Abstract-Magnetic Modulation of Terahertz Waves via Spin-Polarized Electron Tunneling Based on Magnetic Tunnel Junctions

Zuanming Jin, Jugeng Li, Wenjie Zhang, Chenyang Guo, Caihua Wan, Xiufeng Han, Zhenxiang Cheng, Chao Zhang, Alexey V. Balakin, Alexander P. Shkurinov, Yan Peng, Guohong Ma, Yiming Zhu, Jianquan Yao, and Songlin Zhuang

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.014032

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

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Abstract-Nonlinear terahertz emission in the three-dimensional topological insulator Bi2Te3 by terahertz emission spectroscopy

Applied Physics Letters

Zhaoji Fang,  Hangtian Wang,  Xiaojun Wu, Shengyu Shan, Chun Wang,  Haihui Zhao, Chenyi Xia, Tianxiao Nie, Jungang Miao,   Chao Zhang,  Weisheng Zhao,  Li Wang

Characterization of the Bi2Te3 morphology, structure, and terahertz emission. (a) 3D atomic structure illustration of Bi2Te3 on Ge. (b) RHEED pattern of Bi2Te3, in which the streaky lines indicate the flat surface of the film. (c) XRD spectrum of the film grown on the Ge substrate only shows the (003) family of Bi2Te3 diffraction peaks, indicating a high-quality growth. (d) A typical AFM image of the Bi2Te3 film and (e) height profile, showing a step height of ∼1 nm. (f) Experimental setup of the terahertz time-domain emission spectroscopy. HWP: half-wave plate; QWP: quarter-wave plate; OAP: 90° off-axis parabolic mirror; AM: aluminum mirror; SW: silicon wafer; WP: Wollaston prism; and PD: photodiode. (g) The femtosecond (fs) laser pulses induce terahertz (THz) emission from the TI/Ge sample. The inset exhibits the cartoon of the photocurrent induced terahertz radiation. The arrows denote ultrafast currents including the drift current Jdri${\mathbf{J}}_{\text{dri}}$, the diffusion current Jdif${\mathbf{J}}_{\text{dif}}$, and the nonlinear currents Jnl${\mathbf{J}}_{\text{nl}}$. θ represents the incident angle, while α represents the azimuth angle.

https://aip.scitation.org/doi/abs/10.1063/1.5097335

The ultrafast optoelectronic response in topological insulators (TIs) has been recognized as one of the keys for applications on quantum computing and high-speed devices, which thus has attracted great attention recently. In this work, we systematically investigate the ultrafast transient terahertz emission excited by femtosecond laser pulses in Bi2Te3 with terahertz emission spectroscopy serving as an ultrafast and contactless detector. The nonlinear terahertz emission surpasses the terahertz emission from the sum of the drift and diffusion current contributions even at oblique incidence with an incident angle up to 70°, manifesting remarkable surface nonlinear effects on TIs. Quantitatively comprehensive microscopic analysis of the nonlinear terahertz emission origins indicates the 120°-periodic azimuth-angle dependence, which reveals a microscopic picture that the nonlinear current flows along the Bi-Te bonds. Our exploration not only enhances the microscopic understanding of the nonlinear responses in TIs on a femtosecond timescale but also lays a foundation for their applications on high-speed and low-power-consumption devices and systems.

This work was supported by the Beijing Natural Science Foundation (No. 4194083), the National Natural Science Foundation of China (Nos. 61905007, 11827807, 61774013, 11644004, 61775233, and 61731001), the National Key R&D Program of China (Nos. 2018YFB0407602 and 2016YFC0800400), the International Collaboration Project (No. B16001), and the National Key Technology Program of China (No. 2017ZX01032101).

Abstract-Superconducting pair-breaking under intense sub-gap terahertz radiation

Applied Physics Letters

Jie Tian, Jack Zuber, Sunchao Huang, Chao Zhang

The relation between the ratio of superconducting carriers and energy gap (a), temperature (b), and parameters 
(E/ω)2${(E/\omega )}^2$(c).

https://aip.scitation.org/doi/abs/10.1063/1.5098045

We study the effect of a strong and low frequency (ω < Δ, the superconducting gap) electrical field on a superconducting state. It is found that the superconducting gap decreases with the field intensity and wavelength. The physical mechanism for this dependence is the multiphoton absorption by a superconducting electron. By constructing the state of a superconducting electron dressed by photons, we determined the dependence of the superconducting gap on E/ω$E/\omega$ and temperature. We show that the critical temperature is determined by the parameter E/ω$E/\omega$ which is distinct from that induced by the heating effect. The result is consistent with experimental findings. This result can be applied to study terahertz nonlinear superconducting metamaterials.

