electromagnetic field of harmonics propagating along the electric field. Even harmonics appear due to relativistic effects. The efficiency of generation of even and odd harmonics at the leading front of a laser pulse is calculated.     

Measurement of the energy penetration depth into solid targets irradiated by ultrashort laser pulses

The energy penetration depth of a short (100 fs) Ti-sapphire laser pulse (0.8 &mgr;m) of intensity 3x10(16) W/cm(2), in solid density materials has been measured. High-Z (BaF2) and low-Z (MgF2) solid layers targets were used. The penetration depth was determined from the measurement of the x-ray emission spectra, as a function of the target thickness. The investigation of these spectra showed that in the low-Z case, solid density material to a depth of 50 nm was heated to a peak electron temperature of approximately 150 eV. For the high-Z material, the penetration depth corresponding to this temperature exceeded 100 nm. This is evidence of a larger heat penetration depth in a high-Z material in comparison to a low-Z material. A model based on electron heat conduction is used to estimate the energy penetration depth. It is suggested that the larger heat penetration in high-Z material is due to heating of the material, caused by the radiation flux, generated by the electron heat conduction.     

超相对论激光和稠密等离子体作用产生阿秒脉冲的优化

利用一维粒子模拟程序研究了超相对论激光脉冲与稠密等离子体相互作用得到的阿秒脉冲.从超相对论近似的角度分析了电子运动行为和高次谐波的产生,发现当等离子体密度一定时,随着无量纲相似参数S的减小,阿秒脉冲的转换效率呈先增大后减小的趋势,因此选择适当的光强就可以得到转换效率较高的阿秒脉冲.当S一定时,随着等离子体密度的增加,阿秒脉冲转换效率有增大的趋势.这说明用适当的光强照射更稠密度的等离子体靶面,可以产生更强的阿秒脉冲.     

Analysis of electromagnetic pulses generated from ultrashort laser irradiation of solid targets at CLAPA

Electromagnetic pulses(EMPs)produced by the interaction of a TW femtosecond laser with solid targets at the Compact Laser Plasma Accelerator(CLAPA)are measured and interpreted.The statistical results confirm that the intensities of the EMPs are closely related to both target material and thickness.The signal of the titanium target is more abundant than that of the copper target with the same thickness,and the intensity of EMP is positively correlated with the target thickness for aluminium foil.With the boosted EMP radiations,the energy of accelerated protons is also simultaneously enhanced.In addition,EMPs emitted from the front of the target exceed those from the rear,which are also pertinent to the specific target position.The resonant waveforms in the target chamber are analyzed using the fast Fourier transform,and the local resonance and the attenuation lead to changes of the frequency spectra of EMPs with variation of detecting positions,which is well supported by the modeling results.The findings are beneficial to gaining insight into the mechanism of EMP propagation in a typical target chamber and providing more information for EMP shielding design.     

Electron acceleration by a short relativistic laser pulse at the front of solid targets

Acceleration of electrons in a low-density plasma in front of a solid target by a propagating short ultraintense laser pulse is studied. When the laser is reflected at the target surface the accelerated electrons, with energy scaling as the laser intensity, continue to move forward inertially and thus escape from the pulse. Electrons accelerated backwards by the reflected light can attain even higher energies due to their longer acceleration length and their high initial momentum from a relativistic return current.     

Floquet theory for short laser pulses

We develop adiabatic perturbation theory for quantum systems responding to short laser pulses, with or without a frequency chirp. Our approach rests on lifting the time-dependent Schr?dinger equation to an extended Hilbert space, then applying standard perturbational techniques to Floquet states in this extended space, and finally projecting back to the physical Hilbert space. The same strategy also allows us to construct superadiabatic bases for monitoring the quantum evolution in the course of a pulse. These bases provide a diagnostic tool for improving the efficiency of pulse-induced population transfer. The formalism is applied to the selective excitation of molecular vibrational states by chirped laser pulses, which exploit either successive single-photon resonances or a multiphoton resonance, and by a STIRAP-like process. Received: 23 June 1998 / Revised: 18 August 1998 / Accepted: 25 August 1998     

