Attosecond pulse generation from two-electron harmonic emission spectrum

In this paper, we theoretically investigate the high-order harmonic generation and attosecond pulse generation when a two-electron He atom is exposed to the intense laser pulse. It shows that due to the two-electron double recombination mechanism, an extended plateau beyond the classical single-electron harmonic has been obtained on the two-electron harmonic spectrum. Further by using this two-electron harmonic extension scheme combined with the two-color field, two supercontinuum bandwidths with 200 e V have been obtained. As a result, a series of sub-60 as extreme ultraviolet（XUV）pulses have been directly generated.

Effects of dispersion and filtering induced by periodic multilayer mirrors reflection on attosecond pulses

Using temporal and spectral methods,the effects of dispersion and filtering induced by Mo/Si multilayer mirrors reflection on incident attosecond pulses were studied.First,two temporal parameters,the pulse broadening factor,and the energy loss factor,were defined to evaluate the effects of dispersion and filtering.Then,by analyzing these temporal parameters,we investigated and compared the dispersion and filtering effects on attosecond pulses.In addition,we explored the origins of pulse broadening and energy loss by analyzing the spectral and temporal characteristics of periodic Mo/Si multilayer mirrors.The results indicate that the filtering effect induced by Mo/Si multilayer mirrors reflection is the dominant reason for pulse broadening and energy loss.

Optical design of the time-resolved ARPES beamline of the new material spectroscopy experimental station for the update of CAEP THz-FEL facility

The Chinese Academy of Engineering Physics Terahertz Free Electron Laser Facility(CAEP THz FEL,CTFEL)is the only high-average power free electron laser terahertz source based on superconducting accelerators in China.The update of the CTFEL is now undergoing and will expand the frequency range from 0.1–4.2 THz to 0.1–125 THz.Two experimental stations for material spectroscopy and biomedicine will be built.A high harmonic generation(HHG)lightsource based beamline at the material spectroscopy experimental station for time-resolved angle-resolved photoemission spectroscopy(ARPES)research will be constructed and the optical design is presented.The HHG lightsource covers the extreme ultraviolet(XUV)photon energy range of 20–50 eV.A Czerny–Turner monochromator with two plane gratings worked in conical diffraction configuration is employed to maintain the transmission efficiency and preserve the pulse time duration.The calculated beamline transmission efficiency is better than 5%in the whole photon energy range.To our knowledge,this is the first time in China to combine THz-infrared FEL with HHG light source,and this experimental station will be a powerful and effective instrument that will give new research opportunities in the future for users doing research on the dynamic evolution of the excited electron band structure of a material’s surface.

A tunable XUV monochromatic light source based on the time preserving grating selection of high-order harmonic generation

We accomplish a laboratory facility for producing a femtosecond XUV coherent monochromatic radiation with a broad tunable spectral range of 20 eV-75 eV. It is based on spectral selected single-order harmonics from intense laser driven high harmonic generation in gas phase. The time preserving for the selected harmonic radiation is achieved by a Czerny-Turner type monochromator designed with a conical diffraction grating mount for minimizing the time broadening caused by grating diffraction and keeping a relatively high diffraction efficiency. Our measurement shows that the photon flux of the 23-order harmonic（H23） centered at 35.7 eV is 1×10~9 photons/s approximately with a resolving power E/？E ≈ 36.This source provides an ultrashort tunable monochromatic XUV beam for ultrafast studies of electronic and structural dynamics in a large variety of matters.

Isolated attosecond electron wave packet diffraction

Wave-particle duality is one of the most fundamental and mysterious natures of matters. Here, we present an interesting scheme of isolated electron wave packet diffraction with a few-cycle laser pulse and an extreme ultraviolet （XUV） pulse. The diffraction fringes are clearly present in the laser dressed XUV photoelectron spectra, strongly resembling the Airy diffraction pattern of optical waves. This phenomenon suggests a great potential of attosecond diffractometry. According to this scheme we also propose a simple method to determine the XUV pulse duration from the photoelectron spectra with a rather high resolution.

Electron localization of H_2^+ in a dc electric field

A dc electric field is utilized to steer the electron motion after the molecular ion H2-＋ is excited by an ultrashort ultraviolet laser pulse. The numerical simulation shows that the electron localization distribution and the dissociation control ratio are dependent on the polarization direction and amplitude of the dc electric field. Most electrons of the dissociation state move opposite to the dc electric field and stabilize at the dressed-up potential well, for the dressed-down well is occupied by the electrons of the 1 sσg state.

Electron localization of linear symmetric molecular ion H3-(2＋)

Electron localization in the dissociation of the symmetric linear molecular ion H3-（2＋） is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ-2Σu-＋ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ-2Σu-＋ ustate. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current（DC） electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.

Comparison of Nitric Oxide Concentrations in μs-and ns-Atmospheric Pressure Plasmas by UV Absorption Spectroscopy

In this paper,an absorption spectroscopy measurement method was applied on two atmospheric pressure plasma sources to determine their production of nitric oxide.The concentrations are essential for evaluating the plasma sources based on the principle of the Dielectric Barrier Discharge（DBD）for applications in plasma medicine.The described method is based on a setup with an electrodeless discharge lamp filled with a mixture of oxygen and nitrogen.One of the emitted wavelengths is an important resonance wavelength of nitric oxide（λ = 226.2 nm）.By comparing the absorption behaviour at the minimum and maximum of the spectral absorption cross section of nitric oxide around that wavelength,and measuring the change in intensity by the absorbing plasma,the concentration of nitric oxide inside the plasma can be calculated.The produced nitric oxide concentrations depend on the pulse duration and are in the range of 180 ppm to 1400 ppm,so that a distance of about 10 cm to the respiratory tract is enough to conform to the VDI Guideline 2310.