Long-wavelength detection at high-frequency microwave or terahertz band is highly demand nowadays for myriad applications in fields such as security imaging, bioassay, and telecommunication. However, there exists so-called terahertz gap due to lack of efficient manipulation of photodiode (PD) or electrical diode (ED) techniques at this particular spectrum region. PDs for low photon-energy is difficult to operation at room-temperature due to the lack of light-induced effect, and the frequency of EDs is inhibited by the carrier transit-time can hardly reach to THz frequency.

The type-II Dirac semimetal, a van der Waals layered materials hosting tilted Dirac cone, is reported to possess congenital advantages for broadband photon absorption and exotic behavior of carrier transport. Traditionally, semimetal-based materials have not been considered for PDs owing to its drawbacks imposed by large dark current and high noisy when a bias transverse across the material, and device implementation is elusive for semimetal-based materials.

In a study published online in Science Advances, Prof. WANG Lin, Profs. CHEN Xiaoshuang and LU Wei from Shanghai Institute of Technical Physics (SITP) of the Chinese Academy of Sciences, cooperating with Prof. Antonio Politano’s group from University of L'Aquila in Italy and Prof. WAN Xiangang’s group from Nanjing University, developed a new kind of photodetector linked to topological. They realized type-II Dirac semimetal-based PDs with room-temperature capabilities of fast response, broadband as well as high signal-to-noise ratio operational in the high-frequency microwave and terahertz band.

The researchers showed a PdTe₂ single-crystal grown via chemical vapor transport route featuring electronic structure of type-II Dirac semimetal, with tilted Dirac cone forming a large Fermi sea, where the carriers can be driven back and forth by alternating electric field. The tilted Dirac cone cutting at the Fermi level forms a semi-closing electron-hole pocket so that most of the incident photon can be absorbed.

One can imagine that "a Dirac cone has a shape like a funnel, and it has a large projection area when it is tilted, which means the density of states is larger for a tilted Dirac cone." The type-II Dirac semimetal is protected by its C_3V inversion symmetry with atomic-thin thickness, offering greater versatility or flexible than traditional materials for smart system.

There are critical issues for terahertz detection within Dirac semimetal. The terahertz wave is always several hundred micrometers long, and the semimetal materials is only tens of nanometer (nm) thickness, which means that the terahertz wave should be compressed by more than three orders of magnitude in order for efficient coupling or absorbed by materials.

The researchers designed a metallic antenna log-periodic structure which contact symmetrically to PdTe₂, and the antenna is electrically connected to readout electronics for signal recording. With this particular design, the THz field can be successfully localized at metal-material interface, driving the carriers near the interface.

This study validated that the synergistic effect, which is interplayed by the C_3v inversion-symmetry breaking (caused by metal-material charge transferring) and alternative changing (AC) oscillation of surface carriers under localized field, allows the direct detection of THz signal without applying any bias voltage or introducing additional noise. Besides, it showed that detection sensitivity of 2pW/Hz^0.5 and fast response can be derived, rival with commercially available detectors, which verified that the bizarre behavior of semimetal materials may bring people transformative ways toward electromagnetic-power detection without introducing any stringent doping or grown, cooling systems for traditional materials, and thus solving the technical bottlenecks of long-wavelength photon detection.

"This study provides new opportunities to explore performing systems in terahertz band with capabilities of real-time, low-power consumption, by taking advantages of novel transport in topological protected electronic states and their versatile symmetry-breaking operation. Also, it make possible to study the exotic behavior of carriers in topological materials for future information technology," said Prof. WANG from SITP.