Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-09-13
2004-09-14
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S216000
Reexamination Certificate
active
06791071
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for measuring an aperture size of a near-field optical probe, and more particularly, to an apparatus and method for measuring an accurate aperture diameter of a near-field optical probe using a filter. The present application is based on Korean Patent Application No. 2001-57575, which is incorporated herein by reference.
2. Description of the Related Art
Near-field optical probes are generally used in apparatuses for near-field optical microscopy such as high resolution apparatuses for measuring material surfaces or superhigh density recording media.
In an apparatus for measuring resolution of a material surface, resolution R is defined as the distance between two points that can be resolved and is given by equation 1, wherein the resolution R is proportional to a wavelength &lgr; and inversely proportional to an aperture diameter of a lens or an iris.
R
=
1.22
λ
D
(
1
)
In conventional far-field optical microscopy, resolution has to be small as the wavelength of light becomes shorter. However, there is a limit beyond which the resolution cannot be made small due to the diffraction of light. This diffraction limit is not encountered in near-field scanning optical microscopy (hereinafter referred to as “NSOM”). Thus, it is possible to manufacture a high resolution apparatus for measuring a material surface.
In NSOM, it is necessary to accurately know an aperture size of a near-field optical probe in order to measure high resolution of a sample which is positioned at the near-field optical probe of a size smaller than a wavelength, i.e., a sub-wavelength size, using a near-field optical microscope.
An aperture diameter of the optical probe in the NSOM has a subwavelength-size, i.e., a diameter d of about 50-300 nm in near-field optical microscopy using a visible ray with a wavelength &lgr; of 400-1000 nm. This subwavength resolution can be achieved when the sample is positioned at the near-field optical probe.
Conventional NSOM uses a scanning electron microscope (SEM) or a measurement only apparatus (disclosed in U.S. Pat. No. 5,663,798) in order to obtain the aperture diameter of the optical probe.
SEM reduces an electron beam generated from an electron gun to a diameter within a range of several to hundreds Å, using several electron lenses, radiates it onto a sample, detects secondary electrons emitted from the sample or electrons having passed through the sample, modulates brightness in time series on an oscilloscope, and measures the surface of the sample.
SEM can accurately measure an aperture of an optical probe, but it is expensive and takes a long time.
To measure the aperture of an optical probe using SEM, the tip of the optical probe, which is an electrical insulator, has to be coated with a conductive material. When the aperture of the coated optical probe is measured and then the coated optical probe is used in NSOM, the coating of the tip of the optical probe degrades the performance of NSOM. Thus, since it is difficult to reuse the optical probe measured with SEM, a new optical probe should be used in NSOM. However, the new optical probe may have a different diameter than the optical probe measured with SEM.
FIG. 1
is a schematic view of an apparatus for measuring an aperture size of an optical probe disclosed in U.S. Pat. No. 5,663,798. Referring to
FIG. 1
, the apparatus includes a light source
11
for radiating light, a polarizer
13
for polarizing the light, a focusing lens
15
for focusing the light onto an optical probe
10
, a linear analyzer
17
for collecting light transmitted through the optical probe
10
through an optical detector
19
, and the optical detector
19
for converting the light into an electrical signal and detecting the electrical signal.
In the apparatus, to deduce an aperture diameter, the light transmitted through the aperture of the optical probe
10
at a predetermined angle is received, a signal corresponding to light intensity is detected from the optical detector
19
, and the signal is transmitted to a data acquisition unit (DAU)
23
or a computer (PC)
25
.
As shown in
FIG. 1
, a motor
21
in which the linear analyzer
17
and the optical detector
19
are built rotates from −165° to +165° to measure the angular light intensity distribution emitted from the aperture in far-field. As a result, the aperture diameter of the optical probe can be measured.
FIG. 2
is a graph showing an angular distribution light intensity measured by the conventional aperture measuring apparatus. Referring to
FIG. 2
, if the wavelength of the light emitted from the light source
11
is 633 nm and a polarization angle is 90°, the far-field angular intensity distribution of light transmitted through the optical probe having an aperture diameter of 60 nm, 380 nm, or 3.2 &mgr;m (which is pre-measured with SEM) is gaussian with a maximum light intensity value at 0°.
As can be seen from
FIG. 2
, as the aperture diameter decreases, a full width at half maximum (FWHM) becomes increasingly wider. Here, the FWHM is the difference between two angles corresponding to half of the maximum light intensity value.
Referring to
FIG. 2
, if an aperture diameter d (=2a) of the optical probe is 60 nm, the FWHM is the difference between two angles +60° and −60° corresponding to the light intensity of 0.5, i.e., 120°. If the aperture diameter d (=2a) of the optical probe is 380 nm, the FWHM is the difference between +30° and −30°, i.e., 60°.
FIG. 3
is a graph of FWHM according to aperture diameters of the optical probe with reference to FIG.
2
. In
FIG. 3
, the line (a) is predicted based on Kirchoff's theory, the line (b) is given by the small aperture limit theory of Bethe, and the line (c) is that given by the conventional measuring apparatus.
The aperture diameter of an optical probe can be obtained by obtaining the FWHM from the intensity distribution of the light emitted from the optical probe using FIG.
3
and then finding the corresponding diameter from FIG.
2
.
The apparatus shown in
FIG. 2
requires an additional unit which rotates around the optical probe to measure the light intensity transmitted through the optical probe. And, if the rotation of the apparatus is not precise, it is difficult to measure an accurate aperture diameter. Also, since it is difficult to position the tip of the optical probe accurately at the center of rotation, errors easily occur when measuring the aperture diameter.
Moreover, the light intensity should be measured and graphed at a plurality of angles with the rotation of the motor. Thus, it takes a long time to measure aperture diameters and it is difficult to measure aperture diameters smaller than &lgr;/6 due to measurement limit actions of the apparatus.
SUMMARY OF THE INVENTION
To solve the above-described problems, it is an object of the present invention to provide an apparatus for accurately measuring the aperture of an optical probe without damaging the optical probe which can easily be manufactured and configured, and a method thereof.
Accordingly, to achieve the above object, there is provided an apparatus for measuring an aperture of a near-field optical probe. The apparatus includes a light source, an optical detector, and a filter. The light source radiates light to the near-field optical probe. The optical detector is positioned before the near-field optical probe and receives the light transmitted through the near-field optical probe to detect light intensity. The filter is prepared between the light source and the optical detector and transmits only light of wavelengths in a specific mode from the light transmitted through the near-field optical probe.
Here, if a free space or a medium exists between the light source and the filter, the specific mode is a Bessel Gauss mode.
The free space, which has a refractive index of 1, is one of media having uniform refractive indexes.
To achieve the above object, there is provide
Lee Myung-bok
Nikolai Petrov
Woo Gi-myung
Le Que T.
Samsung Electronics Co,. Ltd.
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