Abstract-Generation of highly efficient terahertz radiation in ferromagnetic heterostructures and its application in spintronic terahertz emission microscopy (STEM)

Fengwei Guo, Chandan pandey, Chun Wang, Tianxiao Nie, Lianggong Wen, Weisheng Zhao, Jungang Miao, Li Wang, and Xiaojun Wu

(a) Schematic diagram of STEM. (b) and (c) The definitions of azimuthal angle of the sample, and for incidence angle of the pumping beam. (d) Experimental setup for STEM. P1-4: 90∘${90}^{\circ }$ off-axis parabolic mirrors; M1-5: aluminum reflection mirrors; SW: silicon wafer for combing the probing beam together with terahertz waves; S: sample of W/CoFeB/Pt with 1.8 nm thickness for each layer; QWP: quarter wave plate; WP: Wollaston prism; BD: balanced detector.

https://www.osapublishing.org/osac/abstract.cfm?uri=osac-3-4-893

The laser terahertz emission microscopy (LTEM) technique, which breaks through the resolution limitation of terahertz waves from millimeter to micrometer scales, has been widely used in many real application circumstances, such as contactless chip nondestructive testing, biosensing, imaging, and so on. Recently developed spintronic terahertz emitters featuring many unique properties such as high efficiency, easy integration, low cost, large size and so on, may also have great applications in LTEM, which can be called spintronic terahertz emission microscopy (STEM). To achieve high efficiency and good performance in STEM, we propose and corroborate a remnant magnetization method to radiate continuous and stable terahertz pulses in W/CoFeB/Pt magnetic nanofilms without carrying magnets on the transmitters driven by nJ femtosecond laser pulses. We systematically optimize the incidence angle of the pumping laser and find the emission efficiency is enhanced under oblique incidence, and we finally obtain comparable radiation efficiency and broadband spectrum in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe nonlinear crystals via optical rectification under the same pumping conditions of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not previously demonstrated under such long pulse duration. We believe our observations not only benefit for a deep insight into the physics of femtosecond spin dynamics, but also help develop novel and cost-effective broadband spintronic terahertz emitters for the applications in STEM.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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-Enhancement of Spintronic Terahertz Emission via Annealing in Ferromagnetic Heterostructures

Xiaojun Wu, Yang Gao, Yanbin He, Tianxiao Nie, Chun Wang, Deyin Kong, Chandan Pandey, Bo Wang, Lianggong Wen, Weisheng Zhao, Cunjun Ruan, Jungang Miao, Li Wang, Yutong Li, Kang L. Wang

https://arxiv.org/abs/1812.07113

We systematically investigate the influence of annealing effect on terahertz (THz) generation from CoFeB based magnetic nanofilms driven by femtosecond laser pulses. Three times enhancement of THz yields are achieved in W/CoFeB through annealing effect, and double boosting is obtained in Pt/CoFeB. The mechanism of annealing effect originates from the increase of hot electron mean free path induced by crystallization, which is experimentally corroborated by THz transmission measurement on time-domain spectroscopy. Comparison studies of the thickness dependent THz efficiency after annealing are also implemented, and we eventually conclude that annealing and thickness optimization are of importance for scaling up THz intensity. Our observations not only deepen understanding of the spintronic THz radiation mechanism but also provide normal platform for high speed spintronic opto-electronic devices.

Abstract-Broadband Magnetic-Manipulated Spintronic Terahertz Emitter with Arbitrarily Tunable Polarizations

Xiaojun Wu, Deyin Kong, Tianxiao Nie, Bo Wang, Meng Xiao, Chandan Pandey, Yang Gao, Lianggong Wen, Weisheng Zhao, Cunjun Ruan, Jungang Miao, Li Wang, Yutong Li

https://arxiv.org/abs/1809.10474

Flexible manipulation of terahertz-wave polarization during the generation process is very important for terahertz applications, especially for the next-generation on-chip functional terahertz sources. However, current terahertz emitters could not satisfy such demand, hence calling for new mechanism and conceptually new terahertz source. Here we demonstrate a magnetic-field-controlled, highly-efficient, cost-effective, and broadband terahertz source with flexible switch of terahertz polarization states in ferromagnetic heterostructures driven by femtosecond laser pulses. We verify that the chirality, azimuthal angle, and ellipticity of the generated elliptical terahertz waves can be independently manipulated by delicately engineering of the external applied magnetic fields via effectively manipulating the photo-induced spin currents. Such an ultrafast photomagnetic interaction-based, magnetic-field-controlled, and broadband tunable solid-state terahertz source integrated with terahertz polarization tunability function not only has the capability to reveal physical mechanisms of femtosecond spin dynamics, but also demonstrates the feasibility to realize novel on-chip terahertz functional devices, boosting the potential applications for controlling elementary molecular rotations, phonon vibrations, spin precessions, high-speed terahertz communication, and accelerating the development of ultrafast terahertz opto-spintronics.