Three-color pump of laser-induced plasmas: sum and difference frequency generation.

The generation of strong harmonics and sum and difference frequencies using the propagation of three pulses of different wavelengths and polarizations through Ag, Au, Pb, B, and C plasmas is examined. It is demonstrated that the difference frequency mixing is more efficient compared with the sum frequency mixing. At optimal conditions for the laser-plasma interaction, the intensities of the sum and especially the difference components are almost equal to the intensities of neighboring harmonics attributed to the strong 806 nm pump.

Calculation of high-order harmonic generation in laser-produced lithium plasma.

We report on detailed calculations of high-order harmonic generation (HHG) spectra from Li, Li+, and He using the Lewenstein model. Our model well describes the HHG experiment in Li plasma [J. Phys. B45, 065601 (2012)JPAPEH0953-407510.1088/0953-4075/45/6/065601]. The cutoff position, in the case of neutral lithium atoms (43th harmonic of 800 nm radiation), corresponds to 3.5×1014 W/cm2 laser intensity, as numerically-simulated in the current work. The difference from the experimental value can be well explained by the measurement errors and uncertainty in determining the laser intensity, which is usually around 25%. We found that Li+ ions do not contribute to HHG plateau region of the spectra and that those ions start to contribute only at very high intensities. Our calculations show that the application of materials possessing higher ionization potential does not necessarily leads to the extension of HHG cutoff.

Optical limiting, nonlinear refraction and nonlinear absorption of the associates of Cd0.5Zn0.5S quantum dots and dyes.

The optical limiting properties of alloyed Cd0.5Zn0.5S quantum dots associated with erythrosine dye are analyzed using 532 nm, 40 ps pulses. We show that joint influence of saturable absorption, reverse saturable absorption and two-photon absorption cause the optical limiting of 532 nm radiation at the pulse energies exceeding 1 mJ. The nonlinear refraction and nonlinear absorption of these quantum dots associated with different organic dyes were studied using 1064 nm and 532 nm radiation. The nonlinear refraction index and nonlinear absorption coefficient of Cd0.5Zn0.5S quantum dots were measured at λ = 1064 nm to be 2 × 10-13 cm2 W-1 and 1.2 × 10-8 cm W-1, while the reverse saturable absorption of Cd0.5Zn0.5S quantum dots and erythrosine at λ = 532 nm was almost two orders larger. The potential applications of these quantum dots for high-order harmonic generation are discussed.

Quasi-phase-matching of high-order harmonics in plasma plumes: theory and experiment.

We theoretically analyze the phase-matching of high-order harmonic generation (HHG) in multi-jet plasmas and find the harmonic orders for which the quasi-phase-matching (QPM) is achieved depending on the parameters of the plasma and the generating beam. HHG by single- and two-color generating fields is analyzed. The QMP is studied experimentally for silver, indium and manganese plasmas using near IR and mid-IR laser fields. The theory is validated by comparison with our experimental observations, as well as published experimental data. In particular, the plasma densities and the harmonic phase coefficients reconstructed from the observed harmonic spectra using our theory agree with the corresponding parameters found using other methods. Our theory allows defining the plasma jet and the generating field properties, which can maximize the HHG efficiency due to QPM.

Strong field-induced frequency conversion of laser radiation in plasma plumes: recent achievements.

New findings in plasma harmonics studies using strong laser fields are reviewed. We discuss recent achievements in the growth of the efficiency of coherent extreme ultraviolet (XUV) radiation sources based on frequency conversion of the ultrashort pulses in the laser-produced plasmas, which allowed for the spectral and structural studies of matter through the high-order harmonic generation (HHG) spectroscopy. These studies showed that plasma HHG can open new opportunities in many unexpected areas of laser-matter interaction. Besides being considered as an alternative method for generation of coherent XUV radiation, it can be used as a powerful tool for various spectroscopic and analytical applications.

Isolated sub-fs XUV pulse generation in Mn plasma ablation.

