Femtosecond snapshot imaging of propagating light itself

An ultrafast imaging technique has been developed to visualize directly a light pulse that is propagating in a medium. The method, called femtosecond time-resolved optical polarigraphy (FTOP), senses instantaneous changes in the birefringence within the medium that are induced by the propagation of an intense light. A snapshot sequence composed of each femtosecond probing the pulse delay enables ultrafast propagation dynamics of the intense femtosecond laser pulse in the medium, such as gases and liquids, to be visualized directly. Other examples include the filamentation dynamics in CS2 liquid and the propagation dynamics in air related to the interaction with laser breakdown plasma. FTOP can also be used to extract information on the optical Kerr constant and its decay time in media. This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.     

飞秒激光与金膜作用的脉冲累积效应

采用1kHz飞秒激光（脉宽148fs,中心波长775nm）对石英衬底的金膜的烧蚀过程进行了研究．单脉冲与多脉冲的烧蚀阈值可以通过烧蚀点的直径平方与所用的激光能流的关系曲线获得．通过累积能流和烧蚀脉冲数的关系,可以得到金膜的脉冲累积因子．采用飞秒激光加工材料的一些特点可以合理解释单脉冲阈值附近获得的一些实验现象．     

Clustered gases as a medium for efficient plasma waveguide generation

Clustered gas jets are shown to be an efficient means for plasma waveguide generation, for both femtosecond and picosecond generation pulses. These waveguides enable significantly lower on-axis plasma density (less than 10(18) cm(-3)) than in conventional hydrodynamic plasma waveguides generated in unclustered gases. Using femtosecond pump pulses, self-guided propagation and strong absorption (more than 70%) are used to produce long centimetre scale channels in an argon cluster jet, and a subsequent intense pulse is coupled into the guide with 50% efficiency and guided at above 10(17)W cm(-2) intensity over 40 Rayleigh lengths. We also demonstrate efficient generation of waveguides using 100 ps axicon-generated Bessel-beam pump pulses. Despite the expected sub-picosecond cluster disassembly time, we observe long pulse absorption efficiencies up to a maximum of 35%. Simulations show that in the far leading edge of the long laser pulse, the volume of heated clusters evolves to a locally uniform and cool plasma already near ionization saturation, which is then efficiently heated by the remainder of the pulse.     

Two-colour laser-induced electron dynamics in two-level quantum system

To attempt to control the quantum state of a physical system with a femtosecond two-colour laser field, a model for the two-level system is analysed as a first step. We investigate the coherent control of the two-colour laser pulses propagating in a two-level medium. Based on calculating the influence of the laser field with various laser parameters on the electron dynamics, it is found the electronic state can be changed up and down by choosing the appropriate laser pulses and the coherent control of the two-colour laser pulses can substantially modify the behaviour of the electronic dynamics: a quicker change of two states can be produced even for small pulse duration. Moreover, the oscillatory structures around the resonant frequency and the propagation features of the laser pulses depend sensitively on the relative phase of the two-colour laser pulses. Finally, the influence of a finite lifetime of the upper level is discussed in brief.     

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.     

Time-of-Arrival Fluctuations of a Space-Time Gaussian Pulse in Weak Optical Turbulence: an Analytic Solution.

An analytic expression is derived for the long-term temporal broadening (fluctuations of arrival time) of a collimated space-time Gaussian pulse propagating along a horizontal path through weak optical turbulence. General results are presented for nominal parameter values characterizing laser communication through the atmosphere. Specific examples are calculated for both upper-atmosphere and ground-level cross links. It is shown that, for upper-atmosphere cross links, pulses shorter than 100 fs have considerable broadening, whereas at ground level, broadening is predicted in pulses as long as 1 ps.     

飞秒激光烧蚀MgAl2O4透明陶瓷的实验研究

实验研究了800nm飞秒激光与MgAl2O4透明陶瓷的相互作用，得到其在单脉冲、多脉冲情况下的损伤阈值和损伤面积，用CCD成像技术和扫描电镜观察了烧蚀点的形貌特征，用显微红外光谱仪测试了烧蚀区域的透过光谱．结果表明；单脉冲烧蚀条件下，烧蚀面积与脉冲能量近似为线性关系，而在多脉冲烧蚀条件下，烧蚀面积随着脉冲数量的增加呈近似波尔兹曼（Boltzmann）增大；当激光功率接近损伤阈值时，烧蚀后的区域在波数为2500-7000cm^-1范围内的红外透过率由82％提高到86％，当激光功率超过损伤阈值后，透过率降低20％左右．     

Direct characterization of self-guided femtosecond laser filaments in air

High-power femtosecond laser pulses propagating in air form self-guided filaments that can persist for many meters. Characterizing these filaments has always been challenging owing to their high intensity. An apparently novel diagnostic is used to directly measure the fluence distribution of femtosecond laser pulses after they have formed self-guided optical filaments in air. The diagnostic is unique in that the information contained in the filaments is not lost owing to the interaction with the apparatus. This allows filament characteristics such as energy and size to be unambiguously determined for the first time.     

