Excitonic Insulator Enabled Ultrasensitive Terahertz Photodetection with Efficient Low-Energy Photon Harvesting.

Despite the interest toward the terahertz (THz) rapidly increasing, the high-efficient detection of THz photon is not widely available due to the low photoelectric conversion efficiency at this low-energy photon regime. Excitonic insulator (EI) states in emerging materials with anomalous optical transitions and renormalized valence band dispersions render their nontrivial photoresponse, which offers the prospect of harnessing the novel EI properties for the THz detection. Here, an EI-based photodetector is developed for efficient photoelectric conversion in the THz band. High-quality EI material Ta2 NiSe5 is synthesized and the existence of the EI state at room temperature is confirmed. The THz scanning near-field optical microscopy experimentally reveals the strong light-matter interaction in the THz band of EI state in the Ta2 NiSe5 . Benefiting from the strong light-matter interaction, the Ta2 NiSe5 -based photodetectors exhibit superior THz detection performances with a detection sensitivity of ≈42 pW Hz-1/2 and a response time of ≈1.1 µs at 0.1 THz at room temperature. This study provides a new avenue for realizing novel high-performance THz photodetectors by exploiting the emerging EI materials.

Hybrid Dirac semimetal-based photodetector with efficient low-energy photon harvesting.

Despite the considerable effort, fast and highly sensitive photodetection is not widely available at the low-photon-energy range (~meV) of the electromagnetic spectrum, owing to the challenging light funneling into small active areas with efficient conversion into an electrical signal. Here, we provide an alternative strategy by efficiently integrating and manipulating at the nanoscale the optoelectronic properties of topological Dirac semimetal PtSe2 and its van der Waals heterostructures. Explicitly, we realize strong plasmonic antenna coupling to semimetal states near the skin-depth regime (λ/104), featuring colossal photoresponse by in-plane symmetry breaking. The observed spontaneous and polarization-sensitive photocurrent are correlated to strong coupling with the nonequilibrium states in PtSe2 Dirac semimetal, yielding efficient light absorption in the photon range below 1.24 meV with responsivity exceeding ∼0.2 A/W and noise-equivalent power (NEP) less than ~38 pW/Hz0.5, as well as superb ambient stability. Present results pave the way to efficient engineering of a topological semimetal for high-speed and low-energy photon harvesting in areas such as biomedical imaging, remote sensing or security applications.

High-frequency rectifiers based on type-II Dirac fermions.

The advent of topological semimetals enables the exploitation of symmetry-protected topological phenomena and quantized transport. Here, we present homogeneous rectifiers, converting high-frequency electromagnetic energy into direct current, based on low-energy Dirac fermions of topological semimetal-NiTe2, with state-of-the-art efficiency already in the first implementation. Explicitly, these devices display room-temperature photosensitivity as high as 251 mA W-1 at 0.3 THz in an unbiased mode, with a photocurrent anisotropy ratio of 22, originating from the interplay between the spin-polarized surface and bulk states. Device performances in terms of broadband operation, high dynamic range, as well as their high sensitivity, validate the immense potential and unique advantages associated to the control of nonequilibrium gapless topological states via built-in electric field, electromagnetic polarization and symmetry breaking in topological semimetals. These findings pave the way for the exploitation of topological phase of matter for high-frequency operations in polarization-sensitive sensing, communications and imaging.

Colossal Terahertz Photoresponse at Room Temperature: A Signature of Type-II Dirac Fermiology.

The discovery of Dirac semimetal has stimulated bourgeoning interests for exploring exotic quantum-transport phenomena, holding great promise for manipulating the performance of photoelectric devices that are related to nontrivial band topology. Nevertheless, it still remains elusive on both the device implementation and immediate results, with some enhanced or technically applicable electronic properties signified by the Dirac fermiology. By means of Pt doping, a type-II Dirac semimetal Ir1-xPtxTe2 with protected crystal structure and tunable Fermi level has been achieved in this work. It has been envisioned that the metal-semimetal-metal device exhibits an order of magnitude performance improvement at terahertz frequency when the Fermi level is aligned with the Dirac node (i.e., x ∼ 0.3) and a room-temperature photoresponsivity of 0.52 A·W-1 at 0.12 THz and 0.45 A·W-1 at 0.3 THz, which benefited from the excitation of type-II Dirac fermions. Furthermore, van der Waals integration with Dirac semimetals exhibits superb performance with noise equivalent power less than 24 pW·Hz-0.5, rivaling the state-of-the-art detectors. Our work provides a route to explore the nontrivial topology of Dirac semimetal for addressing targeted applications in imaging and biomedical sensing across a terahertz gap.

Anisotropic ultrasensitive PdTe2-based phototransistor for room-temperature long-wavelength detection.

