Optics: measuring and testing – Of light reflection
Reexamination Certificate
2001-12-26
2004-05-25
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06741352
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 the generation of surface plasmon, and more particularly to a sensor, utilizing ATR, which detects a dark line generated in a measuring light beam due to ATR by the use of photodetection means.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, compression waves called plasma waves are generated. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon resonance sensors for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that surface plasmon is excited 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 that ATR can occur at the interface by surface plasmon resonance; and photodetection means for detecting the state of the surface plasmon resonance, that is, 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 include 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 detectedby a small photodetector that is moved in synchronization with the incidence angle change, 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 rays can be received.
In the surface plasmon resonance sensor mentioned above, if a light beam strikes the metal film at a specific incidence angle &thgr;
sp
equal to or greater than an angle at which total internal reflection occurs, evanescent waves having electric field distribution are generated in the sample in contact with the metal film, whereby surface plasmon is excited in 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 an incident light beam is p-polarized. Accordingly, a light beam must be p-polarized before it strikes the interface.
If the wave number of the surface plasmon is found from an incidence angle &thgr;
sp
at which ATR takes place, the dielectric constant of the sample can be obtained by the following Equation:
K
sp
(&ohgr;)=(&ohgr;/
c
){&egr;
m
(&ohgr;)&egr;
s
}
½
{&egr;
m
(&ohgr;)+&egr;
s
}
½
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 known, then the density of the specific substance within the sample can be derived based on a predetermined calibration curve or the like. As a result, the incident angle &thgr;
sp
at which the aforementioned reflected light intensity drops can be known, thereby the properties relating to the dielectric constant, that is, the refractive index of the sample can be derived.
As a similar sensor making use of ATR, a leaky mode sensor is 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 that ATR can occur at the interface by the excitation of an optical waveguide mode in the optical waveguide layer; and photodetection means for detecting the excited state of the waveguide mode, that is, ATR by measuring the intensity of the light beam satisfying total internal reflection at the interface.
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 at which total internal reflection occurs, 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 can be analyzed by finding the above-mentioned specific incidence angle at which ATR occurs.
In the conventional surface plasmon resonance sensor and leaky mode sensor mentioned above, a laser is generally employed as the light source. Particularly, if a single mode laser is employed, the curve for ATR changes sharply and therefore a measurement can be made with high sensitivity. However, the emission wavelength of the laser is susceptible to influences from the outside and easily fluctuates. Because of this, there is a problem that it will become difficult to make a measurement with a high degree of accuracy. That is, if the emission wavelength of the laser fluctuates, it will have detrimental effects on the condition for generating surface plasmon (or the condition for exciting a waveguide mode) and cause noise to occur in a detection signal (which represents the intensity of light satisfying total internal reflection at the interface between the dielectric block and the thin film layer), resulting in a reduction in the accuracy of measurement.
Hence, to avoid the aforementioned problem, there has been proposed an apparatus employing a light-emitting diode (LED) as a light source. The LED has a great spectral line width and is not affected by wavelength fluctuation. Howeve
Fuji Photo Film Co. , Ltd.
Stafira Michael P.
Sughrue & Mion, PLLC
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