Enhanced XUV harmonics generated in mixed noble gases using three-color laser fields.

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.

Quasi-phase-matching of resonance-enhanced high-order harmonics in laser plasmas.

The resonance-enhanced harmonics in laser-induced arsenic and selenium plasmas are studied at the quasi-phase-matching (QPM) conditions. We demonstrate that the enhancement of these harmonics was significantly smaller than the one of the neighboring harmonics. Though the enhancement factors of the harmonics in the vicinity of resonance-enhanced harmonics were in the range of 5× to 18×, the resonance-enhanced harmonics were almost unenhanced at QPM conditions. The most probable reason for such restriction in the enhancement of specific harmonics at the conditions of QPM was a stronger influence of free electrons on the phase-matching conditions of the resonance-enhanced single harmonic compared to the QPM-enhanced group of harmonics.

High-order harmonics generation in Cd and Pd laser-induced plasmas.

We demonstrate the generation of high-order harmonics of laser pulses in palladium and cadmium plasmas. We adjusted the wavelength of driving pulses to investigate the resonance enhancement in different ranges of extreme ultraviolet region. The summation of incommensurate waves during the two-color pump of Pd and Cd plasmas allowed the generation of a broader range of harmonics. The theoretical aspects of the two-color pump of the laser-induced plasma are discussed.

High-Order Harmonics Generation in MoS2 Transition Metal Dichalcogenides: Effect of Nickel and Carbon Nanotube Dopants.

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.

Application of vector beams for enhanced high-order harmonics generation in laser-induced plasmas.

High-order harmonics driven by phase- and polarization-structured femtosecond pulses are unique sources of the extreme ultraviolet vortex and vector beams, which have various applications. Here, we report the generation of intense high-order harmonics during propagation of the polarization-structured vector beams (radially polarized beam, azimuthally polarized beam, and their superposition) through the laser-induced plasmas (In, C, CdS, Zns, Ag2S). Low-order harmonics became stronger with radially polarized and azimuthally polarized driving beams compared with the linearly polarized beams, which is explained on the basis of phase matching and specific properties of vector beams. Contrary to that, the resonance-enhanced harmonic generated in the indium plasma in the case of radially polarized and azimuthally polarized beams was twice weaker compared with the harmonic generated by the LP beam due to modification in the resonant transition selection rules leading to a decrease of the oscillator strength of ionic transitions. Harmonic cut-off and intensity in the case of superposition of the radially and azimuthally polarized beams were lesser compared with the cases of the individual (radially polarized and azimuthally polarized) beams.

Third-order optical nonlinearities of exfoliated Bi2Te3 nanoparticle films in UV, visible and near-infrared ranges measured by tunable femtosecond pulses.

We characterize the nonlinear optical properties of synthesized Bi2Te3 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 cm2 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 Bi2Te3 nanoparticles are discussed.

High-order harmonics generation in nanosecond-pulses-induced plasma containing Ni-doped CsPbBr3perovskite nanocrystals using chirp-free and chirped femtosecond pulses.

We demonstrate high-order harmonic generation in Ni-doped CsPbBr3perovskite nanocrystals ablated by nanosecond pulses using chirp-free 35 fs, and chirped 135 fs pulses in the case of single-color pump (800 nm) and a two-color pump (800 and 400 nm). We analyzed the spectral shift, cut-off, and intensity distribution of harmonics in the case of chirped drving pulses compared to chirp-free pulses. It is shown that the presence of Ni dopants and CsPbBr3plasma components improves the harmonics emission. Also, we measured the third-order nonlinear optical (NLO) properties of these nanocrystals using 800 nm, 60 fs, 1 kHz pulses. The variations of measured NLO parameters of CsPbBr3perovskite nanocrystals containing different concentrations of nickel correlate with variations of generated high-order harmonics from laser induced plasmas of studied nanocrystals in terms of harmonics intensity, cut-off, and spectral shift (in case of chirped driving pulses). The spectral shift of the harmonics generated from the Ni-doped CsPbBr3perovskite nanocrystals can be used to form tunable extreme ultraviolet sources.

Laser-Induced Molecular Plasma: A Medium for High-Order Harmonics Generation of Ultrashort Pulses.

