飞秒激光等离子体通道传导能量特性的研究进展

基于强飞秒激光诱导等离子体通道的基本性质,分析了等离子体通道在传导电能和射频电磁能时的一些基本问题。综述了飞秒激光等离子体通道传输电能和电磁能方面的新概念和新技术,重点描述了该领域近几年所取得的一些理论及应用的最新成果。最后,展望了飞秒激光等离子体通道在传导能量技术领域中的发展趋势。     

Spectroscopic analysis of femtosecond laser plasma filament in air

We report a spectroscopic analysis of a filament generated by a femtosecond laser pulse in air. In the filament spectra, the characteristic Stark broadened atomic oxygen triplet centered at 777.4 nm has been observed. The measured electron impact Stark broadening parameter of the triplet is larger than the theoretical value by Griem [H.R. Griem, Plasma Spectroscopy, McGraw Hill, New York, 1964] by a factor 6.7. Using the experimental value , the plasma densities derived from Stark broadening agree well with those most recently obtained from Théberge et al.’s measurement of the nitrogen fluorescence calibrated by longitudinal diffraction [F. Théberge, W. Liu, P.T. Simard, A. Becker, S. L. Chin, Phys. Rev. E 74 (2006) 036406]. However, the Stark broadening approach is much simpler and can be used to non-invasively measure the filament plasma density distribution in air under different propagation conditions.     

Transverse evolution of a plasma channel in air induced by a femtosecond laser

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear.     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

等离子体通道的声学诊断方法

超短脉冲强激光在空气中传输可以形成很长的等离子体通道，目前对这种等离子体通道有许多诊断方法，文章介绍了一种十分简易但很有效的方法——声学测量方法．该方法可以根据通道不同位置的声音信号，测量通道的长度和通道内电子密度等一些参数，具有其他传统诊断方法无可比拟的优越性．     

Overheated plasma at the surface of a target with a periodic structure induced by femtosecond laser radiation

A periodic structure is induced at the surface of a metal target exposed to a series of p-polarized 200-femtosecond laser pulses with intensity close to the melting threshold of the target material. The period of the structure is determined by the interference between the incident pump wave and the surface electromagnetic wave. Exposure of the obtained structure to the same laser pulse, but with an intensity of ～10¹⁶ W/cm², provides resonant excitation of the surface electromagnetic waves at the plasma-vacuum interface. This leads to an increase in the X-ray output and the temperature of plasma hot electrons.     

Adaptive control of femtosecond laser ablation plasma emission

The influence of temporal pulse shaping on plasma plume generated by ultrafast laser irradiation of aluminum is investigated. Time resolved plasma emission spectroscopy is coupled with a temporal shaping procedure in a closed loop. The ionic emission is enhanced relative to the neutral one via an adaptive optimization strategy. The plasma emission efficiency in case of optimized and ultrashort temporal shapes of the laser pulses are compared, evidencing an enhancement of the ionization degree of the plasma plume. Simplified temporal shapes of the femtosecond laser pulses are extracted from the optimized shape and their corresponding effect on laser induced plasma emission is discussed.     

Prolongation of the fluorescence lifetime of plasma channels in air induced by femtosecond laser pulses

The lifetime of plasma channels induced by femtosecond laser pulses is investigated by detecting the decay time of the fluorescence signals from ions. It is found that the lifetime of the plasma can be prolonged to the order of microseconds when an additional sub-nanosecond laser pulse is injected into the channel. This prolongation is due to the heating and further ionization through the inverse bremsstrahlung absorption of the post-pulse.     

Microscopic analysis of carbon phases induced by femtosecond laser irradiation on single-crystal SiC

Elemental analysis of femtosecond laser-induced modified region was carried out by transmission electron microscopy and Raman spectroscopy. The relative Raman intensities of a-SiC were higher in the peripheral region of laser irradiated spot where the fine ripple was formed. On the contrary, the relative Raman intensities of a-Si were higher in the central region where the coarse ripple was formed. This result suggests that the material migration has strongly occurred in the higher fluence region. On the other hand, the mapping of carbon atoms in the topmost amorphous layer of laser induced periodic structures did not show any significant segregation. In addition, Raman spectroscopic analysis showed that the domain size of carbon was very small (< 1 nm). From these facts, it was found that the carbon atoms were uniformly distributed in the topmost amorphous layer and were randomly connected without forming any observable fine particles.     

