Laser fragmentation of organic microparticles into colloidal nanoparticles in a free liquid jet

We present a novel approach for laser fragmentation of melamine cyanurate microcrystals suspended in liquid into colloidal nanoparticles. Laser fragmentation is done by irradiating a liquid jet of melamine cyanurate suspended in water with intense picosecond pulses. The free liquid jet is generated by a nozzle with small diameter and provides a thin liquid filament (d _fil<1 mm) perpendicular to the focused laser beam. This geometry allows tight focusing resulting in high intensities without the danger of damaging an optical element like windows necessary in conventional flow cells or cuvettes. It reduces losses of excitation light by avoiding scattering or absorption in front of the focus. We stabilized the nanoparticles electrosterically in-situ with neutral and polyelectrolytic polymers preventing agglomeration and precipitation. The threshold for sufficient stabilization of laser-fragmented nanoparticles (d _hydrodyn≈200 nm) is reached at a mass fraction of 0.25 wt% dextrin as a neutral polymer and 0.01 wt% polyacrylic acid as a polyelectrolytic polymer. Hydrodynamic size and zeta-potential of the nanoparticles can be controlled by mass fraction of the stabilization agent.     

Photodarkening of amorphous Se and Se95Te5 films under μs light pulses

Photodarkening phenomena in the amorphous Se and Se₉₅Te₅ films at the μs light pulses irradiation are investigated. The process of photodarkening during each of the individual 1, 10 or 100 μs pulses and also the processes proceeding between successive pulses are recorded and analyzed. The darkened film was shown to restore initial optical properties after some stay in darkness without any additional irradiation or heating. Photodarkening is very strongly dependent on the light pulse fluence.     

Raman study of phase transformation of TiO2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACT

Semimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (E_g and A_1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals.     

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

The influence of melting on the excitation of Surface Acoustic Wave (SAW) pulses in silicon is studied both theoretically and experimentally. The developed theory of Rayleigh-type SAW laser-induced thermoelastic excitation in a structure composed of a liquid layer on a solid substrate predicts that the SAW is predominantly generated in the solid phase due to the absence of shear rigidity in a liquid. The characteristic changes in the SAW pulse shape as well as the saturation and even the decrease of the SAW pulse amplitude observed above the melting threshold are explained theoretically to be a result of the decrease of the heat flux into the solid phase as well as due to the decrease of the volume of the solid phase caused by melting. Although the heat flux into the solid phase is decreased both as a consequence of the reflectivity increase and the additional energy losses (latent heat of melting) at the phase transition, it is demonstrated that the influence of reflectivity changes on the SAW pulse is negligible in comparison with the effect of melt-front motion. For laser pulses of 7 ns duration at 355 nm, the threshold value of laser fluence for meltingF _m=0.23±0.04 J/cm² and for the ablationF _a=1.3±0.2 J/cm² were determined experimentally as the points of characteristic changes in the observed SAW pulses.     

Compositional stability in GeO x and SbO x thin films for optical-storage applications

Laser-induced transformations (optical and compositional) of amorphousMO _x thin films are studied as a function of the oxygen content and the nature of theM element,M being either a semiconductor (Ge) or a semi-metal (Sb). A high optical contrast in reflectivity is always found associated to the first laser pulses, this process being unaffected by the presence of oxygen. Next pulses lead to a process which may involve optical changes depending on the environmental conditions. Whereas the former optical changes are most likely related to a structural process which is completed before oxidation starts, the latter ones occur when irradiating at high oxygen pressures (1.2 bar) and thus they are easily related to an oxidation process. At low oxygen pressures (close to the oxygen partial pressure in air), the films show a good compositional stability upon irradiation together with a high optical contrast.     

Time resolved spectra of optical and electrical transient response induced by nanosecond laser pulses in amorphous AgInSbTe films

The phase transition dynamics of amorphous Ag₈In₁₄Sb₅₅Te₂₃ (AIST) thin films induced by single nanosecond laser pulses were studied by transient optical reflectivity and electrical resistance measurements with nanosecond resolution. Phase transition driven by nanosecond laser pulses can be achieved in a proper fluence range on AIST thin films. The results show that phase transition dynamics driven by nanosecond laser pulses was a multi-stage optical evolution process involving melt, solidification, recalescence, and recrystallion. However, it was found that the real-time responses of optical and electrical signals were quite different under the same irradiated condition. The recalescence process reflected by the second rising of optical reflectivity will not result in obvious changes in electrical resistance. The dependence of saturated time determined by optical and electrical evolution curve on laser pulse fluence was compared and analyzed. A two-dimensional percolation model was employed to explain the difference between electrical and optical transient responses.     

