Sukosin Thongrattanasiri

Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons.

Plasmons in doped graphene exhibit relatively large confinement and long lifetime compared to noble-metal plasmons. Here, we study the propagation properties of plasmons guided along individual and interacting graphene nanoribbons. Besides their tunability via electrostatic gating, an additional handle to control these excitations is provided by the dielectric environment and the relative arrangement of the interacting waveguides. Plasmon interaction and hybridization in pairs of neighboring aligned ribbons are shown to be strong enough to produce dramatic modifications in the plasmon field profiles. We introduce a universal scaling law that considerably simplifies the analysis an understanding of these plasmons. Our work provides the building blocks to construct graphene plasmon circuits for future compact plasmon devices with potential application to optical signal processing, infrared sensing, and quantum information technology.

Gated Tunability and Hybridization of Localized Plasmons in Nanostructured Graphene

Graphene has emerged as an outstanding material for optoelectronic applications due to its high electronic mobility and unique doping capabilities. Here we demonstrate electrical tunability and hybridization of plasmons in graphene nanodisks and nanorings down to 3.7 μm light wavelength. By electrically doping patterned graphene arrays with an applied gate voltage, we observe radical changes in the plasmon energy and strength, in excellent quantitative agreement with rigorous analytical theory. We further show evidence of an unexpected increase in plasmon lifetime with growing energy. Plasmon hybridization and electrical doping in nanorings of suitably chosen nanoscale dimensions are key elements for bringing the optical response of graphene closer to the near-infrared, where it can provide a robust, integrable platform for light modulation, switching, and sensing.

Quantum Finite-Size Effects in Graphene Plasmons

Graphene plasmons are emerging as an alternative solution to noble metal plasmons, adding the advantages of tunability via electrostatic doping and long lifetimes. These excitations have been so far described using classical electrodynamics, with the carbon layer represented by a local conductivity. However, the question remains, how accurately is such a classical description representing graphene? What is the minimum size for which nonlocal and quantum finite-size effects can be ignored in the plasmons of small graphene structures? Here, we provide a clear answer to these questions by performing first-principles calculations of the optical response of doped nanostructured graphene obtained from a tight-binding model for the electronic structure and the random-phase approximation for the dielectric response. The resulting plasmon energies are in good agreement with classical local electromagnetic theory down to ∼10 nm sizes, below which plasmons split into several resonances that emphasize the molecular character of the carbon structures and the quantum nature of their optical excitations. Additionally, finite-size effects produce substantial plasmon broadening compared to homogeneous graphene up to sizes well above 20 nm in nanodisks and 10 nm in nanoribbons. The atomic structure of edge terminations is shown to be critical, with zigzag edges contributing to plasmon broadening significantly more than armchair edges. This study demonstrates the ability of graphene nanostructures to host well-defined plasmons down to sizes below 10 nm, and it delineates a roadmap for understanding their main characteristics, including the role of finite size and nonlocality, thus providing a solid background for the emerging field of graphene nanoplasmonics.

Plasmons in electrostatically doped graphene

Graphene has raised high expectations as a low-loss plasmonic material in which the plasmon properties can be controlled via electrostatic doping. Here, we analyze realistic configurations, which produce inhomogeneous doping, in contrast to what has been so far assumed in the study of plasmons in nanostructured graphene. Specifically, we investigate backgated ribbons, co-planar ribbon pairs placed at opposite potentials, and individual ribbons subject to a uniform electric field. Plasmons in backgated ribbons and ribbon pairs are similar to those of uniformly doped ribbons, provided the Fermi energy is appropriately scaled to compensate for finite-size effects such as the divergence of the carrier density at the edges. In contrast, the plasmons of a ribbon exposed to a uniform field exhibit distinct dispersion and spatial profiles that considerably differ from uniformly doped ribbons. Our results provide a road map to understand graphene plasmons under realistic electrostatic doping conditions.

Hypergratings: nanophotonics in planar anisotropic metamaterials

We present a technique capable of producing subwavelength focal spots in planar nonresonant structures not limited to the near-field of the source. The approach combines the diffraction gratings that generate the high-wave-vector-number modes and planar slabs of homogeneous anisotropic metamaterials that propagate these waves and combine them at the subwavelength focal spots. In a sense, the technique combines the benefits of Fresnel lens, near-field zone plates, hyperlens, and superlens and at the same time resolves their fundamental limitations. Several realizations of the proposed technique for visible, near-IR, and mid-IR frequencies are proposed, and their performance is analyzed theoretically and numerically. Generalizations of the developed approach for subdiffractional imaging and on-chip photonics are suggested.

Tunable Molecular Plasmons in Polycyclic Aromatic Hydrocarbons

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules.

Strong Plasmon Reflection at Nanometer-Size Gaps in Monolayer Graphene on SiC

We employ tip-enhanced infrared near-field microscopy to study the plasmonic properties of epitaxial quasi-free-standing monolayer graphene on silicon carbide. The near-field images reveal propagating graphene plasmons, as well as a strong plasmon reflection at gaps in the graphene layer, which appear at the steps between the SiC terraces. When the step height is around 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20% of its value at graphene edges, and it approaches 50% for step heights as small as 5 nm. This intriguing observation is corroborated by numerical simulations and explained by the accumulation of a line charge at the graphene termination. The associated electromagnetic fields at the graphene termination decay within a few nanometers, thus preventing efficient plasmon transmission across nanoscale gaps. Our work suggests that plasmon propagation in graphene-based circuits can be tailored using extremely compact nanostructures, such as ultranarrow gaps. It also demonstrates that tip-enhanced near-field microscopy is a powerful contactless tool to examine nanoscale defects in graphene.

Plasmons driven by single electrons in graphene nanoislands

Abstract Plasmons produce large confinement and enhancement of light that enable applications as varied as cancer therapy and catalysis. Adding to these appealing properties, graphene has emerged as a robust, electrically tunable material exhibiting plasmons that strongly depend on the density of doping charges. Here we show that adding a single electron to a graphene nanoisland consisting of hundreds or thousands of atoms switches on infrared plasmons that were previously absent from the uncharged structure. Remarkably, the addition of each further electron produces a dramatic frequency shift. Plasmons in these islands are shown to be tunable down to near infrared wavelengths. These phenomena are highly sensitive to carbon edges. Specifically, armchair nanotriangles display sharp plasmons that are associated with intense near-field enhancement, as well as absorption cross-sections exceeding the geometrical area occupied by the graphene. In contrast, zigzag triangles do not support these plasmons. Our conclusions rely on realistic quantum-mechanical calculations, which are in ostensible disagreement with classical electromagnetic simulations, thus revealing the quantum nature of the plasmons. This study shows a high sensitivity of graphene nanoislands to elementary charges, therefore emphasizing their great potential for novel nano-optoelectronics applications.

Plasmonic mid-infrared beam steering

We present a metal/semiconductor beam steering device for use in the mid-infrared wavelength range. We demonstrate how changing the frequency of the incident light results in a smoothly varying shift in the angular distribution of the transmitted beam, and we present an analysis of the beam profile for a number of different wavelengths. Finally we verify that a similar steering effect is achieved with fixed frequency incident light and a modification of the permittivity of the semiconductor substrate, ultimately resulting in a 3° shift in the transmitted beam angle for minimal shifts in the semiconductor permittivity.