Optics: measuring and testing – Of light reflection
Reexamination Certificate
2002-10-18
2004-09-14
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Of light reflection
C356S448000
Reexamination Certificate
active
06791691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a measuring method and apparatus using attenuation in total internal reflection such as a surface plasmon sensor for analyzing a sample on the basis of generation of surface plasmon.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called “Kretschmann configuration” is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
The plasmon resonance sensor using the Kretschmann configuration basically comprises a dielectric block shaped, for instance, like a prism, a metal film which is formed on one face of the dielectric block and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block to impinge upon the interface of the dielectric block and the metal film at various angles of incidence so that total internal reflection conditions are satisfied at the interface, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of surface plasmon resonance, i.e., a state of attenuation in total internal reflection.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface while deflecting the incident light beam or a relatively thick incident light beam may be caused to impinge upon the interface in the form of convergent light or divergent light so that components of the incident light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the incident light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the incident light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, an area sensor which extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.
In such a plasmon resonance sensor, when a light beam impinges upon the interface at a particular angle of incidence &thgr;sp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution in the sample in contact with the metal film are generated and surface plasmon is excited in the interface between the metal film and the sample. When the wave number vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total internal reflection at the interface of the dielectric block and the metal film sharply drops. The sharp intensity drop is generally detected as a dark line by the photodetector. The aforesaid resonance occurs only when the incident light beam is p-polarized. Accordingly, it is necessary to set the light beam to impinge upon the interface in the form of p-polarized light.
When the wave number of the surface plasmon can be known from the angle of incidence &thgr;sp at which the phenomenon of attenuation in total internal reflection (ATR) takes place, the dielectric constant of the sample can be obtained. That is
K
sp
(
ω
)
=
ω
c
ϵ
m
(
ω
)
ϵ
s
ϵ
m
(
ω
)
+
ϵ
s
wherein 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 a vacuum, and ∈
m
and ∈
s
respectively represent the dielectric constants of the metal and the sample.
A property related to the dielectric constant ∈
s
(refractive index) of the sample can be detected by detecting the angle of incidence &thgr;sp at which the intensity of light reflected in total internal reflection from the interface of the prism and the metal film sharply drops (this angel &thgr;sp will be referred to as “the attenuation angle &thgr;sp”, hereinbelow).
In such a surface plasmon sensor, it has been proposed, in order to measure the attenuation angle &thgr;sp accurately with a wide dynamic range, to use a photodetector in the form of an array of a plurality of photodetector elements arranged in a predetermined direction so that light beam components reflected at different angles at the interface impinge upon different photodetector elements as disclosed in Japanese Unexamined Patent Publication No. 11 (1999)-326194.
In this case, the output signals output from the photodetector elements are generally differentiated in the direction in which the photodetector elements are arranged, and the refractive-index-related property of the material to be measured is generally obtained on the basis of the differentials.
As a similar apparatus utilizing the phenomenon of attenuation in total internal reflection (ATR), there has been known a leaky mode sensor described in, for instance, “Spectral Research” Vol.47, No.1 (1998), pp21 to 23 & pp26 and 27. The leaky mode sensor basically comprises a dielectric block shaped, for instance, like a prism, a clad layer which is formed on one face of the dielectric block, an optical waveguide layer which is formed on the clad layer and is brought into contact with a sample liquid, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block to impinge upon the interface of the dielectric block and the metal film at various angles of incidence so that total internal reflection conditions are satisfied at the interface, and a photodetector means which detects the intensity of the light beam reflected in total internal reflection at the interface and detects a state of waveguide mode excitation, i.e., a state of attenuation in total internal reflection (ATR).
In the leaky mode sensor with this arrangement, when the light beam is caused to impinge upon the clad layer through the dielectric block at an angle not smaller than an angle of total internal reflection, only light having a particular wave number and impinging upon the optical waveguide layer at a particular angle of incidence comes to propagate through the optical waveguide layer in a waveguide mode after passing through the clad layer. When the waveguide mode is thus excited, almost all the incident light is taken in the optical waveguide layer and accordingly, the intensity of light reflected in total internal reflection at the interface of the dielectric block and the clad layer sharply drops. That is, attenuation in total internal reflection occurs. Since the wave number of light to be propagated through the optical waveguide layer in a waveguide mode depends upon the refractive index of the sample on the optical waveguide layer, the refractive index and/or the properties of the sample related to the refractive index can be detected on the basis of the angle of incidence at which the attenuation in total internal reflection occurs.
Also in such a leaky mode sensor, a photodetector in the form of an array of a plurality of photodetector elements can be used to detect the position of the dark line generated due to the attenuation in total internal reflection, and at the same time differentiation of the output signals output from the photodetector elements is often applied.
The surface plasmon sensor and the leaky mode sensor are sometimes used in random screening for finding a
Kimura Toshihito
Ohtsuka Hisashi
Shimizu Hitoshi
Fuji Photo Film Co. , Ltd.
Stafira Michael P.
Sughrue & Mion, PLLC
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