D. A. Fadeev

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.

Thermal mechanism of laser induced THz generation from a metal surface

We propose a new theoretical model of terahertz (THz) generation from a metal surface irradiated by femtosecond laser pulses. The mechanism is based on the inducing of the low-frequency polarization by the inhomogeneous temperature distribution of the electron gas near the surface. We present analytical and numerical results for the characteristic experimental parameters of a laser pulse. The experimentally observed duration of the THz signal (~1 ps) is interpreted as a time of the thermal energy relaxation after the optical radiation action. The total THz energy as a function of the initial laser pulse characteristics and metal parameters is obtained. The most important experimental features are discussed and interpreted on the basis of the thermal mechanism.

Generation of Terahertz Radiation by the Optical Breakdown Induced by a Bichromatic Laser Pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A theoretical model developed for the terahertzradiation generation includes the nonlinear interaction of a femtosecond pulse with an ionized gas and the excitation of transverse oscillations of the plasma column. The results of the theoretical analysis are compared with the experimental data.

Development of a compact hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range

The key elements of a mobile hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range have been designed, produced, and approved in model experiments. These elements are a compact THz oscillator based on an all-fiber femtosecond laser system and a unit for recovering electrodynamic characteristics of layered objects from scattered THz radiation spectra. Generation of 250-fs optical pulses at a wavelength of 1.03 µm with energy of 0.3 µJ and a repetition frequency of 1MHz is demonstrated and the efficiency of optical-THz conversion is found to be 5×10−6. The proposed algorithm is constructed based on an iterative procedure and can be used for dispersive and absorbing media. It has higher operating speed in comparison with the algorithms for solving inverse problems, which are based on functional minimization methods.

Mechanisms of optical-to-THz conversion on metal and semimetal surfaces

The paper is devoted to the very recent experimental and theoretical results on the laser-induced THz generation from metals and semimetals. Despite the relatively low efficiency, study of the optical-to-THz conversion on metal surface are still relevant, mostly because of possible applications in structural diagnostics and due to the fundamental interest in transformation mechanisms. Here we consider a thermal model of THz response and present some new experimental results on the anisotropic THz generation from the surface of mono- and polycrystalline bismuth samples.The paper is devoted to the very recent experimental and theoretical results on the laser-induced THz generation from metals and semimetals. Despite the relatively low efficiency, study of the optical-to-THz conversion on metal surface are still relevant, mostly because of possible applications in structural diagnostics and due to the fundamental interest in transformation mechanisms. Here we consider a thermal model of THz response and present some new experimental results on the anisotropic THz generation from the surface of mono- and polycrystalline bismuth samples.

Optical-to-THz conversion and scattering in metals

Laser induced terahertz (THz) waves generation from metals is a result of thermal nonlinear effects in the electron gas near the surface. We discuss the generation mechanism and the possibility of electron scattering investigation with a help of nondestructive optical-to-THz conversion on the surface. It is shown that measurement of THz response can be used to determine electron scattering frequency for electron gas temperatures up to 1-2 eV.

To the problem of numerical simulation of gas optical breakdown on GPU

Abstract An efficient solution for a key problem of implicit simulations of nonlinear interaction of ultra-short laser pulses with gaseous matter is proposed. The algorithm of fast tridiagonal linear system solver is discussed. The proposed implementation accounts for specific features of 2d data processing on GPU.

Specific features of terahertz radiation emitted from laser sparks induced in gases by femtosecond laser pulses with linear polarization

The increasing interest to the terahertz radiation is connected with its unique potential for lots of applications such as identification and detection of different objects and materials for the problems of security, biology, chemistry (see reviews). Some of the most promising alternative for terahertz waves generation are laser plasma methods. Recently in Institute of Applied Physics RAS new experimental data were obtained. In the common scheme of generation (50 fs, 2.5 mJ linear polarized laser pulse, 80–400mm focal length lens) a radiation pattern with strong cylindrical assymetry depending on optic polarization was obtained (see fig. 1 c)[1]. This effect could not be explained in terms of ponderomotive force like in [2] due to its axisymmetric character.

Generation of THz radiation in a laser spark

A brief review of experimental and theoretical activity in IAP RAS on the generation of THz radiation in a laser spark produced by femtosecond optical pulses has been presented.