Nonlinear Propagation Of Strong Laser Pulses And X-ray Spectroscopy In Molecular Media

Strong field laser has always been the focus of attention of scientists. In the past fewyears, the research fields to use the laser technology have been dramatically promotedand the range of applications has been increasingly extended due to technologydevelopment of the strong field of laser as well as further development of synchrotronradiation sources and X-ray free electron lasers. So the related science of the nonlinearoptics and X-ray scattering develops in more microscopic and fine direction, and leadsto a variety of new phenomena and new theories, which stimulates our researchenthusiasm in this area.The main purpose of this thesis is to study the microscopic mechanism of molecularmedium within the laser, including dynamic propagation of laser pulses in molecularmedium and the research related to X-ray spectroscopic, explanation and prediction of avariety of new nonlinear optical phenomena. We got some new research results and theresearch has important implications for both theoretical and experimental works. Themain contents and the results are summarized as follows:1. Optical limiting effect induced by two-photon absorptionFirst, we studied the optical limiting effect and the dynamic two-photon absorptioncross section of4,4-bis (diphenylamino) stilbene (BDPAS) molecular medium in thenanosecond pulse using finite-difference time domain method and the predictor-corrector algorithm for solving the rate equations which describe the particle numberdensity and the paraxial wave equation which describes the pulse light field. Theresearch results showed that the thickness and density of the medium, the pulse width ofthe incident laser pulse are all important factors to affect the optical limiting effect andthe dynamic two-photon absorption cross section. Further analysis indicated that whenthe pulse width is short, one-step two-photon absorption plays a major role and thedynamic two-photon absorption cross section increases linearly with the pulse widthincreasing, while when the pulse width increases to the range of long pulse, two-steptwo-photon absorption takes over one-step two-photon absorption and becomes themain mechanism to enhance the dynamic two-photon absorption. Secondly, we took4,4-bis(dimethylamino) stilbene (BDMAS) molecular mediumas an example to study the influence of the solvent effect for the optical limiting effectof molecular medium in the nanosecond and femtosecond laser pulses. We used the rateequations and the intensity equation of the light field to study nanosecond pulse case,and selected the Maxwell equations and the density matrix equations to studyfemtosecond pulse case. The study results showed that in nanosecond laser pulses, theoptical limiting effect slightly decreases if the relative conductivity of the solventpolarity increases. In femtosecond pulses, we can observe the saturation phenomenon oftwo-photon absorption with higher intensity, and the optical limiting effect shows abroader range in strong polarity solvent. However, within the region of the opticallimiting, reducing the polarity of the solvent can significantly decrease the rate oftransmittance of the light intensity, and the optical limiting behavior becomes moreobvious. We further did the fitting calculation of the dynamic two-photon absorptioncross section. The analysis showed one-step coherent two-photon absorption plays themajor role with the femtosecond pulses while two-step two-photon absorption becomesthe main absorption mechanism in the nanosecond pulses.Finally, we studied the optical limiting effect induced by reverse saturableabsorption for phthalocyanines with different central metals in long nanosecond pulses.Due to the large range of decay time, the system is divided into fast and slowsub-systems, and the rate equations are reduced to one equation for the description ofthe number of particles on the ground state. Considering the transverse distribution ofthe incident field pulse, we used the Crank-Nicolson difference method for solving theground state population rate equation and the light field paraxial wave equation.Theoretical simulation results showed that phthalocyanines have remarkable opticallimiting characteristics. When the element of the center metal is heavier, thecharacteristic time STof population transferred from the ground stateS0to thelowest triple stateT1is shorter, and the optical limiting behavior is more obvious. Themain reason is that they have high rate of the intersystem crossing transition. Contraryto the role of the central metal elements, peripheral substituents have relatively weakrole. Further analysis indicated that the two-step two-photon absorption channel(S0S1)(T1T2)including one-photon absorption of singlet and triplet states isthe main optical limiting mechanism. Meanwhile, the optical limiting behavior ofphthalocyanines significantly depends on the light field intensity and medium density. 2. Two-photon absorption area theoremWe took one-dimensional asymmetric conjugate molecular (E,E)-4-{2-[p-(N,N-Di-n-butylamino)stilben-p-yl]vinyl}pyridine (DBASVP) as an example, based ontwo-photon area theorem and Maxwell-Bloch equations, to study the propagation ofultrashort pulses in this kind of molecules and the evolution of the two-photonabsorption area. The two-photon area theorem is different with the one-photon areatheorem. According to the two-photon area evolutionary equations deduced fromMaxwell-Bloch equations, we calculated the strict numerical solution of two-photonarea with the evolution of the propagation distance. Strict numerical solution andtheoretical results showed different evolution of the area. The main reason is that thestrict numerical solution considers the high-order harmonic and other factors whichaccompanied with the interaction of the laser pulse and the molecular media, while thetwo-photon area theorem only describes the single-mode propagation under the slowlyvarying amplitude and phase approximation. So the strict numerical solution is moreaccurate which reflects the real evolution of the two-photon area.3. Local field effectBy solving the full-wave Maxwell-Bloch equations with the predictor-correctoralgorithm and finite-difference time domain method, we studied the influence of thelocal field effect to the propagation of ultrashort laser pulses in dense para-nitroaniline(PNA) molecules without slowly varying amplitude approximation and rotating waveapproximation. The one-photon resonant and nonresonant propagations of the laserpulses in the dense molecules were discussed in detail. The results showed that underthe one-photon resonance condition, the local field effect is obvious, and the pulsepropagation speed becomes faster and the time interval between the sub-pulses becomesshorter in the dense medium or the pulse intensity is higher. The results of one-photonnonresonant case are similar, except that the time interval between the sub-pulses doesnot increase when consider the local field effect if increase the incident pulse intensity.This indicated that when the incident pulse intensities are different, the local field effecthas a different impact on the interaction of the pulse and the medium.4. Dissociative X-ray LasingWe took the Cl2p1/26resonant excitation of HCl molecule as an example todemonstrate that this scheme can be used to create ultrashort coherent X-ray pulses. Inthe numerical simulations, the rate equations were solved with the predictor-corrector algorithm and the dissociative X-ray lasing intensity equation was solved with theupwind differencing method. X-ray lasing is predicted to ensue when molecules arepumped into dissociative core-excited states by a free-electron-laser pulse. The lasing isdue to the population inversion created in the neutral dissociation product, and theefficiency of the laser scheme features self-trapping of the X-ray pulse at the gain ridge.The self-trapping effect drastically increases the amplification of the dissociative X-raylasing pulse, which is our new prediction. The dissociative X-ray laser provides analternative way to create femtosecond coherent inner-shell X-ray pulses with narrowspectral width, which is vital for numerous applications, for instance, within the field ofpump-probe time-resolved spectroscopy.5. X-ray Raman scattering: the short-time approximationWe analyzed of the fast scattering in the resonant Raman scattering consideringone-mode and two-mode molecules. The results indicated that the resonant Ramanscattering process has a characteristic duration time, which depends on excited statelifetime and resonant frequency detuning. So we can control the duration time bychanging and. Thus, the shortening of the scattering duration by the detuning givesa very convenient tool in resonant Raman spectroscopy to ‘purify’ the spectrum fromhigher overtones and soft modes and so makes the analysis of the spectrum muchsimpler.