A. N. Stepanov

Highly efficient optical-to-terahertz conversion in a sandwich structure with LiNbO 3 core

We demonstrate experimentally the efficiency of a recently proposed scheme of terahertz generation based on Cherenkov emission from ultrashort laser pulses in a sandwich structure. The structure has a thin nonlinear core covered with a prism of low terahertz absorption. Using an 8 mm long Si-LiNbO(3)-BK7 structure with a 50 microm thick LiNbO(3) core, we converted 40 microJ, 50 fs Ti:sapphire laser pulses into terahertz pulses of approximately 3 THz bandwidth with a record efficiency of over 0.1%.

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.

Terahertz generation by tilted-front laser pulses in weakly and strongly nonlinear regimes

Terahertz generation by femtosecond laser pulses with tilted intensity front in room-temperature and cryogenically cooled LiNbO3 crystals was investigated. The role of the interaction length and pump pulse duration was studied for weak and strong laser pump. It was shown that the optical-to-terahertz conversion efficiency is saturated as a result of the Kerr self-phase modulation of the optical pump.

Efficient terahertz generation by optical rectification in Si-LiNbO3-air-metal sandwich structure with variable air gap

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.

A high-speed quasi-optical wave phase switch based on the induced photoconductivity effect in silicon

A new microwave switch is proposed, in which the active element represents a metal reflector with a semiconductor plate on a mirror surface. The wave beam phase switching is ensured by a conducting layer formed in the semiconductor by laser radiation with quantum energy approximately equal to the bandgap width. Using a disk of high-purity silicon irradiated by a pulsed Ti:sapphire laser, a 180° phase switching in a 30-GHz wave beam has been studied. At a low microwave power level, the wave phase was effectively switched during a time on the order of several nanoseconds.

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.

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.

Filament formation beyond linear focus during femtosecond laser pulse propagation in air

The threshold relation between the strength of laser beam focusing and its peak power is determined for post-focal filamentation on the basis of the experimental data and numerical simulations. The dynamics of the post-focal filamentation can vary (propagating or reconstruction type) depending on the strength of linear beam focusing.

Two-photon luminescence imaging by scanning near-field optical microscopy for characterization of gold nanoparticles

Spatial distribution of a two-photon luminescence signal when gold nanoparticles on the surface are scanned by the tip of an atomic force microscope irradiated by femtosecond laser radiation was studied in an experiment. Comparison of the experimental data with the results of numerical simulation of two-photon luminescence based on a finite-difference time-domain calculation of electric field distribution in the nanoparticle–tip system can give information on the shape and orientation of the nanoparticles.