Stimulated four-photon mixing and efficient polychromatic visible light generation in SiO2−based single-mode optical fibers pumped at 1.319 μm

Abstract

The generation of near-infrared and intense visible light through stimulated multi-wave mixing processes in single-mode silica-based optical fibers pumped by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm is described. The experimental results show that intense infrared light around 1.2 μm is produced via selp-hase-matched four-photon mixing at the minimum group velocity dispersion region of pure SiO₂?core and P₂O₅?doped silica fibers. In the visible spectral region, from 580 nm to 600 nm, 20 W peak power 100-ps pulses were generated by pumping single spans of single-mode P₂O₅?doped and undoped SiO₂?core fibers with 1.319-μm laser pulses. The signal light generated in such fibers propagated in the LP₀₂ fiber mode and exhibited a threshold power that depended upon the fiber length and a critical length that was power dependent. Also, it exhibited an asymmetrical spectrum of a few nanometers bandwidth, with a long tail toward high frequencies. For GeO₂?doped silica-based fibers, a multiple-wavelength visible signal propagating in several high-order fiber modes was generated.     

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

By means of optical pump–probe technique, the ultrafast dynamics of nonlinear optical response of the ferroelectric semiconductor Sn₂P₂S₆ crystal excited with a femtosecond laser pulse has been investigated. It has been shown that, under the action of femtosecond pulses, change in optical second harmonic generation occurs in the sample, which can be due to screening of existing electric polarization.     

Surface ablation оf aluminum and silicon by ultrashort laser pulses of variable width

Single-shot thresholds of surface ablation of aluminum and silicon via spallative ablation by infrared (IR) and visible ultrashort laser pulses of variable width τ_las (0.2–12 ps) have been measured by optical microscopy. For increasing laser pulse width τ_las < 3 ps, a drastic (threefold) drop of the ablation threshold of aluminum has been observed for visible pulses compared to an almost negligible threshold variation for IR pulses. In contrast, the ablation threshold in silicon increases threefold with increasing τ_las for IR pulses, while the corresponding thresholds for visible pulses remained almost constant. In aluminum, such a width-dependent decrease in ablation thresholds has been related to strongly diminished temperature gradients for pulse widths exceeding the characteristic electron-phonon thermalization time. In silicon, the observed increase in ablation thresholds has been ascribed to two-photon IR excitation, while in the visible range linear absorption of the material results in almost constant thresholds.     

Dynamics of phase transitions induced by femtosecond laser pulse irradiation of indium phosphide

The structural transformation dynamics of single-crystalline indium phosphide (InP) irradiated with 150 fs laser pulses at 800 nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150 fs up to 500 ns. The results obtained show that for fluences above a threshold of 0.16 J/cm² thermal melting of the material occurs on the timescale of 1–2 ps. The evolution of the reflectivity on a longer timescale reveals the reflectivity of the liquid phase and shows resolidification times typically around 10–30 ns after which an amorphous layer several tens of nanometers thick is formed on the surface. This amorphous layer significantly alters the optical properties of the surface and finally leads to a reduced ablation threshold for subsequent laser pulses. Single-pulse ablation at higher fluences (>0.23 J/cm²) is preceded by an ultrafast phase transition (non-thermal melting) occurring within 400 fs after the arrival of the pulse to the surface. PACS 79.20.Ds; 78.47.+p; 64.70.-p     

Optical and ultrasonic monitoring of femtosecond laser filamentation in fused silica

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica (FS) by single focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy E_opt = 5 μJ corresponding to a few megawatt power levels pulses collapse due to self-focusing, producing channels filled by electron-hole plasma and luminescent defects, and exhibits predominantly compressive pressure transients. Analysis of the optical and ultrasonic response versus the laser pulse energy suggests that filamentary pulse propagation in the channels occurs with considerable dissipation of about ∼10 cm⁻¹. The predominant ionization mechanism is most likely associated with avalanche ionization, while the main mechanism of optical absorption is free-carrier absorption via inverse Bremsstrahlung interaction with the polar lattice.     