I present a study of laser-induced plasmas (LIPs) produced on the surfaces of molecular targets to create optimal conditions for high-order harmonics generation during the propagation of femtosecond pulses through the LIP. The resonance enhancement of a harmonic, two-color pump of plasma, quasi-phase-matching and nanoparticle-induced growth of the harmonic yield are analyzed, which allows for the formation of sources of coherent extreme ultraviolet radiation based on molecular plasma formation.

Analysis of laser plasma dynamics using the time resolved nonlinear optical response of ablated carbon nanocomposites mixed with epoxy resin.

Nonlinear optical properties of carbon nanostructures attract attention due to the unique response of these materials during interactions with ultrashort laser pulses. Here we probe the carbon nanocomposites mixed with epoxy resin in laser-induced plasmas using the high-order harmonics generation (HHG) method. We analyze the nanosecond pulses induced plasmas containing three carbon nanostructures (fullerenes, multiwalled carbon nanotubes and diamond nanoparticles) using 40 fs pulses propagating through these plasmas. HHG efficiencies in ablated graphite and nanocomposites are compared. We utilize two digitally synchronized (nanosecond and femtosecond) laser sources allowing for the HHG-based analysis of the evolution of different plasma plumes up to 10 µs delay from the beginning of ablation. The role of different carbon-containing nanocomposites is analyzed and the evidence for the presence of various nanomaterials in laser-induced plasma at the moment of propagation of the driving femtosecond pulses is demonstrated.

Synthesis and low-order optical nonlinearities of colloidal HgSe quantum dots in the visible and near infrared ranges.

We synthesize colloidal HgSe quantum dots and characterize their nonlinear refraction and nonlinear absorption using a Nd:YAG laser and its second harmonic. The 7.5 nm quantum dots were synthesized using the hot-injection method. The nonlinear absorption (ß = 9×10-7 cm W-1) and negative nonlinear refraction (γ = -5×10-12 cm2 W-1) coefficients of colloidal quantum dots were determined using the 10 ns, 532 nm laser radiation. The joint influence of above processes was realized at a higher intensity of probe pulses. In the case of 10 ns, 1064 nm radiation, only negative nonlinear refraction dominated during z-scans of these quantum dots. The studies of optical limiting using two laser sources demonstrated the effectiveness of this process at 532 nm. The role of nonlinear scattering is analyzed. We discuss the mechanisms responsible for the nonlinear refraction processes in colloidal HgSe quantum dots.

High-order harmonics generation in the plasmas produced on different rotating targets during ablation using 1 kHz and 100 kHz lasers.

We analyze the high-order harmonics generation using 1 kHz and 100 kHz lasers by ablating different rotating targets. We demonstrate the high average flux of short-wavelength radiation using the latter laser, while comparing the plasma formation conditions at different pulse repetition rates. The analysis of harmonic stability in the case of the 100 kHz experiments showed the two-fold decay of the 27th harmonic generating in silver plasma after 3.5×106 shots. The advantages of shorter pulse-induced ablation for the improvement of harmonic generation stability are demonstrated. Two-color pump of plasma, resonance enhancement of single harmonic, and quasi-phase matching studies are presented for 1 kHz laser applications. The formation of modulated multi-jet plasma on the plane and curved surfaces during ablation by 100 kHz pulses is demonstrated. In the case of the 25th harmonic of 1030 nm radiation (E=30 eV) generated during experiments in carbon plasma, at 100 kHz and 40 W average power of driving pulses, 0.4 mW of average power for single harmonic in the 40 nm spectral range was achieved.

Effective high-order harmonic generation from metal sulfide quantum dots.

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 Ag2S, 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.

Application of mid-infrared pulses for quasi-phase-matching of high-order harmonics in silver plasma.

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.

Two-color high-harmonic generation in plasmas: efficiency dependence on the generating particle properties.

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.

Generation of broadband noise-like pulse from Yb-doped fiber laser ring cavity.

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.

High-Order Harmonics Generation Using Spherical and Non-Spherical Nanoparticles.

