Investigation Of The Detection And Control Mechanism Of Ultrashort Pulse Laser In A Strongly Nonlinear Medium

When the light intensity of the laser beam is low,significant chang es in the material will not be induced by the interaction process between laser and material.When the laser intensity reaches a certain level,significant changes in the material will be occurred（such as the generation of plasma channel）,this will in turn affect the propagation of laser pulses.The study of ultrashort laser pluses evolution at different propagation position based on the pump-probe technique will have great significance for the ultrafast laser technology and laser materials.In this dissertation,we investigated the related problems on the detection and control of ultrashort pulse laser in a strongly nonlinear medium,and we have obtained the following main results:Firstly,based on a pump-probe technique,we investigate the spatial-temporal evolution of ultrashort pulses at different spatial positions of the beam during nonlinear propagation.We also demonstrate an improved method for measuring the spatial-temporal evolution of ultrashort pulses at different spatial positions of the beam.We first demonstrate the spatial evolution of pump beam during small-scale self-focusing（SSSF）.We find that there will be different growths in the different spatial positions,especially the modulation peak and modulation bottom in space,with the increase of input power.Different trends of spatial intensity lead to the emergence of different changing trends of pluse width.Due to the effect of spatiotemporal coupling,the increase of spatial intensity at modulation peak will lead to pulse width compression.However,the pulse width of modulation bottom is broadened with the increase of power,because its relative intensity fall-off.Though we have obtained the spatial-temporal evolution of ultrashort pulse laser,it needs to improve the accurately of measurement.Then,we improve the experimental scheme by using a cylindrical lens to replace the slit,the influence of side-lobes caused by the slit is thoroughly eliminated and the accurately of measurement is also improved.Using this improved method,pulse widths of the pump pulses at different spatial positions are obtained when they are output from an amplified Ti:sapphire laser system,we also investigate the pulse evolution at a fastest growth area for spatial intensity of the beam during SSSF.It is shown that pulse widths at the edge of the beam are longer than that of at the center,pulse widths at the fastest growth area for spatial intensity of the beam decreases as the power increases.This method is conveniently and useful for characterizing and real-time monitoring ultrashort pulses.Secondly,we demonstrate the generation of the plasma,which has a long lifetime and its shape can be controlled.We also demonstrate the realization of spatial light modulation based on the long-lifetime plasma channel.When ultrashort laser pulses propagate in a Kerr media,a plasma channel will be generated if the power is increased continuously.We experimentlly observe a long-lifetime plasma channel in CS₂ induced by femtosecond laser pulses.Though the measurement of the shape changes in the dark spot generated when the probe beam transverse through the plasma channel,we obtain the length of the plasma channel is15 mm.Through the measurement of light intensity at the center of ring-shaped beams generated when the probe beam collinear through the plasma,we obtain the lifetime of the plasma channel is 400 us.It finds that the shape of plasma channels can be controlled conveniently by changing the spatial intensity distribution of the pump beam,we can also monitor its shape changes dynamically by this experimental setup.Then,we experimentally demonstrate the realization of spatial light modulation based on the plasma channel generated in CS₂ by the pump beam.It finds that the modulation direction and the intensity profile of the probe beam can be controlled easily by adjusting the translation mirror and the spatial distribution of the pump beam,respectively.We can also change the modulation depth by adjusting the power of the pump beam.Thirdly,we demonstrate a new method for the generation of ring-shaped beams（RSBs）based on the graded-index plasma lens,we then experimentally display that TI:Sb₂Te₃ can be more conducive for the generation of RSBs,for the first time.Investigations have shown that ring-shaped beams can be used in optical tweezers,cold atom guidance,and superresolution fluorescence microscopy,etc.When the ultrashort pulse laser propagates in a nonlinear media,the electron density distribution in the plasma channel generated by ionization is close to Gaussian because of a Gaussian intensity profile of the femtosecond beam.The refractive index distribution in the plasma is also close to Gaussian,as a resut of a graded difference in refractive index exists between the center of the plasma and its periphery.Similar to a beam passing through a graded-index diverging lens,the probe beam will be deflected when it propagates in the periphery of the plasma,hence,a ring-shaped beam is formed.Good propagation properties in free space are exhibited by the generated RSB.It is found that the dark spot size can be controlled conveniently by adjusting the power of the pump beam.We also investigate the forming process of RSB in theory,and how electronic density,the width of plasma and propagation distance affect the generation of RSB.In addition,we successfully synthesize Sb₂Te₃ hexagonal nanosheets by a solvothermal method,we also demonstrate the generation of RSB in Sb2Te3 dispersion solution.The giant optical nonlinearity and the surface states of Sb₂Te₃ will play an essential role in the generation of plasma channels during laser irradiation in TI:Sb₂Te₃ dispersion solution.Corresponding to that,it is more sensitive and easily to generate an RSB in TI:Sb₂Te₃ dispersion solution based on the effect of graded-index plasma lens.Good propagation properties in free space are exhibited by the RSB generated in TI:Sb₂Te₃dispersion solution.We also find that it is more easily to generate a RSB in the high concentration Sb₂Te₃ dispersion solution and the divergence angle of the RSB increases as the concentration of dispersion solution increase.