介质的非均匀性对高次谐波影响的研究

讨论了气体介质的非均匀性对飞秒激光高次谐波产生过程的可能影响.计算结果表明,采用喷阀技术所提供的气体介质的密度梯度和气体原子运动速度的影响很小,不会产生实验上可观测的效果.这一结果支持了喷气靶在高次谐波产生中的广泛应用 关键词： 高次谐波 气体介质 各向异性 飞秒激光     

双光束相干控制产生多次谐波的实验验证

利用两束相干度可改变的飞秒脉冲光对产生的五次谐波的强度特性进行了研究，发现谐波信号的强度随相干程度的增大而增加，谐波信号的最大值相对于相干零点(双光束相干度最大处)是非对称的，从而证明了合成的激光脉冲波形变化对谐波信号的影响.同时对谐波的受激放大理论进行了初步的实验探索和展望. 关键词： 五次谐波 波形变化 相干 受激放大     

静电场对强激光场非序列双电子电离的影响

利用半经典重碰模型计算并研究了He原子在强激光场与静电场的混合场下的非序列双电子电离过程. 由于静电场破坏了激光场电场的反演对称性，在平行于激光场偏振方向的He²⁺离子动量分布不再具有对称双峰结构. 由于超短脉冲具有相似的非对称性，研究静电场对原子在强场中非序列电离的影响，有助于分析超短脉冲非序列电离的过程，进一步了解非序列双电子电离的机理.     

饱和吸附H原子的Si（100）表面的光学二次谐波研究

测量了Ｓｉ（１００）（２×１）－Ｈ表面和Ｓｉ（１００）（３×１）－Ｈ表面的反射二次谐波强度随温度的变化关系，并与清洁的Ｓｉ（１００）（２×１）表面进行了比较。Ｓｉ（１００）（２×１）表面和Ｓｉ（１００）（３×１）－Ｈ表面的二次谐波强度随温度的上升而单调地减小，Ｓｉ（１００）（２×１）－Ｈ表面二次谐波强度随温度的变化不是单调的，约在４７０Ｋ时信号最大。可以根据二次谐波信号的强度及其随温度的变化关系来确定样品温度和表面结构。     

Laser Mode-Dependent Size of Plasma Zones Induced by Femtosecond Laser Pulses in Fused Silica

We carry out the numerical simulations of femtosecond laser propagation with TEMoo mode, TEM10 mode and a beam combining both the modes in fused silica. It is found that the transverse size of plasma zones induced by laser pulses with the TEM10 mode is smaller than that induced by the TEM00 mode, while the longitudinal size is almost the same, and the saturated plasma density is higher. The transverse size, the longitudinal size and the ratio of the longitudinal to transverse size, for the beam combining both the modes, all could be reduced at the same time in comparison with the TEM00 mode under the same focusing conditions.     

Conical Double Frequency Emission by Femtosecond Laser Pulses from DKDP

Conical double frequency emission is investigated by femtosecond laser pulses at a wavelength of 80Ohm in a DKDP crystal. It is demonstrated that the sum frequency of incident wave and its scattering wave accounts for the conical double frequency emission. The gaps on the conical rings are observed and they are very sensitive to the propagation direction, and thus could be used to detect the small angle deviation of surface direction.     

Transient saturation absorption spectroscopy excited near the band gap at high excitation carrier density in GaAs

Transient saturation absorption spectroscopy in GaAs thin films was investigated using femtosecond pump and supercontinuum probe technique at excitation densities higher than 1×10^{19}cm^{-3}. The Coulomb enhancement factor of the electron－hole plasma results in a spectrum hole at the pump wavelength. Two distinct transmission peaks at two sides of the pump wavelength are observed, arising from the bleaching of transitions from the heavy- and light-hole bands to the conduction band. The dynamic process of the transient saturation absorption is fitted using a bi-exponential function. The fast decay process is dominated by the carrier-phonon scattering and the slow process may be attributed to the electron－hole recombination.     

The Carrier-Envelope Phase of Focused Few-Cycle Laser Pulses

The Gouy phase shift modulates the carrier-envelope phase of a focused few-cycle laser pulse. We investigate the variations of the carrier-envelope （CE） phase of a pulsed Gaussian beam on and off the axis by the first-order approximation and numerical calculation. The CE phase undergoes a π shift from minus infinity to infinity, and it changes fastest with distance at the focus. The variation of CE phase on propagation distance is nearly independent of pulse duration At when At is greater than lO fs and the first order approximation fits this result very well. The maximum difference of CE phase between a pulse with duration of one cycle and the result of first-order approximation is 0.09.     

Controlling of Slope of Carrier-Envelope Phase of Few-Cycle Laser Pulses on Propagation Distance near the Focus

The effect of focusing geometry on slope of carrier-envelope （CE） phase ФCE versus propagation distance from the focus in few-cycle laser pulses is investigated. The slope could be adjusted by changing the distance L between the waist of the incident beam and the lens. At the focus, 偏dФCE/O（Z/ZR） = 0 when L = 0, and 偏dФCE/偏d（z/zR） = -2 when L = ∞. The longer the distance L, the steeper the curve of the CE phase at the focus.     

Measurement of Carrier-Envelope Phase and Field Strength of a Few-Cycle Pulse by Non-sequential Double Ionization

We propose a method to measure the carrier-envelop phase （CEP） and the intensity of a few-cycle pulse by controlling the non-sequentiai double ionization （NSDI） process. By using an additional static electric field, we can change the momentum distribution of the double-charged ions parallel to the laser polarization from an asymmetrical double-hump structure to a nearly symmetrical one. It is found that the ratio between the strength of the static electric field and that of the laser field is sensitive to the CEP but robust against the intensity fluctuation. Therefore we can determine the OEP of a few-cycle pulse precisely by measuring the static electric field. Fhrthermore, if the CEP of the few-cycle pulse is fixed at a certain value, we can also calibrate the intensity of the laser pulse by the static electric field.