基于交叉椭圆飞秒激光光束的多丝阵列全光调控(英文)

Optical Switch of Multifilament Arrays by Two Noncollinear Elliptical Femtosecond Laser Beams

提出了一种基于交叉椭圆飞秒激光双光束的多丝阵列全光调控方法,可产生规则、可重复性的多丝阵列空间分布。多丝阵列的光丝数量、空间分布可通过交叉飞秒激光光束的交叉角度、椭圆率、入射功率以及频率等进行调控。进一步,系统研究了多丝阵列产生的理论机制。对于不同频率的交叉椭圆飞秒激光双光束,交叉相位调制和初始光斑的非对称性是产生二维光丝阵列的主要机制。对于同频率的交叉椭圆飞秒激光双光束,干涉和初始光斑的非对称性导致了光丝阵列的产生。研究结果可为二维全光开关、多色泵浦探测实验等相关应用提供参考。

Femtosecond laser filamentation has attracted extensive interest during the last two decades.When femtosecond laser beam power is much larger than the critical power for self-focusing,the beam breaks up into multiple filaments due to the modulation instability of the wave-front.Multiple filaments are essential for many applications,such as multichannel white-light radiation,terahertz generation,phasematched ultrafast Raman frequency conversion,and waveguiding of microwave radiation.One of the most challenging problems is to obtain high reproducibility and regular localization of the multifilament pattern.Nowadays,the multifilament array has been realized based on amplitude masks,beam ellipticity,focusing with an axicon,diffractive optical elements or a spatial light modulator.These approaches depend on control of beam wave-front amplitude,initial phase distribution or spatial waveshape by a kind of optical tuning devices.However,an all-optical switching approach to control the multifilament array has not been considered up to now,which would be particularly important for 2-D all-optical switching devices or pump-probe experiments based on multifilament arrays.Therefore,a new type of optical control of switching on and off the multifilament array pattern is explored based on two noncollinear elliptical femtosecond laser beams in this work.For the two noncollinear femtosecond laser beams with different frequencies,a simulation model of the two crossing beams propagating in air is set up,considering selffocusing,cross-phase modulation and the plasma defocusing effect based on a multiphoton ionization model.Based on this model,a numerical simulation of two noncollinear elliptical femtosecond laser beams with central wavelengths of 800 nm and 700 nm propagating in air is carried out.The radii of the two input elliptical beams along x long-and y short-axis are both set to be 0.9 mm and 0.6 mm respectively,while the input powers of both two beams are 10 Pcr.The crossing angle is 0.44°.A reproducible and regular multifilament array pattern is generated finally.In order to investigate the effect of beam ellipticity on the multifilament pattern generation,the propagation of two noncollinear Gauss femtosecond laser beams without ellipticity in air is simulated.Comparing with two crossing elliptical beams,the multifilament array generated by two crossing Gauss beams is less regularly distributed.The ellipticity of input beam can dominate the effect of transverse modulational instability,regularizing the nucleation of annular rings and resulting in predictable and highly reproducible multiple filamentation patterns.The propagation of two elliptical femtosecond laser beams individually without any interaction effect between two beams is considered as well.The result indicates that without cross-phase modulation effect between two crossing laser beams,much fewer multifilament laser spots can be observed.Further,the filament cluster number as a function of propagation distance for the above three conditions,including two crossing elliptical beams,two crossing Gauss beams and two individual elliptical beams without any interaction,is investigated.The result confirms that Cross-phase modulation and the cylindrical symmetry breaking in the initial beam profile contribute to the 2-D multifilament array generation from the two noncollinear elliptical femtosecond laser beams with different frequencies.Moreover,the filament cluster number and spatial distribution of the multifilament array can be tuned by the crossing angle,laser beam power,ellipticity and frequency of the second femtosecond laser beam.In addition,multiple filaments can be significantly elongated by using this method.For the propagation of two noncollinear elliptical femtosecond beams with identical frequencies,the total complex amplitude can be obtained by the superposition of the two beams directly.The wavelength of the two beams are both 800 nm.The initial input beam parameters,such as elliptical beam widths,the distance between two input beams,and the crossing angle between two beams are all set to be the same with those of two noncollinear elliptical femtosecond laser beams with different frequencies.The evolution of the beam intensity pattern under this condition is investigated.A regular 2-D multifilament array is generated with a relatively large filament number,resulting from the modulation instability of the wave-front dependent on both the ellipticity and the interference effects of the two beams.Therefore,interference plays a dominant role in the regular multifilament array generation under this condition.