We present a fully three-dimensional theoretical study of the extraordinary transmission of light through subwavelength hole arrays in optically thick metal films. Good agreement is obtained with experimental data. An analytical minimal model is also developed, which conclusively shows that the enhancement of transmission is due to tunneling through surface plasmons formed on each metaldielectric interfaces. Different regimes of tunneling (resonant through a "surface plasmon molecule", or sequential through two isolated surface plasmons) are found depending on the geometrical parameters defining the system. PACS numbers: 78.66. Bz, 73.20.Mf, 42.79.Dj, 71.36.+c In the last few years, and mainly due to advances in nanotechnology, there has been a renewed interest in exploiting the dielectric response of metals to make photonic materials [1][2][3]. For instance, the photonic insulating properties of metals can be used to trap incident radiation, focusing light in very small volumes [4][5][6]. Very recently [7], another interesting effect of light interacting with structured metals has been discovered: the transmission of light through subwavelength hole arrays made in a metal film can be orders of magnitude larger than expected from standard aperture theory [9]. Apart from its fundamental interest, this extraordinary transmission effect has potential applications [10,11] in subwavelength photolithography, near-field microscopy, wavelength-tunable filters, optical modulators, and flatpanel displays, amongst others. While the wavelength at which some transmission features appeared suggested [7,8] that surface plasmons (SP) [12] were involved in the process, the physical mechanism for the huge enhancement has not yet been elucidated. Some calculations have been performed for a simpler geometry: an array of slits [13][14][15], where high transmission was also predicted. However, although interesting in their own right, these results do not apply to the experimental situation.In this letter, we present the first fully threedimensional theoretical study of transmission through hole arrays, obtaining an excellent agreement with experimental data. Moreover, we develop a simplified version of the model that clearly captures the physics involved. Fig. 1 shows the experimental "zero-order" transmittance of light at normal incidence (T 00 ), through an array of holes in a free-standing metal film. The free-standing metal film, of which the fabrication is described elsewhere [8] consisted of a 220nm thick Ni core, perforated with a square array of holes by focused-ion beam milling. The film was subsequently overcoated with 50nm of Ag on both sides by sputter deposition which resulted in a coating of the walls of the holes as well as the in-plane surfaces of the film. The total thickness of the film was h = 320nm and the lattice constant of the hole array was L = 750nm. After coating the holes had an average diameter of 280nm. It has been shown [8] that such a "sandwich" structure has the same transmission properties as an eq...

The optical transmission through a subwavelength aperture in a metal film is strongly enhanced when the incident light is resonant with surface plasmons at the corrugated metal surface surrounding the aperture. Conversely, the aperture acts as a novel probe of the surface plasmons, yielding useful insights for optimizing the transmission enhancement. For the optimal corrugation geometry, a set of concentric circular grooves, three times more light is transmitted through the central subwavelength aperture than directly impinges upon it. This effect is useful in the fabrication of near-field optical devices with extremely high optical throughput.

In order to understand the details of the extraordinary enhancement of the optical transmission through subwavelength holes in metal films, we have fabricated perforated free-standing metal films where the nature of the bulk and surfaces can be controlled independently. We find that the transmission enhancement depends only on the dielectric properties of the metal within a skin depth of the two in-plane surfaces and not on those of the film core, nor on the metal constituting the hole walls. This provides direct and conclusive evidence that the phenomenon is mediated by surface plasmons.

The transmission of light through an aperture in a metal film is extremely small when the aperture diameter is much smaller than the optical wavelength. But when the metal surface surrounding the subwavelength hole is corrugated, the incident light can couple to surface plasmons (SP), excitation modes on the metal surface. A resonant interaction leads to an enhanced transmission at wavelengths determined by the corrugation pitch. We discuss applications of the SP enhanced transmission in near-field scanning optical microscopy and in high-density optical data storage.

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