Outstanding nonlinear optical properties of all-inorganic perovskite CsPbX3 (X=Cl, Br, I) precursor solutions and polycrystalline films.

In this work, we systematically explore the third-order nonlinear optical properties of all-inorganic CsPbX3 (X = Cl, Br, I) perovskite precursor solutions and thin films using femtosecond and nanosecond laser pulses. We show that these samples possess strong two-photon absorption (TPA) and reverse saturable absorption (RSA), which depend on the excitation wavelength. The obtained nonlinear absorption and refraction coefficients for precursor solutions are followed by the relation CsPbCl3 > CsPbBr3 > CsPbI3 for the 800 and 1,064 nm excitation wavelengths, whereas this relation became reverse in the case of 355 and 400 nm laser pulses. It was shown that CsPbCl3 thin film possesses RSA at 400 nm, CsPbCI3 shows RSA+ saturable absorption (SA), and CsPbBr3 demonstrates SA + RSA. In addition, at 800 nm excitation, the CsPbBr3 thin films show SA + RSA at low peak intensity, and the absorption becomes reverse (TPA+SA) with a further increase in the input laser intensity. The suitability of nonlinear absorption depends on the band gap of the thin films with respect to the pumping photon energy. Our studies demonstrate that these perovskites can be used as the excellent materials for the all-optical signal processing.

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.

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.

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.

Harmonics Generation in the Laser-Induced Plasmas of Metal and Semiconductor Carbide Nanoparticles.

Carbon-containing plasma is an attractive medium for generation of harmonics of laser pulses in the extreme ultraviolet range. We ablate two metal carbide (B4C and Cr3C2) nanoparticles and silicon carbide (SiC) nanoparticles and generate harmonics after propagation of 35 fs pulses through the laser-induced plasmas. We analyze the spectra, spectral shifts, and splitting of harmonics from nanoparticles-contained plasmas, which demonstrate the chirp-related harmonic cut-off scaling. In addition, we present the simplified two-color pump model calculations of HHG based on the strong field approximation.

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.

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.

High-order harmonics generation in the laser-induced lead-free perovskites-containing plasmas.

High-order harmonics generation in the laser-induced plasmas produced on the surfaces of lead-free perovskites is studied. We analyze the harmonics generation in (CH3NH3)2CuCl4 and (CH3NH3)2CuBr4 plasmas during their ablation by the femtosecond, picosecond, and nanosecond pulses. The modifications of the high-order harmonics spectra are studied using the -color pump scheme (800 nm and 400 nm, 40 fs pulses). The influence of the variations of laser chirp and pulse duration on the dynamics of high-order harmonics generation is examined. The spectral shift, chirp-related harmonic cutoff scaling, and the role of the pulse duration of converting and heating laser radiation are examined at different conditions of plasma formation and harmonic generation. The dependencies of the pulse duration and the fluence of heating pulses on the harmonic's blue shift are found. The effect of harmonics broadening and splitting on the two red- and blue-shifted components is demonstrated.

Investigation of Nonlinear Optical Processes in Mercury Sulfide Quantum Dots.

The authors report the third-harmonic generation, nonlinear refraction, and nonlinear absorption in HgS quantum dot (QD) suspensions and films using the nanosecond and femtosecond pulses. High conversion efficiency (7 × 10-4) towards the third harmonic (TH) of the 900-1700 nm, 150 fs laser in the thin (70 nm) films containing HgS QDs deposited on the glass substrates is obtained. The authors analyze spectral dependencies of the TH, nonlinear refractive indices, and nonlinear absorption coefficients of QDs in the 500-1700 nm range and discuss the relation between the TH process and the low-order nonlinear optical properties of these quantum dots.

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.

Nonlinear Absorption and Refraction of Picosecond and Femtosecond Pulses in HgTe Quantum Dot Films.

We report measurements of the saturated intensities, saturable absorption, and nonlinear refraction in 70-nm thick films containing 4 nm HgTe quantum dots. We demonstrate strong nonlinear refraction and saturable absorption in the thin films using tunable picosecond and femtosecond pulses. Studies were carried out using tunable laser pulses in the range of 400-1100 nm. A significant variation of the nonlinear refraction along this spectral range was demonstrated. The maximal values of the nonlinear absorption coefficients and nonlinear refractive indices determined within the studied wavelength range were -2.4 × 10-5 cm2 W-1 (in the case of 28 ps, 700 nm probe pulses) and -3 × 10-9 cm2 W-1 (in the case of 28 ps, 400 nm probe pulses), respectively. Our studies show that HgTe quantum dots can be used in different fields e.g., as efficient emitters of high-order harmonics of ultrashort laser pulses or as laser mode-lockers.

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.

Enhanced XUV harmonics generation from diatomic gases using two orthogonally polarized laser fields.

Enhanced high repetition rate coherent extreme ultraviolet (XUV) harmonics represent efficient probe of electron dynamics in atoms, molecules and solids. In this work, we used orthogonally-polarized two-color laser field to generate strong even and odd high order harmonics from molecular gas targets. The dynamics of odd and even harmonics from O2, and N2 gases were investigated by employing single- and two-color laser fields using the fundamental radiation and second harmonic of 1030 nm, 37 fs, 50 kHz pulses. The relative efficiencies of harmonics were analyzed as a function of the thickness of the barium borate crystal used for second harmonic generation. Defocusing-assisted phase matching conditions were achieved in N2 gas for different groups of XUV harmonics.

Expedited Transition in the Wettability Response of Metal Meshes Structured by Femtosecond Laser Pulses for Oil-Water Separation.

Oil-water separation using super-wetting and the selective permeability of membranes for oil or water has great ecological and economic significance. We report on the transition of wettability response, from superhydrophilic underwater-superoleophobic to superhydrophobic-superoleophilic state, by nanostructuring stainless steel and copper meshes using ultrashort femtosecond laser pulses. Our approach is environment-friendly, chemical free, and efficient as it exploits the benefit of aging the processed samples in a high vacuum environment. We optimized the laser scanning parameters, mesh pore size, and aging conditions to produce membranes exhibiting an extraordinary separation efficiency of 98% for the oil-water mixture. A variation in the water and oil contact angles for different meshes is presented as a function of the laser scanning speed. Stainless steel meshes with 150 µm pore size and copper meshes with 100 µm pore size have demonstrated an excellent wettability response for oil and water phases. Vacuum aging causes rapid chemisorption of hydrocarbons on laser-structured surfaces in the absence of water molecules, rapidly transforming the wetting state from superhydrophilic to superhydrophobic.

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.

High-Order Harmonic Generation in Au Nanoparticle-Contained Plasmas.

Gold nanoparticles (NPs) have a wide range of applications in various fields. Here, we present high-order nonlinear optical studies of the plasmas produced from ablation of Au bulk targets and Au NP films deposited on paper and glass substrates. Experimentally, we analyze high-order harmonic generation (HHG) from gold NPs-containing plasmas. The HHG is produced by 35-fs pulses at 800 and 400 nm, while the plasmas are produced by femtosecond (35 fs, 800 nm), picosecond (200 ps, 800 nm), and nanosecond (5 ns, 1064 nm) pulses, respectively. High-order harmonics produced from ablated Au NPs on paper were 40 times stronger than the HHG from that ablated from the Au bulk targets. Through molecular dynamic simulations, we investigate the formation of gold NPs during laser ablation of a metal surface under different conditions.

Effects of Laser Plasma Formation on Quasi-Phase Matching of High-Order Harmonics from Nanoparticles and Atoms.

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.

Nonlinear Optical Studies of Gold Nanoparticle Films.

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.

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.