Low-order harmonic generation in metal ablation plasmas in nanosecond and picosecond laser regimes

Abstract

Low-order harmonics, 3rd and 5th, of IR (1064 nm) laser emission have been produced in laser ablation plasmas of the metals manganese, copper and silver. The harmonics were generated in a process triggered by laser ablation followed by frequency upconversion of a fundamental laser beam that propagates parallel to the target surface.These studies were carried out in two temporal regimes by creating the ablation plasma using either nanosecond or picosecond pulses and then probing the plasma plume with pulses of the same duration. The spatiotemporal behaviour of the generated harmonics was characterized and reveals the distinct composition and dynamics of the plasma species that act as nonlinear media, allowing the comparison of different processes that control the generation efficiency. These results serve to guide the choice of laser ablation plasmas to be used for efficient high harmonic generation of laser radiation. 2 I INTRODUCTIONHarmonic generation (HG) of intense laser pulses in isotropic media such as gases and vapours serves to create coherent radiation in the short wavelength range of the spectrum down to the extreme ultraviolet (XUV) and X-ray regions. Low-order frequency up conversion using laser pulses of nanosecond (ns) duration have provided table-top vacuum ultraviolet (VUV) coherent light for spectroscopy and molecular photodissociation studies. 1,2 On the other hand, high-order harmonic generation 3 (HHG) requires pulses of higher intensities (typically 10 14 -10 15 W/cm 2 ) which can easily be achieved with femtosecond (fs) driving lasers. Although the phenomenon of HG is universal and can be generalized to any material with sufficient non linear response, mostly atomic, 4,5 or in some cases, molecular 6,7 gas jets have been investigated as nonlinear media. In practical applications of coherent short wavelength HG sources, a high conversion efficiency and thus high photon flux of the harmonic radiation are essential.Since the beginning of the nineties, ions generated in laser ablation plasmas have been investigated as alternative isotropic media for HHG. [8][9][10][11][12] The higher ionization potentials of ions as compared with neutral noble gas atoms would result in the extension of the harmonic cutoff towards shorter wavelengths. However, those experiments have shown that ionization-induced defocusing of the fundamental laser beam plays a detrimental effect on the HHG process.Together with atomic ions and neutrals, laser ablation can easily generate other species, such as aggregates, clusters and nanoparticles, with various nonlinear optical properties and a relative abundance that can in principle be controlled by the right choice of solid target and laser wavelength and pulse duration. Akiyama et al 9 noted that control of limiting factors (self-defocusing and wave phase mismatch between the harmonics and the radiation being converted) leads to efficient HHG in alkali ions generated by laser 3 ablation. In the last few years more research has demonstrated the advantages of usin...

Generation of low-order harmonics in laser ablation plasmas

“…This makes possible to extend the techniques of HG and HHG to some nonlinear species based in polar molecules, atomic ions or nanoparticles, which are not readily available in the gas phase [12][13][14][15][16].…”

Ablation of nanoparticles and efficient harmonic generation using a 1-kHz laser

“…For ablation with 160-ps pulses, only when the ablation intensity was substantially increased (>3 × 10 10 W cm targets resulted in extremely inefficient HHG. Production of nanoparticles by laser ablation of metallic targets is a wellstudied phenomenon (see, for example, [33,34]). However, the use of high ablation fluences results in the generation of large free-electron densities, which is a detrimental factor in HHG due to the electron contribution to the phase vector mismatch between the driving and harmonic waves.…”

Use of extended laser plasma for generation of high-order harmonics of picosecond duration

“…While the efficiency of HOHG with picosecond pulses still remains low (due to the low intensity of laser radiation and lower thresholds of multiphoton ionisation of plasma particles), research along this line is being continued by the example of low-order harmonics [23].…”