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-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-Strong Terahertz Radiation from a Liquid-Water Line

Liang-Liang Zhang, Wei-Min Wang, Tong Wu, Shi-Jia Feng, Kai Kang, Cun-Lin Zhang, Yan Zhang, Yu-Tong Li, Zheng-Ming Sheng, and Xi-Cheng Zhang

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.014005

Terahertz radiation generation from liquid water has long been considered impossible due to strong absorption. A few very recent works reported terahertz generation from water, but the mechanism is not clear and the efficiency demands to be enhanced. We show experimentally that strong single-cycle terahertz radiation with field strength of $0.2\mathrm{MV}{\mathrm{cm}}^{-1}$ is generated from a water line (or column) of approximately $200\mu m$ in diameter irradiated by a mJ femtosecond laser beam. This strength is 100-fold higher than that produced from air using single-color pumping. We attribute the mechanism to the laser-ponderomotive-force-induced current with the symmetry broken around the water-column interface. This mechanism can explain our following observations: the radiation can be generated only when the laser propagation axis deviates from the column center; the deviation determines its field strength and polarity; it is always $p$ polarized no matter whether the laser is $p$ or $s$ polarized. This study provides a simple and efficient scheme of table-top terahertz sources based on liquid water.

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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-Reconfigurable topological phases in photoexcited graphene nanoribbon arrays

Weixuan Zhang, Tong Wu,   Xiangdong Zhang

http://iopscience.iop.org/article/10.1088/2040-8986/aada5b

We introduced here a reconfigurable topological phase using optically pumped graphene nanoribbon arrays with homogeneous spacing at terahertz frequencies. By adjusting the profile of imaginary potential, various topologically non-trivial dimer chains composed of graphene nanoribbons can be selectively produced. These dimer chains support different kinds of midgap states. Moreover, we numerically demonstrated that these midgap states are robust against some perturbations, such as non-Hermitian and lattice disorders, and can be further adjusted by varying the pumping strength on the specific graphene nanoribbon. Consequently, the reconfigurable topological phase can be obtained by only tuning the profile of imaginary potential without altering the geometry of the system. These tunable topological phases may provide a convenient method to tune the spatial profile of graphene-based topological lasers and have potential applications in topologically protected waveguiding in terahertz frequency range.

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-Continuous-terahertz-wave molecular imaging system for biomedical applications

Rui Zhang

Peking University, College of Engineering, No. 8 YiHeYuan Road, Beijing 100871, China

Liangliang Zhang, Tong Wu, Cunlin Zhang

Capital Normal University, Department of Physics, No. 105 XiSanHuan BeiLu, Beijing 100048, China

Ruixue Wang, Shasha Zuo, Dong Wu

Peking University, Academy for Advanced Interdisciplinary Studies, No. 8 YiHeYuan Road, Beijing 100871, China

Jue Zhang, Jing Fang

Peking University, College of Engineering, No. 8 YiHeYuan Road, Beijing 100871, China

Peking University, Academy for Advanced Interdisciplinary Studies, No. 8 YiHeYuan Road, Beijing 100871, China

J. Biomed. Opt. 21(7), 076006 (Jul 12, 2016). doi:10.1117/1.JBO.21.7.076006

http://biomedicaloptics.spiedigitallibrary.org/article.aspx?articleid=2534363

Abstract.  Molecular imaging techniques are becoming increasingly important in biomedical research and potentially in clinical practice. We present a continuous-terahertz (THz)-wave molecular imaging system for biomedical applications, in which an infrared (IR) laser is integrated into a 0.2-THz reflection-mode continuous-THz-wave imaging system to induce surface plasmon polaritons on the nanoparticles and further improve the intensity of the reflected signal from the water around the nanoparticles. A strong and rapid increment of the reflected THz signal in the nanoparticle solution upon the IR laser irradiation is demonstrated, using either gold or silver nanoparticles. This low-cost, simple, and stable continuous-THz-wave molecular imaging system is suitable for miniaturization and practical imaging applications; in particular, it shows great promise for cancer diagnosis and nanoparticle drug-delivery monitoring.

Abstract-Contrast-enhanced continuous-terahertz-wave imaging based on superparamagnetic iron oxide nanoparticles for biomedical applications

Rui Zhang, Liangliang Zhang, Tong Wu, Shasha Zuo, Ruixue Wang, Cunlin Zhang, Jue Zhang, and Jing Fang
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-8-7915

We present a novel contrast-enhanced continuous-terahertz-wave imaging modality based on magnetic induction heating of superparamagnetic iron oxide nanoparticles (SPIOs), which yields a highly sensitive increment in the reflection terahertz (THz) signal in SPIO solution upon exposure to an alternating magnetic field. In the differential and relative refection change focal-plane images before and after alternating magnetic field exposure, a dramatic contrast is demonstrated between water with and without SPIOs. This low-cost, simple, and stable contrast-enhanced continuous-THz-wave imaging system is suitable for miniaturization and real-time imaging application.

© 2016 Optical Society of America

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Abstract-Terahertz-to-infrared emission through laser excitation of surface plasmons in metal films with porous nanostructures

Liangliang Zhang, Ji Zhao, Tong Wu, Cunlin Zhang, and X.-C. Zhang
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-13-17185

We report on the investigation of terahertz-to-infrared (THz-to-IR) thermal emission that relies on the excitation of surface plasmons in metal films deposited on a substrate with randomly ordered nanoscale pore arrays. The THz-to-IR radiation was observed both in the direction of laser beam propagation and the reverse direction. The intensity ratio between backward and forward radiation is exponentially dependent on the nominal thickness of the porous metal films. The findings are discussed in view of the proposed generation mechanism based on propagating surface plasmon polaritons on both air/metal and metal/substrate interfaces.

© 2015 Optical Society of America

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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.

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