Optics: measuring and testing – Of light reflection
Reexamination Certificate
2002-04-02
2004-02-24
Evans, F. L. (Department: 2877)
Optics: measuring and testing
Of light reflection
C356S128000, C356S136000
Reexamination Certificate
active
06697158
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a measuring apparatus utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance measuring apparatus for quantitatively analyzing a substance in a sample by utilizing excitation of surface plasmon, and more particularly to a measuring apparatus, utilizing ATR, of a type that detects a dark line occurring in a measuring light beam by ATR with the use of photodetection means consisting of light-receiving elements juxtaposed in a predetermined direction.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, a compression wave called a plasma wave will be generated. The compression wave, generated in the metal surface and quantized, is called surface plasmon.
There have hitherto been proposed various kinds of surface plasmon resonance measuring apparatuses for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that surface plasmon is excited by a light wave. Among such apparatuses, one employing a system called “Kretschmann configuration” is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance measuring apparatus employing the “Kretschmann configuration” is equipped with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on one surface of the dielectric block, for placing a sample thereon; and a light source for emitting a light beam. The measuring apparatus is further equipped with an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the metal film and that ATR due to surface plasmon resonance can occur; and photodetection means for measuring the intensity of the light beam totally reflected at the interface, and thereby detecting surface plasmon resonance.
To obtain various angles of incidence in the aforementioned manner, a relatively thin light beam can be deflected so that it strikes the above-mentioned interface at different angles of incidence, or a relatively thick beam can be emitted convergently or divergently so that the components thereof strike the interface at various angles of incidence. In the former, the light beam whose reflection angle varies with the deflection thereof can be detected by a small photodetector that is moved in synchronization with the light beam deflection, or by an area sensor extending along a direction where the reflection angle varies. In the latter, on the other hand, the light beams reflected at various angles can be detected by an area measuring apparatus extending in a direction where all the reflected light beams are received.
In the surface plasmon resonance measuring apparatus mentioned above, an evanescent wave having electric field distribution is generated in a sample in contact with the metal film, if a light beam strikes the metal film at a specific incidence angle &thgr;
sp
greater than a critical incidence angle at which total internal reflection (TIR) takes place. The generated evanescent wave excites surface plasmon at the interface between the metal film and the sample. When the wave vector of the evanescent wave is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent wave resonates with the surface plasmon and the light energy is transferred to the surface plasmon, whereby the intensity of the light satisfying TIR at the interface between the dielectric block and the metal film drops sharply. This sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when an incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary that a light beam be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from the specific incidence angle &thgr;
sp
at which ATR takes place, the dielectric constant of a sample to be analyzed can be calculated by the following Equation:
K
sp
(&ohgr;)=(&ohgr;/
c
){∈
m
(&ohgr;) ∈
s
}
1/2
/{∈
m
(&ohgr;)+∈
s
}
1/2
where K
sp
represents the wave number of the surface plasmon, &ohgr; represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and ∈
m
and ∈
s
represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant ∈
s
of a sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, the dielectric constant of the sample, i.e., the properties of the sample related to the refractive index thereof can be quantitatively analyzed by finding the specific incidence angle &thgr;
sp
at which the intensity of the reflected light at the interface drops sharply.
In this kind of surface plasmon resonance sensor, photodetection means in the form of an array can be employed with the object of measuring the aforementioned incidence angle &thgr;
sp
with a high degree of accuracy and in a large dynamic range, as disclosed in Japanese Unexamined Patent Publication No. 11(1999)-326194. The photodetection means is constructed of a plurality of light-receiving elements juxtaposed in a predetermined direction. The light-receiving elements are disposed to respectively receive the components of a light beam reflected at the aforementioned interface at various angles of reflection.
In that case, differentiation means is provided to differentiate the photodetection signals output by the light-receiving elements of the aforementioned photodetection means, in the direction where the light-receiving elements are juxtaposed. The properties of the sample related to the refractive index thereof are often analyzed based on differentiated values output by the differentiation means, particularly the differentiated value corresponding to a dark line that occurs in a reflected light beam. The differentiation means can employ, for example, means for detecting the difference between the outputs of two adjacent light-receiving elements.
In the case of detecting the properties of a sample corresponding to the aforementioned incidence angle &thgr;
sp
by obtaining the difference between the outputs of two adjacent light-receiving elements, it is possible to employ a two-piece photodiode instead of the array-shaped photodetection means, if a large dynamic range is not required. In that case, the difference between the outputs of two photodiodes corresponds to the position of the aforementioned dark line in the direction where the photodiodes are arranged. Based on the difference, the properties of a sample corresponding to the aforementioned incidence angle &thgr;
sp
can be detected.
On the other hand, as a similar resonance measuring apparatus making use of ATR, there is known a leaky mode sensor (e.g., see “Spectral Researches,” Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 to 27). This leaky mode measuring apparatus is equipped with a dielectric block formed, for example, into the shape of a prism; a cladding layer formed on one surface of the dielectric block; and an optical waveguide layer, formed on the cladding layer, for placing a sample thereon. The leaky mode sensor is further equipped with a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the cladding layer and so that ATR occurs by a waveguide mode excited in the optical waveguide layer; and photodetection means for measuring the intensity of the light beam totally reflected at the interface between the dielectric block and the
Hayashi Katsumi
Kimura Toshihito
Mori Nobufumi
Ogura Nobuhiko
Evans F. L.
Fuji Photo Film Co. , Ltd.
Geisel Kara
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