Second harmonic generation of a self-focused Hermite-Gaussian laser in plasma

Second harmonic generation (SHG) using intense Hermite-Gaussian laser beam (HGLB) propagating through the plasma for mode-indices m = 0 and m = 1 is reported in the present work. Ponderomotive force induced density perturbation beats with the oscillatory velocity of electrons at incident laser frequency, generate the second harmonic nonlinear current that give rise to SHG. Using paraxial approximations, we have derived the coupled equations for the beam width parameter of HGLB and second harmonic's normalized amplitude. Resonance condition is obtained by considering wiggler magnetic field which providing an extra momentum to the second harmonic photon and this result a significant increase in the amplitude of SHG. Our analysis shows the prominent rise in normalized amplitude of second harmonic on increasing the value of the intensity of fundamental laser pulse, normalized wiggler magnetic field and normalized density of plasma. It is notified that the gain of SHG is more prominent for m = 1. Dependency of laser and plasma parameters on SHG is also reported in the current work.     

High-order harmonic generation in nanoparticle-containing laser-produced plasmas

Nanoparticle-containing media can be used for the efficient high-order harmonic generation (HHG) of laser radiation in the extreme ultraviolet range. We review the results of recent studies of the HHG in laser-produced plasmas containing Ag, Au, Pd, Pt, Ru, GaN, BaTiO₃, and SrTiO₃ nanoparticles. The harmonics of femtosecond radiation up to the 55th order were achieved using the nanoparticle-containing plumes, when the femtosecond radiation propagated through the preformed plasma. These results are compared with the high-order harmonics generated from the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse. We discuss a six-fold enhancement of the HHG yield, which was achieved in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas.     

Relativistic second-harmonic generation in a laser-produced plasma

Summary In this paper the relativistic second-harmonic generation of a high-power laser radiation in a laser-produced plasma has been studied theoretically in the presence of a self-generated magnetic field. The relativistic Vlasov equation has been employed for the nonlinear response of the electrons in the hot magnetized plasma. It is observed that the power conversion efficiency of the generated second harmonic wave is much higher for relativistic calculations than that for nonrelativistic calculations. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Second harmonic generation in plasma in the presence of SBS

The mechanism of second harmonic generation in the presence of intense-radiation SBS in plasma is considered under the condition that the incident beam has no time for filamentation. The mechanism is based on the formation of low-frequency plasma-density perturbations that are optimal for this effect in the field of scattered waves. The theoretical calculations are compared with the experimental data.     

Second harmonic generation in InSe

Experimental evaluation of non-linear susceptibility coefficient d₂₂ for InSe, which is responsible for second harmonic generation in this material at 1.06 and 10.6 μm, is reported and the value compared with GaSe. At the CO₂ laser frequency of oscillation, phase matching has been observed and a value of d₂₂ almost equal to d₁₅ for CdSe has been found. The InSe high transparency in the wavelength range 1.1–22 μm, the sizeable value of d₂₂ and the possibility of reaching phase matching condition in the IR range make InSe a potential device for far IR frequency conversion.     

Second harmonic generation in graphene

Second harmonic generation in graphene has been theoretically studied. Phenomenological analysis based on symmetry arguments is carried out. It is demonstrated that second harmonic generation in ideal graphene samples is possible only if the radiation wave vector or its magnetic field is taken into account. Microscopic theory is developed for the classical regime of radiation interaction with electrons, where the photon energy is much lower than the characteristic energy of charge carriers. It is demonstrated that the emitted radiation can be strongly circularly polarized for the linearly polarized incident wave.     

Second harmonic generation in a polarized Dirac vacuum

The Dirac vacuum, polarized by a strong electric field E₀, is discussed as nonlinear medium for laser radiation (LR). It is shown that such a medium leads to (a) a LR refractive index appearing, (b) LR polarization plane rotation, (c) LR second harmonic generation. It is proposed to use these effects for E₀ field diagnostics.     

Mechanisms for three-halves harmonic emission from laser-produced plasma

Three halves harmonic emission from thin foil targets irradiated with high power ( > 10¹⁶ W cm^-2) Nd laser pulses is reported. Spectral differences between forward and backscattered 3ω₀/2 emission are seen, explained by plasmon/photon and three plasmon recombination.     

Second harmonic generation in order-disorder ferroelectrics

We investigate the temperature dependence of second harmonic generation in order-disorder ferroelectrics. We obtain the qualitative temperature dependence of the second harmonic amplitude; the amplitude goes to zero when the temperature reaches the Curie point. We take into account the disruption of wave number harmony when the temperature is changed. We show the second harmonic is largest at the temperature where the wave numbers are synchronized and there is no detuning. The theoretical conclusions reached here are confirmed by experimental data. Volgogradskii State University, Volgograd. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 55–58, January, 1999.     