Processing of metals by double pulses with short laser pulses

Experimental results related to the influence of time delayed pulses for ablation efficiency with short multi pulses (pulse duration of 5 ps) are reported. A significant improvement of the micro structuring quality at relatively high fluence regime in metals is obtained. Less removed or recast matter is observed and the processed surface appears to be smoother with better roughness. Ablation depths and burr heights are compared for single pulses and double pulses in steel, Al and Cu as a function of scans number. Best results are obtained for weak time delays, typically less than 1 ps. PACS 79.20.Ds; 42.62.Cf; 81.65.Cf     

Absorption of subpicosecond ultraviolet laser pulses in high-density plasma

The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 10¹²–10¹⁴ W cm^–2 and as a function of polarization and angle of incidence for the intensity range of 10¹⁴–2.5×10¹⁵ W cm^–2. As the intensity increases from 10¹² W cm^–2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5–0.61 at 10¹⁴ W cm^–2. For this intensity maximum absorption of 63–80% has been observed for p-polarized radiation at angles of incidence in the range of 54°–57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/, in the range of 0.13–0.15 at critical density and normalized density gradient scale lengths, L/₀, in the range of 0.018–0.053 for a laser intensity of 10¹⁴ W cm^–2.At 2.5×10¹⁵ W cm^–2 a small amount of preplasma is present and maximum absorption of 64–76% has been observed for p-polarized radiation at angles of incidence in the range of 45°–50°.Dedicated to Prof. Dr. Rudolf Wienecke on the occasion of his 65^th birthdayOn leave from: Department of Electrical Engineering, University of Alberta, Edmonton, T6G 2G7, Canada     

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.     

Detuned raman amplification of short laser pulses in plasma

The recently proposed scheme of so-called "fast compression" of laser pulses in plasma can increase peak laser intensities by 10(5) [Phys. Rev. Lett. 82, 4448 (1999)]. The compression mechanism is the transient stimulated Raman backscattering, which outruns the fastest filamentation instabilities of the pumped pulse even at highly overcritical powers. This Letter proposes a novel nonlinear filtering effect that suppresses premature backscattering of the pump in a noisy plasma layer, while the desired amplification of a sufficiently intense seed persists with a high efficiency. The effect is of basic interest and also makes it robust to noise the simplest technologically fast compression scheme.     

On the interaction of femtosecond laser pulses with cluster targets

The heating of clusters by femtosecond laser pulses is studied theoretically and experimentally. Both the formation of a cluster target and the results of experimental studies of the cluster plasma by the methods of X-ray emission spectroscopy are considered. A numerical model of cluster formation in a supersonic gas jet is proposed. It is shown that detailed studies of two-phase gas-dynamic processes in a nozzle forming the jet give the spatial distributions of all parameters required for the correct calculation of the cluster heating by short laser pulses. Calculations of nozzles of different configurations show that in a number of cases an almost homogeneous cluster target can be formed, whereas in other cases the distributions of parameters prove to be not only inhomogeneous but also even nonmonotonic. A simple physical model of the plasma production by a femtosecond laser pulse and a picosecond prepulse is proposed. It is shown that a comparison of X-ray spectra with detailed calculations of the ion kinetics makes it possible to determine the main parameters of the plasma being produced.     

Molecular alignment by trains of short laser pulses

We show that a dramatic field-free molecular alignment can be achieved after exciting molecules with proper trains of strong ultrashort laser pulses. Optimal two- and three-pulse excitation schemes are defined, providing an efficient and robust molecular alignment. This opens new prospects for various applications requiring macroscopic ensembles of highly aligned molecules.     

Interaction of short laser pulses with biological structures

Focusing of light pulses emitted by a Q-switched laser, which have a duration of some nanoseconds, into a medium leads to laser-induced breakdown. The shockwaves produced by this propagate through the medium and can interact with inhomogeneities situated within the medium and consequently cause destruction. Because of the increasing interest in these short light pulses in the medical field the interaction of the shockwaves with biological tissue has been investigated. The observed results indicate the influence of two physical quantities of the shockwave: the peak pressure and the duration.     

Absorption of iodine laser pulses in potassium and cesium vapors

The absorption of ns iodine laser pulses at a wavelength of 1.315 μm in potassium and cesium vapors has been measured. At elevated temperature there is considerable absorption in both vapors. The absorption was not saturable at intensities up to 1 GW/cm², but increased with increasing intensity.