A smooth metal film having a thickness which is larger than its optical skin depth (the depth that the electromagnetic fields from incident light penetrate into the material where the electric field intensity drops to 1/e.sup.2, typically about 20 nm to 30 nm for a metal) is opaque to light at frequencies below the bulk plasma frequency .omega..sub.p, which is given by .omega..sub.p.sup.2 =(4.pi.ne.sup.2)/m*, where n is the electron density, e is the electron charge, and m is the effective mass. A single aperture in such a metal film transmits light with an efficiency which depends on the diameter of the aperture. If the aperture diameter is smaller than the wavelength of light passing through the aperture, the transmission is proportional to (d/.lambda.).sup.4. See H. A. Bethe, "Theory of Diffraction by Small Holes", Physical Review, Second Series, Vol. 66, Nos. 7 and 8, pp. 163-182 (1944). For this reason, the optical throughput of near-field optical devices is extremely low.
Recently, a strong enhancement of optical transmission has been demonstrated using a metal film perforated with an array of subwavelength-diameter apertures. See T. W. Ebbesen et al., "Extraordinary optical transmission through sub-wavelength hole arrays," Nature, Vol. 391, pp. 667-669 (Feb. 12, 1998); see also U.S. patent application Ser. No. 08/979,432 now U.S. Pat. No. 5,973,316, to T. W. Ebbesen et al., filed Nov. 26, 1997, and U.S. Provisional Patent Application Ser. No. 60/051,904 to T. W. Ebbesen et al., filed Jul. 8, 1997, which are both incorporated herein by reference. This enhancement, which can be as large as a factor of 1,000, occurs when light incident on the metal film interacts resonantly with a surface plasmon mode. Surface plasmons (also referred to herein as simply "plasmons") are collective electronic excitations which exist at the interface of a metal with an adjacent dielectric medium. See H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Vol. 111 of Springer Tracts in Modern Physics (Springer-Verlag, Berlin, 1988); A. D. Boardman (ed.), Electromagnetic Surface Modes, Chs. 1, 17, pp. 1-77, 661-725 (John Wiley & Sons, 1982). The periodic structure of the aperture arrays allows the surface plasmons to couple with the incident light.
On the other hand, the periodic array of apertures also has properties similar to those of a diffraction grating (see Ebbesen et al., supra), including the presence of Wood's anomaly (see R. W. Wood, "On a Remarkable Case of Uneven Distribution of Light in a Diffraction Grating Spectrum," Philosophical Magazine, Vol. 4, pp. 396-402 (1902), and R. W. Wood, "Anomalous Diffraction Gratings," Physical Review, Vol. 48, pp. 928-936 (1935)), which causes deep, sharp minima in the zero-order transmission when a higher-order diffracted beam becomes tangent to the metal film. The combination of these two effects (the surface plasmon coupling and Wood's anomaly) gives rise to well-defined maxima and minima in the zero-order transmission spectra. These maxima and minima exist at wavelengths which are determined by the geometry, both of the aperture array and that of the incident light, and the refractive index of the adjacent dielectric media. See Ebbesen et al., supra, and H. F. Ghaemi et al. "Surface plasmons enhance optical transmission through subwavelength holes", Physical Review B, Vol. 58, No. 11, pp. 6779-6782 (Sep. 15, 1998). Optical transmission control apparatus utilizing these properties have been developed. See U.S. patent application Ser. No. 09/168,265, now U.S. Pat. No. 6,040,936, to T. J. Kim et al., filed Oct. 8, 1998, which is incorporated herein by reference.
In previous aperture-array apparatus, every feature in the array is an aperture that transmits light. However, for some light transmission applications (such as near-field optical microscopy), it would instead be desirable to have the high transmission exhibited by the aperture arrays occur in a single aperture or a small set of apertures. It would also be desirable to further increase the transmission of an array of apertures. To date, such enhanced transmission has not been achieved.
The present invention overcomes this problem by providing enhanced light transmission apparatus wherein the metal film is provided with a periodic surface topography so as to allow strong coupling with the incident light. The metal film may be perforated with a relatively small array of apertures or even a single aperture depending on the application. In addition, the invention includes a novel near field scanning microscope, a novel mask technology for sub-wavelength photolithography, a wavelength-selective filter and a light collecting device utilizing the generalized enhanced light transmission apparatus.