Ar plasma waveguide produced by a low-intensity femtosecond laser

Using the interaction of a low-intensity femtosecond laser pulse (30 fs, 6 × 10¹⁵ Wcm^? 2) with argon cluster jet produced from a slit nozzle, we experimentally probe the formation of a uniform plasma waveguide by the interferogram analysis. The results about evolution of plasma channel demonstrate that it is feasible to produce the plasma waveguide for an fs laser pulse of low-intensity. It takes tens of nanoseconds to form a plasma waveguide. The simulation by one-dimensional Gaussian plasma hydrodynamic expansion model indicates that the temperature of plasma channel is not high under this condition. Thus it takes tens of nanoseconds to form a plasma waveguide.     

Time-resolved spectroscopy analysis of Ga atom in laser induced plasma

Time-resolved optical emission spectroscopy was used to characterize Ga atom in plasma produced by 1064 nm, 10 ns pulsed laser. The evolution and the character of the spectra were analyzed. The line width and shift of Ga lines were detected. The Ga lines have two distinct line shift—“red shift” and “blue shift.” It attributed to their level states and the quadratic stark effect. The strong lines at around 425 nm whose upper energy level could be regard as high excited states can be used to explain why the intensities of Ga I spectral line 403.3 and 417.2 nm begin with a increasing dependence on time followed by a decrease. In addition, the effects of materials, laser power densities and the environmental pressure on the plasma were also detected and analyzed.     

Measurement of the collision time of dense electronic plasma induced by a femtosecond laser in fused silica

Electronic plasma induced by a focused femtosecond pulse (130 fs, 800 nm) in fused silica was investigated by use of pump-probe technology. Pump and probe shadow imaging and interferometric fringe imaging were combined to determine electronic collision time tau in the conduction band, and tau was measured to be 1.7 fs at an electron density near 5 x 10(19) cm(-3). The lifetime of the electronic plasma is also measured to be approximately 170 fs by use of the time-resolved shadow imaging technique.     

Colorizing silicon surface with regular nanohole arrays induced by femtosecond laser pulses

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.     

Damage induced by femtosecond laser in optical dielectric films

Both the nature of avalanche ionization （AI） and the role of multi-photon ionization （MPI） in the studies of laser-induced damage have remained controversial up to now. According to the model proposed by Stuart et al., we study the role of MPI and AI in laser-induced damage in two dielectric films, fused silica （FS） and barium aluminum borosilicate （BBS）, irradiated by 780-nm laser pulse with the pulse width range of 0.01 - 5 ps. The effects of MPI and initial electron density on seed electron generation are numerically analyzed. For FS, laser-induced damage is dominated by AI for the entire pulse width regime due to the wider band-gap. While for BBS, MPI becomes the leading power in damage for the pulse width T less than about 0.03 ps. MPI may result in a sharp rise of threshold fluence Fth on r, and AI may lead to a mild increase or even a constant value of Fth on r. MPI serves the production of seed electrons for AI when the electron density for AI is approached or exceeded before the end of MPI. This also means that the effect of initial electron can be neglected when MPI dominates the seed electron generation. The threshold fluence Fth decreases with the increasing initial electron density when the latter exceeds a certain critical value.     

Observation of laser satellites in a plasma produced by a femtosecond laser pulse

Laser satellites are detected in the emission spectra of magnesium and aluminum plasmas produced by femtosecond laser pulses. This is made possible by the realization of picosecond time resolution in a high-luminosity x-ray spectrograph with a spherically curved mica crystal. The temporal characteristics of these newly recorded spectral lines show unequivocally that they are formed as a result of nonlinear processes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 454–459 (10 October 1997)