Abstract Plasmonic excitations, consisting of collective oscillations of the electron gas in a conductive film or nanostructure coupled to electromagnetic fields, play a prominent role in photonics and optoelectronics. While traditional plasmonic systems are based on noble metals, recent work has established graphene as a uniquely suited materials platform for plasmonic science and applications due to several distinctive properties. Graphene plasmonic oscillations exhibit particularly strong sub-wavelength confinement, can be tuned dynamically through the application of a gate voltage, and span a portion of the infrared spectrum (including mid-infrared and terahertz (THz) wavelengths) that is not directly accessible with noble metals. These properties have been studied in extensive theoretical and experimental work over the past decade, and more recently various device applications are also beginning to be explored. This review article is focused on graphene plasmonic nanostructures designed to address a key outstanding challenge of modern-day optoelectronics – the limited availability of practical, high-performance THz devices. Graphene plasmons can be used as a means to enhance light–matter interactions at THz wavelengths in a highly tunable fashion, particularly through the integration of graphene resonant structures with additional nanophotonic elements. This capability is ideally suited to the development of THz optical modulators (where absorption is switched on and off by tuning the plasmonic resonance) and photodetectors (relying on plasmon-enhanced intraband absorption or rectification of charge-density waves), and promising devices based on these principles have already been reported. Novel radiation mechanisms, including light emission from electrically excited graphene plasmons, are also being explored for the development of compact narrowband THz sources.

Graphene provides a promising materials platform for fundamental studies and device applications in plasmonics. Here we investigate the excitation of THz plasmon polaritons in large-area graphene samples on standard oxidized silicon substrates, via diffractive coupling from an overlying periodic array of metallic nanoparticles. Pronounced plasmonic absorption features are measured, whose frequencies can be tuned across a large portion of the THz spectrum by varying the array period. At the same time, the ability to tune these resonances actively via electrostatic doping is found to be strongly limited by the presence of large carrier density variations across the sample area induced by the underlying SiO 2 , which are measured directly by Raman microscopy. These results highlight the importance of minimizing charge "puddles" in graphene plasmonic devices, e.g., through the use of more inert substrates, in order to take full advantage of their expected dynamic tunability for applications in THz optoelectronics.

Graphene is a promising materials system for basic studies and device applications in THz optoelectronics with several key functionalities, including photodetection and optical modulation, already demonstrated in recent years. The use of plasmonic excitations in this context is particularly attractive by virtue of their dynamic gate tunability across the far-infrared spectrum, relatively long lifetimes, and highly subwavelength confinement. Here these favorable properties are exploited for the generation of narrowband tunable THz radiation from current-driven plasmonic oscillations. We employ arrays of graphene nanoribbons, where localized plasmonic resonances are excited by an injected electrical current (through the generation and subsequent energy relaxation of hot carriers) and then radiate into the far field. Pronounced emission peaks are correspondingly measured at tunable frequencies across a wide portion of the THz spectrum (4–8 THz), controlled by design through the ribbon width and actively through the applied gate voltage. These results provide a new path for the study of plasmonic and hot-carrier phenomena in graphene and are technologically relevant for the development of highly miniaturized and broadly tunable THz radiation sources.

In this article, a wide‐band and polarization‐insensitive perfect absorber composed of 4 sandwiched layers of dielectric and metal disks is introduced. Compared to classical perfect absorbers, the system supports near‐unity absorption within a wider spectral window through multiple perfect absorption mechanisms that exist due to a constituting inter‐metal disk, functioning either as a dipolar antenna or a conducting ground for different perfect absorption mechanisms. Circular shape of the antenna makes the working mechanism of the system polarization insensitive. The working principle of the system is investigated through near‐ and far‐field calculations by finite difference time domain (FDTD) simulations. A fine‐tuning mechanism of the wide‐range perfect absorption window is introduced through geometrical device parameters. The multilayer perfect absorber system is fabricated through a high‐quality fabrication method based on electron beam lithography, lift‐off method, and multi‐step deposition of metal and dielectric layers. The spectral behavior of the perfect absorber system is finally experimentally investigated through Fourier transform infrared (FTIR) spectroscopy.

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