Femtosecond laser breakdown of gases and transparent solid states: ultrafast space-time and spectrum-time resolved diagnostics of multicharged microplasma

We present the results of experimental studies of formation and evolution of multiply ionized (multicharged) laser micro-size plasma produced in gases (air, nitrogen, argon and helium) and inside the transparent solids (fused silica) with high intensity (up to ~ 1017 W/cm2), ultrashort (τ ~100 fs), 800nm/400nm laser pulses tightly focused in a region of ~1.5 μm in diameter. The measuring techniques and experimental setups for generation and precise optical diagnostics of laser-induced plasma - pump-probe microinterferometry and ultrafast spectroscopy are described. The measured spatiotemporal distributions of plasma refractive index/electron density and plasma spectra are demonstrated. In the experiments, the main attention was paid to the most intriguing initial stage of ultrafast plasma formation and evolution characterized by strong laser-matter and laser-plasma coupling resulting in efficient photoionization of material and plasma heating. We found out that the almost complete ionization (down to nuclei) of the initial gas occurs even at the initial stage of plasma formation. Besides, it was observed, for the first time, that a characteristic time of laser plasma formation considerably (in times) exceeds the duration of the pump laser pulse. This postionization process is attributed to impact ionization of plasma by hot electrons heated due to inverse bremsstrahlung. A theoretical model describing the mechanism of plasma postionization by hot photoelectrons was proposed. We compare the results of the experiments with what the theory predicts - the results of electron density calculations are in good agreement with the experimental data. The dynamics of plasma emission (spectral continuum and spectral line formation) in UV-visible spectral range was investigated with a picosecond time resolution applying the developed ultrafast streak-camera-based spectrometer. The spatiotemporal distributions of refractive index of laser irradiated fused silica were recorded with pump-probe microinterferometry. It was demonstrated that the induced refractive index of laser-matter interaction area changes its sign from the positive to the negative during the laser irradiation and again to the positive one after the laser pulse ends.

[1]  M. Kishimoto,et al.  Diagnostics of laser-induced plasma with soft X-ray (13.9 nm) bi-mirror interference microscopy , 2004 .

[2]  O. Willi,et al.  Electron-Density Structures in Laser-Produced Plasmas at High Irradiances , 1979 .

[3]  P. Mulser,et al.  Exact field ionization rates in the barrier-suppression regime from numerical time-dependent Schrödinger-equation calculations , 1998, physics/9802042.

[4]  M. Mattioli,et al.  Holographic interferometry of laser produced plasmas using picosecond pulses , 1971 .

[5]  A. A. Malyutin,et al.  Formation and development dynamics of femtosecond laser microplasma in gases , 2006 .

[6]  M. Takeda,et al.  Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry , 1982 .

[7]  K. Tanaka,et al.  Measurement of Density Modification of Laser-Fusion Plasmas , 1977 .

[8]  Kunze,et al.  Electron-impact excitation and recombination into excited states of lithiumlike ions. , 1988, Physical review. A, General physics.

[9]  C. Capjack,et al.  Enhanced inverse bremsstrahlung heating rates in a strong laser field , 2002, physics/0203030.

[10]  Todd Ditmire,et al.  Time-Resolved Study of Nonlocal Electron Heat Transport in High Temperature Plasmas , 1998 .

[11]  D. Attwood.  Diagnostics for the laser fusion program - Plasma physics on the scale of microns and picoseconds , 1978, IEEE Journal of Quantum Electronics.

[12]  N. Badnell,et al.  Calculated cross sections and measured rate coefficients for electron-impact excitation of neutral and singly ionized nitrogen , 1998 .

[13]  Femtosecond laser optical gas breakdown microplasma : the ionisation and postionisation dynamics , 2007 .

[14]  A. Couairon,et al.  Light filaments in air for ultraviolet and infrared wavelengths. , 2002, Physical review letters.

[15]  A M Rubenchik,et al.  Investigation of ultrafast laser-driven radiative blast waves. , 2001, Physical review letters.

[16]  A. Balakin,et al.  Bremsstrahlung in a strong laser field , 2001 .

[17]  T W Barbee,et al.  Picosecond x-ray laser interferometry of dense plasmas. , 2002, Physical review letters.

[18]  G. Dunn,et al.  Measured electron-impact ionization of Be-like ions: B+, C2+, N3+, and O4+ , 1983 .

[19]  V. Tikhonchuk,et al.  Quantum calculations of correlated electron-ion collisions in a strong laser field , 2006 .

[20]  D. Attwood,et al.  Microscopic interferometry of laser‐produced plasmas , 1974 .

[21]  E. M. Lifshitz,et al.  Quantum mechanics: Non-relativistic theory, , 1959 .

[22]  K A Nugent,et al.  Interferogram analysis using an accurate fully automatic algorithm. , 1985, Applied optics.

[23]  Martin Richardson,et al.  Diagnostics for laser plasma EUV sources , 2005, International Congress on High-Speed Imaging and Photonics.

[24]  E. H. Linfoot.  Principles of Optics , 1961 .

[25]  Friedrich Dausinger,et al.  High resolution interferometric diagnostics of plasmas produced by ultrashort laser pulses , 2003 .

[26]  Nikolai S. Vorobiev,et al.  Streak camera diagnostic of femtosecond laser spark , 2005, International Congress on High-Speed Imaging and Photonics.

[27]  Weber,et al.  Electron Density Measurements of High Density Plasmas Using Soft X-Ray Laser Interferometry. , 1995, Physical review letters.

[28]  P. Berger,et al.  Shadowgraphic and interferometric investigations on Nd:YAG laser-induced vapor/plasma plumes for different processing wavelengths , 1999 .

[29]  D. W. Sweeney,et al.  Interferometric Confirmation of Radiation-Pressure Effects in Laser-Plasma Interactions , 1978 .

[30]  G. Mourou,et al.  Self-focusing, channel formation, and high-energy ion generation in interaction of an intense short laser pulse with a He jet. , 1999, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics.

[31]  Friedrich Dausinger,et al.  Ultrafast laser-induced plasma diagnostics with time-spatial-resolved shadow and interferometric techniques , 2002, SPIE LASE.

[32]  Hutchinson,et al.  Multi-keV Electron Generation in the Interaction of Intense Laser Pulses with Xe Clusters. , 1996, Physical review letters.