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) [CrossRef] ; Opt. Lett. 30, 1402 (2005) [CrossRef] ]. 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.

It was recently predicted [ Phys. Rev. A 95(6), 063817 (2017) 10.1103/PhysRevA.95.063817 ] that an intense femtosecond laser pulse propagating in an electro-optic crystal and producing free carriers via multiphoton ionization can generate a unipolar electromagnetic precursor propagating ahead of the laser pulse. Here we report the experimental observation of this phenomenon in a GaP crystal excited by an amplified Ti:sapphire laser.

A record high optical-to-terahertz conversion efficiency of 0.25% was realized with femtosecond laser pulses propagated in a planar Si-LiNbO₃-air-metal structure. Terahertz spectrum tuning was demonstrated by adjusting an air gap between the LiNbO₃ layer and the metal plate. The influence of optical pulse chirp on the efficiency of terahertz generation was investigated.

Terahertz generation by optical rectification of femtosecond laser pulses propagating in a 40-$\mu$m thick LiNbO$_3$ layer attached to an output Si prism has been experimentally investigated for different laser wavelengths from 800 to 2100 nm. For longer wavelengths, the saturation of the optical-to-terahertz conversion efficiency has been observed at higher laser pulse energies, thus enabling higher efficiencies. In particular, record high conversion efficiency of 1.3% has been achieved with 30-$\mu$J laser pulse energy at 2100 nm.

We consider laser nanostructuring of the material surface by means of a colloidal particle lens array. Here, the monolayer of dielectric micro- or nanospheres placed on the surface acts as an array of near-field lenses that focus the laser radiation into the multitude of distinct spots, allowing the formation of many structures in a single stage. We show that conversion of a small part of the energy of the femtosecond beam into the second harmonic (SH) is an efficient way to increase the surface density of obtained nanostructures. By combining the fundamental frequency and the SH, one benefits both from the power of the former and from the focusing ability of the latter. This combination provides an efficient nanostructuring with sphere diameter close to the wavelength of the second harmonic. The possibility to create arrays of nanostructures with surface density above $5\times {10}^8\hspace{0.17em}{\text{cm}}^{-2}$ with femtosecond Ti:sapphire laser operating at 800 nm was demonstrated by employing 0.45 μm spheres.

Implementation of free-space quasi-phase-matched electro-optic detection is reported, based on measuring the energy variation of a femtosecond laser pulse induced by a terahertz field. Narrowband probe-energy type of detection is demonstrated using wide-aperture periodically poled $\text{Mg}:\text{Y}:{\text{LiNbO}}_3$ crystals with as-grown domain gratings. Spectra of air plasma generation measured by this method are compared with the spectrum measured by the conventional ZnTe-based probe-phase ellipsometry scheme. Besides, the conventional method was performed for both types of sources, for the air plasma generation, and for the quasi-phase-matched optical rectification in the same periodically poled $\text{Mg}:\text{Y}:{\text{LiNbO}}_3$ crystals.