Nonlinear optical characteristics of carbon disulfide

The nonlinear refractive index γ of CS₂ is studied using laser pulses of various durations (from 110 fs to 75 ns). It is found that γ increases with increasing pulse duration within the picosecond region (from (3 ± 0.6) × 10^?15 cm² W^?1 at 110 fs to (3.5 ± 0.7) × 10^?14 cm² W^?1 at 75 ns) due to orientational nonlinearities. Variations in the sign of γ caused by the thermal effect at different pulse durations and repetition rates are analyzed. It is demonstrated that the fast electronic component, the component associated with molecular processes, causing positive nonlinear refraction, and the acoustic component, responsible for negative nonlinear refraction, manifest themselves simultaneously. The results of a study of the nonlinear absorption of carbon disulfide are presented. The two-and three-photon absorption coefficients of CS₂ are determined to be (5 ± 1.5) × 10^?11 cm W^?1 and (2.8 ± 0.8) × 10^?23 cm³ W^?2, respectively.     

Spectroscopic diagnostics of a laser erosion plasma formed from CuSbS<Subscript>2</Subscript> polycrystals

The spectra and dynamics of emission from regions of a laser plasma torch located at different distances from a polycrystalline CuSbS₂ target irradiated by a neodymium laser (W=(3–5)×10⁸ W/cm², ?=20 ns, f=12 Hz, 73x03BB;=1.06μm) were investigated. The emission data were used to estimate the average temperature (≤0.82 eV) and the electron density ((1.82?1.92)×10¹⁶ cm^?3) in the laser torch and the recombination times of ions (t _r(S²⁺)=15 ns, t _r(Cu⁺)=65?85 ns), as well as to analyze the efficiency of filling of excited atomic levels. A model describing the target destruction and the evolution of the processes accompanying spread of the laser plasma is proposed.     

Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I～2×10¹⁷ W cm^?2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<10¹⁷ W cm^?2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.     

Laser ablation of silver in different liquids: Optical and nonlinear optical properties of silver nanoparticles

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared using the method of laser ablation in various liquids at wavelengths of 397, 532, and 795 nm with laser pulses of different duration are studied. An analysis of the dimensional and spectral characteristics of the silver nanoparticles revealed a time dynamics of the nanoparticle size distribution in solutions. It is shown that thermal self-defocusing is observed for the case of nanosecond or shorter pulses generated with a high repetition rate. For picosecond and femtosecond pulses with a low repetition rate, the effects of self-focusing (γ = 3 × 10^?13 cm² W^?1) and saturated absorption (β = ?1.5 × 10^?9 cm W^?1) were observed in the solutions under study. The third-order nonlinear susceptibility of the silver nanoparticles was found to be 5 × 10^?8 esu at a wavelength of 397 nm.     

Laser ablation of gallium arsenide in different solutions

The optical, structural, and nonlinear optical characteristics of GaAs nanoparticles obtained by laser ablation in different liquids were investigated. Thermally induced self-defocusing in GaAs solutions was observed using both a high pulse repetition rate and nanosecond pulses. In studying the nonlinear optical characteristics of GaAs solutions using picosecond and femtosecond pulses, two-photon absorption was observed. The nonlinear absorption coefficient of an aqueous GaAs solution measured by the Z-scan technique and the nonlinear susceptibility of GaAs nanoparticles were, respectively, 0.7 × 10^?9 cm W^?1 and 2 × 10^?9 esu at a wavelength of 795 nm.     

Spectral and nonlinear studies of an aniline blue dye in liquid and solid media

Solid-state dye-doped polymers are an attractive alternative to conventional liquid dye solutions. In this paper, the spectral characteristics and nonlinear properties of the Aniline Blue dye has been studied. The third-order nonlinear optical properties of the Aniline Blue dye in ethanol and a dye-doped polymer film were measured by the Z-scan technique using a 632.8-nm He-Ne laser. This material exhibits a negative optical non-linearity. The dye at a 0.4-mM concentration exhibited a nonlinear refractive coefficient (n ₂ = ?4.02 × 10^?8 and ?4.41 × 10^?8 cm²/W in liquid and solid media, respectively), a nonlinear absorption coefficient (β = ?9.7 × 10^?4 and ?11.63 × 10^?4 cm/W in liquid and solid media, respectively), and susceptibility (x ⁽³⁾ = 1.844 × 10^?6 and 2.028 × 10^?6 esu in liquid and solid media, respectively). These results show that the Aniline Blue dye has potential applications in nonlinear optics.     

Nonlinear refraction of silver nanowires from nanosecond to femtosecond laser excitation

In order to investigate the effect of pulse width and solvent on the nonlinear properties of metal nanostructures, silver nanowires were fabricated in a direct current electric field (DCEF) using a solid-state ionic method and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nonlinear refractive index (γ) of silver nanowires suspended in ethanol was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nanosecond) pulse durations. Experimental results indicated that silver nanowires have obvious positive refractive nonlinearities and γ (the Kerr-induced self-focusing) increases as the pulse duration increases from 7.4×10⁻⁸ cm²/GW at 110 fs to 1.6×10⁻⁴ cm²/GW at 8 ns, due to the additional influence of the atomic reorientational Kerr effect in the case of longer pulses. Due to the solvent dependence of the nonlinear behavior of the silver nanowires, the nonlinear absorption and refraction of silver nanowires suspended in de-ionized water are smaller than those of silver samples suspended in ethanol. The thermal nonlinearities are insignificant in our experimental conditions.     

