Spatiotemporal dynamics of femtosecond pulses in nonlinear optical waveguides with material dispersion

The effect of the second-order dispersion on the spatiotemporal dynamics of ultrashort pulses in nonlinear waveguides with infinite parabolic and stepped profiles of the refractive index is studied. It is shown that the leakage of radiation from the core to the cladding of step-index waveguides prevents the formation of a steady spatiotemporal distribution of the field. The possibility of compressing light pulses in a dielectric waveguide with a positive group-velocity dispersion is discussed.     

Time-resolved dynamics analysis of nanoparticles applying dual femtosecond laser pulses

The evolution of a fs laser generated plume on copper was studied by dual laser pulses. The plume generated by the first pulse (620 nm) was excited by the second, delayed pulse (310 nm). The actual state of the plume was monitored by detecting the intensities of the emitted light and the reflected delayed pulse from the interaction range, i.e. 0–150 μm above the surface. Four peaks were observed in the 0–1400 ps time range and assigned to appearance of electrons, ions, atomic species and nanoparticles. Accordingly, the timing of the creation and ejection of these species was determined. By integrating the intensity of the nanoparticle peak, the production yield of the nanoparticles and its dependence on the ablating laser fluence was calculated. The ablation products were deposited on a Si substrate, too, and analyzed by atomic force microscopy and scanning electron microscopy. The results of the deposition and the dual-pulse experiments are in good agreement with the theory. PACS 52.38.Mf; 79.20.Ds; 61.80.Ba     

Spectral and temporal characteristics of metallic nanoparticles produced by femtosecond laser pulses

We study the time of flight optical emission from titanium and tungsten nanosized particles, generated through femtosecond laser-matter interaction in vacuum, in the wavelength spectral range from 300 to 900 nm. Typical spectra consist of broadband structureless signals similar to black body emission from a macroscopic object. Nanoparticles temperature, deduced from their emission spectra, decreases drastically as a function of their time of arrival at a given distance from the target. This behaviour is seen to be independent of individual particle velocities.     

Surface plasmon dynamics of simple metal clusters excited with femtosecond optical pulses

We report a new mechanism on the dynamics of correlated electrons in simple metal clusters which manifests by a strong electron temperature dependence of the surface plasmon resonance spectral profile. This effect is revealed thanks to a theoretical approach based on the time-dependent local-density approximation at finite electronic temperature, and it should be experimentally observable using pump-probe femtosecond spectroscopy techniques.     

Anisotropy in the nonlinear absorption of elongated silver nanoparticles in silica, probed by femtosecond pulses

We present measurements of the anisotropy of the nonlinear absorption of a silica matrix doped with aligned spheroidal silver nanoparticles, produced by a double ion-implantation process of silver nanoparticles followed by an irradiation with Si ions. The nonlinear response was studied using the z-scan technique with fs pulses at 530 nm, which lies very close to the surface-plasmon absorption peak of the sample. The observed saturable absorption is studied for different angles of the input linear polarization of the pulses, showing a strong anisotropy, which is consistent with the fact that the nanorods are aligned.     

Probing nonequilibrium dynamics with white-light femtosecond pulses

Femtosecond pulsed lasers have become an invaluable tool for examining ultrafast nonequilibrium dynamics. With pulsewidths of a few hundred femtoseconds (fs) to less than 10 fs, these lasers can clearly provide unprecedented temporal resolution. By amplifying ultrashort laser pulses to sufficient levels of energy per pulse, it is possible to exploit the nonlinear optical properties of certain materials to generate extremely broadband pulses. These pulses retain the time structure of the incident pulse, but contain a spectral bandwidth extending from the infrared to as far as the ultraviolet. By generating white-light pulses, it becomes possible to probe ultrafast nonlinear processes over a large range of energies. In this paper, the process of generating white-light ultrashort pulses will be presented, along with a discussion of different probing techniques and procedures necessary for modeling the transient optical data. Finally, results from pump-probe measurements using a white-light probe on indium phosphide (InP) films will be presented as a demonstration of this technique.     

Magnetization dynamics in epitaxial films induced by femtosecond optical pulses near the absorption edge

Peculiarities of the magnetization dynamics induced in iron garnet films by laser pulses with a frequency detuning near the absorption edge have been studied experimentally. It has been found that the dependence of the observed signal amplitude on the pumping energy becomes nonmonotonic with an increase in the pumping frequency. At the same time, the pumping energy corresponding to the maximum amplitude of the signal and this maximum signal amplitude decrease. Moreover, the signal amplitude starts to decrease with an increase in the pumping energy at frequencies within the absorption band. The observed phenomena are possibly caused by generation of magnetostatic spin waves and the effect of ultrafast optically induced demagnetization.     

Reverse propagation of femtosecond pulses in optical fibers

We present a numerical technique for reversing femtosecond pulse propagation in an optical fiber, such that given any output pulse it is possible to obtain the input pulse shape by numerically undoing all dispersion and nonlinear effects. The technique is tested against experimental results, and it is shown that it can be used for fiber output pulse optimization in both the anomalous and normal dispersion regimes.     