Intensity limitation for a self-focusing femtosecond laser pulse propagating in a medium with cubic nonlinearity

The self-focusing of an axisymmetric beam propagating in a cubically nonlinear medium has been studied by computer simulation with allowance for the temporal dispersion of the nonlinear response, which is manifested for femtosecond laser pulses. It shown that, depending on the ratio of the medium length and the diffraction length, the dispersion of the nonlinear response can lead either to limitation of the intensity growth rate in the nonlinear focus or its absence (for weakly diffracting beams), to an increase in the range of the nonlinear focus formation (for beams with a diffraction length 2.5–25 times the medium length), or to a decrease in this range (for beams with a diffraction length approximately equal to or shorter than the medium length). These phenomena are related to the dependence of the group velocity of pulse propagation on its intensity.     

Remote atmospheric breakdown for standoff detection by using an intense short laser pulse

A remote atmospheric breakdown is a very rich source of UV and broadband visible light that could provide an early warning of the presence of chemical-biological warfare agents at extended standoff distances. A negatively chirped laser pulse propagating in air compresses in time and focuses transversely, which results in a rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof-of-principle laboratory experiments are performed on the generation of remote atmospheric breakdown and the spectroscopic detection of mock biological warfare agents. We have generated third harmonics at 267 nm and UV broadband radiation in air from the compression and focusing of femtosecond laser pulses. Fluorescence emission from albumin aerosols as they were illuminated by the femtosecond laser pulse has been observed.     

Application of femtosecond-laser induced nanostructures in optical memory

The femtosecond laser induced micro- and nanostructures for the application to the three-dimensional optical data storage are investigated. We have observed the increase of refractive index due to local densification and atomic defect generation, and demonstrated the real time observation of photothermal effect after the femtosecond laser irradiation inside a glass by the transient lens (TrL) method. The TrL signal showed a damped oscillation with about an 800 ps period. The essential feature of the oscillation can be reproduced by the pressure wave creation and propagation to the outward direction from the irradiated region. The simulation based on elastodynamics has shown that a large thermoelastic stress is relaxed by the generation of the pressure wave. In the case of soda-lime glass, the velocity of the pressure wave is almost same as the longitudinal sound velocity at room temperature (5.8 microm/ns). We have also observed the localized photo-reduction of Sm3+ to Sm2+ inside a transparent and colorless Sm(3+)-doped borate glass. Photoluminescence spectra showed that some the Sm3+ ions in the focal spot within the glass sample were reduced to Sm2+ ions after femtosecond laser irradiation. A photo-reduction bit of 200 nm in three-dimensions can be recorded with a femtosecond laser and readout clearly by detecting the fluorescence excited by Ar+ laser (lambda = 488 nm). A photo-reduction bit can be also erased by photo-oxidation with a cw Ar+ laser (lambda = 514.5 nm). Since photo-reduction bits can be spaced 150 nm apart in a layer within glass, a memory capacity of as high as 1 Tbit can be achieved in a glass piece with dimensions of 10 mm x 10 mm x 1 mm. We have also demonstrated the first observation of the polarization-dependent periodic nanostructure formation by the interference between femtosecond laser light and electron acoustic waves. The observed nanostructures are the smallest embedded structures ever created by light. The period of self-organized nanostructures can be controlled from approximately 140 to 320 nm by the pulse energy and the number of irradiated pulses. Furthermore, we have also observed the self-assembled sub-wavelength periodic structures created in silica glass by femtosecond pulses on the plane of the propagation of light.     

Generation of a train of ultrashort pulses from a compact birefringent crystal array

A linear array of n calcite crystals is shown to allow the generation of a high contrast (>10:1) train of 2(n) high energy (>100 microJ) pulses from a single ultrafast laser pulse. Advantage is taken of the pulse-splitting properties of a single birefringent crystal, where an incident laser pulse can be split into two pulses with orthogonal polarizations and equal intensity, separated temporally in proportion to the thickness of the crystal traversed and the difference in refractive indices of the two optic axes. In the work presented here an array of seven calcite crystals of sequentially doubled thickness is used to produce a train of 128 pulses, each of femtosecond duration. Readily versatile properties such as the number of pulses in the train and variable mark-space ratio are realized from such a setup.     

Elastic light scattering from single microparticles on a femtosecond time scale

Experimentally measured and theoretically calculated elastic light-scattering spectra from single microparticles illuminated by 100-fs pulses are presented. Although in the theoretical calculation only a single incoming femtosecond laser pulse was used, the spectral behavior of scattered light shows all the features seen in the experimental spectrum from many femtosecond pulses, including morphology-dependent resonances (MDR's). The good agreement between experimental and theoretical elastic light-scattering data has stimulated a theoretical investigation of the time-dependent behavior of the elastically scattered light from a single microparticle on a femtosecond time scale. Since the spatial pulse length of the incoming laser pulse is smaller than the particle circumference, the temporal behavior of reflection, diffraction, refraction, and coupling into MDR's can be distinguished. Since the time-dependent scattering is strongly dependent on particle size, refractive index, and pulse chirp, it may be possible to encode several bits of information into a single laser pulse and therefore to increase optical data communication rates.     

