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-Parasitic transport paths in two-well scattering-assisted terahertz quantum cascade lasers

Li Wang, Tsung-Tse Lin, Ke Wang,  Hideki Hirayama,

https://iopscience.iop.org/article/10.7567/1882-0786/ab2b56

Using nonequilibrium Green's functions, possible parasitic paths are identified in two-well scattering-assisted terahertz quantum cascade lasers operating at 3.5 THz. The majority of electrons in the upper laser state can escape through these paths, causing a 66% loss of population inversion at 50 K. Three types of paths are clarified: one is responsible for non-selective injection via LO-phonon scattering, the other two lead to leakages via high-lying states to downstream periods by sequential tunneling. Finally, several ways of suppressing these paths are suggested by having small oscillator strength (<0.3), or employing asymmetric structure.

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-Propagation of terahertz waves in a monoclinic crystal BaGa4Se7

Yiwen E, Jiyong Yao,  Li Wang, 

https://www.nature.com/articles/s41598-018-34552-y

The complex symmetric dielectric tensor of a monoclinic crystal cannot be diagonalized by a space rotation operation in general, which poses a serious difficulty in analyzing the propagation of electromagnetic fields in monoclinic crystals so far. This propagation issue is discussed in a general case without using the index ellipsoid scheme. It is found that, when incident waves travel along the mirror plane normal or 2-fold rotation axis of monoclinic crystals, two eigenmodes following specific dispersion relations are elliptically polarized with the same ellipticity and chirality but have spatially orthogonal elliptical principal axes. The frequency independent features are the unique manifestation of the crystal symmetry. Using polarization sensitive terahertz time-domain spectroscopy and our developed data analyzing and processing methods, three complex permittivity tensor elements for a monoclinic crystal BaGa4Se7 are straightforwardly extracted and the properties of the two eigenmodes are characterized in full. It is also interesting that the spectral components beyond 1.7 THz show a very high refraction index (>10) and low dissipation during propagation, which suggests that the bulk phonon-polariton waves may be excited and effectively propagate in the crystal, resulting from the coherent phonon excitations by the incident terahertz waves. Our results may promote to develop novel terahertz devices based on polariton excitation and propagation in monoclinic crystals.

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.

Abstract-Coherent excitation of phonon polaritons in BaGa4Se7 by terahertz pulses

Bo Wang, Yiwen E, Jiyong Yao, and Li Wang

https://www.osapublishing.org/abstract.cfm?uri=fio-2018-JW4A.36&origin=search

Phonon polaritons are generated in BaGa4Se7 by linear excitation of terahertz pulses, and probed using 800 nm femtosecond laser pulses. The observed phonon polaritons can be perfectly reproduced by a damped harmonic oscillator model.

© 2018 The Author(s)

Abstract-Controlling loss of waveguides for potential GaN terahertz quantum cascade lasers by tuning the plasma frequency of doped layers

Ke Wang, Tsung-Tse Lin, Li Wang, Wataru Terashima and Hideki Hirayama

http://iopscience.iop.org/article/10.7567/JJAP.57.081001

We have analyzed the waveguide loss originating from various doping layers in double metal waveguides for potential GaN-based terahertz quantum cascade lasers (THz QCLs) by theoretical calculations. The optical field can be very well confined in the active QCL region. Average electron densities in the QCL active region and n+ contact layers should be controlled carefully. The loss in the low-frequency range (3 THz) can be minimized by decreasing the electron density in the QCL layer. In the middle- and high-THz-frequency ranges (~3 < f < 15 THz), the absorption by the heavily doped n+ contact layer dominates the waveguide loss. Consequently, the bulk plasma frequency, which is determined by electron density and shows a strong absorption peak, must be tuned to deviate from the target QCL frequency. The waveguide loss plus the cavity mirror loss can be controlled to be as low as ~24 cm−1.

Abstract-Towards Ultra-strong Terahertz Field Enhancement in Nanogap Split Ring Resonators

Jiahui Cao, Baogang Quan, Kanglong Chen, Bo Wang, Li Wang, and Xiaojun Wu

https://www.osapublishing.org/abstract.cfm?uri=ISUPTW-2018-WI12

We design and fabricate terahertz split ring resonators with nanogaps for extremely high field enhancement factor of >100000 when the splitting gap is sub-10 nm, and the experimental results agree very with the simulated results.

