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.

We propose a hydrodynamic model describing steady-state and dynamic electron and hole transport properties of graphene structures which accounts for the features of the electron and hole spectra. It is intended for electron-hole plasma in graphene characterized by high rate of intercarrier scattering compared to external scattering (on phonons and impurities), i.e., for intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually monopolar plasma). We demonstrate that the effect of strong interaction of electrons and holes on their transport can be treated as a viscous friction between the electron and hole components. We apply the developed model for the calculations of the graphene dc conductivity, in particular, the effect of mutual drag of electrons and holes is described. The spectra and damping of collective excitations in graphene in the bipolar and monopolar limits are found. It is shown that at high gate voltages and, hence, at high electron and low hole densities (or vice-versa), the excitations are associated with the self-consistent electric field and the hydrodynamic pressure (plasma waves). In intrinsic and optically pumped graphene, the waves constitute quasineutral perturbations of the electron and hole densities (electron-hole sound waves) with the velocity being dependent only on the fundamental graphene constants.

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 ...

We derive the system of hydrodynamic equations governing the collective motion of massless fermions in graphene. The obtained equations demonstrate the lack of Galilean and Lorentz invariance, and contain a variety of nonlinear terms due to quasi-relativistic nature of carriers. Using those equations, we show the possibility of soliton formation in electron plasma of gated graphene. The quasi-relativistic effects set an upper limit for soliton amplitude, which marks graphene out of conventional semiconductors. The mentioned non-invariance of equations is revealed in spectra of plasma waves in the presence of steady flow, which no longer obey the Doppler shift. The feasibility of plasma wave excitation by direct current in graphene channels is also discussed.

We develop an analytically solvable classical kinetic model of spatially dispersive transport in Dirac materials accounting for strong electron-electron (e-e) and electron-hole (e-h) collisions. We use this model to track the evolution of graphene conductivity and properties of its collective excitations across the hydrodynamic-to-ballistic crossover. We find the relaxation rate of electric current by e-e collisions that is possible due to the lack of Galilean invariance, and introduce a universal numerical measure of this non-invariance. We find the two branches of collective excitations in the Dirac fluid: plasmons and electron-hole sound. The sound waves persist at frequencies exceeding the e-e collision frequency, have a small viscous damping at the neutrality point, but acquire large damping due to e-h friction even at slight doping. On the contrary, plasmons acquire strong frictional damping at the neutrality point and become well-defined in doped samples.