Victor G. Bespalov

Femtosecond stimulated Raman scattering in pressurized gases in the ultraviolet and visible spectral ranges

We investigate femtosecond stimulated Raman scattering (SRS) in H2,D2, HD, and CH4 excited with 300-fs-duration pulses at 390-nm wavelength and with as much as 0.1 mJ of energy. We show that the SRS-generation threshold and conversion efficiency are due to the transient nature of SRS in femtosecond regime. We determine optimal conditions for efficient generation in the broad spectral range 289–797 nm and show how self-phase modulation and white-light generation limit the ultimate conversion efficiency.

Stimulated Raman scattering in hydrogen by frequency-doubled amplified femtosecond Ti:sapphire laser pulses

We present results on stimulated Raman scattering in hydrogen excited by 200-fs-duration 1-kHz repetitionrate and 0.6-W average-power Ti:sapphire (780-nm) and 0.2-W average-power second-harmonic (390-nm) laser pulses with linear and circular polarizations. Spectral and temporal measurements of the vibrational and rotational components at Stokes and anti-Stokes frequencies in the wavelength range of 289-1154 nm are reported. We observe threefold shortening of the first vibrational Stokes component pulses, excited by 400-fsduration second-harmonic pulses.

Stimulated Raman amplification of femtosecond pulses in hydrogen gas.

We report efficient amplification of weak femtosecond supercontinuum pulses by a stimulated Raman scattering process in pressurized H(2) gas excited with 350-fs-duration frequency-doubled pulses from a regenerativeamplified Ti:sapphire laser. An amplification factor of 10(9) is obtained at the wavelength of 465 nm for seed pulses produced by supercontinuum generation in glass.

Holography and phase retrieval in terahertz imaging

In this paper, we present review and latest results obtained in the scope of terahertz holographic and other methods for phase retrieval in terahertz imaging. Not only accurate change of amplitude, but also rigorous phase retrieval is essential for precise calculation of optical parameters of the samples in terahertz range. Pulse terahertz holography introduced some years ago shows itself as perfect method for overall-object phase retrieval technique, but in the same time it allows measurement with low signal to noise that leads to less precise derivation of sample optical parameters. And certainly just point-by-point terahertz time-domain spectroscopy provides the most precise information of sample phase, but it is rather time consuming and has low spatial resolution as well. The other possible way assumes, in contrary to pulse terahertz holography and spectroscopy, using narrow-band continuous terahertz source, which tunability might also make the measurement process easier. And diffraction patterns registered with microbolometer array or any other terahertz intensity sensor placed at several different distances from the object and/or taken for several different terahertz frequencies are used for phase retrieval in this case. We present both numerical predictions and experimental results for the proposed methods, estimate the achievable spatial and other limits of the techniques and compare them to the others used in different spectral ranges.

Lasing in the far IR spectral range under femtosecond optical excitation of the InAs semiconductor in a magnetic field

Generation of a coherent electromagnetic radiation in the far IR (THz) spectral range upon excitation of a semiconductor InAs crystal by 70-fs Ti: sapphire laser pulses is studied. The effect of a magnetic field of different orientation on generation in the submillimeter-wavelength range is analyzed. Placing the crystal into the magnetic field of an optimized permanent magnet with a strength of 5 kOe aligned along the surface of the semiconductor increased the power of generated radiation by a factor of six compared with that in the absence of the field. For the average pump-laser output power of 150 mW and repetition rate of 80 MHz, the average power of the THz radiation reached 100 nW. For detection of ultrashort pulses of the THz radiation, we used, for the first time, a highly sensitive uncooled optoacoustic detector, which detected signals with a power lower than 1 nW.

Phase retrieval of THz radiation using set of 2D spatial intensity measurements with different wavelengths

Using the infrared matrix of pyroelectric or other photodetectors along with THz band pass filters with pulsed or CW sources one can record the 2D intensity distribution of THz radiation with a high degree of monochromatization. This allows one to use various approaches to solving the phase problem which were developed for the visible frequencies. In this contribution we present the results of the numerical investigation of the wavefront reconstruction using THz radiation at several wavelengths and taking the intensity distribution at various distances.

Transient stimulated Raman scattering in gas mixtures.

We studied, for the first time to our knowledge, transient stimulated Raman scattering in H(2)- D(2) and H(2)- CH(4) gas mixtures excited with 200-fs and 2-ps, 390-nm pulses. Depending on the composition and partial pressure, of those gases, we observed approximately 40 output lines simultaneously. Under optimum conditions a conversion efficiency of as much as 8% in the combination Raman line at 556 nm was obtained in the H(2)- CH(4) gas mixtures. The difference between picosecond and femtosecond pumping is due to self-phase modulation and to generation of a spectral continuum in the latter. Our calculation of the ratio of the partial pressures is in a good agreement with the experimental results. A new method for measuring the unknown Raman scattering cross section of gas molecules is suggested.

Experimental studies of the possibilities of diagnosing caries in the solid tissues of a tooth by means of terahertz radiation

Spectroscopic and tomographic studies on the solid tissues of a human tooth have been carried out in the terahertz (THz) frequency range. It is experimentally shown that, in samples of dental tissue damaged by caries, the absorption lines of water in the THz region have higher intensity by comparison with the intensity of healthy tissue. The radiation intensity in the THz frequency range reflected from samples of human dental enamel damaged by caries in this case is smaller by a factor of 2.2 and from samples of dentine is smaller by a factor of 4.5 by comparison with samples of healthy tissue.

Nondestructive monitoring of aircraft composites using terahertz radiation

In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts.

Self-action of femtosecond pulses with continuum spectrum

On the basis of the analysis of the wave equations for an electrical field of radiation and without use of slowly varying envelope approximation the self-action of femtosecond light pulses in transparent media is investigated. The results of numerical simulation of spectral supercontinuum evolution, accompanying temporary broadening of intensive pulses with a spectrum in the range of normal group dispersion of medium both with only electronic nonlinearity, and with simultaneous electronic and electronic-vibrational nonlinearities are presented. The opportunity of compression of pulses with supercontinuum spectrum in light formations consisting of one cycle of an electric field is predicted. It is shown that spectral superbroadening of the elliptically polarized radiation is accompanied by nonuniform rotation of a polarization ellipse.