Abstract-Ultrafast photoexcitation dynamics of ZnTe crystals by femtosecond optical pump‐probe and terahertz emission spectroscopy

Jianrui Liu, Xinzhong Chen,   Ziheng Yao, Xincheng Wu,   Mengkun Liu, Alexey V. Balakin,   Alexander P. Shkurinov, Guanjun You, Yiming Zhu

https://onlinelibrary.wiley.com/doi/abs/10.1002/mop.32392

In this work we perform ultrafast optical pump‐optical probe (OPOP) and optical pump terahertz (THz) emission (OPTE) studies on the ultrafast excitation dynamics in <110> ZnTe crystals. Ultrafast two‐photon absorption and coherent phonon are revealed in OPOP measurements. Pump‐power‐ and polarization‐dependent phonon dynamics are characterized in time‐resolved transmission, reflection, and Kerr rotation using OPOP. The phonon polariton‐induced THz emission is directly observed in the time domain of OPTE dynamics. It is clear that the transverse optical phonon at ~3.7 THz and phonon polariton at ~2.6 THz are evident in OPOP measurement while OPTE only reveals part of the polariton dynamics.

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-Generalized framework for determining time origin in terahertz emission spectroscopy on the basis of causality

Takeya Unuma and Kaishu Minami

Fig. 1 Analysis of THz emission data simulated for a damped cosine current. (a) Temporal waveform of THz electric field E(t) convolved with a resolution of τres = 0.30 ps. (b) Fourier spectra of amplitude ρ(ω) and phase θ(ω). (c) Causality-based functions K(δt) versus possible time-origin misplacement δt (curves 1–3) computed with the three different sets of anchor points (ω0/2π, ω1/2π, ω2/2π) = (0.98, 1.22, 2.06), (0.98, 1.50, 2.06), and (0.98, 1.79, 2.06) THz. (d) Magnified view of (c) around the point where causality is best satisfied.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-4-5136

We develop a generalized causality-based framework for determining the time origin in terahertz emission spectroscopy. Our framework is formulated in terms of a multiply subtractive Kramers-Kronig relation and can treat all major mechanisms of terahertz emission, which include the occurrence of a delta-function-like instantaneous polarization observed typically in nonlinear optical processes. We show that a function derived within our framework properly determines the positions of t = 0 both for simulated terahertz waveforms and for a measured one obtained in biased conjugated polymers. This function will be useful for an in-depth understanding of ultrafast phenomena involving terahertz emission in various optoelectronic materials.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Evaluation of semiconductor materials and devices by laser-induced terahertz emissions (Conference Presentation)

Iwao Kawayama

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560N/Evaluation-of-semiconductor-materials-and-devices-by-laser-induced-terahertz/10.1117/12.2322900.short


I introduce recent studies on evaluation of interface and surface of semiconductor materials and devices with laser-induced terahertz emission spectroscopy and imaging that measure and visualize THz emissions from the materials and devices excited by femtosecond laser pulses. The waveforms of lase-induced THz emissions refrect the dinamics of photoexcited carriers at the area excited by the laser pulses, therefore we can extract various physical properties of the samples using this phenomena in principle. In this study, we have applied this technique to characterize local properties of semiconductor materials and devices such as solar cells, wide-gap semiconductors and Metal-Oxide-Semiconductor (MOS) devices. As a result, we demonstrated that it was possible to evaluate electric polarization, surface potentials, defects, damage, performance deterioration, which were difficult with conventional methods.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Terahertz Emission from Compensated Magnetic Heterostructures

Mengji Chen, Rahul Mishra, Yang Wu, Kyusup Lee, Hyunsoo Yang

https://arxiv.org/abs/1806.02517

Terahertz emission spectroscopy (TES) has recently played an important role in unveiling the spin dynamics at a terahertz (THz) frequency range. So far, ferromagnetic (FM)/nonmagnetic (NM) heterostructures have been intensively studied as THz sources. Compensated magnets such as a ferrimagnet (FIM) and antiferromagnet (AFM) are other types of magnetic materials with interesting spin dynamics. In this work, we study TES from compensated magnetic heterostructures including CoGd FIM alloy or IrMn AFM layers. Systematic measurements on composition and temperature dependences of THz emission from CoGd/Pt bilayer structures are conducted. It is found that the emitted THz field is determined by the net spin polarization of the laser induced spin current rather than the net magnetization. The temperature robustness of the FIM based THz emitter is also demonstrated. On the other hand, an AFM plays a different role in THz emission. The IrMn/Pt bilayer shows negligible THz signals, whereas Co/IrMn induces sizable THz outputs, indicating that IrMn is not a good spin current generator, but a good detector. Our results not only suggest that a compensated magnet can be utilized for robust THz emission, but also provide a new approach to study the magnetization dynamics especially near the magnetization compensation point