© 2018 OSA

Abstract-High resolution continuous wave terahertz spectroscopy on solid-state samples with coherent detection

De-Yin Kong, Xiao-Jun Wu, Bo Wang, Yang Gao, Jun Dai, Li Wang, Cun-Jun Ruan, and Jun-Gang Miao

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-14-17964

We systematically investigate the data analysis methods in terahertz frequency domain spectroscopy (THz-FDS) with coherent detection. We demonstrate that the Hilbert transform method is one of the most appropriate for data processing in THz-FDS. By converting frequency-domain signal into time domain with further data processing, the system noise due to Fabry–Pérot (FP) interference is greatly restrained. Accurate permittivity of lactose monohydrate is successfully extracted under the condition of the existence of atmospheric water vapor. Our work greatly promotes the development of THz spectroscopy in practical applications.

© 2018 Optical Society of America

Abstract-Broadband tunable electromagnetically induced transparency analogue metamaterials based on graphene in terahertz band

Yue Wang, Yanbing Leng, Li Wang, Lianhe Dong, Shunrui Liu, Jun Wang and Yanjun Sun

http://iopscience.iop.org/article/10.7567/APEX.11.062001/meta

Most of the actively controlled electromagnetically induced transparency analogue (EIT-like) metamaterials were implemented with narrowband modulations. In this paper, a broadband tunable EIT-like metamaterial based on graphene in the terahertz band is presented. It consists of a cut wire as the bright resonator and two couples of H-shaped resonators in mirror symmetry as the dark resonators. A broadband tunable property of transmission amplitude is realized by changing the Fermi level of graphene. Furthermore, the geometries of the metamaterial structure are optimized to achieve the ideal curve through the simulation. Such EIT-like metamaterials proposed here are promising candidates for designing active wide-band slow-light devices, wide-band terahertz active filters, and wide-band terahertz modulators.

Abstract-Simultaneous excitation of extremely high-Q-factor trapped and octupolar modes in terahertz metamaterials

Shengyan Yang, Chengchun Tang, Zhe Liu, Bo Wang, Chun Wang, Junjie Li, Li Wang, Changzhi Gu,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-14-15938&origin=search

Achieving high-Q-factor resonances allows dramatic enhancement of performance of many plasmonic devices. However, the excitation of high-Q-factor resonance, especially multiple high-Q-factor resonances, has been a big challenge in traditional metamaterials due to the ohmic and radiation losses. Here, we experimentally demonstrate simultaneous excitation of double extremely sharp resonances in a terahertz metamaterial composed of mirror-symmetric-broken double split ring resonators (MBDSRRs). In a regular mirror-arranged SRR array, only the low-Q-factor dipole resonance can be excited with the external electric field perpendicular to the SRR gap. Breaking the mirror-symmetry of the metamaterial leads to the occurrence of two distinct otherwise inaccessible ultrahigh-Q-factor modes, which consists of one trapped mode in addition to an octupolar mode. By tuning the asymmetry parameter, the Q factor of the trapped mode can be linearly modulated, while the Q factor of the octupolar mode can be tailored exponentially. For specific degree of asymmetry, our simulations revealed a significantly high Q factor (Q>100) for the octupolar mode, which is more than one order of magnitude larger than that of conventional metamaterials. The mirror-symmetry-broken metamaterial offers the advantage of enabling access to two distinct high-Q-factor resonances which could be exploited for ultrasensitive sensors, multiband filters, and slow light devices.

© 2017 Optical Society of America

Abstract-Angular dependent anisotropic terahertz response of vertically aligned multi-walled carbon nanotube arrays with spatial dispersion

• Yixuan Zhou
• , Yiwen E.
• , Xinlong Xu
• , Weilong Li
• , Huan Wang
• , Lipeng Zhu
• , Jintao Bai
• , Zhaoyu Ren
•  & Li Wang

http://www.nature.com/articles/srep38515

Spatial dispersion effect of aligned carbon nanotubes (CNTs) in the terahertz (THz) region has significance for both theoretical and applied consideration due to the unique intrinsically anisotropic physical properties of CNTs. Herein, we report the angular dependent reflection of p-polarized THz wave from vertically aligned multi-walled CNT arrays in both experiment and theory. The spectra indicate that the reflection depends on the film thickness of vertically aligned CNTs, the incident angle, and the frequency. The calculation model is based on the spatial dispersion effect of aligned CNTs and performed with effective impedance method and the Maxwell-Garnett approximation. The results fit well with the experiment when the thickness of CNT film is thin, which reveals a coherent superposition mechanism of the CNT surface reflection and CNTs/Si interface reflection. For thick CNT films, the CNTs/Si interface response determines the reflection at small incident angles, while the CNTs surface effect dominates at large incident angles. This work investigates the spatial dispersion effect of vertically aligned CNT arrays in the THz region, and paves a way for potential anisotropic THz applications based on CNTs with oblique incidence requirements.

