A research team lead by ICFO has demonstrated a cavity-enhanced terahertz (THz) photoresponse in a scalable monolayer graphene device, using acoustic graphene plasmons (AGPs) to boost detection efficiency while maintaining a practical fabrication approach.
Terahertz radiation, spanning wavelengths from 15 to 1000 μm (0.3 to 20 THz), is gaining attention for applications such as biomedical imaging, chemical sensing, security screening, and high-speed wireless communications. However, current THz detectors typically face trade-offs between speed, sensitivity, noise, and operating conditions, making it difficult to achieve high performance across all parameters. In this work, the researchers designed a device based on chemical-vapor-deposited (CVD) monolayer graphene integrated with a dipole antenna. This antenna concentrates incoming THz radiation (1.83 to 2.52 THz), acts as a pair of gate electrodes, and launches AGPs - collective oscillations of electrons - into the graphene channel. These plasmons reflect within the device and form standing waves, effectively creating a Fabry–Pérot-type cavity.
This cavity strongly enhances the interaction between light and graphene. AGPs confine the electromagnetic field to extremely small volumes, with confinement factors reaching 165 laterally and 4000 vertically relative to the free-space wavelength. As a result, absorption in the graphene layer is significantly increased despite its atomic thickness.
The absorbed energy generates localized heating in different regions of the graphene, producing a temperature gradient that is converted into an electrical signal via the photothermoelectric (PTE) effect. By tuning the device so that the plasmon resonances align with the peak Seebeck coefficient, the researchers maximize this electrical response.
Measurements show pronounced, gate-dependent photovoltage peaks at temperatures between 6 and 90 K, with the response modulated by up to approximately 40%. These peaks arise from AGP standing waves forming either across the full graphene channel or within half-channel regions. Overall, the plasmonic cavity enhances the photoresponse by about 30% compared to the maximum conventional PTE signal - significantly stronger than in previous graphene-based THz detectors.
Importantly, this performance is achieved using wafer-scale CVD graphene without hexagonal boron nitride (hBN) encapsulation, simplifying fabrication and improving scalability. While the resonances weaken and disappear around 130 K, the results suggest that further improvements in graphene quality could enable operation closer to room temperature.
This work demonstrates a practical route toward scalable, low-power, and frequency-selective THz photodetectors, with potential applications in sensing, imaging, spectroscopy, and communications.
