L. B. Elouga Bom

Pencil lead plasma for generating multimicrojoule high-order harmonics with a broad spectrum

Using the plasma harmonic method, we show the generation of efficient and intense high-order harmonics from plasma of pencil lead. We demonstrate multimicrojoule energy in each harmonic order for the 11th to the 17th order of a Ti:sapphire laser. By analyzing the target morphology and the plasma composition, we conclude that these intense harmonics are generated from nanoparticles of graphitic carbon.

Multi-µJ coherent extreme ultraviolet source generated from carbon using the plasma harmonic method

We demonstrate intense high-order harmonic generation from plasma that is created from different carbon targets. We obtain high-order harmonic energy in the multi-microjoule range for each harmonic order from the 11th to the 17th harmonic. By analyzing the target morphology and the plasma composition, we conclude that the intense harmonics from the bulk carbon targets originate from nanoparticles target.

Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighboring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment.

Femtosecond envelope of the high-harmonic emission from ablation plasmas

We characterize the temporal profile of the high-order harmonic emission from ablation plasma plumes using cross-correlations with the infrared (IR) laser beam provided by two-photon harmonic+IR ionization of rare gas atoms. We study both non-resonant plasmas (lead, gold and chrome) and resonant plasmas (indium and tin), i.e. plasmas presenting in the singly charged ions a strong radiative transition coinciding with a harmonic order. The cross-correlation traces are found to be very similar for all harmonic orders and all plasma targets. The recovered harmonic pulse durations are very similar to the driving laser, with a tendency towards being shorter, demonstrating that the emission is a directly laser-driven process even in the case of resonant harmonics. This provides a valuable input for theories describing resonant-harmonic emission and opens the perspective of a very high flux tabletop XUV source for applications.

Intense multimicrojoule high-order harmonics generated from neutral atoms of In2O3 nanoparticles

We studied high-order harmonic generation from plasma that contains an abundance of indium oxide nanoparticles. We found that harmonics from nanoparticle-containing plasma are considerably more intense than from plasma produced on the In2O3 bulk target, with high-order harmonic energy ranging from 6 μJ (for the ninth harmonic) to 1 μJ (for the 17th harmonic) in the former case. The harmonic cutoff from nanoparticles was at the 21st order, which is lower than that observed using indium oxide solid target. By comparing the harmonic spectra obtained from solid and nanoparticle indium oxide targets, we concluded that intense harmonics in the latter case are dominantly generated from neutral atoms of the In2O3 nanoparticles.

Comparison of high-order harmonic generation from various cluster- and ion-containing laser plasmas

We apply the technique of high-order harmonic generation from laser-produced plasma to investigate the harmonics generated from a medium that contains an abundance of micron- and sub-micron-sized structures. We compare harmonic efficiencies from the plasma plumes containing silver nanoparticles (110 nm spherical), colloidal silver (blocks of 100–1000 nm), aluminium particles (2–14 µm) and copper oxide nanorods (50 nm thick, few µm long). We compare these results with those obtained using solid bulk targets. We show that plasma media that are abundant with small-sized structures have higher harmonic generation efficiencies. Results also reveal that the harmonic cutoff is not affected significantly by using the nanostructured targets. The maximum conversion efficiency we observe is 4 × 10−5 using silver nanoparticles.

Extending the capabilities of ablation harmonics to shorter wavelengths and higher intensity

We study the generation of high-order harmonics from ablation plume, by using the 20 TW, 10 Hz laser of the Advanced Laser Light Source (ALLS). We perform detailed studies on enhancement of single high-order harmonics generated in laser plasma using the fundamental and second harmonic of the ALLS beam line. We observe quasi-monochromatic harmonics for Mn, Cr, Sb, Sn, and In plasmas. We identify most of the ionic/neutral transitions responsible for the enhancement, which all have strong oscillator strengths. We demonstrate intensity enhancements of the 13, 17, 21, 29, and 33 harmonics from these targets using the 800 nm pump laser and varying its chirp. We also observed harmonic enhancement from some targets for 400 nm pump laser. Using Mn plume, we demonstrated the highest harmonic photon energy (52.9 eV) at which enhancement has been observed (17 order, λ = 23.5 nm).

Application of nanoparticle-containing laser plasmas for optical harmonic generation

Nanoparticle-containing media can be used for efficient high-order harmonic generation of laser radiation. We present the results of studies of the harmonics generated in the laser-produced plasmas containing Cr2O3, Ag, Cu, and Au nanoparticles. These results are compared with the harmonics generated in the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse and different intensities of prepulse on the target surface. We discuss the harmonic enhancement, which was observed in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas. Morphological studies of plasma debris confirmed the presence and integrity of nanoparticles in the plumes.

Time-resolved spectroscopy of plasma plumes: A versatile approach for optimization of high-order harmonic generation in laser plasma

The time-resolved studies of laser-produced Ag, In, Pt, V, Mn, and Ga plasmas are presented from the point of view of plasma application as the nonlinear optical medium for high-order harmonic generation of laser radiation. We show that optimization of plasma formation using this technique allows the enhancement of harmonic generation efficiency and extension of maximal harmonic order.

High-order harmonic generation from C{sub 60}-rich plasma

We performed systematic investigation of high-order harmonic generation from fullerene-rich laser-produced plasmas. We studied harmonic generation by varying several experimental parameters, such as the delay between the ablation and driving pulses, and divergence and polarization of the pump laser. Enhancement of harmonic yield is observed near 20 eV, which is attributed to the influence of a broadband plasmon resonance of C{sub 60} on the nonlinear optical response of fullerene-rich plasma. This increase in the harmonic intensity occurs despite the increased absorption by C{sub 60} at these wavelengths. Using simulations based on time-dependent density-functional theory, we confirm that this effect is due to the influence of collective excitations. We compare harmonic generation from fullerenes using lasers with 793 nm and 396 nm wavelengths, which show the influence of plasmon resonance on the conversion efficiency of high-order harmonics for different laser wavelengths.