Sergey V. Garnov

Broadband Light-Induced Absorbance Change in Multilayer Graphene

We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pump-probe measurements in 1100-1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.

Plasma filament investigation by transverse optical interferometry and terahertz scattering

Transverse plasma distribution with 10(17) cm(-3) maximum electron density and 150 μm transverse size in a plasma filament formed in air by an intense femtosecond laser pulse was measured by means of optical interferometry. Two orders of magnitude decay of the electron density within 2 ns was obtained by combined use of the interferometry and newly proposed terahertz scattering techniques. Excellent agreement was obtained between the measured plasma density evolution and theoretical calculation.

All-optical control of ultrafast photocurrents in unbiased graphene.

Graphene has recently become a unique playground for studying light-matter interaction effects in low-dimensional electronic systems. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. In particular, strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field effects and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain. The obtained all-optical control of the photocurrents opens new routes towards graphene based high-speed and broadband optoelectronic devices.

The dependence of terahertz signal and third harmonic amplitudes on mutual polarization of two-color pump components under optical breakdown of air

In this paper we measured the dependence of the signal of the third harmonic and terahertz radiation on the mutual polarization of the first and second harmonic to compare the mechanisms of their generation. For this purpose we used the output radiation of laser system Spitfire pro with the following parameters: pulse duration 35 fs, central wavelength 800nm, repetition rate 1kHz, energy of the first and second harmonic are 2mJ and 240mJ respectively. The radiation was focused by parabolic mirror with focal length 7,5 cm in air. The energy of terahertz and third harmonic radiation was measured by Holey cell and photomultiplier respectively.

Peculiarities of the electron avalanche for the case of relatively large photon energy

Abstract. We applied a Monte Carlo simulation for studies of single-shot laser-induced breakdown in pure transparent dielectrics for the case when the photon energy is close to or exceeds mean electron energy. Avalanche ionization rate dependence on intensity was obtained by solving both the kinetic Fokker–Planck type equation and the quantum kinetic equation. Threshold scaling with pulse width was obtained in the range from 3 ps to 30 ns for several values of laser frequency and initial lattice temperature. Lattice heating influence (during the laser pulse action) on avalanche ionization rate was studied.

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.

The nanosecond optical parametric amplifier of a weak signal based on BBO crystals

Parameters of the optical parametric amplifier (OPA), based on two BBO crystals were studied. The OPA was made with the schematic of the extraordinary wave walk off compensation. Efficient amplification of the weak signal (λ = 1053 nm) in the field of the strong pumping wave (λ = 532 nm) was obtained. The measured value of the amplification was equal to ~106. The noise level of the parametric amplifier was less than 10−3 from the signal level.

All-optical control of photon drag current in graphene

Strong and broadband light absorption in graphene allows one to achieve high carrier densities essential for observation of nonlinear optical phenomena making graphene a unique playground for studying many-body effects. Being of strong fundamental importance, these effects also open a wide range of opportunities in photonics and optoelectronics. Here, we make use of strong photon-drag effect to generate and optically manipulate ultrafast photocurrents in graphene at room temperature. In contrast to the recent reports on injection of photocurrents in graphene due to external or built-in electric field and by quantum interference, we force the massless charge carriers to move via direct transfer of linear momentum from photons of incident laser beam to excited electrons in unbiased sample. Direction and amplitude of the drag-current induced in graphene are determined by polarization, incidence angle and intensity of the obliquely incident laser beam. We also demonstrate that the irradiation of graphene with two laser beams of the same wavelength offers an opportunity to manipulate the photocurrents in time domain.

Femtosecond laser breakdown of gases and transparent solid states: ultrafast space-time and spectrum-time resolved diagnostics of multicharged microplasma

We present the results of experimental studies of formation and evolution of multiply ionized (multicharged) laser micro-size plasma produced in gases (air, nitrogen, argon and helium) and inside the transparent solids (fused silica) with high intensity (up to ~ 1017 W/cm2), ultrashort (τ ~100 fs), 800nm/400nm laser pulses tightly focused in a region of ~1.5 μm in diameter. The measuring techniques and experimental setups for generation and precise optical diagnostics of laser-induced plasma - pump-probe microinterferometry and ultrafast spectroscopy are described. The measured spatiotemporal distributions of plasma refractive index/electron density and plasma spectra are demonstrated. In the experiments, the main attention was paid to the most intriguing initial stage of ultrafast plasma formation and evolution characterized by strong laser-matter and laser-plasma coupling resulting in efficient photoionization of material and plasma heating. We found out that the almost complete ionization (down to nuclei) of the initial gas occurs even at the initial stage of plasma formation. Besides, it was observed, for the first time, that a characteristic time of laser plasma formation considerably (in times) exceeds the duration of the pump laser pulse. This postionization process is attributed to impact ionization of plasma by hot electrons heated due to inverse bremsstrahlung. A theoretical model describing the mechanism of plasma postionization by hot photoelectrons was proposed. We compare the results of the experiments with what the theory predicts - the results of electron density calculations are in good agreement with the experimental data. The dynamics of plasma emission (spectral continuum and spectral line formation) in UV-visible spectral range was investigated with a picosecond time resolution applying the developed ultrafast streak-camera-based spectrometer. The spatiotemporal distributions of refractive index of laser irradiated fused silica were recorded with pump-probe microinterferometry. It was demonstrated that the induced refractive index of laser-matter interaction area changes its sign from the positive to the negative during the laser irradiation and again to the positive one after the laser pulse ends.