We report studies of high-order harmonic generation in laser-produced manganese plasmas using sub-4-fs drive laser pulses. The measured spectra exhibit resonant enhancement of a small spectral region of about 2.5 eV width around the 31st harmonic (~50eV). The intensity contrast relative to the directly adjacent harmonics exceeds one order of magnitude. This finding is in sharp contrast to the results reported previously for multi-cycle laser pulses [Physical Review A 76, 023831 (2007)]. Theoretical modelling suggests that the enhanced harmonic emission forms an isolated sub-femtosecond pulse.

Enhancement of high harmonics from plasmas using two-color pump and chirp variation of 1 kHz Ti:sapphire laser pulses.

We have investigated resonance effects in high-order harmonic generation (HHG) within laser-produced plasmas. We demonstrate a significantly improved harmonic yield by using two-color pump-induced enhancement and a 1 kHz pulse repetition rate. Together with an increased HHG output, the even harmonics in the cutoff region were enhanced with respect to odd harmonics. We report the observation of a resonance-induced growth in intensity of 20th harmonic in silver plasma (2×), 26th harmonic in vanadium plasma (4×), and 28th harmonic in chromium plasma (5×).

Higher-order harmonic generation from fullerene by means of the plasma harmonic method.

We demonstrate, for the first time, high-order harmonic generation from C60 by an intense femtosecond Ti:sapphire laser. Laser-produced plasmas from C60-rich epoxy and C60 films were used as the nonlinear media. Harmonics up to the 19th order were observed. The harmonic yield from fullerene-rich plasma is about 25 times larger compared with those produced from a bulk carbon target. Structural studies of plasma debris confirm the presence and integrity of fullerenes within the plasma plume, indicating fullerenes as the source of high-order harmonics.

Pulsed laser deposition of metal films and nanoparticles in vacuum using subnanosecond laser pulses.

A study of silver, chromium, stainless-steel, and indium thin films prepared by subnanosecond laser deposition in vacuum is reported. We compare the laser ablation in vacuum at the weak- and tight-focusing conditions of a Ti:sapphire laser beam and analyze the nanoparticles synthesized in the latter case using absorption spectroscopy, x-ray fluorescence, atomic force microscopy, and scanning electron microscopy. Our results show that the nanoparticle formation can be accomplished using long laser pulses under tight-focusing conditions.

Highly directive 18.9 nm nickel-like molybdenum x-ray laser operating at 150 mJ pump energy.

We experimentally demonstrate that by longitudinally pumping 2 mm long molybdenum preformed plasma with high-intensity 475 fs duration laser pulse, a highly directive soft-x-ray laser at 18.9 nm wavelength is generated. The divergence of the beam is evaluated to be of the submilliradian order, and only requires a pump laser energy of 150 mJ.

Measurements of blue shifts due to collisionless absorption in harmonic generation from subpicosecond laser-produced plasmas.

Harmonic generation from solid surface plasmas is studied using a subpicosecond Nd:glass laser system. For a 45 degrees angle of incidence, the speculars up to the fifth harmonics are blue shifted when the laser intensity exceeds 2 x 10(16) W cm(-2). The second harmonic is blueshifted by approximately 16 A, and the fifth harmonic is blue shifted by approximately 51 A for p polarization at the intensity of 1 x 10(17) W cm(-2). We observed the blue shift of the fifth harmonics and found that the magnitude of blue shift is higher compared with that for the second harmonics. The blue shift is interpreted as a collisionless absorption due to the anomalous skin effect. It is also found that the divergence of harmonics preserves a smaller divergence when using a shorter pulse length for the driving laser.

Experimental study of harmonic generation from solid surfaces irradiated by multipicosecond laser pulses.

An experimental study is presented on harmonic generation from solid surfaces using 27 ps Nd:glass laser pulses (lambda=1053 nm) in the intensity range of 10(13)-10(15) W cm(-2). Second, third, and fourth harmonics emitted in the specular reflection direction showed intensity scaling exponents of 1.5, 1.8, and 3.8 for an obliquely incident p-polarized laser beam, providing a conversion efficiency of 2x10(-8), 10(-10), and 5x10(-12) at 10(15) W cm(-2), respectively. Second and third harmonic radiation generated using an s-polarized pump was about 10 and 100 times smaller, respectively, compared to that for the p-polarized laser radiation. Faraday rotation observed in the reflected fundamental radiation can explain the relative harmonic yields for the p and s polarizations of the pump beam.