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High repetition coherent extreme ultraviolet (XUV) harmonics offer a powerful tool for investigating electron dynamics and understanding the underlying physics in a wide range of systems. We demonstrate the utilization of combined three-color (ω+2ω+3ω) laser fields in the generation of coherent extreme ultraviolet radiation in mixed noble gases. The three-color field results from the combination of fundamental, second-, and third-order harmonics of the near-infrared laser pulses in the nonlinear crystals. Different noble gases were selected as gas targets based on their ionization potentials, which are important parameters for generating higher cut-offs and intensities for the XUV harmonics. Enhanced XUV harmonic intensities were observed in the mixture of He + Kr gases when using three-color laser fields, compared to harmonics generated in the He + Kr mixture under a single-color pump. On the other hand, suppression of XUV harmonic intensity was observed in the mixture of He + Xe under the three-color pump due to the highest ionization level for these two mixed gases at similar laser conditions. Strong harmonic yields in the range of 25 to 80 eV of photon energy were observed.

We characterize the nonlinear optical properties of synthesized Bi 2 Te 3 nanoparticle-contained thin films using the tunable femtosecond laser in the spectral range of 400-1000 nm. These nanoparticles possess a strong saturable absorption and positive nonlinear refraction (-6.8×10 -5 cm W -1 in the case of 500 nm, 150 fs probe pulses, and 3×10 -10 cm 2 W -1 in the case of 400 nm, 150 fs probe pulses, respectively). The spectral, intensity, and temporal variation of saturable absorption and nonlinear refraction of the thin films containing exfoliated Bi 2 Te 3 nanoparticles are discussed.

In the past, common media for high-order harmonic generation (HHG) has been atoms and molecules. More recently, clusters, and nanoparticles have been introduced as HHG emitting media. Multi-particle media can enhance HHG yields but have more stringent requirements in determining the optimal parameters. Here, we demonstrate, for the first time, the effective application of 1-3 nm metal sulfide quantum dots (QDs) for harmonic generation in the 20 - 115 nm extreme ultraviolet range. We report on the syntheses, ablation of Ag 2 S, CdS, and ZnS QDs, and HHG from laser-produced plasmas by using single- and two-color pumps. We compare HHG efficiency from the ablated QDs to that of bulk metal sulfides and show a seven-fold increase in harmonic yields. Further, the study also allows us to understand the effects of QD-contained plasma spreading dynamics on HHG yield.

The high-order harmonic generation (HHG) in silver, gold, and zinc plasma plumes irradiated by orthogonally polarized two-color field is studied theoretically and experimentally. We find an increase of the HHG efficiency in comparison with the single-color case, which essentially depends on the plasma species and harmonic order. An increase of more than an order of magnitude is observed for silver plasma, whereas for gold and zinc it is lower; these results are reproduced in our calculations that include both propagation and microscopic response studies. We show that the widely used theoretical approach assuming the 1s ground state of the generating particle fails to reproduce the experimental results; the agreement is achieved in our theory using the actual quantum numbers of the outer electron of the generating particles. Moreover, our theoretical studies highlight the redistribution of the electronic density in the continuum wave packet as an important aspect of the HHG enhancement in the two-color orthogonally polarized fields with comparable intensities: in the single-color field the electronic trajectories with almost zero return energy are the most populated ones; in the two-color case the total field maximum can be shifted in time so that the trajectories with high return energies (in particular, the cut-off trajectory) become the most populated ones.

We demonstrate the quasi-phase-matching of a group of harmonics generated in Ag multi-jet plasma using tunable pulses in the region of 1160 - 1540 nm and their second harmonic emission. The numerical treatment of this effect includes microscopic description of the harmonic generation, propagation of the pump pulse, and the propagation of the generated harmonics. We obtained more than 30-fold growth of harmonics at the conditions of quasi-phase-matching in the region of 35 nm using eight-jet plasma compared with the case of imperforated plasma.

We have demonstrated a generation of the noise-like pulse (NLP) with broadband spectrum in a nonlinear polarized evolution-based passive mode-locked Yb-doped fiber (YDF) ring laser. At the cavity dispersion of near zero, the NLP with spectrum bandwidth up to 131 nm (FWHM) was obtained at a central wavelength of 1070 nm with output power of 136 mW and 80 MHz repetition rate. To our best knowledge, this spectrum bandwidth of NLP is the broadest among the reported YDF lasers. The autocorrelation function of pulse contained the short (30 fs) and long (4.6 ps) components. This short coherence light source is well suited for the optical coherent tomography used for ophthalmology at a wavelength of ∼1000  nm.

We have demonstrated a generation of 71(st) harmonic at a photon energy of 110 eV (wavelength 11.2 nm) using low ionized vanadium ions in a laser-ablation plume. The conversion efficiency of this harmonic was estimated to be 1.6x10(-7). Such high harmonic generation occurred from the interaction of a femtosecond laser pulse with a doubly charged ion, which has high third ionization potential. This work shows the extension of HHG cutoff energy can be achieved by using the doubly charged ion.

We demonstrated the intensity enhancement of a single high-order harmonic at a wavelength of 37.67 nm using the lowly ionized antimony laser-ablation plume. The conversion efficiency of this harmonic was 2.5x10(-5) and the output energy was 0.3 muJ. Such an enhancement of single-harmonic was caused by the multiphoton resonance with the strong radiative transition of the Sb II ions. The cutoff energy of the harmonics generated in Sb plasma was 86 eV (55(th) harmonic).

Third-harmonic generation (THG) in air in tight-focusing conditions is presented. Variation of the pulse duration supplied optimal conditions for THG by femtosecond pulses that varied in the range 110-1300 fs. We show that this third-harmonic generation was caused by Kerr-induced phase variations of fundamental and harmonic beams. Various characteristics (laser intensity, focusing conditions, pulse duration, air pressure, etc.) of THG in air were analyzed to optimize this process. The THG conversion efficiency of 795 nm, 300 fs radiation was 1 x 10(-3). The harmonic radiation did not show considerable disruption of its spectral and spatial distribution in tight focusing conditions for intensities as high as 5 x 10(14) W cm(-2).

A hybrid tabletop (2 m x 3 m) terawatt chirped-pulse amplification Ti:sapphire-Nd:glass laser (1054 nm, 475 fs, 500 mJ, 9 x 10(17) W cm(-2)) has been developed for laser-matter experiments. An overall gain factor of 10(10) was achieved for the laser. The results of laser applications in the studies of soft-x-ray Ni-like Mo 18.9-nm lasing and of harmonic generation from solid surfaces are presented.