Abstract-Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave

Sichen Jin, Xinke Wang, Peng Han, Wenfeng Sun, Shengfei Feng, Jiasheng Ye, Chao Zhang,  Yan Zhang,

Fig. 1 (a) Three-dimensional and (b) side schematic views of a moving electron beam atop a graphene layer on a piezoelectric MoS2 flake under an applied surface acoustic wave (SAW) field. The vacuum layer, the MoS2 flake with the applied SAW field, and the substrate layer are labeled as regions I, II, and III, respectively. The distance between the electron beam and the graphene layer and the thickness of the MoS2flake are labeled 𝑏$b$ and d$\text{d}$, respectively. (c) Schematic illustration of the electron and hole distributions in the SAW-induced type-II band-edge modulation of the n-doped MoS2 flake.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-27-8-11137

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene device.https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-27-8-11137

Abstract-Nonlinear optical response of a two-dimensional semi-Dirac system in terahertz regime

Xiaokang Dai, Lifang Liang, Qinjun Chen, Chao Zhang

http://iopscience.iop.org/article/10.1088/1361-648X/aafdd5/pdf


We demonstrate a strong nonlinear optical response in a two-dimensional semi-Dirac system in the terahertz regime. By applying the Boltzmann transport theory for the intra-band process and a quantum mechanics method for the inter band process, we obtained the three-photon current response. It is found that both the intra- and inter-band excitations make significant contributions to the nonlinear response. The third order conductivities (TOCs) σ_yy^((3) ) are about two magnitudes higher than that of σ_xx^((3) ). Interestingly, for the inter-band TOCs, there is a sign change when the chemical potential varies across the saddle point in the conduction band in k_x direction (that is parabolic), due to the competition between the two opposite nonlinear current contributed by the electrons at states k&gt;k_D and k&lt;k_D, respectively. Finally, we show that the nonlinear response in terahertz regime is significant at experimentally accessible field strengths. Our results suggest that this system could be of potential applications in photonic device for frequency up-conversion

Abstract-Strong tunable photomixing in semi-Dirac materials in the terahertz regime

                                                                   

Sunchao Huang, My Hanh Tran, Jack Zuber, Qian Wang, Yiming Zhu, and Chao Zhang

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-36-2-200

We demonstrate a strong and anisotropic photomixing effect in an electronic system whose energy–momentum dispersion is parabolic in the 𝑥$$ direction and linear in the 𝑦$$ direction, such as a TiO2/VO2${}_{\mathrm{}}{}_{\mathrm{}}$ multilayered structure. The third-order photoresponses along the linear and parabolic directions have been analyzed and determined quantitatively. We have found a remarkable tunability of the mixing efficiency along the parabolic direction by a small electric field in the linear direction, up to two orders of magnitude. In the terahertz (THz) regime, the third-order response is comparable to the linear response under an applied field of 103–104  V/cm${\mathrm{}}^{\mathrm{}}{\mathrm{}}^{\mathrm{}}\mathrm{}$. Additionally, the nonlinear response persists at room temperature. The results may have applications where different current responses are required along different directions in the THz regime.

© 2019 Optical Society of America

Abstract-Terahertz radiation in graphene hyperbolic medium excited by an electric dipole

Xiaodong Feng, Sen Gong, Renbin Zhong, Tao Zhao, Min Hu, Chao Zhang, and Shenggang Liu

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-5-1187#Abstract

In this Letter, the enhanced and directional radiation in a wide terahertz (THz) frequency range in a graphene hyperbolic medium excited by an electric dipole is presented. The numerical simulations and theoretical analyses indicate that the enhanced radiation comes from the strong surface plasmon couplings in the graphene hyperbolic medium, consisting of alternative graphene and dielectric substrate layers. The simulation results also show that the peak power flow of the enhanced THz radiation in the graphene hyperbolic medium is dramatically enhanced by more than 1 order of magnitude over that in a general medium within a certain distance from the dipole, and the electromagnetic fields are strongly concentrated in a narrow angle. Also, the radiation fields can be manipulated, and the fields’ angular distributions can be tuned by adjusting the dielectric permittivity and thickness of the substrates, and the chemical potential of graphene. Accordingly, it provides a good opportunity for developing miniature, integratable, high-power-density, and tunable radiation sources in the THz band at room temperature.

© 2018 Optical Society of America

Abstract-Optical bistability induced by nonlinear surface plasmon polaritons in graphene in terahertz regime

Matthew Sanderson¹, Yee Sin Ang¹, Sen Gong^2,3, Tao Zhao^2,3, Min Hu^2,3, Renbin Zhong^2,3, Xiaoxing Chen^2,3, Ping Zhang^2,3, Chao Zhang^1,3,a) and Shenggang Liu^2,3
http://scitation.aip.org/content/aip/journal/apl/107/20/10.1063/1.4936232?TRACK=RSS
a) czhang@uow.edu.au
Appl. Phys. Lett. 107, 203113 (2015); http://dx.doi.org/10.1063/1.4936232

We demonstrate optical bistability in a prism-air-graphene-dielectric structure. Under a moderate electric field in the terahertz frequency regime, the third order nonlinear optical conductivity is comparable to the linear conductivity. The nonlinear conductivity enhances the energy of surface plasmonpolaritons. Both the energy and frequency of the surface plasmonpolaritons depend on the strength of the nonlinear current in the graphene layer. When considering excitation in the Kretschmann configuration, the reflectance as a function of frequency exhibits bistability. The origin of the bistability is the field dependence of the plasmon mode. We have determined the parameter regime for the occurrence of bistability in this structure.