Abstract-Mechanically tunable terahertz graphene plasmonics using soft metasurface

Li Wang, Xin Liu, Jianfeng Zang,
http://iopscience.iop.org/article/10.1088/2053-1583/3/4/041007/meta;jsessionid=EB8BDE24B94CFE0F02A24EBB8E8B9051.c1.iopscience.cld.iop.org

This letter presents a new approach to continuously tune the resonances of graphene plasmons in terahertz soft metasurface. The continuous tunability of plasmon resonance is either unachievable in conventional plasmonic materials like noble metals or requires gate voltage regulation in graphene. Here we investigate a simplest form of terahertz metasurface, graphene nanoribbon arrays (GNRAs), and demonstrate the graphene plasmon resonance modes can be tailored by mechanical deformation of the elastomeric substrate using finite element method (FEM). By integrating the electric doping with substrate deformation, we have managed to tune the resonance wavelength from 13.7 to 50.6μm. The 36.9 μm tuning range is nearly doubled compared with that by electric doping regulation only. Moreover, we observe the plasmon coupling effect in GNRAs on waved substrate and its evolution with substrate curvature. A new decoupling mechanism enabled by the out-of-plane separation of the adjacent ribbons is revealed. The out-of-plane setup of plasmonic components extends the fabrication of plasmonic devices into three-dimensional space, which simultaneously increases the nanoribbon density and decreases the coupling strength. Our findings provide an additional degree of freedom to design reconfigurable metasurfaces and metadevices.

Abstract-Spoof surface plasmon polaritons in terahertz transmission through subwavelength hole arrays analyzed by coupled oscillator model

• 
  • Shan Yin
  • , Xinchao Lu
  • , Ningning Xu
  • , Shuang Wang
  • , Yiwen E.
  • , Xuecong Pan
  • , Xinlong Xu
  • , Hongyao Liu
  • , Lu Chen
  • , Weili Zhang
  •  & Li Wang

http://www.nature.com/articles/srep16440

• Both the localized resonance and excitation of spoof surface plasmon polaritons are observed in the terahertz transmission spectra of periodic subwavelength hole arrays. Analyzing with the coupled oscillator model, we find that the terahertz transmission is actually facilitated by three successive processes: the incident terahertz field first initiates the localized oscillation around each hole, and then the spoof surface plasmon polaritons are excited by the localized resonance, and finally the two resonances couple and contribute to the transmission. Tailoring the localized resonance by hole size, the coupling strength between spoof surface plasmon polaritons and localized resonances is quantitatively extracted. The hole size dependent transmittance and the coupling mechanism are further confirmed by fitting the measured spectra to a modified multi-order Fano model.

Abstract-Dielectric Property of MoS2 Crystal in Terahertz and Visible Region

Xianding Yan, Lipeng Zhu, Yixuan Zhou, Yiwen E, Li Wang, Xinlong Xu

(Submitted on 14 May 2015)

http://xxx.tau.ac.il/abs/1505.03617

Two-dimensional materials such as MoS2 have attracted much attention in recent years due to their fascinating optoelectronic properties. Dielectric property of MoS2 is desired for the optoelectronic application. In this paper, terahertz (THz) time-domain spectroscopy and ellipsometry technology are employed to investigate the dielectric response of MoS2 crystal in THz and visible region. The real and imaginary parts of the complex dielectric constant of MoS2 crystal are found to follow a Drude model in THz region, which is due to the intrinsic carrier absorption. In visible region, the general trend of the complex dielectric constant is found to be described with a Lorentz model, while two remarkable peaks are observed at 1.85 and 2.03 eV, which have been attributed to the splitting arising from the combined effect of interlayer coupling and spin-orbit coupling. This work affords the fundamental dielectric data for the future optoelectronic applications with MoS2.

Comments:	6 pages
Subjects:	Materials Science (cond-mat.mtrl-sci)
Cite as:	arXiv:1505.03617 [cond-mat.mtrl-sci]
	(or arXiv:1505.03617v1 [cond-mat.mtrl-sci] for this version)

Submission history

From: Xinlong Xu [view email] 
[v1] Thu, 14 May 2015 04:34:20 GMT (337kb)