超快光脉冲照射GaAs晶体产生的干涉环

Interference rings induced by ultrafast laser pulse in GaAs crystal

用飞秒脉冲激光照射砷化镓(gallium arsenide,GaAs)晶体,透射光出现了多级的干涉环.利用透射式Z-扫描光路,改变飞秒脉冲激光入射到GaAs晶体表面时的功率密度,观察到干涉环有规律地收缩(或扩张):功率密度越大,出现的干涉环越多,张角也越大.高强度飞秒脉冲激光的非线性效应局部地改变了GaAs晶体的折射率,从而导致光程差的出现,这是一种非线性效应(克尔透镜).超快光脉冲在GaAs晶体里产生的克尔透镜不能像理想的薄透镜那样把光束聚焦,而是让透镜光束形成了干涉环.通过分析干涉环的变化,可以得到GaAs晶体的非线性吸收系数和非线性折射率.

Gallium arsenide(GaAs)is an important semiconductor material with direct bandgap and clear-cut absorption edge.High-quality crystal is available,which has excellent performances in the field of high power transmission and luminescence because of high laser damage threshold and high thermal conductivity.Nonlinear optics is of great significance for developing the laser technology,spectroscopy,and optoelectronic devices.The Z-scan technology is a nonlinear measurement method based on the analysis of single beam transmittance.Previous studies mainly focused on the measurement of the nonlinear absorption coefficient of the sample by open-aperture configuration of Z-scan and the nonlinear refractive index of the sample by shut-aperture configuration of Z-scan.We build a dual-channel Z-scan system with variable optical path difference,equipped together with a high speed camera,to observe the spatial distribution of transmitted light.Besides,we find the interference rings induced by nonlinear absorption in semiconductor GaAs crystals.Using three kinds of lasers(continuous-wave laser,80 MHz femtosecond pulsed laser,1 kHz femtosecond pulsed laser)to illuminate the GaAs crystal separately,multi-level interference rings come out only when a GaAs crystal wafer is illuminated by a femtosecond pulsed laser.In the single-channel Z-scan experiment,we observe that the interference rings contract or expand regularly when changing pulsed laser intensity incident on the surface of GaAs crystal.The higher the intensity of pulse,the more interference rings appear,and the maximum exiting angle becomes larger.Nonlinear effect of high intensity femtosecond pulsed laser locally changes the refractive index of GaAs crystals,resulting in optical path difference(Kerr lens effect).However,the Kerr lens generated by ultrafast light pulse in GaAs crystal cannot focus a beam as done by an ideal thin lens,leading the transmitted light to form interference rings instead.By analyzing the variation of the interference rings,the nonlinear absorption coefficient and refractive index of GaAs crystal can be obtained.In the dual-channel Z-scan experiment,different interference rings are induced in the GaAs crystal as the path difference between the two pulses changes,as done by the nonlinear transmission power.Thus we obtain the formation time of the interference rings and ascribe it to the ultrafast relaxation process of GaAs carriers.