Second-harmonic Yield Research Articles

Influence of self-focused Laguerre-Gaussian laser beam on second harmonic generation in collisionless plasma having density transition

The influence of self-focused Laguerre-Gaussian beam (L-G beam) on second harmonic generation (SHG) in collisionless plasma with density transition is explored in present work. In presence of collisionless nonlinearity, the ponderomotve force causes redistribution of electrons thereby establishing transverse density gradients inside plasma. These density gradients causes production of electron plasma wave(EPW) at main beam frequency. The nonlinear interaction of EPW with main beam produces 2nd harmonics. The authors have employed the moment theory in order to obtain differential equation controlling the growth of beam waist and second harmonic yield (SHY). It is perceived from results that L-G beam modes and density transition play a major role in increasing beam’s focusing behavior and SHY.

Optical guiding of intense Hermite–Gaussian laser beam in preformed plasma channel and second harmonic generation

This work presents the scheme of optical channeling of the intense Hermite Gaussian laser beam and second-harmonic generation in plasma having the preformed plasma channel, where relativistic nonlinearity is operative. Excitation of the electron plasma wave at the incident beam frequency leads its coupling with the latter produces the second harmonics of the beam. For the formulation of differential equations for the beam waists of the Hermite Gaussian laser beam propagating through the channel, the method of moments has been used. The solutions of the coupled differential equations are found using Runge Kutta fourth-order numerical method. Perturbation theory has been applied to find the equation governing the excitation of electron plasma wave and hence the source term for the second-harmonic yield. It has been observed that the preformed plasma channel helps to optically guide the laser beam and enhances the efficiency of second-harmonic generation of various modes of the Hermite Gaussian laser beam in plasma.

Enhanced second harmonic generation of dark hollow Gaussian laser beam in collisionless magneto-plasma

This paper presents an analysis and subsequent discussion of self-focusing and second harmonic generation (SHG) of a dark hollow Gaussian (dhG) laser beam in a magneto plasma, considering ponderomotive non-linearity. As a result of collisionless non-linearity, non-uniform distribution of plasma electrons takes place due to non-uniform irradiance of intensity along the wavefront of the laser beam which leads to the self-focusing of laser beam and production of density gradient in the transverse direction. As a result of density gradient, an electron plasma wave is excited at pump frequency that interacts with the pump beam to produce its second harmonics of frequency twice that of latter. To envision the spot size and dynamics of propagation of dhG laser beam, moment theory in Webtzel-Kramers-Brillouin (WKB) approximation has been used. The nature of self-focusing and second harmonic yield (SHY) is highlighted through critical curves as a plot of dimensionless beam width parameter and power of second harmonic radiation versus the dimensionless distance of propagation. The effect of magnetic field on self-focusing and SHY of various orders of dhG laser beam has also been explored.

Generation of second harmonics by a self-focused Hermite-Gaussian laser beam in collisionless plasma

In this work, the generation of second harmonics of a Hermite–Gaussian laser beam in collisionless plasma has been presented. On incidence of the Hermite–Gaussian laser beam in plasma, the charge carriers shift from the high field region to the low field region on account of the ponderomotive force which results in the generation of a transverse density gradient in the background plasma which in turn generates plasma waves at incident beam frequency ω0. Interaction of this plasma wave with the pump beam generates the second harmonics of the incident laser beam with frequency 2ω0. The moment theory approach has been used to derive the coupled differential equations for the beam widths of the laser beam in the transverse x- and y-directions which are further solved numerically. The effect of different modes and initial beam widths of the Hermite–Gaussian beam in the x- and y-directions has been investigated for the self-focusing and second harmonic yield (SHY) of the laser beam in plasma. Also, the effect of increasing plasma density is visualized on the self-focusing and SHY of the beam. It has been observed that the SHY significantly depends on different modes and initial widths of the Hermite–Gaussian laser beam as well as on plasma density.

Generation of second harmonics of intense Hermite–Gaussian laser beam in relativistic plasma

AbstractIn this paper, the scheme of generation of second harmonics of incident electromagnetic wave having a Hermite–Gaussian intensity profile in an under dense relativistic plasma has been presented. The relativistic mass variation of electrons by the intense electric field of incident beam generates the density gradients in background plasma which further excites the electron plasma wave (EPW) at resonant frequency and coupling of the EPW with the incident beam results in the generation of second harmonics of incident beam. Propagation dynamics of the Hermite–Gaussian laser beam in plasma has been studied by the formulation of differential equation for the spot size of the laser beam with the help of method of moments. Numerical simulations have been carried out to solve the differential equation for the dimensionless beam width parameters. Solution of the nonlinear wave equation for the electric field vector of second harmonics of incident beam gives the expression for second-harmonic yield. It has been observed that second-harmonic yield is affected by the different modes of Hermite–Gaussian laser beam in relativistic plasma.

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

This work presents a theoretical approach for self-focusing as well as second harmonic generation of a Hermite-Gaussian (HG) laser beam in collisional plasma. On incidence in plasma, high electric field of HG laser beam causes the nonuniform distribution of plasma electrons as a result of collisional nonlinearity and thus creates the refractive index gradient which causes the wavefront to collapse about the centre and form a tight focus. Moment theory approach has been used to derive the coupled differential equations governing the spot size of the HG laser beam in both the transverse directions x and y. Second harmonics of the HG laser beam is generated by the excitation of the electron plasma wave at pump frequency which further interacts with the pump beam to generate the radiation with double the frequency of the later. Different mode indices of HG laser beam have significant effect on the self-focusing as well as on second harmonic generation of the laser beam in plasma. It has been observed that for higher mode indices (TEM02) second harmonic yield (SHY) is greater.

