Abstract-Ultrafast all-optical terahertz modulation based on an inverse-designed metasurface

 

Weibao He, Mingyu Tong, Zhongjie Xu, Yuze Hu, Xiang’ai Cheng, Tian Jiang, 

Structure chart of ultrafast all-optical terahertz modulation. (a) Schematic illustration of hybrid structure combining Ge film with inverse-designed metasurface at the pump light of 800 nm for terahertz modulation. (b) Processed inverse-designed metasurface structure without Ge film covering on the face. (c) Processed inverse-designed metasurface structure covering 200 nm thick Ge film on the face. The scale bar in the pictures is 50 μm.

https://www.osapublishing.org/prj/fulltext.cfm?uri=prj-9-6-1099&id=451405

Metasurface plays a key role in various terahertz metadevices, while the designed terahertz metasurface still lacks flexibility and variety. On the other hand, inverse design has drawn plenty of attention due to its flexibility and robustness in the application of photonics. This provides an excellent opportunity for metasurface design as well as the development of multifunctional, high-performance terahertz devices. In this work, we demonstrate that, for the first time, a terahertz metasurface supported by the electromagnetically induced transparency (EIT) effect can be constructed by inverse design, which combines the particle swarm optimization algorithm with the finite-difference time-domain method. Incorporating germanium (Ge) film with inverse-designed metasurface, an ultrafast EIT modulation on the picosecond scale has been experimentally verified. The experimental results suggest a feasibility to build the terahertz EIT effect in the metasurface through an optimization algorithm of inverse design. Furthermore, this method can be further utilized to design multifunctional and high-performance terahertz devices, which is hard to accomplish in a traditional metamaterial structure. In a word, our method not only provides a novel way to design an ultrafast all-optical terahertz modulator based on artificial metamaterials but also shows the potential applications of inverse design on the terahertz devices.

© 2021 Chinese Laser Press

Abstract-Enhanced Terahertz Radiation by Efficient Spin-to-Charge Conversion in Rashba-Mediated Dirac Surface States

 

Mingyu Tong, Yuze Hu, Zhenyu Wang, Tong Zhou, Xiangnan Xie, Xiang’ai Cheng, Tian Jiang

https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03079

The enhancement of terahertz (THz) radiation is of extreme significance for the realization of the THz probe and imaging. However, present THz technologies are far from being enough to realize high-performance and room-temperature THz sources. Fortunately, topological insulators (TIs), with spin-momentum-locked Dirac surface states, are expected to exhibit a high terahertz emission efficiency. In this work, the novel concept of a Rashba-state-enhanced spintronic THz emitter is demonstrated on the basis of ferromagnet/heavy metal/topological insulator (FM/HM/TI) heterostructure. We find that the THz emission intensity changes as a function of HM interlayer thickness, and a 1.98 times higher intensity compared to that of FM/TI can be achieved when a meticulously designed thickness of the HM layer is inserted. The improvement of terahertz radiation is ascribed to the additive effect of Rashba splitting and topological surface states at the HM/TI interface. These results offer new possibilities for realizing spintronic THz emitters in TI-based magnetic heterostructures.

Abstract-Saturated absorption of different layered Bi2Se3 films in the resonance zone [Invited]

Jun Zhang, Tian Jiang, Tong Zhou, Hao Ouyang, Chenxi Zhang, Zheng Xin, Zhenyu Wang, and Xiang’ai Cheng

http://65.202.222.105/prj/abstract.cfm?uri=prj-6-10-C8

Here, we used the micro P-scan method to investigate the saturated absorption (SA) of different layered Bi2Se3${\mathrm{Bi}}_2{\mathrm{Se}}_3$ continuous films. Through resonance excitation, first, we studied the influence of the second surface state (SS) on SA. The second SS resonance excitation (∼2.07  eV$\sim 2.07\mathrm{eV}$) resulted in a free carrier cross section that was 4 orders of magnitude larger than usual. At the same time, we found that the fast relaxation process of the massless Dirac electrons is much shorter than that of electrons in bulk states. Moreover, the second SS excitation resonance reduced the saturation intensity. Second, we studied the effect of the thickness on the SA properties of materials. The results showed that the saturation intensity was positively correlated to the thickness, the same as the modulation depth, and the thicker the Bi2Se3${\mathrm{Bi}}_2{\mathrm{Se}}_3$ film was, the less the second SS would influence it. This work demonstrated that by using Bi2Se3${\mathrm{Bi}}_2{\mathrm{Se}}_3$ as a saturable absorber through changing the thickness or excitation wavelength, a controllable SA could be achieved.

© 2018 Chinese Laser Press