Search

Thin films containing small-sized quantum dots (QDs) and nanoparticles have shown strong optical nonlinearities caused by the confinement effect. Here, we report the study of third-order optical nonlinearities of silver sulfide (Ag2S) QD thin films using 800 and 400 nm, 30 fs pulses. The absorption spectrometry and transmission electron microscopy are used to characterize the synthesized 80 and 500 nm Ag2S QD films. The giant enhancement of nonlinearities is observed up to three to six orders of magnitude larger compared to those for the bulk and liquid Ag2S samples. We also demonstrate the efficient high-order harmonic generation in the plasmas produced during ablation of the Ag2S QD thin films. The analysis of the dynamics of the QD-containing plasma spreading allowed optimization of the delay between the heating and the driving pulses for an enhancement of harmonics conversion efficiency.

The mechanism of resonance enhancement of a single harmonic during high-order harmonic generation in indium-, zinc-, and chromium-contained atomic and molecular plasmas is reexamined using single-color and two-color pumps of ablated species. We demonstrate that oxides, selenides, and phosphides of these metals notably reduce the enhancement of a single harmonic compared with purely atomic plasma due to either a shift of the ionic transitions possessing strong oscillator strength out of the wavelength of those harmonics or a reduction of the oscillator strength of these transitions. The role of oscillator strength in harmonic enhancement is discussed and compared with improvements in phase-matching conditions for some harmonics in the region of anomalous dispersion of ionic transitions. The application of different wavelengths of driving pulses drastically changes the enhancement of a single harmonic in indium-contained plasmas. [ABSTRACT FROM AUTHOR]

The transition metal dichalcogenides have instigated a lot of interest as harmonic generators due to their exceptional nonlinear optical properties. Here, the molybdenum disulfide (MoS2) molecular structures with dopants being in a plasma state are used to demonstrate the generation of intense high-order harmonics. The MoS2 nanoflakes and nickel-doped MoS2 nanoflakes produced stronger harmonics with higher cut-offs compared with Mo bulk and MoS2 bulk. Conversely, the MoS2 with nickel nanoparticles and carbon nanotubes (MoS2-NiCNT) produced weaker coherent XUV emissions than other materials, which is attributed to the influence of phase mismatch. The influence of heating and driving pulse intensities on the harmonic yield and cut-off energies are investigated in MoS2 molecular structures. The enhanced coherent extreme ultraviolet emission at ~32 nm (38 eV) due to the 4p-4d resonant transitions is obtained from all aforementioned molecular structures, except for MoS2-NiCNT. [ABSTRACT FROM AUTHOR]

As high-order harmonic emitters, quantum dots are produced through laser-induced plasmas. Subsequently, we generate high-order harmonics with 800-nm and 30-fs pulses from laser-produced plasmas containing quantum dots of different metal sulfides (Ag2S, CdS, and Cd0.5Zn0.5S). The high-order harmonic generation is analyzed using different approaches, including two-color (800 nm + 400 nm) pump, application of alloyed quantum dots, and quasiphase matching of interacting waves. We discuss the self-phase modulation induced splitting of harmonics, the difference in the application of thick and thin crystals for second harmonic (400 nm) emission during two-color pumping of the quantum dot plasma, the spatial modulation of the quantum dot plasma for quasiphase matching, and the comparison of harmonic yields from monomer and quantum dot plasmas. This study allows us to determine the mechanisms of coherent extreme ultraviolet radiation generation using a few nanometer-sized emitters as well as optimal methods for further enhancing the high-order harmonic generation efficiency. [ABSTRACT FROM AUTHOR]

The application of advanced methods for high-order harmonics enhancement allows the formation of conditions for phase matching of the generated harmonics in different ranges of extreme ultraviolet. We demonstrate quasi-phase matching (QPM) of the high-order harmonics generated in the plasmas containing silver microparticles and atoms, and silver sulfide molecules. The groups of harmonics in the range of 30th to 40th orders at QPM conditions prevail over the lower orders of harmonics. In most cases, the lower-order harmonics disappeared from the generated spectra at QPM conditions. The 22× enhancement factor of QPM harmonics and the 3 × 10−5 conversion efficiency for a maximally enhanced harmonic were demonstrated in the case of silver microparticle-contained plasma in the 47–67 eV energy range. [ABSTRACT FROM AUTHOR]

Thin films containing small-sized quantum dots (QDs) and nanoparticles have shown strong optical nonlinearities caused by the confinement effect. Here, we report the study of third-order optical nonlinearities of silver sulfide (Ag2S) QD thin films using 800 and 400 nm, 30 fs pulses. The absorption spectrometry and transmission electron microscopy are used to characterize the synthesized 80 and 500 nm Ag2S QD films. The giant enhancement of nonlinearities is observed up to three to six orders of magnitude larger compared to those for the bulk and liquid Ag2S samples. We also demonstrate the efficient high-order harmonic generation in the plasmas produced during ablation of the Ag2S QD thin films. The analysis of the dynamics of the QD-containing plasma spreading allowed optimization of the delay between the heating and the driving pulses for an enhancement of harmonics conversion efficiency. [ABSTRACT FROM AUTHOR]

The application of nanoparticles (NPs) and quasi-phase matching (QPM) each play an important role in the enhancement of high-order harmonics (HHG) of ultrashort laser pulses. We analyze various regimes of nanoparticle plasma formation for the creation conditions for maximal QPM-induced enhancement of the groups of harmonics in the extreme ultraviolet (XUV). Laser plasmas were formed on the surfaces of NPs- and microparticle (MPs)-contained targets using ablation by nanosecond, picosecond, and femtosecond pulses. Different conditions of laser plasma formation (extended and perforated plasma) and variable concentrations of free electrons in these three cases of laser ablation led to modifications of QPM conditions. We demonstrate novel approaches in the optimization of QPM at the conditions of laser ablation of NPs and MPs by pulses of different durations. The formation of QPM conditions using femtosecond and picosecond heating pulses during HHG in such plasmas allowed the growth of conversion efficiency of the groups of harmonics, with the enhancement factors exceeding 25× in different ranges of XUV, contrary to less efficient QPM in the case of nanosecond pulse-induced ablation. [ABSTRACT FROM AUTHOR]

Gold films are widely used for different applications. We present the results of third- and high-order nonlinear optical studies of the thin films fabricated from Au nanoparticle solutions by spin-coating methods. These nanoparticles were synthesized by laser ablation of bulk gold in pure water using 200 ps, 800 nm pulses. The highest values of the nonlinear absorption coefficient (9 × 10−6 cm W−1), nonlinear refractive index (3 × 10−11 cm2 W−1), and saturation intensity (1.3 × 1010 W cm−2) were achieved using 35 fs, 400 nm pulses. We also determined the relaxation time constants for transient absorption (220 fs and 1.6 ps) at 400 nm. The high-order harmonic generation was studied during propagation of 35 fs, 800 nm pulses through the plasma during the ablation of gold nanoparticle film and bulk gold. The highest harmonic cutoff (29th order) was observed in the plasma containing gold nanoparticles. [ABSTRACT FROM AUTHOR]