## Ultrafast and Strong Field Physics

This topic is focused on the study of intense, ultrafast laser pulses interacting with atoms, molecules, solids, and plasmas. Atoms and molecules in strong field have induced abundant physical phenomena, such as above threshold ionization (ATI), non-sequential double ionization (NSDI), high-order harmonic generation (HHG) and attosecond physics. The extremely non-perturbative and extremely non-linear characterization, beyond the suitable scope of classical pertubative theory, has aroused great enthusiasm among researchers looking for new theory. Meanwhile, using femtosecond laser pulse, high resolution measurements on temporal (attosecond) and spatial (sub-angstrom) can be performed by generating tabletop coherent extremely-ultraviolet source, with which we can real-time measure atomic and molecular structure and even manipulate them.

Time evolution of the electronic wave packets and revealing the underlying connection between terahertz radiation ang high-order harmonic generation . |

Photoelectron angular distributions of CO for (a) parallel and (b) antiparallel orientation. |

Transient absorption of an isolated attosecond pulse by laser-dressed
bound states of neon near the first ionization threshold. |

## Terahertz Technology

Terahertz (THz) radiation is a category of electromagnetic radiation in a frequency interval from 0.1 to 10 THz, which occupies a large portion of the electromagnetic spectrum between the infrared and microwave bands. For the shortage of mature generation and detection methods, the THz band remains unexplored compared to the relatively well-developed science and technology in the microwave and optical frequencies. Thus it is of great chance and challenge in filling up this “THz Gap”. Our main goal is to become the leading platform for THz science and engineering, dedicated to the following: research and development of new materials and their electromagnetic characterization for THz applications; exploration of new applications in the areas of telecommunication and electromagnetic sensing, THz security and defense applications; research on generation and of new intense THz source; THz time-domain spectra system (TDS) and coherently THz image and manipulation. Recently, we proposed attosecond synchronization of THz wave and harmonics, arbitrary polarized THz wave detection, difference frequency generation of THz radiation in GaSe crystal and so on, showing the progress and potential in THz generation, detection and application.

Experiment apparatus for joint measurement of terahertz waves and high-harmonics generation. |

Polarization-sensitive air-biased-coherent-detection for terahertz wave. |

FEMTOPOWER compact PRO Laser (@790nm,25fs,1kHz,1.5mJ) |

Terahertz Air-Biased-Coherent-Detection (THz-ABCD) Apparatus |

## High Energy Density Physics

In the field of high energy density physics (HEDP), the pressures of matter can be reached above 1 Mbar. The objectives in HEDP include inertial confinement fusion (ICF), interactions between super-strong laser and matter, the interiors of giant planets and stars, where the temperatures can be from a few eV to a few thousand eV (1 eV = 11604.5 K), the densities can be from 0.1 g/cm3 to 1000 g/cm3. The studying of structures and the dynamics of dense matter, in particular, the warm dense matter, is the one of the most important project in our group. For warm dense matter, there is great challenge in both theory and experiment, because of the ionic strong coupling and electronic partial degeneracy, and the fast dynamics for the structures.

In order to study the structures and dynamics of warm dense matter, (also hot dense matter), we developed models based on the first principles molecular dynamics including electron-ion collisions dynamics, path integral molecular dynamics, quantum Monte Carlo models. Within the framework of these methods, the strong correlation of electrons, degeneracy of electrons, coupling of electron-ion, can be researched in details. Furthermore, combing theoretical methods, we are developing experimental methods based on Terahertz reflective spectra to measure the electric conductivity of warm dense matter.

The novel structures characterized by the ionic clusters with electron bubbles are found using quantum Langevin molecular dynamics. |

The average proton distribution function P(d,dOO) as a function of the proton position relative to the bond midpoint d and thecorresponding oxygen-oxygen separation dOO in classical (left panels) and quantum (right panels) simulations at 34.5 GPa. |

## Atomic Structure and Collisions

Atoms and molecules are the basic units of materials, and the research on their structure is of important basic significance, and has the important application value. The emerging science and technology, such as quantum control, quantum information and processing, precision spectroscopy and optical frequency standard, strongly depends on precise data of atomic structure and energy levels on a single-atom level. Experiments on the interactions between atoms, collision between electrons and atoms, and photon absorption and scattering by atoms are the basic tools to investigate structures and properties of atoms and molecules. After many-year development, our group stays in a high level on theoretical study, including self-consistent calculations, configuration-interaction calculations and R-matrix calculations for the atomic structure and continuum process. Such theoretical studies play an important role in the investigations of physical effects of resonance states and electron correlations，and provide accurate atomic data such as energy levels, oscillator strengths, electron collision cross sections and photoionization cross sections.

Auger spectra for the single Auger decay of Ar (a) 2p−13/2,(b) 2p−11/2, and direct double Auger decay of (c) 2p−13/2, and (d) 2p−11/2. |

Contributions of continuum channels to the 1S partial cross section
of 1s2s22p5 1Po in the vicinity of the resonance 1s02s22p6 1S |

Total photodetachment cross sections of B− for the length form (black solid line)and the velocity form (black
dot-dashed line). |

## Atomic Processes in Plasma

The radiative properties of atoms and plasmas are of great significance in the research field such as inertial confinement fusion (ICF), laser-plasma interaction and astrophysics. The radiative opacity is a key parameter to determine the radiative transfer in the plasmas and many efforts are devoted to obtain the radiative opacity as accurate as possible. To do this, detailed atomic model, i.e., detailed term/level accounting (DTA/DLA) models are developed and are utilized to study the radiative opacity of hot dense plasmas. For plasmas in local thermodynamic equilibrium (LTE), the physical effects on radiative opacity are investigated systematically, the DLA model is utilized to investigate the radiative opacity of high-Z plasmas and a method is developed to diagnose the electronic temperature and density based on accurate radiative opacity. For plasmas in non-LTE, a kinetic model is developed to investigate the population distributions, which is utilized to study the dynamics of atomic populations and energy relaxations in X-ray and atom interactions. For dense plasmas, the plasma screening effects on atomic structure and atomic processes are investigated.

The CSD of a gold plasma at (a) Te = 2200 eV and ne = 6×1020 cm−3. In panel (b), the present calculation forTe = 2400 eV and ne = 6×1020 cm−3 is plotted (green) and compared with that for Te = 2200 eV (red). |

Effects of IRA 1s → 4p of Ne6+ on CSD produced by 1050-eV photon beams. Circles:With ionizationmechanisms and inner-shell resonance 1s → 4p of Ne6+. Squares |