Optics: measuring and testing – Of light reflection
Reexamination Certificate
2001-12-27
2003-11-25
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06654123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sensor utilizing attenuated total reflection (hereinafter referred to as ATR), such as a surface plasmon resonance sensor for quantitatively analyzing a substance in a sample by utilizing excitation of a surface plasmon, and more particularly to a sensor, utilizing ATR, of a type that detects a dark line occurring in a reflected light beam due to ATR by the use of photodetection means consisting of a plurality 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, compression waves called plasma waves will be generated. The compression waves generated in a metal surface are quantized and called a surface plasmon.
A variety of surface plasmon resonance sensors have been proposed for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that a surface plasmon is exited by light waves. Among such sensors, 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 sensor employing the “Kretschmann configuration” is equipped mainly with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on a surface of the dielectric block, for placing a sample thereon; 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 the condition for total internal reflection is satisfied at the interface between the dielectric block and the metal film; and photodetection means for detecting the state of the surface plasmon resonance, that is, the state of ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface.
In order to obtain various angles of incidence, as described above, a relatively thin light beam may be caused to strike the above-mentioned interface at different angles of incidence, or relatively thick convergent or divergent rays may be caused to strike the interface so that they contain components incident at various angles. In the former, the light beam whose reflection angle varies with a change in the incidence angle of the incident light beam can be detected by a small photodetector that is moved in synchronization with the variation in the reflection angle, or by an area sensor extending in the direction in which the angle of reflection varies. In the latter, on the other hand, rays reflected at various angles can be detected by an area sensor extending in the direction in which all of the reflected rays can be received.
In the surface plasmon resonance sensor mentioned above, 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 &egr;
m
and &egr;
s
represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant &egr;
s
of the sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, by finding the incidence angle &thgr;
sp
at which the intensity of reflected light drops, the dielectric constant of the sample, that is, the properties of the sample related to the refractive index thereof can be specified.
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 formed by 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 satisfying total internal reflection at various angles of reflection at the aforementioned interface.
In that case, differentiation means is provided for differentiating the photodetection signals output by the light-receiving elements of the aforementioned photodetection if a light beam strikes the metal film at a specific incidence angle &thgr;
sp
equal to or greater than a critical angle of incidence at which total internal reflection takes place, evanescent waves having electric field distribution are generated in the sample in contact with the metal film, whereby a surface plasmon is excited at the interface between the metal film and the sample. When the wave vector of the evanescent light is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light satisfying total internal reflection at the interface between the dielectric block and the metal film drops sharply. The 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 the 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 a specific incidence angle &thgr;
sp
at which ATR takes place, the dielectric constant of a sample can be obtained by the following Equation:
K
sp
(&ohgr;)=(&ohgr;/
c
){&egr;
m
(&ohgr;)&egr;
s
}
1/2
/{&egr;
m
(&ohgr;)+&egr;
s
}
1/2
means, in the direction in which 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.
In addition, a leaky mode sensor is known as a similar sensor making use of ATR, as disclosed, for instance, in “Spectral Researches,” Vol. 47, No.1 (1998), pp. 21 to 23 and pp. 26 and 27. The leaky mode sensor is constructed mainly of a dielectric block in the form of a prism, for example; a cladding layer formed on a surface of the dielectric block; an optical waveguide layer, formed on the cladding layer, for placing a sample thereon; 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 the condition for total internal reflection is satisfied at the interface between the dielectric block and the cladding layer; and photodetection means for detecting the excited state of the waveguide mode, that is, the state of ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface between the dielectric block and the cladding layer.
In the leaky mode sensor with the construction mentioned above, if a light beam falls on the cladding layer through the dielectric block at angles of incidence equal to or greater than an angle of incidence at which total internal reflection takes place, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific angle, is propagated in the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light satisfying total internal reflection at the above-mentioned interface drops sharply. Since the wave number of light propagating in the optical waveguide layer depends on the refractive index of the sample on the optical waveguide layer, the refractive index of the sample and/or the properties of the sample related to the refractive index thereof c
Fuji Photo Film Co. , Ltd.
Stafira Michael P.
Sughrue & Mion, PLLC
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