S. V. Chekalin

Generation of 30 μJ single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification

We report the generation of 30 microJ single-cycle terahertz pulses at 100 Hz repetition rate by phase-matched optical rectification in lithium niobate using 28 mJ femtosecond laser pulses. The phase-matching condition is achieved by tilting the laser pulse intensity front. Temporal, spectral, and propagation properties of the generated terahertz pulses are presented. In addition, we discuss possibilities for further increasing the energy of single-cycle terahertz pulses obtained by optical rectification.

Fabrication of fiber Bragg gratings with 267 nm femtosecond radiation

Strong high-quality fiber Bragg gratings with photoinduced refractive-index modulation of more than 10-(3) were written in a Corning SMF-28 fiber, a P(2)O5-doped-core fiber and a pure-silica-core fluorine-doped-cladding fiber by third-harmonic radiation (267 nm, 150 fs and 1.2-1.8x1011 W/cm(2)) of a femtosecond Ti:sapphire laser using a phase mask. We compare the 267-nm photosensitivity responses with the results of irradiation by 193-nm ArF and 157-nm F(2) excimer lasers. The dependence of the refractive-index change on the exposure dose and the annealing characteristics of the fabricated gratings are typical for Type-I UV-written fiber gratings.

Spectral Modification of Femtosecond Laser Pulses in the Process of Highly Efficient Generation of Terahertz Radiation via Optical Rectification

The narrowing and “red” shift of the spectrum of a femtosecond laser pulse due to the high-efficiency transformation to terahertz radiation by means of optical rectification in a LiNbO3 crystal are experimentally observed. These changes are in good agreement with the description of the appearance of the terahertz radiation as the difference-frequency generation and make it possible to determine the energy of terahertz pulses. A photon-number transformation coefficient larger than 50% is obtained.

Light bullets from near-IR filament in fused silica

The formation of light bullets during femtosecond laser pulse filamentation in the presence of anomalous group velocity dispersion has been recorded for the first time. The minimum experimentally detected width of the light bullet autocorrelation function is 27 fs, which corresponds to a duration of about 13.5 fs. The duration of the light bullet at a wavelength of 1800 nm is about two periods of the light field oscillation. The numerically calculated width of the autocorrelation function for such a light bullet is 23 fs, which is in good agreement with the experimental value.

Anti-Stokes wing of femtosecond laser filament supercontinuum in fused silica.

We have demonstrated that in the IR pulse filament the anomalous dispersion of fused silica leads to the formation of an isolated anti-Stokes wing (ASW), which is located in the visible region of the supercontinuum (SC). It is shown that the isolated ASW is formed by the interference of the light field of a SC undergoing anomalous group velocity dispersion.

Light bullets from a femtosecond filament

The scenario of the formation of light bullets in the presence of anomalous group velocity dispersion is presented within the same general scenario for condensed matter and humid air. The temporal and spectral parameters of light bullets during filamentation in fused silica and humid air are obtained. A light bullet (LB) is a short-lived formation in a femtosecond filament with a high spatiotemporal light field localization. The sequence formation of the quasi-periodical LB is obtained numerically and is confirmed experimentally by autocorrelation measurements of the LBs duration. The estimation of the LB duration reaches few-cycle value. It is established that the generation of each LB is accompanied by the ejection of a supercontinuum (SC) in the visible spectrum and an isolated anti-Stokes wing is formed in the visible area of the SC as a result of destructive interference of broadband spectral components. It was found that the energy of a visible SC increases discretely according to the number of LBs in the filament. We demonstrated that the model of ionization in solid dielectric which is used in numerical simulation fundamentally affects the obtained scenario of LB formation. The possibility of the formation of LBs under the filamentation of middle-IR pulses in the atmosphere was shown with numerical simulation.

Two-photon absorption of powerful femtosecond pulses in C60 film

Abstract Irradiance-dependent transmission measurements were conducted for thin C60 film irradiated by 300 fs laser pulse at 612 nm in the intensity range from 108 W/cm2 to the values ≈ 1012 W/cm2 corresponding to the film damage. The upper limit for the two-photon absorption coefficient was derived from these measurements as (3 ± 1.5) × 10−8 cm/W, which corresponds to an Im χ(3) values of (2.6 ± 1.3) × 10−11 esu.

Giantically blue-shifted visible light in femtosecond mid-IR filament in fluorides.

A giant blue shift (more than 3000 nm) of an isolated visible band of supercontinuum was discovered and studied in the single filament regime of Mid-IR femtosecond laser pulse at powers slightly exceeding critical power for self-focusing in fluorides. At the pulse central wavelength increasing from 3000 nm to 3800 nm the spectral maximum of the visible band is shifted from 570 nm and 520 nm up to 400 nm and 330 nm for BaF(2) and CaF(2), respectively, its spectral width (FWHM) being reduced from 50 - 70 nm to 14 nm. It is shown that the formation of this narrow visible wing is a result of the interference of the supercontinuum components in the anomalous group velocity dispersion regime.

Investigation of coherent phonons in bismuth by femtosecond laser and X-ray pulse probing

Using femtosecond laser pulses, coordinated oscillations (coherent phonons) of Bi single-crystal atoms have been excited and recorded. The comparison with experimental results on the X-ray probing of coherent phonons at the same excitation energy density demonstrates that the observed lifetime and frequency shift of the oscillations are similar in both cases. Moreover, it has been revealed that the relaxation (incoherent) contributions are identical. This coincidence of the photoinduced response parameters indicates that probing in the visible spectrum correctly reflects the coherent dynamics of the Bi crystal lattice.

Reactions induced in (CF3I)n clusters by femtosecond UV laser pulses

The excitation and ionization of CF3I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF3I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF3I)n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I2+ ion is produced in reactions induced in (CF3I)n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I2+ ions with characteristic times τ1 ≈ 1 ps and τ2 ≈ 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.