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-The active modulation of flexible terahertz tube

Jing Liu, Hongyu Ji, Jingling Shen, Cunlin Zhang, Yuejin Zhao,


https://www.sciencedirect.com/science/article/abs/pii/S0925346719304756

We demonstrated a flexible polyimide terahertz tube that can modulate Terahertz (THz) wave efficiently based on indium oxide (In2O3) nanoparticle. The transmission of the THz pulse can be modulated using optical control over 0.2 to 2.6 THz, and the modulation depth reached up to 14%. By combining nano-indium oxide layer with metal periodic metamaterial structure, the modulation was optimized to 35%. The results show that a photo-excited tunable terahertz modulator can be realized by irradiating the structure under different intensity of the laser beam.

Abstract-Using Markov constraint and constrained least square filter to develop a novel method of passive terahertz image restoration

Yuanmeng Zhao, Xiao Sun, Cunlin Zhang, Yuejin Zhao,

https://iopscience.iop.org/article/10.1088/1742-6596/1187/4/042094

In recent years, passive terahertz imaging has gained significant attention in both research and practice. One big challenge with passive terahertz imaging is its low-quality images with high level of noise. State-of-the art image restoration methods have been developed for image denoising, such as methods based on Markov constraint and regular filter methods. Building upon these two methods, this paper develops a novel method for passive terahertz image restoration which preserves well both high frequency and low frequency information of the images. Performance of our method is evaluated using two common image criteria of the image sharpness, i.e. edge intensity and definition. Experimental results showed our method outperform state-of-the art methods for passive terahertz image restoration.

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 signal analysis for biological tissues based on empirical mode decomposition

Rui Zhang, Tong Wu, Yuejin Zhao,



https://www.sciencedirect.com/science/article/pii/S0030402619302591

We propose an empirical mode decomposition (EMD) based method for terahertz (THz) signal analysis to differentiate different biological tissues. The THz signals reflected from fresh porcine muscle and skin tissues were observed. The time-domain THz signals of the tissues are decomposed into a series of components called the intrinsic mode functions (IMFs). It is found that the decomposed low-order IMFs, especially the first-order IMF, have better performance to distinguish different tissues. Because the first-order IMF contains most of the energy from the original signal in the higher frequency range, in which tissue water has lager absorption of THz wave. The proposed strategy provides a supplementary and more efficient way for biological tissues characterization.

Abstract-Excitation-wavelength dependent terahertz wave polarization control in laser-induced filament

Liangliang Zhang, Shijing Zhang, Rui Zhang, Tong Wu, Yuejin Zhao, Cunlin Zhang, and X.-C. Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-26-32346

We examine the terahertz (THz) emission from air filament driven by two-color lasers with relatively longer wavelengths than 800 nm. The THz energy dependence on the input laser energy increases more rapidly with a longer laser wavelength, and the scaling laws of THz energy as a function of fundamental wavelength vary for different optical powers, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, the THz polarization undergoes a continuous rotation as a function of the laser wavelength, since the relative phase and polarization of the two pulses are adjusted through changing the excitation wavelength in the frequency doubling crystal. Our results contribute to the understanding of THz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.

© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Photomechanical meta-molecule array for real-time terahertz imaging

Yongzheng Wen, Delin Jia, Wei Ma, Yun Feng, Ming Liu, Liquan Dong, Yuejin Zhao,  Xiaomei Yu

https://www.nature.com/articles/micronano201771


Real-time terahertz (THz) imaging offers remarkable application possibilities, especially in the security and medical fields. However, most THz detectors work with scanners, and a long image acquisition time is required. Some thermal detectors can achieve real-time imaging by using a focal plane array but have the drawbacks of low sensitivity due to a lack of suitable absorbing materials. In this study, we propose a novel photomechanical meta-molecule array by conveniently assembling THz meta-atom absorbers and bi-material cantilevers together, which can couple THz radiation to a mechanical deflection of the meta-molecules with high efficiency. By optically reading out the mechanical deflections of all of the meta-molecules simultaneously, real-time THz imaging can be achieved. A polyimide sacrificial layer technique was developed to fabricate the device on a glass wafer, which facilitates the transmission of a readout light while the THz wave radiates onto the meta-molecule array directly from the front side. THz images and video of various objects as well as infrared images of the human body were captured successfully with the fabricated meta-molecule array. The proposed photomechanical device holds promise in applications in single and broadband THz as well as infrared imaging.

Abstract-Composite multiscale entropy analysis of reflective terahertz signals for biological tissues

Rui Zhang, Yuezhi He, Kai Liu, Liangliang Zhang, Shijing Zhang, Emma Pickwell-MacPherson, Yuejin Zhao, and Cunlin Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-20-23669

We demonstrate a composite multiscale entropy (CMSE) method of terahertz (THz) signal complexity analysis to distinguish different biological tissues. The THz signals reflected from fresh porcine skin and muscle tissues were measured and analyzed. The statistically significant difference and separation of the two tissues based on several parameters were analyzed and compared for THz spectroscopy and imaging, which verified the better performance of the CMSE method and further enhancement of the contrast among THz signals that interact with different tissues. This process provides a better analysis and discrimination method for THz spectroscopy and imaging in biomedical applications.

© 2017 Optical Society of America

Abstract-Terahertz wave polyethylene lens based on wave front phase modulation

Tielin Lu; Xiaohu Guo; Lingqin Kong; Yuejin Zhao; Jingshui Zhang; Shijing Zhang

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10383/103830M/Terahertz-wave-polyethylene-lens-based-on-wave-front-phase-modulation/10.1117/12.2273282.short


Terahertz(THz) wave modulator technology, due to its important value of imaging and detecting research. In the paper, we discuss the Polyethylene lens based the terahertz wave front modulation, which is benefit the terahertz wave image technology. Simulation results show that the optical system can extent the depth of imaging field of test objects based on continuous terahertz source. The way to get the image has significant meaning for detection and large image quality.