Nonequilibrium motion of a metal surface exposed to submicrosecond laser pulses

The results of experimental studies and theoretical analysis of the displacement of a metal mirror surface upon heating by submicrosecond laser pulses (t ₀ = 15 × 10⁻⁹ s) are reported. The dynamics of the surface displacement in the irradiation zone was monitored by a Michelson interferometer with photoelectron count of fringes. A substantial delay of the motion of the exposed surface relative to a heating pulse was observed in experiments. It is shown that under thermal perturbations with high temperature gradients corresponding to large optical absorption coefficients, the contribution of nonequilibrium processes to the thermomechanical response of metals becomes decisive even for a submicrosecond duration of heating. The allowance for nonequilibrium processes in theoretical analysis leads to the generalized Duhamel law with thermal memory, which can be used for describing the observed effect correctly. The resultant characteristic time of a transient process (∼(2–6) × 10⁻⁸ s) considerably exceeds the available estimates of the phonon-phonon interaction time and is due to the effect of nonequilibrium processes on the scale of structural elements, which is considerably larger than the atomic scale.     

Optical limiting action of C60 doped poly(dimethylacetylendicarboxylate)

The optical limiting action of poly(dimethylacetylendicarboxylate) polymer doped with fullerene C₆₀ has been investigated under irradiation with 10 ns laser pulses at 532 nm. The optical limiting measurements were performed at four different dopant concentrations. The threshold limiting fluence at 0.3 J/cm² was observed at high doping concentrations, with transmission of about 55%. An explanation based on the combination of two-photon absorption and reverse saturable absorption was proposed for its nonlinear optical absorption behavior.     

Formation of defects in a GaAs crystal under the action of picosecond laser radiation near the optical damage threshold

The nonlinear optical properties of a GaAs crystal are investigated by the z-scan method. An increase (by a factor of more than 10) in the concentration of defects was observed in the GaAs crystal subjected to long-term pulsed laser irradiation (more than 10⁴ pulses, τ = 35 ps, λ = 1.064 μm) near the optical damage threshold.     

周期量级激光脉冲的Thomson散射

用经典辐射理论对线偏振周期量级激光脉冲的线性Thomson散射进行分析，从理论上得到它可产生亚阿秒脉冲的结论. 计算显示，在电子相对论因子为50、激光脉冲中心波长为1μm、归一化光场强度为0.01的情况下，用包含1.5个光周期的激光脉冲，可获得0.2as(半高全宽)的散射脉冲输出. 还对光场载波包络初相φ_ce和电子进入光场的初相φ_in对散射脉冲的影响作了分析讨论，结果表明，在适当的φ_ce和φ_in条件下，能实现单个阿秒脉冲输出，并可对脉冲宽度和频率进行调谐. 关键词： 线性Thomson散射 周期量级激光脉冲 载波包络初相 阿秒脉冲     

Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.     

Dynamics of a Peierls system in a light field

Equations describing the temporal dynamics of the order parameter ξ(t) of a metal-semiconductor phase transition and the density n(t) of electron-hole pairs in a Peierls system in a light field are obtained on the basis of the Lagrange equation for the phonon mode and the Liouville equation for the density matrix of the electronic subsystem. The equations obtained are analyzed for a stationary state (with adiabatically slow variation of the light intensity I) and for a transient process near the initial and final states of dynamic equilibrium (with the light field switched on abruptly). It is shown that for adiabatically slow growth of the intensity I up to a certain critical value I _c the band gap of the electronic spectrum decreases but the semiconductor phase of the Peierls system remains stable. For I>I _c the stationary semiconductor state (ξ≠0) becomes unstable. When the light is switched on abruptly, the deviation of the system parameters from the initial values is described by an exponential law with a characteristic reciprocal of the rise time of the process linearly dependent on the irradiation intensity I. As a new position of equilibrium is approached, three qualitatively different regimes of behavior of the order parameter ξ and density n are possible. For low intensities I(I< I ₁) a purely relaxational aperiodic process occurs. For intermediate intensities I(I ₁<I<I _c) damped oscillations of ξ and n are observed near a new stationary semiconductor state with a smaller band gap. For I>I _c the stationary semiconductor state with ξ≠0 is absent. The experimental data on the irradiation of a vanadium dioxide film with a powerful laser pulse is interpreted on the basis of the theory developed. Zh. éksp. Teor. Fiz. 116, 2154–2175 (December 1999)