Luis Martín-Moreno

Surfaces with holes in them: new plasmonic metamaterials

In this paper we explore the existence of surface electromagnetic modes in corrugated surfaces of perfect conductors. We analyse two cases: one-dimensional arrays of grooves and two-dimensional arrays of holes. In both cases we find that these structures support surface bound states and that the dispersions of these modes have strong similarities with the dispersion of the surface plasmon polariton bands of real metals. Importantly, the dispersion relation of these surface states is mainly dictated by the geometry of the grooves or holes and these results open the possibility of tailoring the properties of these modes by just tuning the geometrical parameters of the surface.

Evanescently coupled resonance in surface plasmon enhanced transmission

The optical transmission through subwavelength holes in metal films can be enhanced by several orders of magnitude by enabling interaction of the incident light with independent surface plasmon (SP) modes on either side of the film. Here, we show that this transmission is boosted by an additional factor of ∼10 when the energies of the SP modes on both sides are matched. These results, confirmed by a three-dimensional theoretical analysis, give a totally new understanding of the phenomenon of SP enhanced transmission. It is found that the holes behave like subwavelength cavities for the evanescent waves coupling the SPs on either side of the film. In this unusual device, the reflection at either end of the cavity is provided by the SP modes which act as frequency dependent mirrors.

Efficient unidirectional nanoslit couplers for surface plasmons

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components1,2. But in order that such potential can be realized, it is vital that the relatively poor light–SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3,4,5,6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system’s size. Back-side illumination of subwavelength apertures in optically thick metal films7,8,9,10,11,12,13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.

Transmission of Light through a Single Rectangular Hole

We show that a single rectangular hole in a metallic film exhibits transmission resonances that appear near the cutoff wavelength of the hole waveguide. For light polarized with the electric field pointing along the holes short axis, it is shown that the normalized-to-area transmittance at resonance is proportional to the ratio between the long and short sides, and to the dielectric constant inside the hole. Importantly, this resonant transmission process is accompanied by a huge enhancement of the electric field at both entrance and exit interfaces of the hole.

Edge and waveguide terahertz surface plasmon modes in graphene microribbons

The authors acknowledge support from the Spanish MECD under Contract No. MAT2009-06609-C02 and Consolider Project “Nanolight.es.” A.Y.N. acknowledges the Juan de la Cierva Grant No. JCI-2008-3123

Surface plasmon enhanced absorption and suppressed transmission in periodic arrays of graphene ribbons

Resonance diffraction in the periodic array of graphene microribbons is theoretically studied following a recent experiment [L. Ju et al., Nature Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters are presented. It is shown that a much richer resonant picture would be observable for higher relaxation times of charge carriers: More resonances appear and transmission can be totally suppressed. The comparison with the absorption cross-section of a single ribbon shows that the resonant features of the periodic array are associated with leaky plasmonic modes. The longest-wavelength resonance provides the highest visibility of the transmission dip and has the strongest spectral shift and broadening with respect to the single-ribbon resonance, due to collective effects.

Entanglement of Two Qubits Mediated by One-Dimensional Plasmonic Waveguides

We investigate qubit-qubit entanglement mediated by plasmons supported by one-dimensional waveguides. We explore both the situation of spontaneous formation of entanglement from an unentangled state and the emergence of driven steady-state entanglement under continuous pumping. In both cases, we show that large values for the concurrence are attainable for qubit-qubit distances larger than the operating wavelength by using plasmonic waveguides that are currently available.

Focusing light with a single subwavelength aperture flanked by surface corrugations

In this letter, we show theoretically how a single subwavelength aperture surrounded by a finite array of grooves made on a metallic film can act as a “lens” for electromagnetic radiation within a certain frequency range. The dependence of this resonant focusing ability with the geometrical parameters defining the structure is extensively analyzed. Universal curves for the depth, length and width of the focus as a function of the number of grooves are also given.

Enhanced transmission and beaming of light via photonic crystal surface modes

Surface modes are generally believed to be an undesirable feature of finite photonic crystals (PC), unlike point or line defect modes. However, it is possible to make the surface mode radiate by appropriate corrugation of the PC interface. In this paper we show theoretically that the coherent action of these surface indentations can be engineered to collimate within a few degrees the light exiting a PC waveguide, or to funnel light coming from free space into the waveguide.

Transformation optics for plasmonics

A new strategy to control the flow of surface plasmon polaritons at metallic surfaces is presented. It is based on the application of the concept of transformation optics to devise the optical parameters of the dielectric medium placed on top of the metal surface. We describe the general methodology for the design of transformation optical devices for surface plasmons and analyze, for proof-of-principle purposes, three representative examples with different functionalities: a beam shifter, a cylindrical cloak, and a ground-plane cloak.