Chip-Scalable, Room-Temperature, Zero-Bias, Graphene-Based Terahertz Detectors with Nanosecond Response Time

Quantum Graphene Asymmetric Devices for Harvesting Electromagnetic Energy

We present here the fabrication at the wafer level and the electrical performance of two types of graphene diodes: ballistic trapezoidal-shaped graphene diodes and lateral tunneling graphene diodes. In the case of the ballistic trapezoidal-shaped graphene diode, we observe a large DC current of 200 µA at a DC bias voltage of ±2 V and a large voltage responsivity of 2000 v/w, while in the case of the lateral tunneling graphene diodes, we obtain a DC current of 1.5 mA at a DC bias voltage of ±2 V, with a voltage responsivity of 3000 v/w. An extended analysis of the defects produced during the fabrication process and their influences on the graphene diode performance is also presented.

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Highly integrated automatic injection terahertz microfluidic biosensor based on metasurface and LT-GaAs photoconductive antenna

In this paper, a highly integrated terahertz (THz) biosensor is proposed and implemented, which pioneered the preparation of low-temperature gallium arsenide (LT-GaAs) thin film photoconductive antenna (PCA) on the sensor for direct generation and detection of THz waves, simplifying complex terahertz time-domain spectroscopy (THz-TDS) systems. A latch type metasurface is deposited in the detection region to produce a resonance absorption peak at 0.6 THz that is independent of polarisation. Microfluidics is utilised and automatic injection is incorporated to mitigate the experimental effects of hydrogen bond absorption of THz waves in aqueous-based environment. Additionally, cell damage is minimised by regulating the cell flow rate. The biosensor was utilised to detect the concentration of three distinct sizes of bacteria with successful results. The assay was executed as a proof of concept to detect two distinct types of breast cancer cells. Based on the experimental findings, it has been observed that the amplitude and blueshift of the resonance absorption peaks have the ability to identify and differentiate various cancer cell types. The findings of this study introduce a novel approach for developing microfluidic THz metasurface biosensors that possess exceptional levels of integration, sensitivity, and rapid label-free detection capabilities.

Monolayer Graphene Terahertz Detector Integrated with Artificial Microstructure

Graphene, known for its high carrier mobility and broad spectral response range, has proven to be a promising material in photodetection applications. However, its high dark current has limited its application as a high-sensitivity photodetector at room temperature, particularly for the detection of low-energy photons. Our research proposes a new approach for overcoming this challenge by designing lattice antennas with an asymmetric structure for use in combination with high-quality monolayers of graphene. This configuration is capable of sensitive detection of low-energy photons. The results show that the graphene terahertz detector-based microstructure antenna has a responsivity of 29 V·W^-1 at 0.12 THz, a fast response time of 7 μs, and a noise equivalent power of less than 8.5 pW/Hz^1/2. These results provide a new strategy for the development of graphene array-based room-temperature terahertz photodetectors.

Magnetic induced terahertz modulation characteristics based on ferromagnetic nematic liquid crystals

In recent years, solid state terahertz (THz) modulators have obtained rapid progress with the widespread use of two-dimensional (2D) materials in the field of THz; however, challenges remain in preparing flexible THz modulators. In this study, flexible ferromagnetic nematic materials were prepared by doping thermotropic nematic liquid crystals 5CB into magnetic fluids, and the influence of water was reduced by a self-made cyclic olefin copolymer (COC) microfluidic chip. THz modulation characteristics of magnetic fluid and ferromagnetic nematic liquid crystal (FNLC) under the induction of external magnetic field were compared using a THz time domain spectroscopy system. Under the action of a 91 mT magnetic field, the magnetic fluid has a maximum modulation depth (MD) of 54%. Under the same magnetic field, the maximum MD of the FNLC materials increase to 78% because of the rearrangement of Fe₃O₄ nanoparticles induced by the topological defect of the liquid crystal. We demonstrate that the magneto-optical effect is significantly enhanced in the ferromagnetic nematic liquid crystal hybrid system. This strategy of doping thermotropic nematic liquid crystals to enhance the magneto-optical effect has great potential for THz filtering, modulation, and sensing applications.

