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-Directly Observe Charge Injection of Graphene in the Graphene/WSe2 heterostructure by Optical-pump THz-probe Spectroscopy

 Liangliang Zhang, Zefeng Chen, Rui Zhang, Yong Tan, Tong Wu, Mostafa Shalaby, Rui Xie, Jianbin Xu

https://pubs.acs.org/doi/pdf/10.1021/acsami.9b13996

Charge transfer across the interface and interlayer coupling in graphene van der Waals heterostructure, which is constructed by graphene and semiconducting transition metal dichalcogenides (TMDCs), is critical for their electronic and optoelectronic applications. Photo-induced charge injection from TMDC to graphene has been studied in several heterostructure photodetector. However, the response time significantly varies among different reports, ranging from microseconds to milliseconds. In this work, using graphene/WSe2 heterostructure as an example, we directly observe the carriers density change of graphene by time-resolved optical-pump terahertz (THz)-probe spectroscopy and show ultrafast picosecond photoresponse of graphene. In the absence of photoexcitation, THz time-domain spectroscopic measurements show that WSe2 can transfer holes to graphene and pull down the Fermi level of graphene. After excitation by the ultrafast laser pulse, the transient THz response shows a rapid (∼0.35 ps) increase in the graphene conductivity mainly due to the holes injection from the WSe2 into graphene. Unlike previous reports on band bend as the guidance mechanism for charge transfer, our results show that the relevant mechanism is band offset across the atomically sharp interface.

Abstract-Deep Electrical Modulation of Terahertz Wave Based on Hybrid Metamaterial-Dielectric-Graphene Structure

Liangping Xia, Xin Zhang, Man Zhang, Suihu Dang, Shijian Huang, Yong Tan, Wenjuan Yan, Hong-Liang Cui

https://www.mdpi.com/2076-3417/9/3/507

A terahertz modulation structure based on hybrid metamaterial and graphene is proposed and demonstrated in this work. The metamaterial with a square slit ring array excites terahertz resonance in the slits and enhances the interaction between the terahertz wave and graphene. The graphene layer acting as the active material is tuned by the applied electrical field. With the separation by a dielectric layer between the graphene and the metallic structure, the resonant frequency and transmitted energy are both modulated by the graphene. Experimental result indicates that the modulation depth of the terahertz transmitted amplitude is 65.1% when the applied modulation voltage is tuned 5 V.