Research On The Generation And Characterization Of Attosecond Pulse Driven By Few-cycle Femtosecond Ti: Sapphire Laser

Ever since lasers were proved in laboratories,people have had a profound understanding and control over light.Laser in multiple industries shows important values,such as the laser processing,medical imaging,laser scalpel,laser based research on chemistry,biology,materials science,etc.Different scenarios for laser put forward different requirements for lasers,such as energy,wavelength range,and pulse width.In the field of ultrafast scientific research,pump-probe is widely used in the study of molecular and atomic dynamics,through recording transient state with stable approach.Attosecond pump-probe is the fastest ?camera‘ available that captures electron transitions,interference,and so on.To record ultrafast dynamics with attosecond resolution,attosecond pulse intensity should be strong enough;meanwhile,the pump-probe apparatus and phase of pulses should be stable enough.The stability control,multipurpose attosecond light source generation and characterization for attosecond pump-probe are studied in this paper.This dissertation focuses on the attosecond pump-probe experiment,and has done a relatively complete study from the generation of the stepwise light source to the stability control,among which the main work is described as follows1.In the process of attosecond pulse generation,it is often necessary to use the few-cycle driving pulse.Because the conversion rate of attosecond pulse produced by gas is very low,to obtain the attosecond pulse of higher energy,the first thing is to have a high energy few-cycle driving pulse.The traditional method of spectrum broadening with hollow core fiber has been well studied.To maintain the waveguide structure of hollow core fiber,the core diameter should not be too large,so the output energy meets the bottleneck.What‘s more,the hollow core fiber requires high beam pointing stability of the incident beam.When choosing the solid bulks as spectrum broadening material,because the third-order nonlinear coefficient of solid material is too strong,the incident pulses easily get self-focused,and then form filaments in the bulk material,resulting in the irreversible destruction of the material.Professor A H Kung‘s group in National Tsinghua University figured out and realized a fictional method to broaden the spectrum to span over an octave.They take advantage of the high third-order nonlinear coefficient of solid material while replacing the bulk material with a set of separated solid thin plates to avoid damaging the material.The output pulse energy is 76 μJ with an efficiency of 54%.The output pulse shows high mode quality and good spectral coherence.Inspired by that work,we conducted spectrum broadening experiment with 7 pieces of 0.1 mm-thick solid plates with upgraded optical geometry and higher input pulse energy,the output spectrum covers 450-960 nm,with 0.7 m J pulse energy.After carefully chirp compensation with chirped mirror pairs and wedges,5.4 fs pulse was obtained,which is the first high power as well as chirp compensated result using spectrum broadening with solid thin plates.Compared with the hollow core fiber setup,our method is a promising technique of spectrum broadening,which is easy to adjust,insensitive to beam pointing stability and high output efficiency.2.We conducted slow loop carrier envelope phase(CEP)locking by applying f-2f spectrum interference using the spectrum broadened by solid thin plates when the fast loop CEP in the oscillator is locked.The RMS of CEP noise is 227 mrad under 3 ms integration time.We also compared the solid thin plates and hollow core fiber method by applying f-2f spectrum interferometer after them at the same condition.The spectrum interference modulation depth after solid thin plates is higher than that after hollow core fiber,when the RMS of CEP noise is 346 mrad using the solid thin plates broadened spectrum,it is 540 mrad for hollow core fiber.Those consist with our simulations,and show the spectrum after solid thin plates is more coherent.3.Based on the above two experiments,we can tell that solid thin plates can realize high power output pulse and high output spectral coherence in few cycle pulse generation.These properties are very beneficial in attosecond pulse generation as well as application.We utilized the 5.4 fs pulse described above in high order harmonic generation,the spectrum of a single harmonic is too broad to apply Reconstruction of Attosecond Beating by Interference in Two-Photon Transitions(RABITT),so we reduced the input pulse energy to 0.53 m J to get a less broad spectrum to make the sidebands recognizable.We compared the high order harmonic generation driven by few cycle pulse from solid thin plates and hollow core fiber with the same pulse energy and focus diameter.The harmonic signal is significant brighter driven by the solid thin plates output pulse than hollow core fiber output pulse,that is probably caused by the high coherence of solid thin plates output pulse mentioned before.We used RABITT to measure the attosecond pulse trains,the pulse width is 209 as and the high order harmonic spectrum is stable within 110 min.We also did simulation of high order harmonic generation spectrum under strong field approximation using the experimental parameters,and the simulation results consist with experimental ones.So we applied the simulation to prove the spectrum tailor proposal,which is to use band pass filter after solid thin plates to make the output pulse quasi-tunable.With this proposal,one can study more physical dynamics using RABITT.4.The attosecond beam line in our group is collinear,which means the pump and probe propagate collinear,they pass through the same mechanic components.This makes the beam line very stable,but the multi-layer coated mirror strongly limited the reflected central wavelength and band width of attosecond pulse(usually in XUV region).Another popular layout is to separate the femtosecond pulse into two arms,one arm is for attosecond pulse generation,then the two arms combined with a dilled mirror,this is called non-collinear beam line.Because the XUV is reflected by metal coated mirror with grazing incidence,the XUV spectrum can be better reflected.But this layout suffers relative arm length jitter that brings in strong noise in attosecond experiments.We built Mach-Zender interferometer and the relevant labview program to lock the relative arm length of the pump and probe beam line.By analyzing the 532 nm laser interference fringes with fast Fourier transformation,and locked the phase of the first order signal by applying the feedback signal onto a Piezo transducer whose resolution is better than 0.1 nm.The RMS of the relative arm length offset was 4.1 as after lock within 12 hours.This result is outstanding in our field and can provide sufficient stability for attosecond experiments.5.Experiments were carried out to generate high order harmonics using coherent synthesized laser pulse under carrier envelope phase locking.Use the hollow core fiber to broaden the output pulse spectrum of the Ti: Sapphire chirped pulse amplifier,the output pulse was divided into two parts with dichroic mirror.The short wavelength part contained pulse with spectrum from 450 nm to 750 nm,while the spectrum of long wavelength part started from 650 nm to 980 nm.The chirp of the pulses was compensated separately.Then another dichroic mirror combined the two pulses using coherent synthesis technique.It’s necessary to mention that short wavelength part it not strong enough to generate high order harmonics independently.The effect of carrier envelope phase of long wavelength part on the high order harmonic generation is studied.We also performed the high order harmonic generated by long and short wavelength parts simultaneously at different delays.The influence of short wavelength part on high order harmonics is explained by strong field approximation based on the modulation of ionization electron motion by electric field modulation.