Breakthrough in Terahertz Photodetection using Cobalt Ditelluride

Terahertz (THz) radiation has unique properties such as no radiation damage to human tissues, penetrability to most dielectric materials, and fingerprint identification of innumerable molecules through their characteristic spectroscopic peaks. As a result, THz radiation has shown great potential in a wide variety of applications such as homeland security and imaging, materials diagnostics, biology and medical sciences, and information and communication technology. However, due to the lack of reliable THz sources and detectors working at room temperature, the THz region is the most elusive in the electromagnetic wave spectrum, known as the “THz gap”. Overall, the development of new materials for THz photodetectors is an active area of research with many challenges remaining.

A recent breakthrough in the field of THz photodetection has been made by an international team involving researchers from Italy, China, Spain, India, and Taiwan, that has demonstrated the nonlinear Hall effect (NLHE) in cobalt ditelluride (CoTe₂), a topological type-II Dirac semimetal. The NLHE is a phenomenon in which a transverse current is generated in response to an alternating electric field, and it can indicate the topological properties of inversion-symmetry-breaking crystals. This effect is of particular interest in the field of photodetection because it can lead to high-efficiency photoconversion. However, its practical application has been limited to very low driving frequencies and cryogenic temperatures. The researchers investigated the electronic structure of CoTe₂ using spin- and angle-resolved photoemission spectroscopy (spin-ARPES) at the APE-LE beamline of Elettra and found that the Dirac points are tilted, leading to a Berry curvature dipole that can induce a nonlinear Hall effect. The NLHE in CoTe₂ was found to be disorder-induced and extrinsic in nature, arising from the broken inversion symmetry on the surface.

A second-harmonic generation (SHG) experiment was used to study the second-order response at microwave and THz frequencies and determine the symmetry of the material. SHG measurements were performed using femtosecond laser pumping technology (Fig. 1a). A comparative measurement of the SHG from CoTe₂ and the mica substrate was conducted under 860 nm illumination (Fig. 1b). The observation of an SHG signal indicates symmetry breaking at the surface. The polarization-dependent second-harmonic signal indicates strong anisotropy due to inversion symmetry breaking. The presence of type-II Dirac fermions in bulk CoTe₂ was inferred from the presence of topologically protected surface states with time-reversal symmetry in spin-ARPES (Fig. 1g-j). The orbital-resolved band structure along the Γ–A direction indicates that the Dirac band crossing is ascribed to Te-5p orbitals (Fig. 1d).

Figure 1: (a) The SHG measurement setup. (b) Measured spectra of the SHG from CoTe₂ and the mica substrate. (c) Dependence of SHG emission on the polarization of the pumping laser. (d) Orbital resolved bulk band dispersion of CoTe₂ along the Γ–A direction. (e) Schematic of the calculated Berry curvature distribution of the topological surface states on the whole Brillouin zone. (f) The calculated Berry curvature distribution over the Fermi surface. (g) Density functional theory-calculated momentum-resolved spectral function plots along the direction. (h) Experimental ARPES spectra. (i,j) The calculated and experimental spin texture of the surface bands.

Importantly, the team observed room-temperature THz rectification without the need for semiconductor junctions or bias voltage, demonstrating the potential of CoTe₂ for nonlinear photodetection applications. The fabricated CoTe₂-based detectors enable a photoresponsivity of over 0.1 A W−1, a response time of approximately 710 ns, and a mean noise equivalent power of 1 pW Hz−1/2, showing the potential of CoTe₂ for low-energy photon harvesting in the infrared and THz range. These results open a new pathway for high-efficiency photodetection using materials with strong spin-orbit coupling and inversion symmetry breaking, with potential applications in communication, sensing, and infrared/THz photonics.

The peculiar properties of CoTe₂, such as strong spin-orbit coupling and nontrivial topologically protected surface and bulk bands, could lead to further advancements in our understanding of topological materials and their potential applications. The generation of a nonlinear THz photoresponse based on a second-order nonlinear Hall rectifier without the need for semiconductor junctions could provide a new approach to overcoming the limitations of infrared/THz photonics. The superb performance of the THz nonlinear Hall rectifier in terms of speed, sensitivity, and bandwidth could have practical applications in various fields. In conclusion, the results of this study represent an exciting development in the field of topological physics and infrared/THz photonics. Further research is needed to fully understand the potential implications and applications of these findings. The exploration of the NLHE in newly emerging topological quantum materials could lead to even more exciting discoveries in the future.
 

This research was conducted by the following research team:

Zhen Hu^1,2,*, Libo Zhang^1,3,*, Atasi Chakraborty⁴, Gianluca D’Olimpio⁵, Jun Fujii⁶, Amit Agarwal⁴, Ivana Vobornik6, Daniel Farias^7,8, Changlong Liu³, Chia-Nung Kuo⁹, Chin Shan Lue⁹, Li Han^1,3, Kaixuan Zhang¹, Zhiqingzi Chen^1,2, Chenyu Yao^1,2, Anping Ge^1,2, Yuanchen Zhou¹, Antonio Politano⁵, Weida Hu^1,2,3, Shao-Wei Wang^1,2, Lin Wang^1,2, Xiaoshuang Chen^1,2,3,10 and Wei Lu^1,2,10
* Zhen Hu and Libo Zhang contributed equally to this work
¹ State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China
² University of Chinese Academy of Sciences, Beijing, China
³ College of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, China
⁴ Department of Physics, Indian Institute of Technology Kanpur, Kanpur, India
⁵ Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila, Italy
⁶ Consiglio Nazionale delle Ricerche (CNR)- Istituto Officina dei Materiali (IOM), Laboratorio TASC, Basovizza, Trieste, Italy
⁷ Departamento de Física de la Materia Condensada and Instituto “Nicolás Cabrera”, Universidad Autónoma de Madrid, Madrid, Spain
⁸ Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid, Spain
⁹ Department of Physics, Cheng Kung University, Tainan, Taiwan, China
¹⁰ School of Physical Science and Technology, ShanghaiTech University, Shanghai, China

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