A. M. Kiselev

Laser pulse amplification upon raman backscattering in plasma produced in dielectric capillaries

Femtosecond-pulse amplification upon stimulated Raman scattering is experimentally demonstrated for the case of a counter-propagating femtosecond laser pulse and a frequency-modulated broadband pump pulse with the same carrier frequency in a dielectric capillary filled with gas plasma. A value of ∼103 obtained for the spectral intensity amplification and ∼102 for the output energy are the highest ever achieved for these quantities to date. Numerical simulation demonstrates good agreement with the experimental results. Based on the experimental data and the results of theoretical calculations, we propose a hydrodynamic mechanism for plasma-wave breaking as the mechanism playing an important role in the amplification restriction in the scheme considered.

Nonlinear absorption of intense femtosecond laser radiation in air

A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

Femtosecond laser-induced nanofabrication in the near-field of atomic force microscope tip

Femtosecond laser-induced formation of nanostructures in the near field of an atomic force microscope tip is demonstrated. Mechanism of nanofabrication will be discussed.

Study of the plasma wave excited by intense femtosecond laser pulses in a dielectric capillary

Laser wakefield in a gas-filled capillary driven by a 1-TW femtosecond Ti:Sa laser pulse is studied experimentally by observing driving pulse spectrum modifications, which are caused by the combined action of the optical field ionization and the plasma density oscillations. Good agreement between the results of extensive numerical simulations and the experimental data allows us to estimate the accelerating gradients in the wake, which range from 5 to 10 MV/cm for typical experimental conditions.

Frequency stabilization of a primary subterahertz oscillator by a frequency comb of a femtosecond laser

The frequency of a primary subterahertz oscillator has been phase locked with the use of the equidistant components of a broad spectrum produced by a femtosecond laser. The optical-to-terahertz down-conversion of the laser pulse train and its mixing with subterahertz radiation has been performed at a Schottky diode. This work provides the opportunity of creating a principally new generation of frequency synthesizers with the desired power and phase noise a few orders of magnitude lower than that of their traditional analogues.

Shock-wave generation upon axicon focusing of femtosecond laser radiation in transparent dielectrics

It is shown experimentally that the axicon focusing of intense femtosecond laser pulses in transparent dielectrics leads to efficient excitation of shock waves. A method is developed for measuring the dynamics of shock waves, which uses a frequency-chirped probe pulse and has high spatial (∼1 μm) and time (∼10 ps) resolutions. The initial stage of the evolution of an intense (up to 10 GPa) shock wave is studied by this method.

Femtosecond laser comb based subterahertz synthesizer

Functioning layout of a frequency comb-based subterahertz synthesizer is demonstrated. A primary subterahertz oscillator was phase-locked against the Ti:Sapphire femtosecond laser frequency comb down-converted to the subterahertz range by the semiconductor mixer. Synthesizer operation is demonstrated through Fabri-Perot resonator response curve recording experiment. Spectral purity of the synthesizer was estimated. The advantage of the comb-based synthesizer over the synthesizer based on the Agilent E8257D device was shown.

Ionization spectrum transformation and compression of powerful femtosecond laser pulses in experiments on the propagation in gas-filled dielectric capillaries

The propagation of an intense (I≤106 W/cm2) femtosecond laser radiation with a duration of ∼100 fs through gas-filled dielectric capillaries was studied. The radiation with a power up to 0.2 TW propagates along the paths up to 20 cm with a transmission efficiency of ∼45%. The beam transverse structure at the output is close to the capillary fundamental mode under gas-ionization conditions. The transformation of pulse spectrum was studied as a function of input intensity. It is demonstrated experimentally that the pulse is compressed to a duration of ∼30 fs due to the compensation of ionization-induced self-phase modulation in a linear dispersive element at the capillary output.

Formation of nanostructures on the surface of metal films by the probe of an atomic-force microscope during the action of laser pulses

When the probe of an atomic-force microscope is irradiated with pulses from a titanium-sapphire laser, nanostructures with characteristic diameter 30-70 nm and depth 5-15 nm are obtained on the surface of metal films. The formation thresholds of the structures have been measured, and studies have been made of how the thresholds depend on the polarization of the laser radiation, the widths of the laser pulse, and the distance between the point of the probe and the sample surface. It is found that the main cause for the formation of the nanostructures is the mechanical pressure of the probe on the sample when it elongates because of being heated by absorbed laser radiation. The use of pulses of femtosecond width made it possible to extend the range of materials subject to processing.

Femtosecond laser-induced nanofabrication in the near field of an atomic force microscope tip

Femtosecond laser-induced formation of nanostructures in the near field of an atomic force microscope tip is demonstrated. Mechanism of nanofabrication will be discussed.