Strain-Induced Plasmon Confinement in Polycrystalline Graphene

Terahertz spectroscopy is a perfect tool to investigate the electronic intraband conductivity of graphene, but a phenomenological model (Drude-Smith) is often needed to describe disorder. By studying the THz response of isotropically strained polycrystalline graphene and using a fully atomistic computational approach to fit the results, we demonstrate here the connection between the Drude-Smith parameters and the microscopic behavior. Importantly, we clearly show that the strain-induced changes in the conductivity originate mainly from the increased separation between the single-crystal grains, leading to enchanced localization of the plasmon excitations. Only at the lowest strain values explored, a behavior consistent with the deformation of the individual grains can instead be observed.

Photoresponse enhancement in a cavity-antenna-coupled graphene terahertz detector

The unique and excellent optoelectronic properties of graphene make it a promising material for THz detection. However, the huge gap between the atomic thickness of graphene and the long wavelength of THz radiation severely limits the efficiency of light absorption and the photoresponse. Although optical antennas are commonly used to concentrate THz waves into deep subwavelength regions, a large amount of power is re-radiated out as waste rather than being absorbed by the active material. Here, we propose a cavity-antenna hybrid structure to enhance the THz absorption in a graphene flake through interference manipulation and impedance matching. The photoresponse of the device under an impedance-matched condition is 15 times higher than that under an impedance-mismatched condition. The cavity-antenna-coupled graphene THz detector exhibits a responsivity of 10.2 mA W^-1 and a noise-equivalent power of 0.92 nW Hz^-0.5. The excellent performance of our device makes it one of the best room temperature sub-THz graphene detectors that we investigated. A theoretical model was established to analyze the interaction between the antenna-coupled graphene and the cavity structure. With a combination of both electrostatic doping and interference manipulation, the light coupling management by impedance matching can enhance the absorptance of graphene by more than two orders of magnitude. This work experimentally and theoretically revealed that the cavity-antenna hybrid structure can prominently enhance the responsivity of a tiny piece of graphene in the sub-THz regime. Meanwhile, this strategy can also be applied to enhance the absorptance of other low-dimensional or bulk materials.

Non-Additive Optical Response in Transition Metal Dichalcogenides Heterostructures

Van der Waals (vdW) heterostructures pave the way to achieve the desired material properties for a variety of applications. In this way, new scientific and industrial challenges and fundamental questions arise. One of them is whether vdW materials preserve their original optical response when assembled in a heterostructure. Here, we resolve this issue for four exemplary monolayer heterostructures: MoS₂/Gr, MoS₂/hBN, WS₂/Gr, and WS₂/hBN. Through joint Raman, ellipsometry, and reflectance spectroscopies, we discovered that heterostructures alter MoS₂ and WS₂ optical constants. Furthermore, despite the similarity of MoS₂ and WS₂ monolayers, their behavior in heterostructures is markedly different. While MoS₂ has large changes, particularly above 3 eV, WS₂ experiences modest changes in optical constants. We also detected a transformation from dark into bright exciton for MoS₂/Gr heterostructure. In summary, our findings provide clear evidence that the optical response of heterostructures is not the sum of optical properties of its constituents.

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.

Monolithic Metamaterial-Integrated Graphene Terahertz Photodetector with Wavelength and Polarization Selectivity

The frequency spectra and polarization states of terahertz waves can convey significant information about physical interactions and material properties. Compact and miniaturized on-chip platforms for effective capturing of these quantities are being extensively investigated because of their promising potential for paramount applications of terahertz technology such as in situ sensing and characterization. Here, we present a metamaterial-graphene hybrid device that integrates the functions of photodetection, wavelength, and polarization selectivity into a monolithic architecture. Leveraging the ultrahigh design freedom of metamaterial optical properties and the electronically controllable hot-carrier-assisted photothermoelectric effect in graphene, our detector shows resonantly enhanced photoresponse at two specific target wavelengths with orthogonal polarizations. We demonstrate its versatile capabilities for spectrally selective and polarization resolved imaging on a single-chip platform that is free from advanced optical components. Our strategy is beneficial to the future development of multifunctional, compact, and low-cost terahertz sensors.