Experimental observation of second-harmonic generation and diffusion inside random media

We have experimentally measured the distribution of the second-harmonic intensity that is generated inside a highly scattering slab of porous gallium phosphide. Two complementary techniques for determining the distribution are used. First, the spatial distribution of second-harmonic light intensity at the side of a cleaved slab has been recorded. Second, the total second-harmonic radiation at each side of the slab has been measured for several samples at various wavelengths. By combining these measurements with a diffusion model for second-harmonic generation that incorporates extrapolated boundary conditions, we present a consistent picture of the distribution of the second-harmonic intensity inside the slab. We find that the ratio l2ω/Lc of the mean free path at the second-harmonic frequency to the coherence length, which was suggested by some earlier calculations, cannot describe the second-harmonic yield in our samples. For describing the total second-harmonic yield, our experiments show that the scattering parameter at the fundamental frequency k1ωl1ω is the most relevant parameter in our type of samples.

Second harmonic generation from resonantly excited arrays of gold nanoparticles

We show that second harmonic generation from lithographically prepared arrays of symmetric gold nanorods can be increased by two orders of magnitude by choosing the nanoparticle size to be resonant with the 800-nm wavelength of the 50-fs pump laser. The angular variation of the second-harmonic yield, which is defined by the pitch of the nanorod array, can be predicted using standard diffraction theory. This in turn makes it possible to bound approximately the relative contributions of dipole and quadrupole oscillations to the total second-harmonic yield; the two contributions appear to be of similar magnitude. Resonant ultrafast irradiation also changes the nanorod morphology, apparently due to surface melting and refreezing. At higher fluence, the intensity dependence of the second-harmonic yield changes from quadratic to cubic, an indication that the reshaping influences the mechanism of second-harmonic generation.

Second-harmonic generation in strongly scattering porous gallium phosphide

Second-harmonic generation in a strongly scattering macroporous GaP layer with typical sizes of GaP nanocrystals ranging from 200 to 500 nm is studied using nanosecond pulses of an optical parametric oscillator. As the pump wavelength decreases from 1.6 to 1.0 μm the second-harmonic yield from porous GaP increases more than one order of magnitude relative to the second-harmonic yield from crystalline GaP. This enhancement of the second-harmonic generation for shorter wavelengths correlates with the increase in the efficiency of light scattering, indicating the importance of scattering effects in nonlinear-optical processes in strongly scattering random media .

Plasma wave and second-harmonic generation of intense laser beams due to relativistic effects

This article studies the relativistic self-focusing of an intense laser beam in underdense plasma. Due to relativistic self-focusing of the laser beam, a transverse intensity gradient is created, and this can generate a plasma wave at the pump wave frequency. The mechanism of the plasma wave, second-harmonic generation, and the parameters which govern the second-harmonic yield, have been studied in detail. For a typical laser with wavelength (λ=1.053 μm), intensity (1018 W cm−2) and a plasma having density (ωp/ω0=0.25), the second-harmonic intensity was found to be around (1014 W cm−2).

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Optical Second-Harmonic Generation (SHG) in reflection from a polycrystalline copper surface in air was studied using femtosecond time-resolved pump and probe measurements at λ=625 nm. The observed time dependence of second-harmonic yield from the probe beam demonstrates, that SHG is a very sensitive technique for measuring transient electron temperatures of metals even when these are covered by an oxide layer. For polycrystalline copper, an electron-phonon energy transfer time of 2 ps was observed, corresponding to a coupling constant of 3.75×1017 W/m3 K at average lattice temperatures of about 500 K. The analysis of experimental data indicates that the time dependence of SHG is governed by the linear dielectric function e which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility χ(2).

Water adsorption on (111) surfaces of BaF2 and CaF2.

Water adsorption on polished (111) surfaces of ${\mathrm{BaF}}_{2}$ and ${\mathrm{CaF}}_{2}$ was studied at room temperature under ultrahigh vacuum conditions by optical second-harmonic generation. Above ${10}^{4}$ langmuirs, the total second-harmonic yield decreases with increasing exposure. The azimuthal anisotropy is influenced differently on each substrate. Below ${10}^{4}$ langmuirs the 3m symmetry of the crystal surface is preserved in both cases, but more pronounced for ${\mathrm{CaF}}_{2}$. At high doses the symmetry remains for ${\mathrm{BaF}}_{2}$, but vanishes for ${\mathrm{CaF}}_{2}$. The results point to pseudomorphous epitaxial growth of water on ${\mathrm{BaF}}_{2}$ but not on ${\mathrm{CaF}}_{2}$.

Acoustic wave second-harmonic generation in materials with strain-dependent dielectric constants

Acoustic wave second-harmonic generation in materials with strain-dependent dielectric constants is investigated in the presence of static electric and magnetic fields. It is shown that for such materials with large dielectric constants, the second-harmonic yield is much larger in magnitude than in materials exhibiting piezoelectricity.