On harmonic generation in a photoionized gas

The laws characterizing the radiation of high harmonics due to the coherent bremsstrahlung effect are indicated in the limit of high intensity of the laser pump photoionizing a gas in regime of suppression of the ionization barrier. It is shown that the intensity of the harmonics is determined by the quantum properties of the electron distribution in an atom before it is ionized. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 486–490 (10 April 1999)     

Controlling the divergence of high harmonics from solid targets: a route toward coherent harmonic focusing

Harmonic generation from relativistically oscillating plasma surfaces formed during the interaction of high contrast lasers with solid-density targets has been shown to be an efficient source of extreme ultraviolet (XUV) and X-ray radiation. Recent work has demonstrated that the exceptional coherence properties of the driving laser can be mirrored in the emitted radiation, permitting diffraction limited performance and attosecond phase locking of the harmonic radiation. These unique properties may allow the coherent harmonic focusing (CHF) of high harmonics generated from solid density targets to intensities on the order of the Schwinger limit of 10²⁹ W cm^-2 with laser systems available in the near future [Phys. Rev. Lett. 93, 115002 (2004)] and thus pave the way for unique experiments exploring the nonlinear properties of vacuum on ultra-fast timescales. In this paper we investigate experimentally as well as numerically the prospect of focusing high harmonics under realistic experimental conditions and demonstrate, using particle in cell (PIC) simulations, that precise control of the wavefronts and thus the focusability of the generated harmonics is possible with pre-shaped targets.     

Interference and spectral broadening of higher harmonics of laser radiation in plasma torches containing ions,nanoparticles, and fullerenes

We analyze the interference between two processes of higher harmonic generation (HHG) in plasma containing mixtures of different materials (silver and gold nanoparticles, as well as graphite and boron). We find that, for mixtures and individual ingredients, the limiting orders of generated harmonics of laser radiation approximately coincide with one another. At the same time, for plasma torches formed by the ablation of mixtures of materials, the HHG efficiency is considerably reduced compared to the case of the frequency transformation of laser radiation in individual ingredients of these mixtures as a result of destructive interference in the former case. We demonstrate a considerable spectral broadening of harmonics generated in laser plasma with pulses passed through filaments formed in air. In this case, the HHG efficiency increases fourfold (from 3 × 10⁻⁶ to 1.2 × 10⁻⁵) compared to the case of radiation free of phase and frequency modulation. The generation of harmonics is also observed upon the passage of 120-fs laser pulses through plasma containing fullerenes. In this case, the limiting value of generated harmonics achieves the 33rd order. The efficiency of harmonics in fullerene plasma considerably exceeds a similar process in silver plasma.     

High harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

New methods for determining the polarization state of vacuum ultraviolet radiation

Two new methods are proposed for determining the polarization of vacuum ultraviolet radiation which permit the determination of an arbitrary polarization mode for photons with energies of 10–100 eV. The essence of these methods is to create and detect a nonequilibrium population of the magnetic sublevels of atoms and molecules excited by the original VUV radiation and then determine the polarization of this radiation based on these measurements in accordance with known formulas. Zh. Tekh. Fiz. 69, 115–122 (September 1999)     

Generation of a continuum and stimulated Raman scattering harmonics during scattering of electromagnetic radiation of relativistic intensity

A theory of the propagation instability of plane, monochromatic, circularly polarized electromagnetic waves of relativistic intensity in matter is developed for a spatially three-dimensional geometry including arbitrary polarization of the scattered radiation. Harmonic generation owing to striction and relativistic nonlinearity is examined, as well as scattering owing to electron recoil, the decay instability of the harmonics with formation of scattered electromagnetic waves (Stokes components of the stimulated Raman scattering and plasmons), the interaction of electromagnetic waves in the plasma (antistokes stimulated Raman scattering), and the generation of a radiative continuum. The transition of the three-dimensional theory to a one-dimensional problem in the nonrelativistic limit is discussed. Zh. éksp. Teor. Fiz. 113, 2034–2046 (June 1998)     

Gain coefficient of a high harmonic emitted during the above-threshold ionization of atoms

The problem of the amplification of high harmonics generated during the above-threshold ionization of atoms in a high-power laser wave field is examined for the first time. An estimate of the gain coefficient as a function of the parameters of the atom beam and the pump wave is given. Zh. éksp. Teor. Fiz. 115, 1630–1641 (May 1999)     

Enhanced harmonic generation in C60-containing plasma plumes

The results of enhanced harmonic generation in fullerene-containing plasma plumes using laser radiation of different wavelengths, pulse durations, and phase modulation are presented. Odd and even harmonics up to the 29^th order of 800 nm radiation have been extended in the case of two-color orthogonal polarization pump of C₆₀-containing plasma plumes. The conversion efficiency for the odd and even harmonics in the vicinity of SPR of the C₆₀-containing plasma (40–70 nm) was estimated to be in the range of 10⁻⁴.     