© (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-The birefringence of two liquid crystals in terahertz band

Huijuan Sun, Lingqin Kong, Yuejin Zhao

Beijing Institute of Technology (China)

Qingli Zhou, Chenyu Li, Cunlin Zhang

Capital Normal Univ. (China)

Proc. SPIE 10030, Infrared, Millimeter-Wave, and Terahertz Technologies IV, 100302D (December 8, 2016); doi:10.1117/12.2246343

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2593079


Compared with the wide application of liquid crystals (LCs) in the visible frequency band, their properties in the terahertz band have not been investigated extensively yet. In this paper, we have investigated the optical anisotropy of LCs TEB30A and 9023 at room temperature using terahertz time-domain spectroscopy (THz-TDS). The extraordinary and ordinary refraction indices of LC TEB30A are ne≈1.84 and no≈1.65, or a birefringence of 0.19 from 0.5 to 2.2 THz. The extraordinary and ordinary refraction indices of LC 9023 are ne≈1.83 and no≈1.62, or a birefringence of 0.21 from 0.5 to 2.2 THz. LC 9023 exhibits a little larger terahertz birefringence than that of LC TEB30A.
 © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Broadband terahertz metamaterial absorber based on sectional asymmetric structures

Cheng Gong, Mingzhou Zhan, Jing Yang, Zhigang Wang, Haitao Liu, Yuejin Zhao & Weiwei Liu

http://www.nature.com/articles/srep32466

We suggest and demonstrate the concept and design of sectional asymmetric structures which can manipulate the metamaterial absorber’s working bandwidth with maintaining the other inherent advantages. As an example, a broadband terahertz perfect absorber is designed to confirm its effectiveness. The absorber’s each cell integrates four sectional asymmetric rings, and the entire structure composed of Au and Si3N4 is only 1.9 μm thick. The simulation results show the bandwidth with absorptivity being larger than 90% is extended by about 2.8 times comparing with the conventional square ring absorber. The composable small cell, ultra-thin, and broadband absorption with polarization and incident angle insensitivity will make the absorber suitable for the applications of focal plane array terahertz imaging.

Abstract-Coherent detection for continuous terahertz wave

Hui Yuan, Tielin Lu, Jingshui Zhang, Yuejin Zhao

Beijing Institute of Technology (China)

Liangliang Zhang, Ji Zhao

Capital Normal Univ. (China)

Proc. SPIE 9585, Terahertz Emitters, Receivers, and Applications VI, 95850O (August 31, 2015); doi:10.1117/12.2187465

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2434792

In this paper we demonstrated a coherent raster-scan imaging system that can acquire phase information based on continuous terahertz imaging. It mixes the terahertz with a Fs-laser by a electro-optic crystal of ZnTe to make a hybrid modulation on the crystal to achieve continuous terahertz detection. In this way, it can not only propagate for a long distance but also achieve phase detection for continuous terahertz imaging. The surface images of objects that are under test can be obtained by the Backward-Wave Oscillator, which the output power is 10mW at 205.994GHz. With the repetition frequency of 80MHz, the output power of the MaiTai is 1.65W and 100fs pulse light at 800nm. The images can achieve diffraction-limited resolution approximately. And the simulated results show that the system can obtain phase imaging of test objects based on continuous terahertz source. The way to get the phase of the signal has significant meaning for coherent detection of continuous terahertz source.
 © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Terahertz wave absorption via femtosecond laser-filament concatenation

Ji Zhao ; LiangLiang Zhang ; Tong Wu ; XiaoYan Xu ; CunLin Zhang ; YueJin Zhao

http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=2272700

Ji Zhao, YueJin Zhao

Beijing Institute of Technology, School of Optoelectronics, Beijing Key Laboratory for Precision Optoelectronic Measurement Instrument and Technology, Room 6014, Optoelectronics Building, South Street of Zhongguancun, Beijing 100081, China

LiangLiang Zhang, Tong Wu, XiaoYan Xu, CunLin Zhang

Capital Normal University, Ministry of Education, Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Key Laboratory of Terahertz Optoelectronics, Beijing 100048, China

Opt. Eng. 54(4), 046104 (Apr 09, 2015). doi:10.1117/1.OE.54.4.046104

Abstract.  Terahertz (THz) emission from laser-induced air plasma is a well known and widely used phenomenon. We report that when two laser beams from the laser creating two plasma filaments interact with each other, THz absorption is observed. We believe that a change in the refractive index of the plasma causes the THz-wave absorption. The following experimental results reveal that the THz absorption becomes more pronounced with increasing pump power and that the gas species surrounding the femtosecond laser filament can also influence the THz absorption rate.

• 
• 
• 

Abstract-Frequency spectrum characterization of terahertz emission from laser-induced air plasma

Ji Zhao   Liangliang Zhang   Yiman Luo   Tong Wu   Cunlin Zhang   Yuejin Zhao 

http://www.opticsjournal.net/abstract.htm?id=OJ140723000036cIfLhO

Terahertz (THz) emission from laser-induced air-plasma is presented. The frequency spectra of THz wave are investigated using an air-biased-coherent-detection method. The frequency spectra are measured under different pump-pulse and probe-pulse energies. The frequency spectra become narrow with the increasing pump power and we speculate it caused by collision behavior. Meanwhile, the bandwidth of the frequency spectra is broadened by the increasing probe power, which can be explained by pulse compression. Based on this finding, the optimal frequency spectrum of THz can be achieved by regulating the probe and pump beam.