Direct growth of SnS2 nanowall photoanode for high responsivity self-powered photodetectors

Tin sulfide (SnS₂) has attracted growing attention due to its environmental friendliness, tunable band gap and potential applications for high-sensitivity photodetectors. However, the low responsivity and slow response speed severely hinder its further applications. In this work, SnS₂ nanowalls have been successfully fabricated on FTO substrates by a facile hydrothermal approach. The prepared SnS₂ nanowalls were used as a photoanode material for photoelectrochemical (PEC)-type photodetectors. The SnS₂ based PEC-type photodetectors exhibit excellent photocurrent density (39.06 μA cm^-2), responsivity (1460 μA W^-1), long-term cycling stability and self-powered behavior. The responsivity of the detector is higher than that of most reported SnS₂ based PEC-type photodetectors and even some SnS₂ based photoconductive photodetectors. The high responsivity and self-powered behavior enable the extended potential applications of SnS₂ in PEC-type photodetectors.

Self-Powered Lithium Niobate Thin-Film Photodetectors

Thin-film lithium niobate platform, namely lithium-niobate-on-insulator (LNOI), brings new opportunities for integrated photonics, taking advantages from both outstanding crystalline properties and special structural features. The excellent properties of LNOI have triggered development of a variety of on-chip photonic devices for light generation and manipulation. However, as an indispensable component for photonic circuit with full functionalities, the thin-film photodetector lacks in portfolios of LNOI-based devices due to standing obstacles of low electrical conductivity and poor photoelectric conversion ability. Here, a self-powered broadband LNOI photodetector based on enhanced photovoltaic effect, benefitting from encapsulated plasmonic nanoparticles and doped silver ions, is reported. Maximum responsivity of 0.25 A W^-1 and detectivity (1.56 × 10¹⁴ Jones) are achieved. First-principle calculations and electric-field simulation reveal intrinsic mechanisms and crucial roles of plasmonic nanoparticles and silver ions on photocurrent generation and collection. This work opens an avenue to develop high-performance on-chip LNOI photodetectors, offering a conceivable means toward multiple-functional photonic circuits.

Terahertz modulation characteristics of three nanosols under external field control based on microfluidic chip

Recently, with the widespread application of metamaterials in the terahertz (THz) modulation field, solid-state THz modulators have made breakthrough progress; however, there are still challenges in preparing flexible THz modulators with wide modulation bandwidths. In this study, a THz microfluidic chip was fabricated using cycloolefin copolymers with high transmission (90%) of THz waves. The THz modulation characteristics of TiO₂, Ag, and Fe₃O₄ nanosols under the control of an optical field, electric field, and magnetic field, respectively, were investigated. Under the action of photogenerated carrier migration, colloidal electrophoresis, and magneto-optical effect, all three nanosols exhibit broadband modulation performance in the frequency range of 0.3–2.4 THz, and the maximum modulation depth is 24%, 33%, and 54%, respectively. Contrary to previous studies based on traditional solid-state materials, this study innovatively explores the possibility of modulating THz waves with liquid materials, laying the foundation for the application of flexible liquid-film THz modulators.

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THz broadband amplitude modulation of liquid nanosols under external fields

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Using a microfluidic chip to reduce the absorption of THz waves by hydrogen bonds

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The experimental results lay a foundation for liquid-film THz modulators

Dynamic and Active THz Graphene Metamaterial Devices

In recent years, terahertz waves have attracted significant attention for their promising applications. Due to a broadband optical response, an ultra-fast relaxation time, a high nonlinear coefficient of graphene, and the flexible and controllable physical characteristics of its meta-structure, graphene metamaterial has been widely explored in interdisciplinary frontier research, especially in the technologically important terahertz (THz) frequency range. Here, graphene’s linear and nonlinear properties and typical applications of graphene metamaterial are reviewed. Specifically, the discussion focuses on applications in optically and electrically actuated terahertz amplitude, phase, and harmonic generation. The review concludes with a brief examination of potential prospects and trends in graphene metamaterial.

Semiconductor Nanowire Field-Effect Transistors as Sensitive Detectors in the Far-Infrared

Engineering detection dynamics in nanoscale receivers that operate in the far infrared (frequencies in the range 0.1-10 THz) is a challenging task that, however, can open intriguing perspectives for targeted applications in quantum science, biomedicine, space science, tomography, security, process and quality control. Here, we exploited InAs nanowires (NWs) to engineer antenna-coupled THz photodetectors that operated as efficient bolometers or photo thermoelectric receivers at room temperature. We controlled the core detection mechanism by design, through the different architectures of an on-chip resonant antenna, or dynamically, by varying the NW carrier density through electrostatic gating. Noise equivalent powers as low as 670 pWHz-1/2 with 1 µs response time at 2.8 THz were reached.