Abstract-Circular-Photon-Drag-Effect-Induced Elliptically Polarized Terahertz Emission from Vertically Grown Graphene

Lipeng Zhu, Zehan Yao, Yuanyuan Huang, Chuan He, Baogang Quan, Junjie Li, Changzhi Gu, Xinlong Xu, and Zhaoyu Ren

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.12.044063

Circular photon drag effect (CPDE) is important for helicity-dependent optoelectronic emitters and detectors yet is less studied in graphene due to the relatively weak light-matter interaction and is submerged by other nonlinear optical effects. We give experimental evidence of CPDE in vertically grown graphene (VGG) by terahertz (THz) emission spectroscopy. The emitted THz polarization states can be tuned to linear, left-handed, and right-handed elliptical polarizations by changing the helicity of the pump laser. Polarity reversal of the THz time-domain signal occurs with the opposite helicity of pump laser excitation due to the CPDE. Theory analysis suggests that both the linear photon drag effect and CPDE-induced transient photocurrents contribute to the THz emission from which the contribution weight of CPDE can be tuned by different elliptical states of the excitation light. The photon-helicity-dependent THz emission from VGG based on CPDE offers an alternative thought of graphene-based polarization sensitive THz sources for chiral analysis in the THz field.

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Abstract-Terahertz surface and interface emission spectroscopy for advanced materials

Yuanyuan Huang, Zehan Yao, Chuan He, Lipeng Zhu, Longhui Zhang, Jintao Bai, Xinlong Xu

https://iopscience.iop.org/article/10.1088/1361-648X/ab00c0/pdf

Surfaces and interfaces are of particular importance for optoelectronic and photonic materials as they are involved in many physical and chemical processes such as carrier dynamics, charge transfer, chemical bonding, transformation reactions and so on. Terahertz (THz) emission spectroscopy provides a sensitive and nondestructive method for surface or interface analysis of advanced materials ranging from graphene to transition metal dichalcogenides, topological insulators, hybrid perovskites, and mixed-dimensional heterostructures based on two-dimensional materials. In this review paper, we start with the THz radiation mechanisms under ultrafast laser excitation. Then we concentrate on the recent progresses of THz emission spectroscopy on the surface and interface properties of advanced materials, including transient surface photocurrents, surface nonlinear polarization, surface states, interface potential, and gas molecule adsorption/desorption processes. This novel spectroscopic method can not only promote the development of new and compact THz sources, but also provide a nondestructive optical method for surface and interface characterization of photocurrent and nonlinear polarization dynamics of materials.

Abstract-Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe2 Crystal with Time‐Resolved Terahertz Spectroscopy

Chuan He, Lipeng Zhu,  Qiyi Zhao,   Yuanyuan Huang,  Zehan Yao,   Wanyi Du,  Yuhang He,  Sujuan Zhang,  Xinlong Xu,


https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800290

The dynamics of photoexcited species is quite important for the development of next‐generation ultrafast optoelectronic devices based on transition metal dichalcogenides (TMDs). Herein, time‐resolved optical pump terahertz (THz) probe spectroscopy, which is sensitive to both bounded excitons and free electrons/holes, is employed to study the dynamics of photo‐induced carriers in the typical layered TMDs crystal tungsten diselenide (WSe2). Initial photoexcitation could generate both free carriers and excitons. The free carriers decay followed by phonon‐assistance (≈30 ps) and defect‐assistance (≈200–280 ps). The excitons decay followed by the phonon‐assisted recombination (≈100 ps) and the defect‐induced exciton separation (≈40–200 ps). With the increasing of pump fluence, more free electrons and holes will bind to form excitons by many‐body effect, while with the decay of time, more excitons will dissociate into free carriers by defects. The frequency‐dependent transient complex THz photoconductivity of layered WSe2crystal can be well described by Drude–Smith–Lorentz model, which suggests preferential free carriers backscattering due to the defects. The ratio of free carriers to excitons suggests that free carriers dominate after the photoexcitation, which is important for the optoelectronic devices such as solar cells and photodetectors.

Abstract-Terahertz Surface Emission from Layered MoS2 Crystal: Competition Between Surface Optical Rectification and Surface Photocurrent Surge

Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Longhui Zhang, Chuan He, Qiyi Zhao, Jintao Bai, and Xin Long Xu


http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.7b09723?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry–breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary condition. The maximum of surface depletion field is estimated to be 1.45×104 V/cm with 130 nm in depth under -40oincidence. Interestingly, when the incident angle is tuned from -40o to 0o, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs (100). The results afford not only comprehensive understanding of THz radiation from layered materials like MoS2, but also put forward THz emission spectroscopy for characterizing the surface and interface properties of two-dimensional materials.