The conversion efficiency of 800 nm, 65 fs radiation toward high-order harmonic generation (HHG) in laser-induced plasmas containing spherical and non-spherical nanoparticles (NPs) produced during the laser ablation of different metals in water using 1064 nm, 70 ps pulses was analyzed. Non-spherical NPs of different forms (triangle, cubic, bowtie, rod, rectangular, ellipsoid, etc.) were synthesized during the aging of some spherical NPs (In, Al, and Cu) in water. These NPs were then dried on the glass substrates and ablated to produce plasmas comprising nanostructured species of different morphologies. It was shown that harmonic generation in all synthesized non-spherical NPs was less efficient by a factor of at least five than in the initial spherical NP. Meanwhile, the spherical NPs that maintained the morphology state during aging (Ni, Ag, Mn, and Au) showed almost similar HHG conversion efficiency compared to the fresh spherical NPs. In all cases, the HHG conversion efficiency using spherical and non-spherical nanoparticles was notably larger compared to the atomic and ionic single-particle plasmas of the same elemental composition. NP plasmas demonstrated featureless harmonic distributions, contrary to the indium and manganese atomic/ionic plasmas, when the resonance enhancement of harmonics was observed.

Comparison of high-order harmonic generation in uracil and thymine ablation plumes.

We present studies of high-order harmonic generation (HHG) in laser ablation plumes of the ribonucleic acid nucleobase uracil and its deoxyribonucleic acid counterpart thymine. Harmonics were generated using 780 nm, 30 fs and 1300 nm, 40 fs radiation upon ablation with 1064 nm, 10 ns or 780 nm, 160 ps pulses. Strong HHG signals were observed from uracil plumes with harmonics emitted with photon energies >55 eV. Results obtained in uracil plumes were compared with those from thymine, which did not yield signs of harmonic generation. The ablation plumes of the two compounds were examined by collection of the ablation debris on a silicon substrate placed in close proximity to the target and by time-of-flight mass spectrometry. From this evidence we conclude that the differences in HHG signal are due to the different fragmentation dynamics of the molecules in the plasma plume. These studies constitute the first attempt to analyse differences in structural properties of complex molecules through plasma ablation-induced HHG spectroscopy.

High-order harmonics enhancement in laser-induced plasma.

The methods of enhancement of the strong high-order harmonics of femtosecond pulses in laser-induced plasma are demonstrated. It comprises the application of the four techniques allowing the enhancement of harmonics in different spectral ranges. Among them are the selection of targets for ablation to create the conditions for resonance enhancement of single harmonic, formation of the quasi-phase-matching of a spectrally tunable group of harmonics, application of the two-color pump of plasma, and the formation of nanoparticles-contained plasmas. The number of generated coherent XUV photons increased in the region of single resonantly enhanced harmonic (62 nm) and the shorter-wavelength region (30-50 nm). The above techniques of harmonics enhancement allowed a significant (up to 50 times) growth in a whole harmonic yield in the case of indium plasma. We discuss the reasons preventing the joint implementation of the four methods of harmonics enhancement in the same spectral region.

Variation of Nonlinear Refraction and Three-Photon Absorption of Indium-Tin Oxide Quantum Dot Thin Films and Solutions in Near Infrared Range.

We characterize the nonlinear optical properties of indium-tin oxide (ITO) quantum dots (QDs) in the IR range using the Z-scan method. We present results of three-photon absorption (3PA), third harmonic generation (3HG), and Kerr-effect-induced nonlinear refraction in ITO QDs. Z-scan measurements were carried out for the QDs solution, while 3HG was demonstrated using QD thin films. The Kerr-induced nonlinear refractive index was analyzed along the 800-950 nm range showing an increase in this parameter from -6.7 × 10-18 to -1.5 × 10-17 m2 W-1. At longer wavelengths (1000-1100 nm), the higher-order effects started to contribute to a nonlinear refractive index. The 3PA coefficient at 950 nm was measured to be 1.42 × 10-25 m3/W2. We discuss the peculiarities in the wavelength-dependent variation of the coefficient of nonlinear absorption responsible for 3PA in the range of 800-1150 nm. Third harmonic generation was analyzed in the 1200-1550 nm spectral range. The absolute value of 3HG conversion efficiency in the 150 nm thick film at the wavelength of laser radiation (1350 nm) was estimated to be ~10-5.

Quantum-Dot-Induced Modification of Surface Functionalization for Active Applications of Whispering Gallery Mode Resonators.

Quantum dots can modify the properties of the whispering gallery mode resonators (WGMRs) used in various potential applications. A deposition of a suitable nanomaterial for the surface functionalization of WGMRs allows for the achievement of high quality (Q) factors. Here, we show that the WGMR surface can be functionalized using quantum dots. We demonstrate that WGMRs covered with thin layers of HgS and PbS quantum dots are suitable for third-harmonic generation due to the high Q factor of the developed microresonators, thus significantly lowering the pumping power required for nonlinear optical interactions.