Harmonic Emission Spectrum Research Articles

Quantitative Modeling of the Third Harmonic Emission Spectrum of Plasmonic Nanoantennas

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation.

Modulation of harmonic emission spectra in relativistic laser interactions with overdense plasma

We have recently reported effects of plasma waves on high harmonic emission from laser-overdense plasma interactions [1] and found that the emission spectrum is modulated at the plasma frequency and identified circumstances favourable for modulation, in the plasma density profile, pulse shape and duration. Here we study the ultra-relativistic regime and have characterised the modulation observed in simulations. We find that the harmonic spectra show a behaviour distinct from the power law dependence Im ∼ m−8/3 recently proposed by Baeva et al. [2] on the basis of relativistic similarity theory.

Modulation of harmonic emission spectra from intense laser-plasma interactions

We report results from a series of PIC simulations carried outto investigate the interaction of intense laser light with overdense plasma and in particular, aspects of the harmonic emission spectrum generated in these interactions. Recent observations of emission in this range by Watts et al. [I] show emission above the 20th harmonic extending to the 37th with conversion efficiencies of order 10 -6 with this spectrum showing distinct, if variable, modulation of between two and four harmonics. This spectral feature proved highly sensitive to the incident intensity, with more pronounced modulation at higher intensities. These authors also reported results from a set of model PIC simulations which showed a modulated emission spectrum across the entire harmonic range, but with a wider modulation bandwidth. In the results to be presented we report on features of the emission spectrum across a wide parameter range and its dependence on the detailed structure of the plasma density profile. While we do see evidence of a modulated spectrum, this appears to be sensitive not only to the incident intensity but to the plasma density profile. In addition plasma emission present further complicates the harmonic emission spectrum. Our results lead us to conclude that attempts to interpret modulation in emission spectra in terms of one particular theoretical model and from this to infer the dynamics of the critical surface may be misguided.

Dynamics of the critical surface in high-intensity laser-solid interactions: modulation of the XUV harmonic spectra.

The generation of harmonics from the interaction of an intense (I>or=10(18) W cm(-2)) laser with a solid surface is investigated. Modulation of the harmonic emission spectrum with a periodicity of 2 to 4 harmonics is observed at higher laser intensities. A similar modulation is predicted by a particle-in-cell simulation. The modulation is shown to be caused by the higher modes of oscillation of the critical surface during the interaction. As a result, the dynamics of the critical surface can be inferred from the shape of the harmonic spectrum.

Lattice-parameter determination from the interaction of short femtosecond laser pulses with crystalline solids.

A novel method for determining the lattice parameter in a crystalline solid from the harmonic emission spectra reflected in laser-solid layers interactions is proposed. Radiated emission at high-order harmonic numbers is observed from thin crystalline layers illuminated by short femtosecond elliptically polarized laser light. The radiation mechanism presented here accounts for optical emission effects when free electrons in the material, ionized by the action of an applied external radiation field, are driven to large oscillation amplitudes and harmonics are generated by the electronic response to the periodic lattice potential. Perturbations in the electron motion inside the material due to the ion cores give rise to harmonic emission. In this work, the condition for radiation--the resonance condition--is found analytically through an exact expression for the electron displacement. The analytic prediction is supported by results obtained by solving numerically the electromagnetic force equations for electrons moving in the lattice potential.

Temporal phase control of soft-x-ray harmonic emission

We report a strong dependence of the soft-x-ray spectra generated by high harmonic emission on the chirp of the excitation pulse, when an ultrashort laser drives the process. For identical pulse durations, distinct harmonic peaks can be observed for positively chirped excitation pulses, while for negatively chirped pulses, the harmonic peaks become irregular. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. This work resolves an outstanding discrepancy between theory and experiment by demonstrating that high-order harmonic generation driven by short-duration pulses can result in distinct harmonic peaks. This work conclusively demonstrates the role of the intrinsic phase in determining harmonic emission spectra, and control this phase during the emission process.

CAVITON FORMATION AND SECOND HARMONIC EMISSION DURING THE INTERACTION OF MICROWAVE WITH INHOMOGENEOUS PLASMA

The experimental simulations of laser-plasma interaction process by means of the microwave-plasma interaction have been performed in a steady state plasma device. The evolution of cavitom in the nonlinear interaction is experimentally investigated. The good agreement between experimental results and numerical calculation of Zakharov equation is obtained. The second harmonic emission spectrum is measured, and the mechanism of red-shift of second harmonic frequency is discussed.

Complex second harmonic emission spectra from plasmas produced using short pulse, high intensity, 1μ m laser radiation

Complex second harmonic spectra emitted from plasmas generated using short pulse high intensity 1 μm laser radiation can be most easily explained by a generation mechanism involving a decay instability of resonance absorption operating in combination with modulation of the spectrum by return current driven ion acoustic waves.

Observations of ω 0/2 harmonic emission from 0.35μm laser-irradiated plasmas

Observations of ω 0/2 harmonic emission from both spherical and plane targets irradiated by 0.35 μm, 800 ps laser pulses have been obtained with simultaneous high spectral and temporal resolutions of 16 Å and 20 ps respectively. The ω 0/2 harmonic emission spectrum is interpreted as providing a direct measurement of the frequency of the ω 0/2 plasma waves and hence can be used to estimate the electron temperature.

Time dependence of the second harmonic emission spectrum of a two-level spin system

Abstract The time dependence of the second harmonic emission spectrum of a two-level spin system, excited by a step-modulated radiation at double quantum resonance, is experimentally investigated in a dilute ruby sample.