High-order Harmonic Generation In Plasmas Research Articles

Divergence of High-Order Harmonic Generation by a Convex Plasma Surface

The electron density profile on a plasma surface has a decisive influence on the mechanism and characteristics of the plasma high-order harmonic generation. When the pre-pulse has a similar spatial and temporal distribution as the main laser pulse, the plasma surface on the target will expand to form a convex profile of the similar size as the focal spot of the main pulse. We experimentally observed that the divergence of the harmonics generated by the relativistic laser light incident on a silica target has a saddle-shaped structure. The two-dimensional particle-in-cell simulation with convex plasma surfaces explains the experimental results very well and infers a 0.12λL plasma scale length around the center of the convex profile. Further, we qualitatively explained that the asymmetry of the saddle-shaped harmonic divergence is caused by oblique incidence.

Why plasma harmonics?

We discuss the emergence of interest in the high-order harmonic generation (HHG) of ultrashort pulses propagated through laser-produced plasmas. It is shown that, during the last few years, substantial amendments of plasma HHG allowed in some cases the characteristics of gas HHG to be surpassed. The attractiveness of a new approach in coherent extreme ultraviolet radiation generation is demonstrated, which can also be used as a tool for laser-ablation-induced HHG spectroscopy of a giant class of solids. We present general ideas and prospects for this relatively new field of nonlinear optics.

High-order harmonic generation in plasmas from nanoparticle and mixed metal targets at 1-kHz repetition rate

High-order harmonic generation in laser-produced plasmas from nanoparticle and mixed metal targets is studied using a femtosecond Ti:sapphire laser operating at 1-kHz repetition rate. Mixed plasmas from brass and plasmas from nanoparticles (Al, carbon) are investigated and compared to their monatomic counterparts of Zn, Cu, and Al. The dependence on the delay between the heating ablation pulse and the driving converting pulse is studied for a light (carbon) and a heavy atom (silver) target. Further, we report on a comparative measurement of the efficiencies and the photon flux from argon gas harmonics and carbon plasma harmonics. For our conditions, the 13th harmonic is generated in a carbon plasma with an efficiency of η = 5.3 × 10−7 and yields a flux of about 9.8 × 1010 photons s−1, about 40 % higher than the corresponding harmonic flux in Ar gas under otherwise identical conditions.

Strong Field-Induced Frequency Conversion of Laser Radiation in Plasma Plumes: Recent Achievements

New findings in plasma harmonics studies using strong laser fields are reviewed. We discuss recent achievements in the growth of the efficiency of coherent extreme ultraviolet (XUV) radiation sources based on frequency conversion of the ultrashort pulses in the laser-produced plasmas, which allowed for the spectral and structural studies of matter through the high-order harmonic generation (HHG) spectroscopy. These studies showed that plasma HHG can open new opportunities in many unexpected areas of laser-matter interaction. Besides being considered as an alternative method for generation of coherent XUV radiation, it can be used as a powerful tool for various spectroscopic and analytical applications.

Harmonic generation of picosecond Nd:YAG laser radiation in ablation-produced plasmas

High-order harmonic generation (HHG) up to the 21st order of picosecond Nd:YAG laser radiation in the plasmas produced on the surfaces of metal targets is reported. The peculiarity of these studies was the observation of enhanced single (11th) harmonic generating from some plasmas in the vacuum ultraviolet range (80–200 nm), which may originate from the resonance-induced growth of the nonlinear optical response of the medium in the vicinity of strong neutral and ionic transitions causing an increase of single harmonic conversion efficiency. Our studies showed that picosecond pulses can be efficiently converted towards the 50–220 nm wavelength range using HHG in plasma. This also allowed high fluencies of harmonic emission to be achieved by application of longer (picosecond) pulses compared with the commonly used femtosecond pulses.

High-order harmonic generation in plasma with ultraintense laser and ion beams

In the scope of relativistic quantum theory the high-order harmonic generation (HHG) in underdense plasma with the copropagating ultraintense laser and fast ion beams is considered. It is shown that in the proposed scheme the impeding factor of relativistic magnetic drift of a strong wave is fully eliminated (at the same velocities of the laser and ion beams the laser magnetic field affecting on the ions in plasma exactly is zero), which makes possible the effective generation of huge number of laser harmonics.

Resonance-Induced Enhancement of the High-Order Harmonic Generation in Plasma

Resonance-induced enhancement of the single harmonic generating in laser-produced plasmas allows achieving the microJoule energies for the coherent pulses in the extreme ultraviolet region using the moderate-intensity lasers. We review various approaches for improving the enhancement of single harmonic, which paves the way for a growth of har- monic yield in the short-wavelength range for various applications. enabled the overlap of these harmonics with the ionic transi- tions with strong oscillator strengths. Another approach for the tuning of harmonic wavelength based on the chirp varia- tions of driving radiation has been reported during gas HHG studies (15-17). Moreover, the tuning of harmonics and their sharpness were demonstrated using a combination of the external control of laser chirp and the intensity-induced variation of laser chirp inside a nonlinear medium (16, 17). In this mini-review, we discuss recent studies of the en- hancement of a single harmonic intensity in different parts of plateau in the case of plasma HHG. After brief overview of the experimental details, we analyze the giant single har- monic generation in indium plasma and other plasmas where the harmonic enhancement was reported. We also discuss the conditions of single harmonic enhancement at strong excita- tion of the plasma by femtosecond radiation and using the ultraviolet laser.

High-order harmonic generation in plasmas

The phenomenon of harmonic generation by electrons oscillating in high-intensity laser fields is surveyed and assessed as a means of producing short-wavelength radiation. Starting from the seminal early work by Sarachik and Schappert (1970), simple motivatory examples are given of incoherent harmonic generation via nonlinear scattering from single electrons. More recent studies aimed at observing the coherent version of this effect in underdense plasmas are then reviewed and some problems noted in distinguishing these harmonics from those produced via the analogous nonlinear mechanism from bound electrons in rare gases. Finally, the revival of interest in harmonics reflected from overdense plasmas is considered. Short-pulse laser-generated "surface" harmonics appear to offer a very promising, compact, and efficient means of upshifting coherent radiation to sub-10-nm wavelengths.

Noncollinear phase-matched difference-frequency mixing for high-order harmonic generation in plasmas

Propagation effects in the high-order harmonic generation of high-intensity subpicosecond KrF-laser radiation in carbon vapour and low charged carbon plasma were studied. The fifth harmonic showed an anomalous behaviour for generation in the plasma. Its continous growth with the plasma length is explained by defocusing of the laser radiation and noncollinear phase-matched difference-frequency mixing.