Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-05-21
2003-11-18
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S306000
Reexamination Certificate
active
06649894
ABSTRACT:
This application claims priority to Japanese Patent Application No. 2001-308153 filed on Oct. 4, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical near field generator for generating a near field light and methods for generating near field light.
2. Description of the Background
In a conventional optical microscope, light is condensed using a lens. Therefore, resolution is limited by the wavelength of the light. Alternatively, in a near-field optical microscope, light is condensed using a microstructure having a size on the order of nanometers, e.g., an aperture having a diameter of not larger than the wavelength of light, instead of lens. When light is applied to such a microstructure, a localized light called “near field light” is generated near the microstructure. By approximating this near field light to a sample and allowing it to scan an upper surface of the sample, it is possible to measure the shape and optical characteristics of the sample with a resolution which is determined by the size of the microstructure. Recently, microscopes of this type have been applied to various fields, including measuring the shape and spectroanalysis of biosamples, semiconductor quantum structures, and polymers, as well as in high-density optical recording. “Near field light” as referred to herein means a localized light, i.e., light that has a wave number (k) with an imaginary component.
A widely used optical near field generator (hereinafter referred to as “near field optical probe” is a tapered optical fiber (optical fiber probe) having an aperture with a diameter that is not larger than the wavelength of light. The optical fiber probe is fabricated by stretching one end of an optical fiber under heating or tapering it by chemical etching and subsequently coating the optical fiber with metal except at the tip portion of the optical fiber. By introducing light into the optical fiber it is possible to generate a near field light in the vicinity of the aperture formed in the tip of the optical fiber.
However, the above fiber probe is disadvantageous in that the light utilization efficiency is low. For example, when the aperture diameter is 80 nm, the ratio between the intensity of light incident on the optical fiber and that of light output from the fiber tip is 10
5
or less (See, Applied Physics Letters, Vol. 68, No. 19, pp. 2612-2614, 1996).
In view of this, there has been proposed a probe using a plane metal scatterer. This probe, shown in
FIG. 34
, includes a plane metal scatterer
341
having a triangular shape formed on a plane substrate. In FIG.
34
(
a
) a probe is shown having one metal scatterer
341
, and in FIG.
34
(
b
) a probe is shown having two metal scatterers
341
. Upon incidence of X-polarized light, a near field light is localized at a vertex
342
. Particularly, by making the wavelength of incident light match the plasmon resonance it is possible to generate a very powerful near field light (See, Technical Digest of 6
th
international conference on near field optics and related techniques, the Netherlands, Aug. 27-31, 2000, p55). In FIG.
34
(
a
), a near field light is generated from the vertex
342
of the metal scatterer
341
, and in FIG.
34
(
b
), the two metal scatterers
341
are disposed such that a vertex-to-vertex spacing is several tens of nanometers or less, with a localized near field light being generated in the vertex-to-vertex spacing
343
(the gap).
The above probes using triangular plane scatterers may attain a higher near field light generation efficiency compared to other methods, especially if the frequency of light and the resonance frequency of plasmon generated in the metal are made coincident with each other. In this conventional example, however, the size and shape of the scatterer are not optimized.
SUMMARY OF THE INVENTION
The present invention preferably provides improved shape and size characteristics for a plane scatterer for the efficient generation of plasmon in a probe using the scatterer. A near field optical probe of high resolution and high efficiency may therefore be achieved.
In at least one presently preferred embodiment, a near field light is generated by an electrically conductive scatterer which is tapered toward a near field light generating vertex, and the area of the scatterer is adjusted so as to be smaller than the area of a light spot radiated to the scatterer (herein, the “light spot” is a cross-section of the light taken in the plane that is parallel to and at the surface of the scatterer) or smaller than the square of the wavelength of light radiated to the scatterer. Additionally, the distance between the near field light generating vertex of the scatterer and a point which is most remote (i.e., furthest) from said vertex is smaller than the diameter of a light spot radiated to the scatterer or smaller than the wavelength of the radiated light. With this constraint, the phase of the light radiated into the scatterer becomes uniform at various points, and it is thus possible to efficiently excite plasmon resonance.
It is preferable that the area of the light spot radiated to the scatterer be set at a value of not more than one hundred times the area of the scatterer (or, if expressed in terms of length, the light spot diameter on the scatterer be not more than ten times the distance between the near field light generating vertex of the scatterer and a point furthest from the vertex). As a result, it is possible to decrease the amount of light passing without impinging on the scatterer and to improve the light utilization efficiency.
To maximize the energy of light introduced into the scatterer, it is preferable that a central position of light incident on the scatterer be aligned with a central position of the scatterer. Alternatively, in order to maximize a single portion of the near field light, the central position of incident light may be made substantially coincident with the position of the near field light generating vertex. The “substantial coincidence” means that the distance between the central position of the incident light and the vertex is no more than half of the full width at half maximum of the incident light spot. The “vertex” as referred to herein indicates not only an actually intersecting point of a first line (side) and a second line (side) but also a point having a predetermined curvature.
In application, the surface of the scatterer and that of a recording medium are preferably set so as to be substantially in parallel with each other. This “substantially parallel” limitation means that the angle between the surface of the scatterer and that of the recording medium be within 5°. By satisfying this condition, it becomes possible for light to be incident in a direction perpendicular to the surface of a sample or recording medium and hence possible to use an optical system which is employed in a conventional microscope or recording/reproducing apparatus.
The aforesaid angle between the surface of the scatterer and that of a sample or recording medium may also be set at a value in the range of 0° to 90°. In this case, the scatterer is disposed in such a manner that the distance from the near field light generating vertex to the sample or the medium is shorter than the distance from another portion of the scatterer to the sample or the medium. With this arrangement, it is possible to diminish the influence of a near field light generated at an edge portion which lies on the side opposite to the near field light generating vertex.
The above-described scatterer may be formed as a film having, for example, three or more vertices. In this case, for preventing a near field light from being generated at a vertex other than the near field light generating vertex, the radius of curvature of this other vertex is set larger than that of the near field light generating vertex. The scatterer may be in a combined shape of a tapered film and a circular film or in a shape having a curvilinear portion, such as a sectorial shape. By making the radius of
Kiguchi Masashi
Matsumoto Takuya
Sukeda Hirofumi
Hitachi , Ltd.
Le Que T.
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