Non-envelope formulation for femtosecond optical pulses in semiconductors

We analyze the response of an ensemble of 1s-excitons driven by a femtosecond optical pulse, beyond traditional “slowly varying amplitudes” approach. For optical pulses of a given duration it is shown that the off-resonance optical field can evolve into a stable soliton with nonzero asymptotic behavior. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 380–384 (10 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Damped precession of the magnetization vector of superparamagnetic nanoparticles excited by femtosecond optical pulses

The ultrafast magnetization and electron dynamics of superparamagnetic cobalt nanoparticles, embedded in a dielectric matrix, have been investigated using femtosecond optical pulses. Our experimental approach allows us to bypass the superparamagnetic thermal fluctuations and to observe the trajectory of the magnetization vector which exhibits a strongly damped precession motion. The magnetization precession is damped faster in the superparamagnetic particles than in cobalt films or when the particle size decreases, suggesting that the damping is enhanced at the metal dielectric interface. Our observations question the gyroscopic nature of the magnetization pathway when superparamagnetic fluctuations take place as we discuss in the context of Brown's model.     

Change in the resonance frequency of surface plasmons in copper nanoparticles excited by femtosecond laser pulses

Changes in the optical density ΔD(t), halfwidth ΔH/2(t), and spectral position of the maximum Δλ_SP(t) of the surface plasmon band in Cu nanoparticles after their excitation by femtosecond laser pulses have been investigated. The Δλ_SP(t) dependence appears to be alternating and is accompanied by a nonmonotonic variation in ΔH/2(t) in the time interval 0–5 ps. The results are explained in a model based on the evolution of the dielectric response of such a composite medium excited by intense laser pulses.     

金属靶和绝缘靶对飞秒激光吸收的比较

利用脉宽为150fs、强度为8×10¹⁵W/cm²的P偏振飞秒激光研究了与 金属靶和绝缘靶 相互作用过程中的激光能量吸收、超热电子产额及超热电子能谱. 实验发现，由于绝缘靶电 导率小，因此其电荷分离势大于金属靶，从而导致绝缘靶比金属靶具有较小的激光能量吸收 、较少的超热电子发射和较高的超热电子温度. 关键词： 金属靶 绝缘靶 激光吸收 超热电子     

Electron dynamics and spatial characteristics of emission from electron oscillation driven by femtosecond laser pulses

The dynamics and characteristics of spatial distribution of emission are analyzed with a single electron model in the cases of different intensities and different polarized femtosecond laser pulses. It is discovered that with the increase of laser intensity, for circularly polarized laser pulse, the angular distribution is tipped forward more and more; for linearly polarized laser pulse, the radiation pattern is changed from the fourfold rotational symmetry bifoliate pattern same as that from a dipole antenna to twofold rotational symmetry trefoil pattern.     

Probing molecular dynamics at attosecond resolution with femtosecond laser pulses

The kinetic energy distribution of D+ ions resulting from the interaction of a femtosecond laser pulse with D2 molecules is calculated based on the rescattering model. From analyzing the molecular dynamics, it is shown that the recollision time between the ionized electron and the D+2 ion can be read from the D+ kinetic energy peaks to attosecond accuracy. We further suggest that a more precise reading of the clock can be achieved by using shorter fs laser pulses (about 15 fs).     

Spatially-uniform temporal recompression of intense femtosecond optical pulses

A specially designed telescope with defocusing lens and off-axis parabolic mirror, which is working as a nonlinear element and producing self-phase modulation, was implemented for intense (3.1?TW/cm²) Fourier Transform Limit femtosecond laser pulses with Gaussian beam profiles. The pulse spectrum was broadened quasi- homogeneously over the beam cross-section due to the change in the lens thickness compensating for the reduction of the beam intensity from its center to periphery. In experimental demonstrations a set of chirped mirrors allowed for the spectral phase correction to a final pulse compression of 20?fs from 40?fs.     

Generation and measurement of 200 femtosecond optical pulses

Pulses as short as 0.2 ps have been obtained from a passively mode locked dye laser. The technique is simpler than those previously used, but one must pay the price of a loss of tunability. The pulse durations were determined from their second order correlation functions measured with a highly stable interferometer which is described.     

Microjoule level femtosecond optical pulses with double-cladding fiber-based nonlinear chirped-pulse amplification

An all-fiber high-energy nonlinear chirped-pulse amplification system with two-stage double-cladding fiber amplifiers is presented in this paper. It generates pulses with the energy up to 10 μJ at 20 kHz repetition rate and total accumulated nonlinear phase shift as large as 12.8π. The pulses can be compressed to 137 fs with the compensation of mismatched third-order dispersion by self-phase modulation in the fiber amplifiers.     

Dynamics of femtosecond optical pulses. Solitons with initial phase modulation

The dynamics of optical solitons with initial phase modulation is investigated in the framework of the modified nonlinear Schrödinger equation. Fluctuations of the generated soliton parameters are studied. Statistical characteristics of the excited optical noise in fiber are obtained.