Graphene Detector Achieves 0.13THz Plasmon Resonance, Enabling Tunable Sub-Terahertz Photovoltage Generation

The detection of subtle electromagnetic signals at sub-terahertz frequencies presents a significant challenge for modern technology, yet holds promise for applications ranging from security screening to medical imaging. Researchers, including I. M. Moiseenko, E. Titova, and M. Kashchenko, alongside D. Svintsov, now demonstrate a novel graphene-based detector capable of identifying these signals through the excitation of plasmon resonance. Their combined experimental and theoretical work reveals that this detector functions by harnessing the thermoelectric effect within a specially designed bilayer graphene transistor, creating a tunable p-n junction. Crucially, the team achieves a record-low frequency of plasmon resonance, at just 0. 13 terahertz, by carefully controlling the charge carrier density within the graphene, thereby enhancing the detector’s sensitivity and opening new avenues for sub-terahertz technology.

Bilayer Graphene Exhibits Sub-Terahertz Rectification Effects

Scientists have demonstrated that bilayer graphene efficiently converts subterahertz radiation into a measurable voltage, achieving a record-low resonant frequency of 0. 13 terahertz. This breakthrough stems from carefully engineered devices and a detailed understanding of electron behaviour within the graphene structure, opening possibilities for highly sensitive terahertz detectors. The research confirms that collective electron oscillations, known as plasmons, play a crucial role in this process, even at frequencies previously considered too low for significant interaction. The team fabricated a bilayer graphene transistor with a unique design, incorporating a global back gate and split top gates.

This configuration allows precise control over both the band gap and the concentration of charge carriers within the graphene, enabling the creation of a tunable p-n junction. A thin layer of hafnium dioxide serves as the back gate, remaining transparent to the subterahertz radiation, while a layered structure of hexagonal boron nitride encapsulates the graphene. Measurements reveal that the observed voltage arises from a thermoelectric effect, where the absorption of subterahertz radiation heats the p-n junction. This heating, coupled with the unique electronic properties of graphene, generates a measurable voltage.

The team developed a theoretical model that accurately predicts the temperature distribution within the graphene channel, showing a maximum at the p-n junction when electron and hole concentrations are balanced. Calculations indicate that even small amounts of incident power, just a few nanowatts, can raise the junction temperature by a significant fraction of a Kelvin. The key to this achievement lies in the ability to reduce the concentration of charge carriers within the graphene, which lowers the resonant frequency of the plasmons. By inducing a band gap through electrical control, the team successfully excited plasmons at a record-low frequency, enabling efficient subterahertz detection. This research demonstrates the potential of bilayer graphene as a platform for highly sensitive and tunable terahertz detectors, with implications for a wide range of applications, including security screening, medical imaging, and materials science.

Tunable Graphene p-n Junction for THz Photovoltage

Researchers engineered a bilayer graphene transistor to investigate the generation of voltage from subterahertz radiation, achieving a record-low resonant frequency of 0. 13 terahertz. The device incorporates a global back gate and split top gates, enabling independent control of both the band gap and the concentration of charge carriers within the graphene, and facilitating the formation of a tunable p-n junction. A thin layer of hafnium dioxide, deposited using atomic layer deposition, serves as the back gate and remains transparent to subterahertz frequencies. To deliver focused subterahertz radiation to the micrometer-scale graphene transistor, researchers employed a bow-tie antenna directly connected to the graphene channel.

The radiation source, an impact ionization avalanche transit-time diode operating at 0. 13 terahertz, delivers approximately 70 nanowatts of power to the detector area. Electrical formation of the band gap in graphene was confirmed through exponential increases in graphene resistance at the charge neutrality point as the average perpendicular electric field increased, controlled by the top and back gate voltages. Photovoltaic measurements, conducted at a cryogenic temperature of 7 Kelvin, revealed the origin of the observed voltage. The team demonstrated that the observed photovoltage arises primarily from a thermoelectric mechanism driven by heating of the p-n junction, and characterized the plasmonic resonances that enhance the electromagnetic field and carrier temperature within the junction region.

Thermoelectric Photovoltage From Low-Frequency Plasmons

This work presents a detailed investigation of voltage generation in a bilayer graphene transistor exposed to subterahertz radiation, revealing a thermoelectric mechanism driven by heating at a strategically positioned p-n junction within the device channel. Measurements demonstrate that the observed voltage arises primarily from this thermoelectric effect, with the team successfully identifying and characterizing the excitation of two-dimensional plasmons at a record-low frequency of 0. 13 terahertz. These plasmons, activated by a reduction in charge carrier concentration achieved through electrical control of the band gap, manifest as characteristic oscillations in the measured voltage signal.

The research team developed a model to accurately predict the temperature distribution within the graphene channel, showing a symmetrical profile with a maximum at the p-n junction when electron and hole concentrations are equal. Calculations indicate that the temperature increase at the junction reaches several fractions of a Kelvin for incident terahertz power levels of just a few nanowatts, with the heat diffusing towards the source and drain contacts. Crucially, the model predicts an oscillatory structure in the junction temperature dependent on graphene parameters like band gap and carrier density, directly linked to the excitation of these two-dimensional plasmons. The frequency of these plasmons can be tuned to sub-terahertz frequencies at charge carrier concentrations as low as 1×10 11 cm -2 , enabling wavelengths comparable to the detector channel length even at these low frequencies. This achievement is particularly significant given the relatively large channel length of 6 micrometers, and demonstrates the potential for efficient subterahertz detection.

Plasmonic Rectification in Graphene P-N Junctions

This research demonstrates the significant role of plasmonic effects in sub-terahertz voltage generation within bilayer graphene p-n junctions, even at very low frequencies. Scientists successfully observed plasmonic oscillations in the generated voltage, a phenomenon enabled by inducing a band gap within the bilayer graphene, which reduces carrier density and lowers the resonant frequency to hundreds of gigahertz. The study combined experimental measurements with theoretical modelling to explain how these oscillations arise from the interaction of subterahertz radiation with the p-n junction. The developed model accurately captures the experimentally observed plasmonic oscillation of the generated voltage, particularly at larger induced band gaps. Researchers acknowledge that the model relies on certain approximations, including assumptions about electron scattering and energy relaxation times, as well as the behaviour of metal-graphene Schottky junctions. Future work will focus on optimising device geometry and electrical parameters to further enhance performance and bring these findings closer to practical applications, potentially leading to highly sensitive and tunable terahertz detectors based on bilayer graphene and other two-dimensional materials.