Scheme For Controlling Attosecond Pulse Generation And Its Application

The interaction between intense laser pulses and matter provides an essential tool for attosecond pulse generation,which supports ultra-high temporal resolution in precision measurement,and is accompanied by the birth of attosecond time resolved spectroscopies such as attosecond photoelectron spectroscopy and attosecond transient absorption spectroscopy.Attosecond photoelectron spectroscopy which combines attosecond pulse and photoelectron detection has been successfully used for applications like observing the electronic wave packet dynamics,measuring the photoionization time delay and monitoring electron correlation dynamics.Furthermore,the phase information of attosecond pulse is also encoded in photoelectron spectroscopy,which implies attosecond photoelectron spectroscopy can be applied in attosecond pulse diagnosis.However,attosecond photoelectron spectroscopy relies on sophisticated particle detection techniques for the efficient generation and accurate analysis of photoelectrons.In addition,attosecond photoelectron spectroscopy is not appropriated for measuring the attosecond pulses with low photon energy near the ionization threshold of matter or ultra-broad spectral bandwidth due to the breakdown of central momentum approximation,which limits its application.As for attosecond transient absorption spectroscopy,it is an all-optical scheme which avoids photoelectron detection and brings great convenience in experiment.Attosecond transient absorption spectroscopy can be widely used in probing processes such as observing valance electron motion and nonadiabatic processes in chemical reactions.Nevertheless,transient absorption experiment generally involves core excitation induced by attosecond pulse,which indicates that the photon energy of the attosecond pulse should be large enough and sometimes needs being extended to "water window",which is a technical challenge.On account of the mentioned issues in existing attosecond time resolved spectroscopies,in this thesis,a novel method based on perturbing attosecond pulse generation is proposed and demonstrated.A week femtosecond pulse is introduced to perturb the process of attosecond pulse generation driven by an intense laser field,and the week femtosecond pulse will modify the attosecond pulse spectrum.In consequence,a two-dimensional spectrogram is obtained via scanning the delay between the two femtosecond pulses,from which important information of both electronic structure and ultrafast processes can be precisely retrieved.It is an all-optical scheme and avoids the detection of photoelectrons.This thesis is focusing on the principle and applications of the perturbed attosecond pulse generation spectroscopy.The main contents and innovations of our works are as follows:(1)We propose a novel scheme for isolated attosecond pulse measurement—all-optical frequency resolved optical gating.In the scheme,attosecond pulse spectrum can be modulated via introducing a week femtosecond pulse to perturb the attosecond pulse generating process,and the attosecond pulse can be reconstructed with the modulated spectrum,which theoretically avoids the central momentum approximation in attosecond streaking spectroscopy.The full width at half maximum of isolated attosecond pulse generated in our lab is measured to be around 270 attoseconds with our scheme.Finally,we have studied the feasibility of our scheme in attosecond pulse trains reconstruction.(2)We propose a new all-optical attosecond time domain interferometry.When an intense fewcycle laser pulse is interacting with atoms,the interference of high harmonics generated from different quantum channels leads to odd order harmonics emerging,which implies an improved resolution in frequency domain as compared to isolated attosecond pulse.The generation of high order harmonics will be altered while a week femtosecond pulse is introduced,and the individual harmonic peak will experience a blue or red shift.Hence,the spectrum of harmonics can be controlled via scanning the relative delay between two femtosecond pulses.We find that a wealth of information can be extracted from the modulated spectrum.We apply the mentioned scheme to reconstruct the waveform of an arbitrarily polarized optical pulse,and utilize the provided energy resolution to interrogate the abnormal character of the transition dipole near the Cooper minimum in argon.(3)We investigate the attosecond pulse generation perturbed by a weak second harmonic field.An intense laser pulse drives high order harmonics generation,and even order harmonics will emerge when a weak second harmonic is introduced to perturb high harmonics generation process.Base on the quantum paths model,we have theoretically proved that the group delay dispersion of high harmonics is related to the generating target,that is,the time-phase curve for harmonics reconstruction depends on atomic ionization potential.High harmonic quantum paths information can be extracted quantitatively via scanning the relative delay between the intense laser pulse and the weak second harmonic.We find that short and long quantum paths contributing to high harmonics generation can be separated in appropriate phase matching conditions,and experimentally demonstrate the extraction of the ratio of the short and long quantum paths.