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-Terahertz generation from reduced graphene oxide

Huan Wang, Yixuan Zhou, Zehan Yao, Lipeng Zhu, Yuanyuan Huang, Xinlong Xu, Zhaoyu Ren,

https://www.sciencedirect.com/science/article/pii/S0008622318303609


We firstly investigate the generation of terahertz (THz) wave from reduced graphene oxide(RGO) illuminated with femtosecond near-infrared laser pulse. Experiment results show that the THz generation from RGO can be enhanced by increasing the reduction degree and reducing the film thickness. The former can be attributed to the increase of sp2 carbon region, which has much smaller band gap and graphene-like photoelectric properties. The latter is due to the suppression of the light-induced lateral currents in the surface of RGO layers. The linear dependency of the THz electrical field on the pump power confirms that the THz emission from RGO is governed by second-order nonlinear properties. When exciting laser irradiates from opposite sample sides, π phase shift of the generated THz wave has been observed, suggesting the transient photocurrent related to THz emission is induced by the photon drag effect. The conclusion has been further confirmed by the well fitting of the experiment and theoretical calculation based on the symmetry of RGO. This work makes it clear the THz generation mechanism of RGO and paves a way for developing new THz sources.

Abstract-Terahertz surface emission from layered semiconductor WSe2

Keyu Si,  Yuanyuan, Huang, Qiyi Zhao, Lipeng Zhu, Longhui Zhang, Zehan Yao, Xinlong Xu,

https://www.sciencedirect.com/science/article/pii/S0169433218310651

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggests that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this semiconductor for THz devices.

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 semiconductor WSe2

Keyu Si, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Longhui Zhang, Zehan Yao, Xinlong Xu, 

https://www.sciencedirect.com/science/article/pii/S0169433218310651

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe₂ due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is due to the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggested that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe₂ crystal is 3.11 mJ/cm² confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe₂ and promise the potential applications of this semiconductor for THz devices.

Abstract-Terahertz generation from reduced graphene oxide

Huan Wang, Yixuan Zhou, , Zehan Yao, Lipeng Zhu, Yuanyuan Huang, Xinlong Xu, Zhaoyu Ren,

https://www.sciencedirect.com/science/article/pii/S0008622318303609

We firstly investigate the generation of terahertz (THz) wave from reduced graphene oxide (RGO) illuminated with femtosecond near-infrared laser pulse. Experiment results show that the THz generation from RGO can be enhanced by increasing the reduction degree and reducing the film thickness. The former can be attributed to the increase of sp² carbon region, which has much smaller band gap and graphene-like photoelectric properties. The latter is due to the suppression of the light-induced lateral currents in the surface RGO layers. The linear dependency of the THz electrical field on the pump power confirms that the THz emission from RGO is governed by second-order nonlinear properties. When exciting laser irradiates from opposite sample sides, π phase shift of the generated THz wave has been observed, suggesting the transient photocurrent related to THz emission is induced by the photon drag effect. The conclusion has been further confirmed by the well fitting of the experiment and theoretical calculation based on the symmetry of RGO. This work makes it clear the THz generation mechanism of RGO and paves a way for developing new THz sources.

Abstract-Terahertz emission from vertically aligned multi-wall carbon nanotubes and their composites by optical excitation

Shan Huang, Weilong Li.  Lipeng Zhu, Mi He, Zehan Yao, Yixuan Zhou, Xinlong Xu, Zhaoyu Ren,  Jinbo Bai,

https://www.sciencedirect.com/science/article/pii/S000862231830191X

Terahertz (THz) emission has been successfully observed from vertically aligned multi-wall carbon nanotube (VAMCNT) materials excited with polarized laser pulse in a reflection configuration. The effects of experimental parameters on THz generation are investigated systematically. The results indicate that the linear dependence of THz electric field on the pump power shows a typical second-order nonlinear optical effect and the transient photocurrent for generating THz waves is mainly originated from photon drag effect. By comparing with THz emission from polymer-coated VAMCNT composites, the detailed generation process of THz waves from VAMCNT materials is clarified: the free carriers in VAMCNT films could transport both along single nanotube and between the adjacent nanotubes, while free carriers in VAMCNT composite films transport mainly along the tube axis due to the isolated carrier transport between the adjacent nanotubes by the coated polymer. Thus, transient photocurrent for generating THz emission from VAMCNT composite films is only along the tube axis and its intensity depends on the component of the pump light electric field along the tube axis. In addition, THz emission from VAMCNT composites shows more regular response than that from pure VAMCNT films, which paves a way for stable performance of VAMCNT-based THz emitter.

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.

Abstract-Terahertz surface emission of d-band electrons from a layered tungsten disulfide crystal by surface field

Longhui Zhang, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Zehan Yao, Yixuan Zhou, Wanyi Du, and Xinlong Xu

https://journals.aps.org/prb/accepted/ad078Od4Tf91333d54c29f0414038d0387a0cecfe

Terahertz (THz) time-domain emission spectroscopy in both transmission and reflection configurations has been employed to understand the THz radiation property and surface properties of tungsten disulfide (WS2). We observed only one polarization of THz radiation under different polarization of pump beam and a saturation effect with the increasing of pump power. The results are different from that of MoS2\thinspace based on optical rectification in spite of similar physical and optoelectronic properties of them. The nonlinear optical coefficient calculation based on first-principle method combined with the azimuthal angle dependence of THz radiation implies that THz radiation is insensitive to the azimuthal angle in WS2. From the pump polarization angle dependence of THz radiation, we find that the contribution due to the nonlinear effect is only 12{\%} approximately. All these suggest the main THz mechanism from WS2 is due to the surface depletion field induced by the surface states. We also analyzed the surface field features of WS2 with the maximum surface depletion field of approximate 1.2 \texttimes 105 V/cm. Fresnel law combined with the dipole radiation model is also used to analyze the angular dependence of THz radiation. The results can not only afford a fundamental THz radiation property of layered materials, but also promote the development of THz devices based on layered materials.

Abstract-Enhanced Polarization-sensitive Terahertz Emission from Vertical Grown Graphene by Dynamical Photon drag Effect

Lipeng Zhu,  Yuanyuan Huang,  Zehan Yao,  Baogang Quan,  Longhui Zhang ,  Junjie Li,  changzhi gu,  Xin Long Xu  and  Zhaoyu Ren 

http://pubs.rsc.org/en/content/articlelanding/2017/nr/c7nr02227a#!divAbstract

Improving terahertz (THz) emission from graphene is a challenge for graphene-based THz photonics as graphene demonstrates a weak light-matter interaction. With an unique ultra-black surface structure, vertical grown graphene (VGG) is proposed to enhance the light-matter interaction and further enhance THz emission. Herein, enhanced THz radiation is observed by THz time-domain emission spectroscopy from VGG compared with single-layer graphene. The radiated THz amplitude shows a linear dependence on the pump power, which demonstrates a second order nonlinear effect. Considering the symmetry of VGG on substrate, we can exclude the optical rectification effect and photogalvanic effect (PGE) by the D6h point group with centrosymmetry. Thus we analyze the transient photocurrent related to THz emission only by the photon drag effect (PDE). The polarization-sensitive THz radiation signals are wave-vector reliance and demonstrate cos2φ and sin2φ dependence on the polarization angles of the pump laser. This is consistent with the theoretical analysis of PDE. Our results show the enhanced, ultrafast, broadband THz radiation property of VGG, which paves the way for high performance of THz emitter and THz detector based on graphene materials.