Highly selective population of two excited states [2mm]in nonresonant two-photon absorption

A nonresonant two-photon absorption process can be manipulated by tailoring the ultra-short laser pulse. In this paper, we theoretically demonstrate a highly selective population of two excited states in the nonresonant two-photon absorption process by rationally designing a spectral phase distribution. Our results show that one excited state is maximally populated while the other state population is widely tunable from zero to the maximum value. We believe that the theoretical results may play an important role in the selective population of a more complex nonlinear process comprising nonresonant two-photon absorption, such as resonance-mediated (2+1)-three-photon absorption and (2+1)-resonant multiphoton ionization.     

小孔衍射和近场散射数值计算的格林函数方法

从简谐光波满足的亥姆霍兹方程出发,将由格林定理得到的介质分界面上的积分方程转化为以表面上的光波及其导数为未知量的线性方程组,并对其进行数值求解,实现了光场的数值计算。然后将这一方法应用于亚波长尺度的小孔衍射的光波以及自仿射分形表面产生的随机光场及其在近场区域范围内的传播的计算。在随机表面产生的光场计算中．提出了类比推导夫琅禾费面上散斑场自相关函数的方法产生随机表面,以及计算其导数的傅里叶变换方法。对光场的计算结果表明,在近场范围内,光场随离开表面的距离的增加而迅速变化,其传播特性完全不同于光场在远场范围内的传播特性。     

X射线衍射增强相衬成像的实验结果

报道了X射线衍射增强相衬成像的最新实验结果,简要论述了其原理,证明了衍射增强X射线相衬成像是一种高信噪比的成像方法,它比普通的X射线成像和层析(CT)具有高得多的分辨力,约为10μm,这在临床医学、材料科学及其它一些生物研究领域具有巨大的应用潜力。     

三区复振幅光瞳滤波器

利用光瞳滤波器来改变光在焦斑处的分布,是实现光学超分辨的有效途径。由于受到制作上的影响,现有的光瞳滤波器大都是纯相位型或者纯振幅型的。与纯相位或者纯振幅型光瞳滤波器相比较,复振幅型光瞳滤波器能够实现更好的滤波效果。不过由于制作上的困难,影响了复振幅的光瞳滤波器在实际中的应用。为此给出了一种相对容易实现的复振幅型光瞳滤波器,通过计算表明：在横向具有相同的第一零点比和斯特雷尔比下,该复振幅型光瞳滤波器的旁斑强度比优于纯相位型光瞳滤波器50％,并且有效地降低了纯相位型光瞳滤波器的轴向加宽。     

Experimental Investigation on Selective Excitation of Two-Pulse Coherent Anti-Stokes Raman Scattering

Selective excitation of coherent anti-Stokes Raman scattering from the benzene solution is achieved by adaptive pulse shaping based on genetic algorithm, and second harmonic generation frequency-resolved optical gating （SHG-FROG） technique is adopted to characterize the original and optimal laser pulses. The mechanism for two-pulse coherent mode-selective excitation of Raman scattering is experimentally investigated by modulating the pump pulse in the frequency domain, and it is indicated that two-pulse coherent mode-selective excitation of Raman scattering mainly depends on the effective frequency components of the pump pulse related to specific vibrational mode. The experimental results suggest that two-pulse CARS has good signal-to-background ratio and high sensitivity, and it has attractive potential applications in the complicated molecular system.     

An Ultra-Broadband Spatially Dispersed Regenerative Amplifier Free from Spatial Chirp

An ultra-broadband Ti：sapphire regenerative amplifier based on spatially dispersed amplification is demonstrated experimentally. Departing from previous reports, a new design of the cavity gets the amplified pulse free from spatial chirp. Utilizing this new regenerative amplifier, chirped pulses with bandwidth （FWHM） of about 80 nm are obtained, and the bandwidth is limited only by that of the incident seed pulses.     

Control of high-order harmonic generation with two-colour laser field

We numerically investigate the high-order harmonic generation with two-colour optical field,taking into considera-tion the propagation effects.Some harmonics can be dramatically enhanced at a certain delay between the fundamental pulse and its second harmonics.Choice of the enhanced harmonics can be realised by changing the time delay between the two laser pulses.     

Polarization-dependent femtosecond laser filamentation in air

We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser's ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.     

Theoretical study on the lasing plasmon of a split ring for label-free detection of single molecules and single nanoparticles

This paper reports the plasmonic lasing of a split ring filled with gain material in water. The lasing mode(1500 nm)is far from the pump mode(980 nm), which can depress the detection noise from the pump light. The laser intensities of the two modes simultaneously increase by more than 10~3 in amplitude, which can intensify the absorption efficiency of the pumping light and enhance the plasmonic lasing. The plasmonic lasing is a sensitive sensor. When a single protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap of the split ring, the lasing spectrum moves by 0.031 nm, which is much larger than the detection limit of 10~(-5) nm. Moreover, the lasing intensity is also very sensitive to the trapped nanoparticle. It reduces to less than 1/600 when a protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap.