Abstract-Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence

Hang Zhao, Yong Tan, Liangliang Zhang, Rui Zhang, Mostafa Shalaby, Cunlin Zhang, Yuejin Zhao & Xi-Cheng Zhang

TKE measurement of liquid water.

https://www.nature.com/articles/s41377-020-00370-z

The fundamental properties of water molecules, such as their molecular polarizability, have not yet been clarified. The hydrogen bond network is generally considered to play an important role in the thermodynamic properties of water. The terahertz (THz) Kerr effect technique, as a novel tool, is expected to be useful in exploring the low-frequency molecular dynamics of liquid water. Here, we use an intense and ultrabroadband THz pulse (peak electric field strength of 14.9 MV/cm, centre frequency of 3.9 THz, and bandwidth of 1–10 THz) to resonantly excite intermolecular modes of liquid water. Bipolar THz field-induced transient birefringence signals are observed in a free-flowing water film. We propose a hydrogen bond harmonic oscillator model associated with the dielectric susceptibility and combine it with the Lorentz dynamic equation to investigate the intermolecular structure and dynamics of liquid water. We mainly decompose the bipolar signals into a positive signal caused by hydrogen bond stretching vibration and a negative signal caused by hydrogen bond bending vibration, indicating that the polarizability perturbation of water presents competing contributions under bending and stretching conditions. A Kerr coefficient equation related to the intermolecular modes of water is established. The ultrafast intermolecular hydrogen bond dynamics of water revealed by an ultrabroadband THz pump pulse can provide further insights into the transient structure of liquid water corresponding to the pertinent modes.

Abstract-Terahertz Wave Modulation by Pre-plasma Using Different Laser Wavelength

Tong Wu, Liquan Dong, Rui Zhang, Hang Zhao, Yuejin Zhao, Cunlin Zhang, Liangliang Zhang

https://link.springer.com/article/10.1007%2Fs10762-019-00618-9

We report the terahertz (THz) wave modulation from a pre-plasma using different laser wavelengths, which is intersected orthogonally to the two-color laser filament produced by 800-nm laser pulse. When the pre-plasma exists, the THz radiation excited by the two-color field decreases significantly and the modulation depth increases with the increasing modulation pulse wavelength. Moreover, the amplitude reduction at high frequency in THz spectrum and the THz wave polarization change also have the modulation pulse wavelength dependence. These results can be explained by a photocurrent model considering wavelength-dependent ionization rate. The work contributes to further understand the theoretical mechanism of THz wave generation and enrich the practical application of ultrafast THz modulator.

Abstract-Modulation of terahertz wave generation from laser-induced filament based on a preionized plasma

Tong Wu, Liquan Dong, Shijing Zhang, Hang Zhao, Kai Kang.Cunlin Zhang, Rui Zhang, Yuejin Zhao, Liangliang Zhang



https://www.sciencedirect.com/science/article/abs/pii/S0030401819302718

We demonstrate the modulation of terahertz wave generation from two-color femtosecond laser filament on the basis of a preionized air plasma background created by a modulation pulse using an orthogonal pumping geometry. This is achieved by adjusting and optimizing the phase difference between the two-color laser fields, which is introduced when the two-color fields goes through the preionized air plasma. In experiments, terahertz time domain waveform is observed using electro-optics sampling setup. Both the energy and the waveform of terahertz wave change significantly with the increase of modulation pulse energy. The results are reasonably coincident with our theoretical simulation based on the plasma photocurrent model. We also experimentally observe the additional relative phase between the two-color laser fields due to the presence of the preionized air plasma. Our results can contribute to the further understanding of the generation mechanism of terahertz wave, enhance the generation efficiency and expand the practical application of terahertz wave.

Abstract-Terahertz Wave Generation From Noble Gas Plasmas Induced by a Wavelength-Tunable Femtosecond Laser

Hang Zhao, Liangliang Zhang, Suxia Huang, Shijing Zhang, Cunlin Zhang

https://ieeexplore.ieee.org/document/8340859

We investigated the effects of pump wavelength on terahertz wave generation via two-color laser-induced plasma in various gas targets including nitrogen and noble gases. The terahertz energy was measured as functions of pump wavelength, input pulse energy, gas species, and pressure. The results suggest that the plasmas in the heavier gases induced by relatively longer wavelength lasers are more likely to generate higher energy terahertz waves. However, the existence of phase slippage in the neutral gases causes distinct fluctuations in the terahertz energy as the gas pressure increases. Meanwhile, the terahertz waves produced in the heavier gas are more likely to saturate at relatively higher pressures or incident optical powers. We propose an ionization rate formula associated with the gas species and pump wavelength to theoretically explain the validity of the experimental results. As a result, the terahertz pulse energy approaches a maximum value of 0.099 μJ per pulse in xenon with a 1500-nm excitation laser, and the terahertz wave to optical pulse energy conversion efficiency reaches 5.6 × 10 -3 , which is an order of magnitude higher than that of a conventional 800-nm pump laser.