Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-12-09
2001-05-22
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S234000
Reexamination Certificate
active
06236033
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of optics, and in particular to the enhanced transmission of light through a thin metal film having a periodic surface topography and perforated with one or more apertures.
BACKGROUND OF THE INVENTION
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
2
, typically about 20 nm to 30 nm for a metal) is opaque to light at frequencies below the bulk plasma frequency &ohgr;
p
, which is given by &ohgr;
p
2
=(4&pgr;ne
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/&lgr;)
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.
SUMMARY OF THE INVENTION
Generally speaking, in accordance with the invention, an apparatus for enhanced light transmission is provided. The apparatus comprises a metal film having a first surface and a second surface, at least one aperture provided in the metal film and extending from the first surface to the second surface, and a periodic surface topography provided on at least one of the first and second surface of the metal film. Light incident on one of the surfaces of the metal film interacts with a surface plasmon mode on at least one of the surfaces of the metal film thereby enhancing transmission of light through the at least one aperture in the metal film. The apparatus may have a single aperture or a plurality of apertures arranged on a periodic lattice.
As noted above, the enhanced light transmission apparatus of the present invention includes a metal film with at least one surface having a periodic surface topography. The periodic surface topography includes surface features, such as periodically arranged protrusions or depressions in the metal film surface. The periodic surface topography allows strong coupling of the metal surface plasmons with incident light. The aperture(s) in the metal film thereby provide enhanced transmission for wavelengths in relation to the period and lattice symmetry of the periodic surface topography. The enhanced transmission is much greater than the transmission through apertures of the same size and number in the absence of the periodic surface topography, and this is due to the active participation of the surface plasmons of the metal film.
If subwavelength-diameter apertures are viewed, as has previously been the case, as merely geometrical openings, then the transmission efficiency (defined as the transmitted intensity divided by the intensity of the light impinging on the aperture) can be very small (e.g. about 2×10
−3
when the ratio of the aperture diameter to the wavelength is about 0.1). However, in the present invention, as will be described below, the transmission efficiency is greatly increased and the resulting apparatus may be considered as a high efficiency subwavelength light source with the additional capability of tailoring the transmission properties of the light by adjusting the periodicity and the symmetry of the surface features comprising the periodic surface topography.
The practical effect of this result is that the general enhanced light transmission apparatus of the present invention may be applied as a wavelength-selective optical filter (particularly in the ultra-violet, visible and near-infrared wavelengths), a spatial optical filter, a light collector, a near field optical microscopy probe, and a photolithographic mask, all of which are set forth herein and are included in the invention.
Accordingly, an object of the invention is to provide an apparatus which provides enhanced light transmission through one or more apertures utilizing a periodic surface topography, such that the transmission is enhanced in comparison to the t
Ebbesen Thomas W.
Grupp Daniel E.
Lezec Henri J.
Thio Tineke
Allen Stephone B.
Isztwan Andrew G.
NEC Research Institute Inc.
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