We report the demonstration of strong resonance enhancement of a single high-order harmonic in the extreme ultraviolet (XUV) region generated from the interaction of a femtosecond pulse with low-ionized In ablation. A strong 13th harmonic $\left(61.2\mathrm{nm}\right)$ of Ti:sapphire laser radiation with output intensity almost two orders of magnitude higher than neighboring harmonics was observed in these studies. The high conversion efficiency of the 13th harmonic $(8\times {10}^{-5})$ is attributed to multiple collisions of electron trajectories with the origin due to multiphoton resonance with the In ion.

We have successfully demonstrated intensity enhancement of a single high-order harmonic at a wavelength of $46.76\mathrm{nm}$ by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium. The ablation plume was produced by irradiating a solid tin target with a $10\mathrm{mJ}$ energy picosecond laser pulse. Strong 17th-harmonic generation at a wavelength of $46.76\mathrm{nm}$ was observed with a conversion efficiency of about ${10}^{-4}$. We attribute the strong enhancement of the single high-order harmonic to multiphoton resonance with a strong radiative transition of the $\mathrm{Sn}\mathrm{II}$ ion.

The results of enhanced high-order harmonic generation (HHG) in laser-produced plasmas using a two-color laser pump (98% of 800 nm and 2% of 400 nm) are presented. Even (up to the 38th order) and odd (up to the 45th order) harmonics of 800 nm radiation were generated in the case of a two-color orthogonal polarization pump of various plasmas. The comparison between HHGs using parallel and orthogonally polarized 400 and 800 nm radiations showed better conversion efficiency for the latter case. We have also used circularly polarized 800 nm pump for second-harmonic generation and HHG. We also present the results of the influence of the chirp and phase modulation of the fundamental radiation in laser plasma on the spectral properties of odd and even harmonics.

We demonstrate the generation of up to the 63rd harmonic $(\lambda =12.6\mathrm{nm})$ of a Ti:sapphire laser pulse $(150\mathrm{fs},10\mathrm{mJ})$, using a prepulse- $(210\text{-}\mathrm{ps},24\text{-}\mathrm{mJ})$ produced boron plasma as the nonlinear medium. The influence of various parameters on the harmonic conversion efficiency was analyzed. The steep decrease in intensity for low-order harmonics (up to 19th order) was followed by a plateau. Typical conversion efficiencies were evaluated to be ${10}^{-4}$ (for a 3rd harmonic) to ${10}^{-7}$ (within the plateau region). Harmonic generation appeared to be efficient for the plasma that comprised neutral atoms and singly ionized boron.

Harmonic generation of laser radiation from ionized particles in the extreme ultraviolet range at the conditions of the phase matching between the femtosecond driving field and harmonics is an advanced concept for the development of efficient coherent sources in this spectral region. Here we present experimental evidence of quasi-phase-matched high-order harmonic generation in multiple plasma jets produced during laser ablation of the silver target. We observed both the enhancement of some groups of harmonics along the plateau range and the variation of maximally enhanced harmonic order; additionally we analyzed the influence of the number of plasma jets on the harmonic yield. The enhancement factor of 13 was achieved for the 33th harmonic of Ti:sapphire laser using the five-jet plasma configuration.

Recently, nonspherical nanoparticles took attention due to advanced properties of these structures. We report the study of the nonlinear optical properties of copper oxide nanoellipsoids using 800 nm and 400 nm, 60 fs pulses. The optical limiting effect of copper oxide nanoellipsoids is analyzed. The influence of band gap of copper nanoparticles and copper oxide nanoellipsoids on their nonlinear optical response was studied. For the first time, the low- and high-order nonlinear optical responses of copper nanoellipsoids were studied. The magnitudes of nonlinear optical parameters of the suspension of copper oxide nanoellipsoids were measured to be γ = 1.23 × 10⁻¹⁵ cm² W⁻¹, and β = 1.0 × 10⁻¹¹ cm W⁻¹ respectively. We observed the four-fold enhancement of the nonlinear optical refraction of copper oxide nanoellipsoids at the wavelength of 400 nm, 60 fs probe pulses compared to 800 nm radiation. We also analyzed the high-order nonlinear response of CuO nanoellipsoids through generation of high-order harmonics of 800 nm, 60 fs pulses in the plasmas produced during laser ablation of the nanoellipsoid-contained targets. We demonstrated the harmonics up to the 35th order (E = 50 eV) in case of single-color pump and 24th (30 eV) in case of two-color pump.