Li Yu-Tong

High power terahertz pulses generated in intense laser—plasma interactions

Terahertz (THz) radiation has attracted much attention due to its wide potential applications. Though radiation can be generated with various ways, it is still a big challenge to obtain strong tabletop sources. Plasma, with the advantage of no damage limit, is a promising medium to generate strong THz radiation. This review reports recent advances on strong THz radiation generation from low-density gases and high-density solid targets at different laser intensities.

Generation of surface electrons in femtosecond laser-solid interactions

The characteristics of hot electrons produced by p-polarized femtosecond laser-solid interactions are studied. The experimental results show that the outgoing electrons are mainly emitted in three directions: along the target surface, the normal direction and the laser backward direction. The electrons flowing along the target surface are due to the confinement of the electrostatic field and the surface magnetic field, while the electrons in the normal direction due to the resonant absorption.

Acoustic Diagnostics of Plasma Channels Induced by Intense Femtosecond Laser Pulses in Air

Long plasma channels induced by femtosecond laser pulses in air are diagnosed using the sonographic method. By detecting the sound signals along the channels, the length and the electron density of the channels are measured. Refocusing is also observed at different laser energies and different focal lengths. We find that the sonographic method has manifest advantages compared to other techniques.

High Coupling Efficiency Generation in Water Confined Laser Plasma Propulsion

High coupling efficiency generation in water confined laser plasma propulsion is investigated. It is found that the coupling efficiency is enhanced over thirty times in water confined ablation compared to that of direct ablation. From calculation of the ablation pressure induced by the plasma on the target surface, it is realized that high coupling efficiency is attributed to the confinement of the water layer on the plasma expansion.

Double Jet Emission of Hot Electrons from a Micro-droplet Spray

Spatial distribution of hot electrons with energies above 50 keV are investigated by an ethanol micro-droplet spray irradiated by linearly and elliptically polarized 150 fs laser pulses at an intensity of 1016W/cm2. Two symmetric hot electron jets with respect to the laser propagation direction are observed in the polarization plane for a linearly polarized laser field and in the plane of the long electric vector for an elliptically polarized laser field, respectively. Particle-in-cell simulations suggest that the resonance absorption on the spherical surface of the droplets is mainly responsible for the generation of the double-jet emission of hot electrons.

Measurement of Plasma Density Produced in Dielectric Barrier Discharge for Active Aerodynamic Control with Interferometer

We utilize an interferometer to investigate the changes of the refractive index caused by dielectric barrier discharge plasma. The electronic density of the plasma produced is measured and analyzed tentatively. The results show that density of the plasma increases linearly with exciting voltages.

Evolution of shock waves formed by laser-induced breakdown in air

The evolution of shock waves produced by 7 ns laser pulses in air is investigated by time-resolved shadowgraph. A nodular structure of the shock wave is observed. It is found that the origin of the structure is the multi-longitudinal-microfocus caused by the astigmatism of the laser beam. The spherical shock waves formed by each microfocus expand gradually and collide with each other, resulting in the nodular structure of the shock wave.

Effects of Polarization on Super-hot Electron Generation in Femtosecond Laser-Plasma Interaction

The effects of laser polarization on super-hot electron (> 100 keV) generation have been studied in the interaction of femtosecond laser light (800 nm, 150 fs, 6 x 10(15) W(.)cm(-2)) with a pre-formed plasma from a slab Cu target. For p-polarized laser pulses, high-energy gamma -rays of the energy similar to 400keV were detected. The electron temperatures deduced from the gamma -ray spectra were 66 and 52 ke V, respectively, in normal and reflective directions of the solid target, and hot electrons were emitted out of the plasma mainly in the normal direction. In contrast, there were nearly no gamma -rays > 100keV found for s-polarized laser pulses. The hot electron temperature was 26 keV and the emission of hot electrons was parallel to the laser field. The superposition of resonant field with electrostatic field excited by escaping electrons may contribute to the high-energy gamma -ray or super-hot electron (> 100keV) generation.

Single-Shot Measurement of Broad Bandwidth Terahertz Pulses

We propose a new single-shot method for measuring terahertz pulses using a linearly chirped optical pulse interferogram. Modulated frequency domain phase information can be extracted by the interferogram recorded on imaging spectrographs. The terahertz pulse waveform is obtained from the phase information. We overcome the energy fluctuation problem by using the phase information, making a reference shot unnecessary and the terahertz detection more flexible and convincing.

A 100-TW Ti:Sapphire Laser System at a Repetition Rate of 0.1 Hz

We demonstrate a 100-TW-class femtosecond Tr:sapphire laser running at a repetition rate of 0.1 Hz based on a 20 TW/10 Hz laser facility (XL-II). Pumping the new stage amplifier with a 25J green Nd:glass laser, we successfully improve the laser energy to 3.4 J with duration of 29 fs, corresponding to a peak power of 117 TW.