F. J. García-Vidal

TRANSMISSION RESONANCES ON METALLIC GRATINGS WITH VERY NARROW SLITS

For decades, it has been thought that subwavelength apertures have a very low transmission efficiency of light [1]. However, very recently, several experiments have shown that, if holes are structured forming a 2D periodic array in a metallic film, extraordinary optical transmission can be obtained at wavelengths up to 10 times larger than the diameter of the holes [2]. It has already been proposed that this effect could be exploited in different important technological areas such as photolithography or near field microscopy, or even to extract light from light emitting diodes [3]. Although experiments suggest that the excitation of surface plasmon polaritons (SPPs) in the metallic interfaces of the film plays a crucial role in this effect, a detailed understanding of the physical mechanism behind the enhanced transmission has not been reported yet. In this Letter we propose a simpler alternative structure in which similar extraordinary optical transmission effects can also been found and hence used for practical purposes: transmission metallic gratings with very narrow slits. We will show how, for particular wavelengths, incident light can excite surface electromagnetic modes of the gratings that are able to reemit the absorbed light in the forward direction with almost 100% efficiency. Moreover, a detailed study of these transmission resonances will provide physical insight into the mechanism of the extraordinary transmission in 2D hole arrays. Reflection metallic gratings have been analyzed for many years, mainly in connection with the study of SPPs andor localized electromagnetic modes of the grooves [4 ‐ 9]. With regard to transmission gratings, there have been some theoretical works in the past few years [10,11]. However, transmission gratings with very narrow slits remain unstudied, except for a very recent calculation of the optical transmission properties of a silver grating having the same geometrical parameters of the 2D hole arrays of Ref. [2] and with a special geometry for the slits [12]. The scope of this Letter is, however, different: we do not pretend to fit the experiments on 2D hole arrays because this is a different geometry. Instead, we are interested in analyzing the coupling between the SPPs of the two metallic interfaces of the grating as a possible mechanism to enhance optical transmission through perforated metallic films. We also study, for the first time, the transmission properties of waveguide modes excited in very narrow slits that are periodically structured. On top of Fig. 1 we show a schematic view of the structures under study with the definition of the different parameters: the period of the grating d, the width a, and height h of the slits. The substrate is characterized by a dielectric constant, e. Advances in material technology have allowed the production of transmission gratings with well-controlled profiles [13]. In this Letter we consider metal gratings made of gold and we use a fixed value for the grating period d 3.5 mm. We will only show the results for a 0.5 mm, although the dependence of the transmission resonances on a is also addressed. The thickness of the metallic grating h will be varied between 0 and 4 mm. We believe this range of geometrical values can be reached using present day technology, as reflection gratings with similar parameters have already been prepared [9]. Nevertheless, it should be pointed out that the effects discussed in this Letter do appear for any other range provided a is very small in comparison to d and the frequency of the incident light is well below the plasma frequency of the metal. The dielectric function of gold is described using the tables reported in Ref. [14]. We have analyzed the electromagnetic properties of these gratings by means of a transfer matrix formalism [15]. Within this formalism it is possible to calculate transmission and reflection coefficients for an incoming plane wave. Subsequently, the transmittance and reflectance of the grating as well as real-space electromagnetic fields can be calculated. Figure 1 shows zero-order transmittance for p-polarized normal incident radiation on metallic gratings in vacuum as a function of the wavelength of the incoming

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

Conformal surface plasmons propagating on ultrathin and flexible films

Surface plasmon polaritons (SPPs) are localized surface electromagnetic waves that propagate along the interface between a metal and a dielectric. Owing to their inherent subwavelength confinement, SPPs have a strong potential to become building blocks of a type of photonic circuitry built up on 2D metal surfaces; however, SPPs are difficult to control on curved surfaces conformably and flexibly to produce advanced functional devices. Here we propose the concept of conformal surface plasmons (CSPs), surface plasmon waves that can propagate on ultrathin and flexible films to long distances in a wide broadband range from microwave to mid-infrared frequencies. We present the experimental realization of these CSPs in the microwave regime on paper-like dielectric films with a thickness 600-fold smaller than the operating wavelength. The flexible paper-like films can be bent, folded, and even twisted to mold the flow of CSPs.

Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime.

Carbon nanotubes1,2 are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge3. Defects are known to modify the electrical resistance of carbon nanotubes4; they can be present in as-grown carbon nanotubes, but controlling their density externally opens a path towards the tuning of the electronic characteristics of the nanotube. In this work, consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room-temperature resistance versus SWNT length (R(L)) along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation, and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a SWNT of 400 nm length.

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.