Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2002-03-04
2004-10-19
Lee, John D. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S141000
Reexamination Certificate
active
06807323
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to Korean Application No. 2001-73283 filed Nov. 23, 2001, the disclosure of which is incorporated herein by reference in its entity.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sensor for use in sample analysis and its applications, and more particularly, to a surface plasmon resonance sensor and an imaging system based on the principle of the surface plasmon resonance sensor.
2. Description of the Related Art
Surface plasmon is a quantized oscillation of free electrons that propagates along the surface of a conductor such as a metal thin film. Surface plasmon is excited to cause resonance by an incident light beam entering a metal thin film through a dielectric medium such as a prism at an incident angle greater than a critical angle. This phenomenon is referred to as “surface plasmon resonance” (SPR). The incident angle of an incident light beam that causes resonance is very sensitive to changes in the refractive index of a material closest to the metal thin film. SPR sensors developed based upon the above principle have been widely used for quantification and qualification of a sample or measurement of a sample (thin film) thickness from changes in the refractive index of the sample displaced closest to the metal thin film.
FIG. 1
shows a typical SPR sensor based on the Kretschmann configuration. Referring to
FIG. 1
, the SPR includes a unit U composed of a dielectric medium
10
and a metal thin film
22
that induces SPR. A half-cylindrical or triangular prism made of transparent glass, such as BK7 and SF10, is often used for the dielectric medium
10
. The metal thin film is formed of gold or silver with a thickness of 40-50 nm. The unit U is supported by a rotary plate
50
capable of rotating around a fixed shaft. A sample
23
of interest to be measured for changes in its refractive index within the surface plasmon field is brought into contact with the metal thin film
22
of the unit U.
In
FIG. 1
, reference numeral
30
denotes a light source fixed to emit an incident light beam
31
toward the metal thin film
22
, and reference numeral
40
denotes a photodetector for measuring the intensity of the reflected light from the surface of the metal thin film
22
. A monochromatic laser, a monochromic light emitting diode (LED), a white light source of multiple wavelengths, or a multiple-wavelength LED is often used as the light source
30
.
SPR occurs when the wave vector of the incident light beam
31
parallel to the surface of the metal thin film
22
is equal to the wave vector of the surface plasmon wave. Thus, the following formula (1) is satisfied:
n
sin
θ
re
=
ϵ
1
ϵ
2
ϵ
1
+
ϵ
2
(
1
)
where n is the refractive index of the dielectric medium, &thgr;
re
is the resonance angle, and ∈
1
and ∈
2
are the dielectric constants of the metal thin film
22
and the sample
23
, respectively.
As is apparent from the formula (1) above, if the resonance angle &thgr;
re
is given, the dielectric constant of the sample
23
can be calculated using the formula (1) and thus changes in the refractive index of the sample
23
or with respect to a reference sample can be observed. As a consequence, measurement of the thickness of the sample
23
if it is a thin film, or quantification and qualification of the sample adsorbed onto the metal thin film
22
can be implemented from the changes in the refractive index.
Resonance angle &thgr;
re
can be measured using a variety of methods.
First, the fact that the intensity of the reflected light (or reflectance)
39
has a minimal value when the metal thin film
22
is excited to induce SPR by the incident light beam
31
is used. In this method, the intensity of the reflected light (or reflectance)
39
is measured while changing the incident angle &thgr; of the incident light beam
31
, and the resonance angle &thgr;
re
, the incident angle at which resonance occurs, is read from a plot of the intensity of the reflected light (or reflectance)
39
as a function of the incident angle &thgr;. The intensity of the reflected light (or reflectance)
39
is measured while rotating the rotary plate
50
to vary the incident angle &thgr;, in which a monochromic light source as the light source
30
and a prism with a constant refractive index as the dielectric medium
10
are used.
In a second method, a wavelength where SPR occurs is found by emitting the incident light beam
30
at a fixed incident angle &thgr; using a white light source of multiple wavelengths as the light source
30
. As a result, the resonance angle &thgr;
re
and resonance wavelength can be obtained simultaneously.
In a third method, the resonance angle &thgr;
re
is measured by emitting a monochromic light from the light source
30
within the range of the incident angle to the center of the dielectric medium
10
and by receiving the light reflected from the surface of the metal thin film
22
with the same range of angles as the incident angle using a multi-channel photodetector, such as a photodiode array (PDA), as the photodetector
40
. This method is disclosed in U.S. Pat. Nos. 4,889,427; 5,359,681; and 4,844,613.
The method of measuring the resonance angle &thgr;
re
using a monochromatic light as in the first and third methods described above has about 10 times higher sensitivity than the second method using a white light source at a fixed incident angle. For this reason, the first and third methods have been used widely, and products based on the third method are available from Biocore and Texas Instrument.
FIG. 2
shows reflectances as a function of the incident angle of light measured using the SRP sensor of
FIG. 1
for samples of different refractive indices. In
FIG. 2
, (1) is for water, (2) is for a sample with a refractive index difference of 10
−6
from water, and (3) is for a sample with a refractive index difference of 10
−3
from water.
An inset for a portion A in
FIG. 2
shows changes in resonance angle with respect to changes in the refractive index of samples. A change in resonance angle (&Dgr;&thgr;) by about 0.0001° occurs between samples (1) and (2) having a refractive index difference of 10
−6
. In measuring the resonance angle by the first and third methods described above, the rotary plate
50
used in the first method to vary the incident angle has an angular resolution limit of about 0.0001° and the photodetector
40
such as a PDA which spatially splits the light reflected within a predetermined range of angles has a resolution limit of about 0.0001°. Thus, it is difficult for the SPR sensor with such a resolution limit to detect a minor change in refractive index less than 10
−6
or equivalent physical quantities, for example, protein adsorbed onto the surface of a metal thin film in an amount of less than several picograms per 1 mm
2
. In addition, adsorption of a material having a molecular weight less than 200 cannot be detected.
In the method of measuring reflectance at a fixed incident angle &agr;, a change in reflectance (&Dgr;R) for a refractive index difference of 10
−6
between samples is only 0.03% at &agr;=65.0304°. In consideration of the 0.2% resolution of a measuring system commonly used in the field, this method has a lower refractive index resolution than the methods for directly measuring the resonance angle.
To address the limitations of the SPR sensor, a coupled plasmon-waveguide resonance (CPWR) sensor, as shown in
FIG. 3
, has been developed. In
FIG. 3
, the same elements as those in
FIG. 1
are denoted by the same reference numerals as those in FIG.
1
.
Referring to
FIG. 3
, the CPWR sensor with improved sensitivity is a modification of the SPR sensor of FIG.
1
. The CPWR sensor, which is disclosed in U.S. Pat. No. 5,991,488, includes a dielectric thin film
60
between the metal thin film
22
and the sample
23
. The dielectric thin film
60
Beom Shin Yong
Hee Park Seon
Jeong Ji-wook
Kim Min-Gon
Pyo Hyeon-Bong
Blakely & Sokoloff, Taylor & Zafman
Electronics and Telecommunications Research Institute
Lee John D.
Song Sarah
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