Two-frequency mutual coherence function of electromagnetic waves in random media: a path-integral variational solution

By use of path-integral methods, a general expression is obtained for the two-frequency, two-position mutual coherence function of an electromagnetic pulse propagating through turbulent atmosphere. This expression is valid for arbitrary models of refractive-index fluctuations, wide band pulses, and turbulence of arbitrary strength. The approach presented in this paper was examined in the cases of plane-wave, spherical wave, and Gaussian beam propagation in power-law turbulence and compared with existing numerical and exact results. A number of new results were obtained for the Gaussian beam pulse. Expressions derived here should be applicable to a wide range of practical pulse propagation problems.     

Energy dependent saturable and reverse saturable absorption in cube-like polyaniline/polymethyl methacrylate film

Solid films of cube-like polyaniline synthesized by inverse microemulsion polymerization method have been fabricated in a transparent PMMA host by an in situ free radical polymerization technique, and are characterized by spectroscopic and microscopic techniques. The nonlinear optical properties are studied by open aperture Z-scan technique employing 5 ns (532 nm) and 100 fs (800 nm) laser pulses. At the relatively lower laser pulse energy of 5 μJ, the film shows saturable absorption both in the nanosecond and femtosecond excitation domains. An interesting switchover from saturable absorption to reverse saturable absorption is observed at 532 nm when the energy of the nanosecond laser pulses is increased. The nonlinear absorption coefficient increases with increase in polyaniline concentration, with low optical limiting threshold, as required for a good optical limiter.     

The Generation of a Dense Hot Plasma by Intense Subpicosecond Laser Pulses

A simple model is developed for describing the interaction of intense femtosecond laser pulses with solid-state targets which is based on a set of equations of two-temperature hydrodynamics for electrons and ions of a plasma formed upon ionization of the target matter, equations describing the variation of ion composition of plasma upon ionization and the heat energy expenditure on thermal ionization, and equations defining the energy contribution by laser radiation to the target matter. A self-similar solution is suggested which well describes the heating of plasma electrons during the time of effect of a femtosecond laser pulse in a wide range of its parameters. Lagrangian computer codes developed for this purpose are used to derive, in a one-dimensional approximation, a numerical solution for the set of equations for the parameters corresponding to femtosecond experimental facilities under development in Germany and Russia. Profiles of hydrodynamic quantities (electron and ion temperature, plasma pressure and density, mean ion charge) obtained in the numerical solution at different moments of time may be used for preliminary assessment of the results of future experiments with a view to optimizing their parameters.     

基于三维双温模型的飞秒激光烧蚀面齿轮材料形貌研究

在传统双温模型基础上,建立考虑变离焦量效应和随电子温度变化的动态吸收率效应的三维双温模型。分析烧蚀过程中电子、晶格温度的变化情况和烧蚀凹坑的形貌。对面齿轮材料18Cr2Ni4WA进行飞秒激光烧蚀实验结果表明：随着单脉冲飞秒激光能量密度的增大,电子的峰值温度升高,烧蚀凹坑的深度和直径增大;单脉冲飞秒激光的脉宽对烧蚀形貌的影响并不显著;烧蚀凹坑的直径和深度不会随脉冲数目一直增大,脉冲数目存在阈值,超过该阈值对于烧蚀凹坑的形貌影响反而不利。     

Reconstruction of polarization-shaped laser pulses after a hollow-core fiber using backreflection

We present a method to reconstruct the pulse shape of polarization-shaped femtosecond laser pulses after a hollow-core photonic crystal fiber by reflecting the pulses back through the fiber. First, a procedure is introduced to receive the optical fiber properties and generate parametrically shaped pulses after propagation through the fiber. Changes of the fiber's birefringence by mechanical stress are examined to investigate the correlation between the pulse shapes after one and two passes through the fiber. Finally, we demonstrate the characterization of the pulse after one pass through the fiber by calculating the pulse shape from the measured pulse after two passes.     

Comparative study of femtosecond and nanosecond laser-induced breakdown spectroscopy of depleted uranium

We present spectra of depleted uranium metal from laser plasmas generated by nanosecond Nd:YAG (1064 nm) and femtosecond Ti:sapphire (800 nm) laser pulses. The latter pulses produce short-lived and relatively cool plasmas in comparison to the longer pulses, and the spectra of neutral uranium atoms appear immediately after excitation. Evidence for nonequilibrium excitation with femtosecond pulses is found in the dependence of spectral line intensities on the pulse chirp.     

Gauss-Legendre quadrature method used to evaluate the spatio-temporal intensity of ultrashort pulses in the focal region of lenses

We analyze the spatio-temporal intensity of sub-20 femtosecond pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achromatic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. The numerical errors in this method are negligible by taking 96 nodes and the computational time is reduced by 95% compared to the integration method by rectangles. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). This last effect, however, is also significant. For sub-20 femtosecond pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect. The design of an apochromatic doublet is presented in this paper and the spatio-temporal intensity of the pulse at the focal region of this doublet is compared to that given by the achromatic doublet.