Plasmons, collective oscillations of electron systems, can efficiently couple light and electric current, and thus can be used to create sub-wavelength photodetectors, radiation mixers, and on-chip spectrometers. Despite considerable effort, it has proven challenging to implement plasmonic devices operating at terahertz frequencies. The material capable to meet this challenge is graphene as it supports long-lived electrically tunable plasmons. Here we demonstrate plasmon-assisted resonant detection of terahertz radiation by antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot cavities and rectifying elements. By varying the plasmon velocity using gate voltage, we tune our detectors between multiple resonant modes and exploit this functionality to measure plasmon wavelength and lifetime in bilayer graphene as well as to probe collective modes in its moiré minibands. Our devices offer a convenient tool for further plasmonic research that is often exceedingly difficult under non-ambient conditions (e.g. cryogenic temperatures) and promise a viable route for various photonic applications.

Graphene is considered as a promising platform for detectors of high-frequency radiation up to the terahertz (THz) range due to graphene ′ s superior electron mobility. Previously it has been shown that graphene field effect transistors (FETs) exhibit room temperature broadband photoresponse to incoming THz radiation thanks to the thermoelectric and/or plasma wave rectification. Both effects exhibit similar functional dependences on the gate voltage and therefore it was found to be difficult to disentangle these contributions in the previous studies. In this letter, we report on combined experimental and theoretical studies of sub-THz response in graphene field-effect transistors analyzed at different temperatures. This temperature-dependent study allowed us to reveal the role of photo-thermoelectric effect, p-n junction rectification, and plasmonic rectification in the sub-THz photoresponse of graphene FETs.Over the last decade, graphene has attracted a considerable attention in the fields of photonics 1 , plasmonics 2 , and optoelectronics 3 . The interest is motivated by graphenes unique gate-tuneable physical properties that allow realization of radiation detectors operating in a wide range of frequencies 4-7 .Electromagnetic radiation in the terahertz (THz) range deserves a special attention as it allows fast and non-destructive imaging of objects with a strong potential in medical and security sectors 8 . With this potential, the development of efficient THz generators and sensitive detectors is an important technological problem.Recently, it has been shown that graphene field-effect transistors (FETs) can act as THz detectors exhibiting a dc photoresponse to impinging radiation 7,[9][10][11][12][13][14][15][16] . A broadband photodetection in the sub-THz range with the responsivity reaching tens of V/W and noise equivalent power of hundreds of pW/Hz 1/2 has been demonstrated in graphene FETs designed in the configuration where the incoming radiation is coupled between the source and the gate terminals 9,10,14,15 . In this configuration, the photoresponse is usually attributed to the so-called Dyakonov-Shur (DS) rectification arising as a result of the plasma waves excitation in the FET channel 17,18 . However, other effects can also impact the photoresponse. For instance, photo-thermoelectric effect (PTE) arising from the temperature gradient in a FET can provide an additional rectification of the incoming high-frequency signal 5,7 . As we show below, both PTE and DS effects exhibit similar functional dependence on the gate voltage and result in the same sign of the photoresponse, that makes it challenging to point to the origin of the observed rectification. Further improvement of graphene-based THz photodetectors requires a deeper understanding of the rectification mechanisms governing the photoresponse.In this work, we analyze the sub-THz photoresponse of graphene-based FET by comparing its responsivity at liquid nitrogen and room temperatures. Such temperaturedependent measurements allowed us to point ...

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