Optics: measuring and testing – Of light reflection
Reexamination Certificate
2001-12-26
2004-12-07
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06829052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sensor which utilizes attenuated total reflection (ATR), such as a surface plasmon sensor which enables quantitative analysis of a specific material contained in a specimen by utilizing generation of surface plasmons.
2. Description of the Related Art
In metal, free electrons move collectively to produce a compressional wave called a plasma wave. When a plasma wave generated at a surface of the metal is quantized, the plasma wave is regarded as surface plasmons.
The surface plasmons can be produced by exciting a surface of a metal by an optical wave. Conventionally, various surface plasmon sensors are proposed for performing a quantitative analysis of a material contained in a specimen by utilizing the excitation by an optical wave. In particular, surface plasmon sensors which use a system called Kretschmann's arrangement are well known (Refer to Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon sensors which use the above system basically include: a dielectric block having a form of a prism; a metal film formed on a face of the dielectric block and in contact with a specimen; a light source producing a light beam, an optical system injecting the light into the dielectric block at various incident angles which are greater than a critical angle for total reflection, and attenuated total reflection (ATR) due to a surface plasmon resonance occurs; and a light detection unit which can detect the state of the attenuated total reflection (i.e., the state of the surface plasmon resonance) by measuring the intensity of the light beam totally reflected from the above boundary.
The above various incident angles can be realized by deflecting a relatively thin light beam so that the deflected beam is incident on the boundary at desired incident angles. Alternatively, the various incident angles can be realized by letting a relatively thick light beam be incident on the boundary so that the thick light beam converges or diverges at the boundary, and therefore the converging or diverging beam contains components incident on the boundary at the various incident angles. When the relatively thin light beam is deflected, the light beam reflected at a reflection angle which varies with the deflection of the incident light beam can be detected by a small light detector which moves corresponding to the deflection of the incident light beam, or by an area sensor extending in the direction of the variation of the reflection angle. When the relatively thick light beam is incident on the boundary, the reflected light beam can be detected by an area sensor which extends in the direction of the variation of the reflection angle so that substantially all the reflected light beam can be detected.
When a light beam is incident on the metal film in the surface plasmon sensor having the above construction at a specific incident angle &thgr;
SP
which is greater than a critical angle for total reflection, an evanescent wave is generated, where an electric field of the evanescent wave is spread in the vicinity of the metal film in the specimen. By the evanescent wave, surface plasmons are generated at the boundary between the metal film and the specimen. When the wave number of the evanescent wave equals the wave number of the surface plasmons, i.e., these wave numbers match, the evanescent wave is resonant with the surface plasmons, and the energy of the evanescent wave is transferred to the surface plasmons. Therefore, the intensity of the light totally reflected by the boundary between the dielectric block and the metal film sharply decreases. The decrease in the intensity of the light is detected as a dark line by the light detection unit.
The above resonance occurs only when the incident light beam is a p-polarized light beam. Therefore, it is necessary to arrange the surface plasmon sensor so that the light beam is incident on the boundary as a p-polarized light beam.
When the wave number of the surface plasmon is obtained from the incident angle &thgr;
SP
at which the attenuated total reflection (ATR) occurs, the permittivity of the specimen can be obtained from the wave number of the surface plasmons. That is,
K
SP
(
ω
)
=
ω
c
ϵ
m
(
ω
)
ϵ
s
ϵ
m
(
ω
)
+
ϵ
s
,
where the wave number of the surface plasmon is denoted by K
SP
, the angular frequency of the surface plasmon is denoted by &ohgr;, the velocity of light in vacuum is denoted by c, and permittivities of the metal and the specimen are denoted by &egr;
m
and &egr;
s
, respectively.
When the permittivity &egr;
s
of the specimen is obtained, the concentration of the specific material in the specimen can be obtained based on a predetermined calibration curve or the like. Therefore, properties relating to the permittivity (i.e., the refractive index) of the specimen can be obtained by detecting the incident angle &thgr;
SP
at which the intensity of the reflected light decreases.
In addition, the leakage mode sensor is known as another sensor which is also utilizes the attenuated total reflection and similar to the surface plasmon sensor. For example, the leakage mode sensor disclosed in “Spectral Researches”, Vol. 47, No. 1 (1998) pp. 21-23 & 26-27 includes: a dielectric block having a form of a prism; a cladding layer formed on a face of the dielectric block; an optical waveguide layer formed on the cladding layer and in contact with a specimen; a light source producing a light beam, an optical system which injects the light beam into the dielectric block at various incident angles so that the light beam is totally reflected at the boundary between the dielectric block and the cladding layer, and attenuated total reflection (ATR) due to excitation of a propagation mode in the optical waveguide layer can occur; and a light detection unit which can detect the state of the attenuated total reflection, i.e., the state of the excitation of the propagation mode, by measuring the intensity of the light beam totally reflected from the above boundary.
When the laser beam is incident through the dielectric block on the cladding layer in the above leakage mode sensor at a incident angle which is greater than the critical angle for total reflection, only a portion of light being incident on the cladding layer at a specific incident angle and having a specific wave number can propagate in the propagation mode in the optical waveguide layer. Therefore, when the propagation mode is excited, almost all portions of the incident light can enter the optical waveguide layer, i.e., the attenuated total reflection, in which the intensity of light totally reflected from the boundary sharply decreases, occurs. At this time, the wave number of the propagated light depends on the refractive index of the specimen placed on the optical waveguide layer. Therefore, it is possible to obtain the refractive index of the specimen and analyze other properties of the specimen relating to the refractive index.
Incidentally, in the conventional sensors utilizing the attenuated total reflection such as the surface plasmon sensors and leakage mode sensors, semiconductor laser devices are used as the light sources. However, in the conventional sensors using the semiconductor laser devices as the light sources and utilizing the attenuated total reflection, sometimes, the output of the light detection unit, which detects the state of the attenuated total reflection, suddenly varies, and resultantly the precision of measurement deteriorates.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a sensor utilizing the attenuated total reflection, in which sudden variation in the output of a light detection unit is prevented so that high precision of measurement is achieved.
(1) According to the first aspect of the present invention, there is provided a sensor comprising: a dielectric block; a thin film formed on a face of the dielectric block and in contact
Fuji Photo Film Co. , Ltd.
Rosenberger Richard A.
Sughrue & Mion, PLLC
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