Generation of high harmonics and attosecond pulses with ultrashort laser pulse filaments and conical waves

Results illustrating the nonlinear dynamics of ultrashort laser pulse filamentation in gases are presented, with particular emphasis on the filament properties useful for developing attosecond light sources. Two aspects of ultrashort pulse filaments are specifically discussed: (i) numerical simulation results on pulse self-compression by filamentation in a gas cell filled with noble gas. Measurements of high harmonics generated by the pulse extracted from the filament allows for the detection of intensity spikes and subcycle pulses generated within the filament. (ii) Simulation results on the spontaneous formation of conical wavepackets during filamentation in gases, which in turn can be used as efficient driving pulses for the generation of high harmonics and isolated attosecond pulses.     

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.     

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.     

Microablation with ultrashort laser pulses

This paper reports the micromachining results of different materials (Al, Si, InP and fused silica) using a Ti : sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to 10 ps. The hole arrays with a diameter up to a few μm through microdrilling are presented. We discussed how an effective suppression of the thermal diffusion inside the ablated materials and an effective microablation could be realized. If the laser fluence is taken only slightly above the threshold, a hole array can be drilled with diameters even smaller than the wavelength of the laser. Some examples are presented in the paper.     

Generation of ultrashort pulses in solid media with a large gain coefficient

Lebedev Physics Institute, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 3, pp. 369–379, March, 1989.     

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.     

Generation of the fifth and sixth harmonics of iodine laser pulses

High-power coherent radiation at 263 nm and 219 nm was generated by converting iodine laser pulses (λ = 1.315 μm) into fifth- and sixth-harmonic radiation. For 5ω-generation 3ω and 2ω were mixed in a KDP crystal. 6ω was produced by second-harmonic conversion of 3ω radiation in a KPB crystal.     

Modeling cluster jets as targets for high-power ultrashort laser pulses

A hydrodynamic model is formulated that describes the formation of clusters in atomic gas jets expanding into vacuum, which are used as laser plasma targets. Detailed model calculations performed for an argon gas jet describe spatial distributions of the density of gas and cluster phases formed in the Laval nozzle at room temperature in a broad range of entrance gas pressures. The cluster density distribution is significantly inhomogeneous. The cluster distribution features revealed by the model calculations were qualitatively confirmed by the X-ray spectroscopic measurements of the spatial distribution of emission from the plasma created in the jet tar-gets by high-power ultrashort laser pulses.     

Micromachining of quartz with ultrashort laser pulses

Received: 6 January 1997/Accepted: 1 April 1997     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

Measurement of ultrashort laser pulses

A proposal for measuring the pulse shape of ultrashort laser pulses is described. The proposed method consists of a Gamo triple intensity-interferometer to measure the triple-correlation function of the light pulses. A subsequent signal processing algorithm is used to reconstruct the true shape of the light pulses out of their triple-correlation function. The suggested method is quite insensitive to noise, as shown by means of computer simulations.     

Generation of hard x rays by femtosecond laser pulse interaction with solid targets in atmosphere

X ray radiation as high as 50 keV, including K(α) of Ba and Mo, have been observed from a solid target during the interaction of low energy ~0.65 mJ, 1 kHz 40 femtosecond laser pulses focused in air at atmospheric pressure. Energetic electrons generating such x rays are possibly produced when the field strength in laser pulse wake exceeds the runaway threshold in air. Two dimensional particle-in-cell simulations that include optical field ionization of air and elastic collisions support this mechanism.     

Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses

Superstrong shock waves of multimegabar level generated during ablation of an aluminum surface by intense (<1 PW/cm²) femtosecond laser pulses have been detected by observing the propagation of a shock wave in air from the ablated surface to a broadband piezoelectric receiver. The estimated initial pressure and velocity of the shock wave (ablation plume) agree well with data obtained earlier by various methods for shock waves propagating inside ablated targets.     

Study of opto-mechanical characteristics of interaction of ultrashort laser pulses with polymer materials

The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy efficiency, of the laser ablation of flat polymer targets ((C₂F₄) _n , (CH₂O) _n ) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs) of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I ₀ up to 10¹⁵ W/cm²) under normal atmospheric and vacuum (p ∼ 10⁻⁴ mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation.     

Vortices in ultrashort laser pulses

The propagation of optical vortices nested in broadband femtosecond laser beams was studied both numerically and experimentally. Based on the nonlinear Schrödinger equation, the dynamics of different multiple-vortex configurations with varying topological charge were modelled in self-focussing and self-defocussing Kerr media. We find a similar behavior in both cases regarding the vortex–vortex interaction. However, the collapsing background beam alters the propagation for a positive nonlinearity. Regimes of regular and possibly stable multiple filamentation were identified this way. Experiments include measurements on pairs of filaments generated in a vortex beam on an astigmatic Gaussian background with argon gas as the nonlinear medium. Spectral broadening of these filaments leads to a supercontinuum which spans from the visible range into the infrared. Recompression yields <19 fs pulses. Further optimization may lead to much better recompression.     

Photoionization of helium with ultrashort XUV laser pulses

Coupled-channel calculations for multiphoton ionization probabilities of helium through interaction with intensive short laser pulses are presented. Besides Slater-like orbitals we use regular Coulomb wavepackets in our configurational interaction basis to describe the continuum. Linearly polarized laser pulses of 3.8 fs duration and 2.96 x 10¹⁴ Wcm^-2 peak intensity have been used for frequencies between 0.2-1.2 a.u. The results are compared with other ab initio calculations.Received: 8 April 2003, Published online: 9 September 2003PACS: 32.80.Fb Photoionization of atoms and ions - 32.80.Wr Other multiphoton processes     

Phase-matched high-order harmonics by interaction of Ar atoms with high-repetition-rate low-energy femtosecond laser pulses

High-order harmonics are generated by coherent interaction of an intense laser and atoms or molecules[1]. With the development of the intense ultrashort pulse laser, the research of high-order harmonic generation has reached the water-window in spectral region[2] and subfemtosecond in time domain[3]. Especially, the generation and application of subfemtosecond pulse led the study of high-order harmonic generation into a completely new world[4, 5]. It has made the study of ultrafast science fro…     

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