Second harmonic generation in squaric acid

From the temperature-dependent optical second harmonic generation in squaric acid we conclude that no local symmetry breaking occurs in the high temperature phase of this material.     

Second harmonic generation in centro-symmetric semiconductors

Second harmonic generation is observed in Si and Ge at electron-hole plasma densities of 10²¹-10²² cm^?3 and found to depend strongly on crystal orientation. We suggest that a lower crystal symmetry within the absorption volume is consistent with a model in which depletion of covalent bond charge, and subsequent lattice softening, is brought about by laser excitation of a high density of electrons into antibonding states.     

Second harmonic generation in deposited clusters

We analyse theoretically the generation of second and higher harmonics for sodium clusters deposited on an insulating surface. To this end, we use the time-dependent local-density approximation solved on a three-dimensional grid. We explore the impact of the various laser parameters (intensity, frequency, polarisation) on the efficiency of second harmonic generation. The success sensitively depends on a proper tuning of these parameters. We find optimum conditions for which the laser frequency is half the resonance frequency of the system, if the polarisation is directed orthogonal to the surface of the substrate, and if the intensity is large but safely below the critical value for destruction of the electron cloud. Received 22 October 1999 and Received in final form 14 December 1999     

Phase-matched third harmonic generation in a plasma

Relativistic third-harmonic generation in a plasma is investigated. The growth of a third-harmonic wave is limited by the difference between the phase velocity of the pump and driven waves. This phase velocity mismatch results in a third-harmonic amplitude saturation and oscillation. In order to overcome this saturation, a phase-matching scheme based on a resonant density modulation is described. The limitations of this scheme are analyzed     

Second harmonic generation of high power Cosh-Gaussian beam in cold collisionless plasma

The purpose of this study is to explore the second harmonic generation (SHG) of a high power Cosh-Gaussian beam in cold collisionless plasma. The ponderomotive force causes carrier redistribution from high field to low field region in presence of a Cosh-Gaussian beam thereby producing density gradients in the transverse direction. The density gradients so produced the results in electron plasma wave (EPW) generation at the frequency of the input beam. The EPW interacts with the input beam resulting in the production of 2nd harmonics. WKB and paraxial approximations are employed for obtaining the 2nd order differential equation describing the behavior of the beam's spot size against normalized distance. The impact of well-established laser-plasma parameters on the behavior of the beam's spot size and SHG yield are also analyzed. The focusing behavior of the beam and SHG yield is enhanced with an increase in the density of plasma, the radius of the beam and the decentred parameter, and with a decrease in the intensity of the beam. The results of the current problem are really helpful for complete information of laser-plasma interaction physics.     

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.     

Second harmonic generation in birefringent optical fibers

Second harmonic generation in optical fibers is investigated. A scheme to combine material, modal and geometrical dispersion to achieve phase matching is proposed. Conversion efficiency, as a function of the fiber's parameters and the incoming electric field strength, is computed.     

Second harmonic generation in zinc oxide nanorods

Second-order optical nonlinearities of zinc oxide (ZnO) nanorods grown on quartz substrate were determined by optical second harmonic generation (SHG) measurements at 1064 nm fundamental wavelength. The average length of the zinc oxide nanorods ranged from 50 nm to 700 nm. By employing the Maker fringes technique, we obtained the second-order nonlinear optical coefficients d₃₃₃ and d₃₁₁. Their magnitudes and ratio are compared with that of zinc oxide thin film fabricated by different techniques. We see variations of the second-order nonlinear optical coefficients with respect to the aspect ratio of the nanorods. This is attributed to local field effects. PACS 42.65.Ky; 78.67.-n; 81.07.-b     

Second harmonic generation in Ga nanoparticle monolayers

Ga nanoparticle monolayers formed by evaporation-condensation in ultrahigh vacuum and embedded in a transparent SiO_x matrix generate second harmonic (SH) signals in transmission and reflection when illuminated by a 150 fs, 800 nm laser pulses. The observed SH light exhibits a critical dependence on input and output polarizations, angle of incidence and azimuthal orientation of the samples. The results lead to a consistent picture of shape and orientation of the nanoparticles. Linear transmittance spectra in the visible range support these findings and the observed size dependence of the SH signal. Received 29 November 2000     

Frequency conversion in laser-produced boron plasma using longitudinal pump scheme

The high-order harmonic generation from boron plasma was studied using the longitudinal pump scheme. The harmonics up to the 57th order (λ=13.96 nm) were observed. The characteristics of plateau in the range of 15th to 57th harmonics were analyzed depending on fundamental radiation intensity and the saturation of harmonic cutoff was observed. The harmonic cutoff energy was restricted by the self-defocusing of femtosecond radiation. A comparison of longitudinal and orthogonal pump schemes of boron plasma excitation was presented.