Contribution Invited Oral
High-intense femtosecond radiation with gas medium and gas cluster beams
Interactions of laser radiation with gas cluster targets have been investigated in the past two decades. Cluster jet, obtained by the adiabatic condensation of gas flow, has pronounced nonlinear optical properties and combines the advantages of solid-state and gas target. Cluster inherits high local density and this implies high value of nonlinear response. At the same time cluster beams are not exposed to ablation and renew their properties before each act of interaction with the laser pulse. There were observed and predicted various nonlinear effects during interaction of intense laser pulses with cluster jets: generation of X-ray radiation, generation of optical harmonics, self-focusing, Stimulated Raman Scattering. It was shown theoretically and experimentally that absorption of laser radiation in cluster beam can reach high value (up to 95%) which is related to linear (Mie) and nonlinear resonance interactions. Resonance absorption of pulse energy results in efficient production of X-Ray and fast charged particles. In this way, further studies of interaction of intense laser pulses with clusters seems reasonable due to the possibility of usage in various practical applications and solving fundamental problems of behavior of matter under intense laser fields.
In some recent publications, intense THz generation in cluster beam excited by ultrashort laser pulses was reported. It was observed more than two orders of magnitude enhancement of THz pulse intensity in Ar cluster jet compared to that in gaseous Ar with equal average atomic density. With increasing of excitant pulse energy up to its maximum value of 70 mJ (corresponding to the vacuum intensity ~ 1017 W/cm2) THz pulse energy increased by the square law without saturation. There was reported in that the directivity pattern of terahertz radiation from clustered plasma has four-lobed structure.
At the present moment there is no clear theoretical interpretation of the experimental results and complete understanding of the mechanism of low-frequency emission in cluster plasma. It seems reasonable to carry out further study of terahertz generation in a clustered plasma, and examine this process in both ways: as a fundamental issue of laser-matter interaction on the way to solving a problem of the dynamics of laser-cluster interaction, and as a practical goal of obtaining an effective source of pulsed THz radiation. In addition, there is possible to apply two-color excitation scheme, which has been successfully used in the past to increase the efficiency of the optical to terahertz conversion in laser-induced plasma of gas media. In this scheme, the fundamental laser frequency at ω is mixed with its second harmonic at 2ω. Two-color scheme allows to increase THz yield by 2...3 orders of magnitude and nowadays this scheme is widely used for THz generation in gaseous media. Nevertheless, optical to terahertz conversion efficiency in laser-plasma generation method is still low. In addition, there was observed a saturation of THz yield in a two-color scheme at high excitant pulse energy that originates from THz absorption in dense plasma. Cluster target seems to be attractive to solve these problems.
In this paper we present the results of experimental and theoretical studies of generation of terahertz emission in the laser-induced clustered plasma. We have performed experiments using both single-color and two-color excitation schemes. Simultaneously with the control of terahertz emission of clustered plasma we measured the power of accompanying X-ray radiation, which is an important source of information about the processes that occur in a cluster plasma. In the theoretical section we made an attempt to explain the experimental results.