Femtosecond laser-induced sub-wavelength plasma inside dielectrics. II. Second-harmonic generation

Abstract

Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser–plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two-surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.

Femtosecond laser-induced sub-wavelength plasma inside dielectrics: I. Field enhancement

“…We anticipate that measurement of the second harmonic can provide further insights into the absorption mechanisms at play during the interaction of femtosecond pulses with solid dielectrics. In a forthcoming work, Ardaneh et al 49 , we will explicitly compare the second harmonic generation from the experiments with the PIC simulations. Further work will be dedicated to simulations of the ionization dynamics within PIC simulations to provide the plasma evolution during the interaction of femtosecond pulses with solid dielectrics.…”

Femtosecond laser-induced sub-wavelength plasma inside dielectrics. III. Terahertz radiation emission

“…In the outward propagation of hot electrons, electrostatic ambipolar fields form at the plasma surface due to the different inertia of the electron and ion. Moreover, in the second article of this series (Ardaneh et al, 39 Paper II hereafter), we reported the second-harmonic generation by a second-order current of hot electrons near the critical surface. The electron currents form by the resonance absorption and radiation force of the incident laser wave.…”

“…In our simulation, we used the plasma parameters that could reproduce our experimental measurements (far-field, near-field, absorp-tion) as reported in Paper I, 38 and II. 39 The simulation setup is summarized in Table I. The plasma is fully ionized and composed of electrons and ions with equal densities (to preserve electric neutrality) given by n = n max exp(−x 2 /w 2 x ) exp(−y 2 /w 2 y ) tanh(z µm ) with FWHM x = √ 2 ln 2w x = 250 nm, and FWHM y = √ 2 ln 2w y = 600 nm.…”

“…The THz radiation is mainly polarized in the x−direction because E x component is dominant in the radiated field. This polarization is the same as the incident pulse and second-harmonic detailed in Paper II 39 . In Sec.…”

“…with density above the reflection density for the incident laser wavelength, are generated by focusing Bessel beams with moderate intensities on the order of 10 14 W/cm 2 inside sapphire. [37][38][39] A Bessel beam is a solution of the wave equation in which the wave amplitude is defined by the Bessel function of the first kind. 40 Importantly, the axial intensity profile of the Bessel beam is propagation invariant.…”