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.

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We present the results on generating terahertz radiation in the plasma of an optical discharge arising in the atmosphere during the focusing of femtosecond laser radiation. Different generation schemes related to focusing of the optical radiation by spherical and axicon lenses, with a constant electric field imposed on the laser spark region, as well as with the use of bichromatic laser radiation, are studied. Directivity patterns and polarization distributions of the terahertz radiation are analyzed in detail for different generation techniques. Comparison with the experimental results obtained by other research groups is given. Possible nonlinear mechanisms of the terahertz radiation generation are discussed.

Strong terahertz (THz) radiation can be generated by intense femtosecond laser pulses propagating in air. The excitation of transient current induced in the wake just behind the laser pulse is studied in detail using numerical simulations on the basis of Maxwell's equations for THz-band fields and hydrodynamic model for the plasma motion. It is shown that the thermal effects, anisotropic in character in the case of linear polarized laser field, can explain observed quadrupole-type THz radiation pattern in the experiment performed by Akhmedzhanov [Radiophys. Quantum Electron. 52, 482 (2009)]. Taking into account the transverse structure of the plasma filament, our numerical code enables us to calculate the spatial distribution and temporal evolution of terahertz electron current, its spectrum, and angular emission pattern. It is shown that an expansion of full fields in terms of azimuthal modes is a useful tool for research of THz generation in many situations of practical interest.

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.

We demonstrate theoretically that ultrafast heating of metal nanoparticles by the laser pulse should lead to the generation of coherent terahertz (THz) radiation during the heat redistribution process. It is shown that after the femtosecond laser pulse action, the time-dependent gradient of the electronic temperature induces low-frequency particle polarization with the characteristic timescale of about fractions of a picosecond. In the case of the directed metallic pattern, the THz pulse waveform can be controlled by changing the geometry of the particle. The proposed THz generation mechanism can be the basis for interpretation of recent experiments with metallic nanoparticles and nanostructures.

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