We investigate through analytic calculations the surface plasmon dispersion relation for monolayer graphene sheets and a separated parallel pair of graphene monolayers. An approximate form for the dispersion relation for the monolayer case was derived, which was shown to be highly accurate and offers intuition to the properties of the supported plasmon mode. For parallel graphene pairs separated by small gaps, the dispersion relation of the surface plasmon splits into two branches, one with a symmetric and the other with an antisymmetric magnetic field across the gap. For the symmetric (magnetic field) branch, the confinement may be improved at reduced absorption loss over a wide spectrum, unlike conventional SP modes supported on metallic surfaces that are subjected to the trade-off between loss and confinement. This symmetric mode becomes strongly suppressed for very small separations however. On the other hand, its antisymmetric counterpart exhibits reduced absorption loss for very small separations or long wavelengths, serving as a complement to the symmetric branch. Our results suggest that graphene plasmon structures could be promising for waveguiding and sensing applications in the mid-infrared and terahertz frequencies.

We study plasmonic resonances in electrostatically gated graphene nanoribbons on silicon dioxide substrates. Absorption spectra are measured in the mid-far infrared and reveal multiple peaks, with width-dependent resonant frequencies. We calculate the dielectric function within the random phase approximation and show that the observed spectra can be explained by surface-plasmon-phonon-polariton modes, which arise from coupling of the graphene plasmon to three surface optical phonon modes in the silicon dioxide. a)

An active plasmonic switch based on single-and few-layer doped graphene ribbon array operating in the mid-infrared spectrum is investigated with theoretical and numerical calculations. It is shown that significant resonance wavelength shifts and modulation depths can be achieved with a slight variation of the doping concentration of the graphene ribbon. The few-layer graphene ribbon array device outperforms the single-layer one in terms of the achievable modulation depth. Our simulations reveal that, by modulating the Fermi-energy level between 0.2 eV and 0.25 eV, a fourlayer graphene ribbon array device can achieve a modulation depth and resonance wavelength shift of ∼13 dB and 0.94 µm respectively, compared to ∼2.8 dB and 1.85 µm for a single-layer device. Additionally, simple fitting models to predict the modulation depth and the resonance wavelength shift are proposed. These prospects pave the way towards ultrafast active graphene-based plasmonic devices for infrared and THz applications. a) Electronic mail: chuhs@ihpc.a-star.edu.sgRecently, there is tremendous scientific and technological interest in the mid-infrared spectral range of 2-20 µm. This range provides many potential applications for optics/photonics such as spectroscopy, materials processing, chemical and biomolecular sensing, remote explosive detection and covert communication systems 1,2 . Thanks to the integration with electronic devices and possibility to design devices with active control over the surface plasmon resonance 3 at metal/dielectric interfaces, the mid-infrared spectral region is also attractive for the study of plasmonic devices 4,5 . However, due to a relatively weak refractive index change with electrical-bias, mechanical force or temperature, the active plasmonic devices typically exhibit low optical performance such as high power consumption or slow response time. Graphene, a single layer of carbon atoms gathered in a honeycomb lattice, exhibits many unique physical features and has been recently investigated for nanoscale optoelectronic integrated circuits. Graphene-based plasmonic nanostructures support highly confined plasmonic modes that can be tuned via chemical or eletrostatic doping, and are promising to serve as future platforms for highly integrated active plasmonic devices ranged from the infrared to THz frequencies 6-14 . Therefore, it is important to develop broadly tunable plasmonic devices, either as an enabling technology or to add functionality to current plasmonic technologies. In this paper, we demonstrate with numerical simulations that an effective active plasmonic switch operating in the mid-infrared wavelength range can be real-

It is shown how surface plasmons that travel between the slits in Young's interference experiment can change the state of spatial coherence of the field that is radiated by the two apertures. Surprisingly, the coherence can both be increased and decreased, depending on the slit separation distance. This results in a modulation of the visibility of the interference fringes. Since many properties of a light field-such as its spectrum, polarization, and directionality - may change on propagation and are dependent on the spatial coherence of the source, our results suggest that the use of surface plasmons provides a new way to alter or even tailor the statistical properties of a light field.