Theory of gyro devices with thin electron beams (large-orbit gyrotrons)

It is shown for a precisely aligned electron beam of small thickness in a large-orbit gyrotron that high cyclotron harmonics (up to s⩽5) can be selectively excited with an electronic efficiency that is acceptable for some applications (1–10%). When the quality of the electron beam is high, the selective properties are maintained even for modes with high radial indices, raising hopes that this method can be used to obtain coherent radiation not only in the millimeter but also in the entire submillimeter wavelength range. A method for taking into account the space-charge effects is developed, and it is shown that the corresponding effects can be important at relatively small values of the electron pitch factor. The results of the analysis and preliminary experiments reveal the possibility of creating high-power compact sources of submillimeter radiation for the spectroscopy of various media, the diagnostics of dense plasmas, and some other applications. Fiz. Tekh. Poluprovodn. 68, 91–98 (October 1998)     

Parameters for the inclined-path sensing of molecular hydrogen in the atmosphere by lidar with a YAG:Nd laser

The lidar equation for the vibrational backscattering of neodymium laser radiation and its harmonics by hydrogen molecules is solved numerically. Inclined paths in the atmosphere are investigated with the aim of selecting the transmitter wavelength for detecting the lowest concentrations of hydrogen. Zh. Tekh. Fiz. 68, 20–22 (January 1998)     

Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1×10^-7 to 8×10^-6, depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (λ=61.2 nm) at a plateau region with the efficiency of 8×10^-5 in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved. PACS 42.65.Ky; 52.35.Mw; 52.38.-r     

Relativistic Nonlinear Optical Phenomena in the Field of Subterawatt Laser Pulses

Experiments on the generation of high optical harmonics demonstrating that relativistic regimes of interaction of radiation with matter can be implemented in the field of mid-infrared laser pulses with a peak power of 0.3 TW have been reported. The observation of relativistic phenomena at such extraordinary low peak powers of the laser field becomes possible because of the formation of a high-quality space-time mode of the laser field with an exactly specified polarization state. Such a field structure ensures a high intensity of radiation in the focus of the beam and the effective acceleration of electrons by a low-frequency electromagnetic field of a high-contrast laser pulse with an exactly specified polarization at the extremely sharp vacuum-solid interface.

     

Optical absorption in a nanotube with helical symmetry upon longitudinal polarization of radiation

The absorption of linearly polarized radiation and the magnetically induced photogalvanic effect in a nanotube are investigated using a model in which the helical symmetry of the nanotube is determined by the extended δ potential. The plane of polarization of radiation is oriented along the axis of the nanotube. Analytical expressions are derived for the steady-state current generated upon absorption of linearly polarized radiation with low and high intensities.     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

Dynamic polarization of nuclei in a self-organized ensemble of quantum-size n-InP/InGaP islands

Dynamic polarization of ³¹P nuclei is observed in a self-organized system of InP islands grown by metalorganic-hydride epitaxy in an InGaP matrix. The polarized nuclei produce an effective magnetic field which acts on the polarization of the excitonic radiation. Optical detection of the magnetic resonance signal from ³¹P nuclei in the crystal lattice of nanosize InP islands is successfully carried out. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 9, 711–714 (10 November 1998)     

Polarization of radiation of distributed-feedback dye lasers with one- and two-component gain media

Polarization of the radiation generated by a distributed-feedback laser on one-component solutions and binary mixtures of coumarin dyes is investigated. It is established that, with a marked excess of the generation threshold, the degree of polarization P of the radiation generated by one-component solutions is practically independent of the wavelength. For binary mixtures, the spectral behavior of P depends on the donorlacceptor concentration ratio. Here, with the anisotropy of the distribution of excited molecules being partially disrupted in the process of energy transfer (from the donor to the acceptor), the degree of polarization decreases appreciably when radiation frequency is tuned from the short-wavelength region to the long-wavelength one. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 340–343, May–June, 2000.     

Nonlinear dielectric susceptibility in SrTiO3 and possible nature of the low-temperature phase

The nonlinear dielectric susceptibility of SrTiO₃ is studied experimentally. Intense higher-order harmonics of the polarization, whose amplitudes grow with decreasing temperature and saturate at T<4 K, are observed in the temperature range T<50 K. Hysteresis phenomena, differences in the zero-field cooling (ZFS) and field cooling (FC) regimes for the higher-order harmonics, are observed when a constant electric field is applied. The existence in SrTiO₃ of a strongly frustrated state, which arises when the interaction of the structural and ferroelectric order parameters is taken into account, is proposed. The low-temperature state of SrTiO₃ is analyzed on the basis of a phase diagram which is similar to that of a spin glass [S. Kirkpatrick and D. Sherington, Phys. Rev. B 17, 4384 (1978); J. R. L. De Almeida and D. J. Thouless, J. Phys. A 11, 983 (1978)]. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 634–639 (25 April 1996)