Emergent topological Dirac semimetals afford fresh pathways for optoelectronics, although device implementation has been elusive to date. Specifically, palladium ditelluride (PdTe2) combines the capabilities provided by its peculiar band structure, with topologically protected electronic states, with advantages related to the occurrence of high-mobility charge carriers and ambient stability. Here, we demonstrate large photogalvanic effects with high anisotropy at terahertz frequency in PdTe2-based devices. A responsivity of 10 A/W and a noise-equivalent power lower than 2 pW/Hz0.5 are achieved at room temperature, validating the suitability of PdTe2-based devices for applications in photosensing, polarization-sensitive detection, and large-area fast imaging. Our findings open opportunities for exploring uncooled and sensitive photoelectronic devices based on topological semimetals, especially in the highly pursuit terahertz band.

Sensitive Terahertz Detection and Imaging Driven by the Photothermoelectric Effect in Ultrashort-Channel Black Phosphorus Devices.

Terahertz (THz) photon detection is of particular appealing for myriad applications, but it still lags behind efficient manipulation with electronics and photonics due to the lack of a suitable principle satisfying both high sensitivity and fast response at room temperature. Here, a new strategy is proposed to overcome these limitations by exploring the photothermoelectric (PTE) effect in an ultrashort (down to 30 nm) channel with black phosphorus as a photoactive material. The preferential flow of hot carriers is enabled by the asymmetric Cr/Au and Ti/Au metallization with the titled-angle evaporation technique. Most intriguingly, orders of magnitude field-enhancement beyond the skin-depth limit and photon absorption across a broadband frequency can be achieved. The PTE detector has excellent sensitivity of 297 V W-1, noise equivalent power less than 58 pW/Hz0.5, and response time below 0.8 ms, which is superior to other thermal-based detectors at room temperature. A rigorous comparison with existing THz detectors, together with verification by further optical-pumping and imaging experiments, substantiates the importance of the localized field effect in the skin-depth limit. The results allow solid understanding on the role of PTE effect played in the THz photoresponse, opening up new opportunities for developing highly sensitive THz detectors for addressing targeted applications.

PtTe2 -Based Type-II Dirac Semimetal and Its van der Waals Heterostructure for Sensitive Room Temperature Terahertz Photodetection.

Recent years have witnessed rapid progresses made in the photoelectric performance of two-dimensional materials represented by graphene, black phosphorus, and transition metal dichalcogenides. Despite significant efforts, a photodetection technique capable for longer wavelength, higher working temperature as well as fast responsivity, is still facing huge challenges due to a lack of best among bandgap, dark current, and absorption ability. Exploring topological materials with nontrivial band transport leads to peculiar properties of quantized phenomena such as chiral anomaly, and magnetic-optical effect, which enables a novel feasibility for an advanced optoelectronic device working at longer wavelength. In this work, the direct generation of photocurrent at low energy terahertz (THz) band at room temperature is implemented in a planar metal-PtTe2 -metal structure. The results show that the THz photodetector based on PtTe2 with bow-tie-type planar contacts possesses a high photoresponsivity (1.6 A W-1 without bias voltage) with a response time less than 20 µs, while the PtTe2 -graphene heterostructure-based detector can reach responsivity above 1.4 kV W-1 and a response time shorter than 9 µs. Remarkably, it is already exploitable for large area imaging applications. These results suggest that topological semimetals such as PtTe2 can be ideal materials for implementation in a high-performing photodetection system at THz band.

Graphene-based broadband terahertz detector integrated with a square-spiral antenna.

Raising interest in terahertz radiation (loosely defined as the 0.1∼10 THz frequency range) for the application-oriented issues in everyday life requires progressive development of fast, sensitive, and portable photodetectors. In this Letter, a broadband graphene-based terahertz detector with good integrability and sensitivity at room temperature is proposed. It is based on the chemical vapor deposited-grown graphene integrated with a square-spiral metal antenna which, on one hand, improves the efficiency for electromagnetic coupling and, on the other hand, facilitates the hot-electron photo-thermoelectric process for photodetection. Sensitivity over 28 V/W at room temperature and noise-equivalent power of less than 0.35 nW/Hz0.5 are demonstrated in reference to the incident power. The presented results appealingly open an alternative way to realize chip-level graphene-based terahertz optoelectronics with good scalability and expected performance for targeted terahertz applications.

Top-gated black phosphorus phototransistor for sensitive broadband detection.

The present work reports on a graphene-like material that is promising for photodetection applications due to its high optical absorption and layer-dependent properties. To date, only narrowband photodetectors have been realized; therefore, extending the working wavelength is becoming more imperative for applications such as high-contrast imaging and remote sensing. In this work, we developed a novel detection technique that provides enhanced performance across the infrared and terahertz bands by using an antenna-assisted top-gated black phosphorus phototransistor. By using the proposed sophisticated design, the adverse effect due to the back-gate that is generally employed for a long-wavelength photon coupling can be eliminated. Moreover, the antenna-assisted near-field and dark current can be further tailored electromagnetically and electrostatically by employing a gate finger, thus resulting in improved detection efficiency. Various detection mechanisms such as thermoelectric, bolometric, and electron-hole generation are differentiated on the basis of the device geometry and incident wavelength. The proposed photodetector demonstrated superior performance-excellent sensitivity of more than 10 V W-1, a noise equivalent power value of less than 0.1 nW Hz-0.5, and a fast response time across disparate wavebands. Thus, the photodetector can satisfy diverse application requirements.