A study of the nonlinear refractive index in amorphous Ge–As–S layers and nanocomposites with gold nanoparticles

Nonlinear refractive indices of simple and composite chalcogenide glasses with gold nanoparticles are measured by the Z-scan method using a femtosecond Ti:sapphire laser with a central wavelength of 800 nm and a pulse duration of 40 fs. It is shown that introduction of nanoparticles into thin layers of amorphous GeS₂, As₃₀Ge₂₀S₅₀, and As₃Ge₃₅S₆₂ leads to a decrease in the total nonlinear refractive index due to additional absorption of femtosecond laser radiation without efficient excitation of localized plasmons outside the resonance region. For example, the nonlinear refractive index decreases with addition of gold nanoparticles from 16.1 × 10^–12 to 13.0 × 10^–12 cm²/W for GeS₂, from 3.9 × 10^–12 to 3.2 × 10^–12 cm²/W for As₃₀Ge₂₀S₅₀, and from 5.8 × 10^–12 to 4.7 × 10^–12 cm²/W for As₃Ge₃₅S₆₂.     

Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer

We report the study of extended nanoripple structures formed during the interaction of high-intensity 3.5 fs pulses with a moving silicon wafer. The optimization of laser intensity (8×10¹³ W?cm^?2) and sample moving velocity (1 mm?s^?1) allowed the formation of long strips (～5 mm) of homogeneous nanoripples along the whole area of laser ablation. The comparison of nanoripples produced on the silicon surfaces by few- and multi-cycle pulses is presented. We find that few-cycle pulses produce sharp and more homogenous structures than multi-cycle pulses.     

Determination of optical constants and nonlinear optical coefficients of Violet 1-doped polyvinyl alcohol thin film

The optical properties of Violet 1-doped polyvinyl alcohol (PVA) have been investigated using Wemble and Didomenico (WD) method. The optical constants such as refractive index n, the dispersion energy E _d, the oscillation energy E ₀, the lattice dielectric constant \(\varepsilon _{\infty } \), light frequency dielectric constant ε ₀ and the ratio of carrier concentration to the effective mass N/m* have been determined using reflection spectra in the wavelength range 300–900 nm. The single- beam Z-scan technique was used to determine the nonlinear optical properties of Violet 1:polyvinylalcohol (PVA) thin film. The experiments were performed using continuous wave (cw) laser with a wavelength of 635 nm. The calculated nonlinear refractive index of the film, n ₂ = ?2.79×10^?7 cm²/W and nonlinear absorption coefficient, β = 6.31×10^?3 cm /W. Optical limiting characteristics of the dye-doped polymer film was studied. The result reveals that Violet 1 can be a promising material for optical limiting applications.     

Photoinduced semiconductor-metal phase transition in a peierls system

The dynamical equation for the order parameter of the metal-semiconductor phase transition, as well as the kinetic equation for the density of nonequilibrium electron-hole pairs of a Peierls system in a light field, has been derived. An expression for the time τ of the nonthermal photoinduced semiconductor-metal phase transition has been obtained from these equations for the case of an ultrashort light pulse. It has been shown that, to initiate the phase transition, the energy density W of the light pulse must be higher than the critical value W _c. The W _c, τ, and optical absorption coefficient γ₀ that are calculated in the framework of the proposed model are in agreement with the experimental data (W _c ≈ 12 mJ/cm², τ ≈ 75 fs, and γ₀ ≈ 10⁵ cm^?1) on the irradiation of a vanadium dioxide film by a laser pulse with a duration of τ_p ≈ 15 fs, a photon energy of ?θ₀ = 1.6 eV, and an energy density of W = 50 mJ/cm².     

A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond,picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from magnesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10⁴ J cm^− 2. It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10¹⁷ W cm^− 2 for the 45 fs pulses and 6.2 × 10¹⁴ W cm^− 2 for the 25 ps pulses are much higher than 5 × 10¹² W cm^− 2 for the 3 ns laser pulse. The results were explained in terms of the ionization equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.     

The study on the nonlinear optical response of Sudan I

The nonlinear optical properties of Sudan I were investigated by a single beam Z-scan technique. The Sudan I ethanol solution exhibited large nonlinear refractive indices under both CW and pulse laser excitations. The nonlinear refractive indices of Sudan I were in the order of ?10^?8 cm²/W under CW 633 nm excitation and ?10^?6 cm²/W under CW 488 nm excitation, respectively. Under the excitation of a pulse 532 nm laser, the nonlinear refractive index n₂ was calculated to be 1.19 × 10^?14 cm²/W. It was discussed that the mechanism accounting for the process of nonlinear refraction was attributed to the laser heating for the CW laser excitation and the electronic effect for the pulse excitation. Moreover, the second hyperpolarizability of Sudan I was also estimated in this paper.     

