Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
2002-08-06
2004-09-14
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S572000, C369S112040, C369S044240
Reexamination Certificate
active
06791755
ABSTRACT:
This application is based on Japanese Patent Application No. 2001-241124 filed on Aug. 8, 2001, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device for making light converge, and particularly to an optical device for producing a convergent light beam with a great numerical aperture for use in, for example, an optical system of a microscope or optical recording apparatus.
2. Description of the Prior Art
In an optical microscope that permits the observation of a test sample by the use of light, or an optical recording apparatus that permits the recording, reproducing, and erasing of information by the use of light, to achieve a high resolution or high recording density, it is essential to make light converge in a minute area on a test sample or recording medium. The size of the spot formed by a convergent light beam is inversely proportional to the numerical aperture (NA) of the light beam, and therefore, the greater the NA of a light beam is made, the smaller its spot diameter can be made.
The NA of a convergent light beam is given by equation (1) below, where n represents the refractive index through which the convergent light beam passes and &thgr;max represents the maximum angle that the convergent light beam makes with its own optical axis (i.e., the angle between the outermost ray and the optical axis of the convergent light beam).
NA=n
·sin(&thgr;max) (1)
Accordingly, effective ways to reduce the diameter of the spot formed by a convergent light beam is to increase the maximum angle &thgr;max and to increase the refractive index n of the medium in addition. Immersion techniques used in microscopes depend on the latter, and achieve a greater NA by filling the space between an objective lens and a test sample with a liquid having a high refractive index. An immersion technique using oil as a high-refractive-index liquid is called oil immersion, one using water is called water immersion, and one using a solid instead of a liquid is called solid immersion.
Optical devices exploiting solid immersion are best exemplified by solid immersion lenses (SILs) and solid immersion mirrors (SIMs). A SIL is generally used in combination with another objective lens.
FIG. 15
shows such a structure. The SIL
51
is hemispherical, and a convergent light beam emanating from an objective lens
52
enters the SIL
51
through a spherical surface
51
a
thereof and exits the SIL
51
through a flat surface
51
b
thereof. The SIL
51
and the objective lens
52
are so arranged that all the rays of the convergent light beam from the objective lens
52
are incident substantially perpendicularly on the spherical surface
51
a
. Thus, the light enters the SIL
51
without being refracted by the spherical surface
51
a
, and converges on the flat surface
51
b
. In this way, the SIL
51
, although a lens, is used in such a way as not to exert any power that makes light converge.
A SIM is produced by forming a convex reflective surface on a surface of a base material, and on this reflective surface, which is a concave surface when seen from inside, light is shone from inside so as to be made to converge by reflection. Thus, the SIM, although a single device, has the functions of both the SIL
51
and the objective lens
52
described above. In addition, the SIM does not produce aberrations as are inevitable when light is made to converge by refraction, and thus more readily permits light to converge at one point on the optical axis. However, simply forming a convex reflective surface on a surface of a base material results in a reflective device, i.e., a device on which light is shone from the same direction in which the light is reflected by the device. This makes effective use of solid immersion difficult, and imposes severe constraints on the use of such a device.
A SIL that is used in such a way as to make light converge has also been proposed (Japanese Patent Application Laid-Open No. H11-45455).
FIG. 16
shows this structure. Light enters the SIL
53
through an aspherical surface
53
a
thereof and exits the SIL
53
through a flat surface
53
b
thereof. The thickness of the SIL
53
(its dimension in the direction perpendicular to the flat surface
53
b
) is made equal to its focal length as a lens. Thus, the SIL
53
refracts a parallel light beam with the entrance surface
53
a
and thereby converges the light beam onto the flat surface
53
b.
This SIL
53
configured as described above is easy to use, because it does not need to be combined with another objective lens, nor does it require alignment of optical axes or adjustment of a distance. However, with this SIL
53
, it is impossible to obtain a convergent light beam with a NA greater than a certain limit. For example, to form a parallel light beam into a convergent light beam with a NA of 1, if the base material is assumed to have a refractive index of 1.8, the angle of incidence of the outermost ray with respect to the entrance surface
53
a
needs to be 63.4°. This makes the fabrication of the SIL
53
very difficult.
With the advantages and disadvantages of both SILs and SIMs in mind, the inventors of the present invention have proposed a solid immersion device that makes light converge by both refraction and reflection (Japanese Patent Application Laid-Open No. 2000-162503).
FIG. 17
shows this structure. This optical device
54
, like a SIM, has a convex surface formed on a surface of a base material, but a reflective surface is formed only in a central portion of this convex surface
54
a
, with a peripheral portion thereof left as a transmissive surface. The surface of the base material opposite to the convex surface
54
a
is formed into a flat surface
54
b
. Light enters the optical device
54
through the peripheral portion of the convex surface
54
a
, and is thereby formed into a convergent light beam by refraction. The light is then reflected on the flat surface
54
b
so as to be directed to the central portion of the convex surface
54
a
, where the light is reflected again and is thereby formed into a more convergent light beam. Thus, the light is eventually made to converge on the flat surface
54
b
so as to exit the optical device
54
through the flat surface
54
b.
This optical device
54
, although a transmissive device like a SIL, i.e., a device on which light is shone from the direction opposite to the direction in which the light exits the device, produces a convergent light beam with a great NA without an undue increase in the angles of incidence of rays with respect to the convex surface
54
a
. Moreover, this optical device
54
can be made thinner than a comparable SIL.
An optical device that makes light converge by diffraction has also been proposed (Japanese Patent Application Laid-Open No. H10-92002). This optical device has a diffraction grating formed on one surface of a base material shaped like a flat plate. Light enters the optical device through this diffraction grating, and is thereby formed into a convergent light beam, which exits the optical device through the opposite surface thereof.
An optical device that makes light converge by diffraction can be formed as a solid immersion device by giving it an appropriate thickness.
However, even when an optical device that makes light converge by diffraction is formed as a solid immersion device, it is still impossible to obtain a convergent light beam with a NA greater than a certain limit. To obtain a large angle of diffraction, the pitch of the diffraction grating needs to be reduced. However, reducing the pitch of the diffraction grating too much causes a phenomenon called anomaly, which extremely lowers diffraction efficiency.
FIG. 18
shows an example of the relationship between the ratio of the pitch d of a diffraction grating to the wavelength &lgr; of light and diffraction efficiency. Diffraction efficiency drops abruptly starting from a d/&lgr; of about 1.7 down, which phenomenon is referred to as anom
Hatano Hiroshi
Takada Kyu
Jr. John Juba
McDermott Will & Emery LLP
Minolta Co. , Ltd.
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