Extreme Ultraviolet Light Source Based On High Harmonic Generation And Its Radiation Properties

Femtosecond extreme ultraviolet light source shows their ability not only when it comes to resolving spatial scale but also the inherent temporal scale in atoms,molecules and solids,which is of great significance for understanding and controlling the matter.Among a variety of methods generating ultrafast XUV light sources,high-order harmonic generation(HHG)can produce a table-top tunable coherent light source with a high repetition rate(kHz or MHz),high spatial coherence and femtosecond or even attosecond time resolution characteristics.The interaction between an intense ultrashort pulsed laser with gaseous atoms or molecules can result in HHG,which generally is a sequence of peaks corresponding to the odd harmonics of the fundamental laser wavelength with an intensity distribution characterized by a plateau.HHG can be explained by the three-step model.Firstly,the laser field is strong enough to liberate an electron from an atom or molecule,which is the essential difference between HHG and low-order nonlinear process.After the ionization,the electron is governed classically by the oscillating electric field in laser field.When the electric field reverses,the electron decelerates and accelerates back to the nucleus.After a certain time,the electron has a probability to recombine with the nucleus,which results in the emitting of the HHG photons.Owing to the recombination occurs on attosecond time scales,the radiation is an attosecond pulse,whose spectrum is a continuous spectrum.Every half cycle of the fundamental laser,the attosecond light pulses interfere with each other in the frequency domain,giving rise to peaks separated by 2?0 according to the Fourier transform.The main drawback of the HHG is the low conversion efficiency usually in the order of 10-6~10-7.However,more applications would be possible,if the process is made more efficient by phase-matching,that is,the fundamental and HHG light must have the same phase velocity as they travel through the generating medium.When achieving the phase matching,high-order harmonic generation of different atoms in laser field can interfere constructively,leading to the increase of harmonic intensity.This table-top XUV light source can be employed for ultrafast electronic and structural dynamics in a variety of systems at an energy range and temporal resolution previously unattainable.In this study,we design and build a tunable ultrafast extreme ultraviolet coherent light source based on HHG driven by intense femtosecond laser pulse.All optical mirrors and the grating are used at grazing incidence in order to ensure high reflectivity and diffraction efficiency in the short wavelength regime.The apparatus features by using the plane grating in conical diffraction and with an XUV spectrometer attached.The monochromator for HHG uses a C-T type design,that is,the incident light diverging from the HHG source is usually collimated by a toroidal mirror,and the diffracted light from the plane grating is focused by a second toroidal mirror.For the monochromator,the incident and diffracted wave vectors are almost parallel to the grooves in the off plane mount.The advantages of this method are the high diffraction efficiency and the time preserving of the XUV pulses.In the present design,we use a vacuum rotation stage and the wavelength scanning is performed by rotating the grating around an axis that is tangent to the surface,passes through the grating centre and is parallel to the grooves.The light source can be tuned in the photon energy range of 20~75 eV,with a flux of 109 photons/s centered at H23 and the pulse width about 100 fs.In order to facilitate detection of the full spectrum of HHG,a XUV spectrometer has also been designed and attached in the system which is based on a spherical flat field grating and has a spectral detection range of 5~50 nm,and with a detector composed of a MCP detector,a P46 phosphor screen,and a CCD camera for capturing the imaging on the screen for whole HHG spectrum.The HHG spectra of rare gas atoms(Ne,Ar,Kr,and Xe)and simple molecules(N2)are investigated using the XUV spectrometer.Specifically,the relationship between HHG intensity and the driving laser intensity,wavelength,gas pressure,laser elliticity is studied.With the increase of laser intensity,high-order harmonic intensity exponentially increases.When it reaches a certain level,harmonic intensity begins to decrease due to the bad phase matching caused by the high free electron density.For a certain intensity,the yield for each harmonic order increase first to reach a maximum and then to drop with the increase of gas pressure.As the laser intensity increasing,we note that the highest harmonic yield occurs at a lower gas pressure.In order to explain the experimental data,we perform the phase matching calculation using the 1D model provided by Constant et al.The results stem from that gaussian beam phase gradient and atomic dipole phase become the key factors in influencing the phase matching in tight focusing regime.The atomic dipole phase and neutral gas dispersion both have the positive effect on the phase matching after the focus.The atomic dipole phase increases with the increase of the laser intensity.Therefore,a lower pressure which arises from a less contribution of neutral gas dispersion is needed to achieve a better phase matching condition.As the laser intensity increases,plasma defocusing effect can significantly reduce the peak intensity of the 800 nm driven pulse and diverge the driven pulse energy within a few hundred microns near the focus,which confine the increase of light intensity.Thus,the optimal pressure is almost the same in the high laser intensity.We investigate the high harmonic spectra of the Ar/N2?Ar/Kr?Ne/Xe mixed gases and find that the intensity of the high harmonic spectrum of mixed gas is lower than that of one pure gas,which stems from the destructive interference of high harmonic generation from the pure gas atom in the mixed gas.These studies provide a method to understand the radiation properties and the generation of bright table-top XUV light source.