Control of cluster multielectron ionization in ultraintense laser fields

We performed classical molecular dynamics simulations to explore the controllability of the inner ionization process in Xe_n clusters (n = 2?2171), driven by ultraintense infrared Gaussian laser fields (peak intensity I _M = 10¹⁵?10¹⁸ W cm^?2, temporal pulse length τ = 10?100 fs, and frequency ν = 0.35 fs^?1). Controllability of ion charge abundances and of their spatial distributions inside the cluster emerges from the different pulse length dependences of classical barrier suppression ionization (BSI) and of electron impact ionizations (EII), as well as from the time scale of the Coulomb explosion (CE). For large clusters (Xe₂₁₇₁), low intensities (10¹⁵ W cm^?2), and long pulses (τ = 100 fs), EII is the dominating ionization channel, which favors the formation of maximum charged ions (Xe¹⁰⁺, Xe¹¹⁺) in the cluster center. In contrast, BSI forms an inverse radial charge ordering with the highest charges in the exterior cluster shells. This suggests that the production of the two inverse radial charge distributions with an equal average ion charge can be forced by the choice of multiple pulses with different intensities and pulse lengths. At high intensities (10¹⁷?10¹⁸ W cm^?2), where EII is insignificant and CE sets in much earlier, the BSI radial charge ordering and the enhancement of the ion charges beyond the single-atom limit by the ignition effect is observed only for short pulses.     

The study of spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces

Spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces interacting with laser radiation (t=27 ps and I≤1.5×10¹⁵ W/cm²) are studied. Spectral broadening and a long-wavelength shift of the second harmonic were observed for laser radiation intensities exceeding 5×10¹⁴ W/cm². Results of the study of the conversion of spectral parameters and polarization features for the generation of second and third harmonics are presented. Conversion efficiencies for the second, third, and fourth harmonic are 2×10^?8, 10^?10, and 5×10^?12, respectively. The results obtained are compared with data of analogous studies utilizing shorter pulses.     

Hot electrons produced from long scale-length laser-produced droplet plasmas

Microplasmas produced from 15 μm methanol droplets irradiated by 100 fs laser pulses in the intensity range 10¹⁴–10¹⁶ W cm^?2 are investigated via measurements of the hot electron temperature and x-ray yields under different conditions of intensity, polarization state, and plasma scale-length. The scale length of the drop-let plasma is increased with an intentional prepulse that is 10 ns ahead of the main pulse. Hot electron temperatures up to 48 keV have been measured at intensities of 2.5 × 10¹⁵W cm^?2 and the scaling of temperature as a function of intensity is determined for a long scale-length droplet plasma. The polarization and ellipticity dependence of the hard x-ray yield from the microdroplet plasmas are used to probe the shape of the droplet after irradiation by a prepulse.     

Nonlinear optical characteristics of CdS and ZnS nanoparticles implanted into zirconium oxide thin films

The nonlinear refractive indices γ and nonlinear absorption coefficients of ZrO₂ films doped with CdS or ZnS nanoparticles, as well as with various metals, are measured. The effects of semiconductor and metal nanoparticles and annealing on the nonlinear optical properties of films are studied. The structural parameters of films, determined by electron microscopy and x-ray dispersion spectroscopy, are compared to the optical and nonlinear optical characteristics of these media. The high magnitude of γ of the films ((3±0.6)×10^?11 cm² W^{? 1}) is attributed to the surface enhancement effect in semiconductor nanoparticles. On the basis of Z-scan data obtained at different intensities of radiation, it is shown that the variations in γ of the ZrO₂:CdS(Cr) and ZrO₂:ZnS(Mn) films are related to the generation of free carriers.     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

Effect of the pulse leading-edge time on the dielectric strength of a vacuum gap

Optimal regimes for electrode conditioning in a vacuum by applying voltage pulses with different waveforms are considered. For nanosecond pulses with a constant duration (t _p = const), the impulse dielectric strength for an oblique voltage wave is shown to be more than four times higher than for a rectangular pulse with an infinitely short leading-edge duration. The dependences of the dielectric strength on the conditioning pulse duration in the range 10^?10 < t _p < 10^?3 s for pulses with different rise rates are obtained. The dielectric strength increases from 2 × 10⁷ V/m for microsecond pulses to 10¹⁰ V/m for subnanosecond pulses.     

Nonlinear refraction in CS2

The nonlinear refractive index () of CS₂ was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nano-second) pulse durations. We observed the growth of with the increase of the pulse duration (from (3±0.6)×10^-15 cm²W^-1 at 110 fs to (4±2)×10^-14 cm²W^-1 at 75 ns) due to the additional influence of the molecular reorientational Kerr effect in the case of longer (picosecond and nanosecond) pulses. Acoustic wave induced negative nonlinear refraction was observed using wavefront analysis. We analyzed the simultaneous influence of both electronic and molecular processes leading to the positive nonlinear refraction and acoustic processes leading to the negative nonlinear refraction in carbon disulfide. Variations of the refractive index due to the thermal effect at high pulse repetition rates were also investigated. PACS 42.65.An; 42.65.Jx; 78.66.Nk