JP2011017576A - Refractive index measuring instrument, surface plasmon sensor and refractive index measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive index measuring instrument capable of performing the measurement of a refractive index or surface plasmon of a wide range, without having to exchange a prism, and to provide a surface plasmon sensor and a refractive index measurement method.SOLUTION: The refractive index measuring instrument is equipped with a rod-shaped optical element, having a circular arc part and a straight-line shaped part in the orthogonal cross section in the longitudinal direction thereof; a light source part for emitting the light linearly condensed to the straight line shaped part of the rod-shaped optical element; and an imaging element for imaging the intensity distribution of the reflected light from the rod-shaped optical element. The rod-shaped optical element has a refractive index distribution in the longitudinal direction thereof.

Description

  The present invention relates to a refractive index measuring device, a surface plasmon sensor, and a refractive index measuring method.

  Abbe's refractometer is known as a conventional refractive index measuring device. In this refractometer, a light beam is irradiated onto a test object arranged on a prism having a triangular prism shape or a hemispherical shape. The divergent light reflected from the test object has a critical angle detected by a sensor, and the refractive index of the test object is calculated based on the critical angle (Patent Document 1).

Here, a specific method for measuring the refractive index of the test object will be described.
First, the refractive index of the prism is np, and the refractive index of the test object is ns. The light beam for refractive index measurement is incident from the prism side so that the angle formed with the normal line of the boundary surface between the prism and the test object is θ1. When the incident light beam is refracted at an angle θ2 with respect to the normal of the boundary surface, np, ns, θ1, and θ2 satisfy the following expression (1).
np · sin θ1 = ns · sin θ2 (1)
Here, when the value of “np · sin θ1 / ns” is 1 or more, there is no corresponding θ2. At this time, since the light beam cannot enter the test object, the light is totally reflected.

In addition, the angle θ1 when the following equation (2) is satisfied is called a critical angle (θc).
np · sin θ1 / ns = 1 (2)
If the refractive index np of the prism is known, the refractive index ns of the test object can be obtained from the following equation (3) by changing the incident angle of the light beam and measuring the critical angle θc.
np · sin θc = ns (3)

JP 2004-170218 A

  However, in the above-described refractive index measurement method, the possible range of the angle θ1 formed between the ray for measuring the refractive index of Abbe's refractometer and the normal of the boundary surface between the prism and the test object is θ1a to θ1b, and the refraction of the prism When the rate is np, the measured refractive index range of the test object is limited to np · sin θ1 to np · sin θ2. Therefore, when measuring a test object outside the measurement range, it is necessary to replace the prism with an optimum refractive index, which requires time. In addition, in a surface plasmon sensor using a refractometer, it is necessary to replace the prism with an optimum refractive index that matches the test object.

  The present invention has been made in view of the above, and can perform a wide range of refractive index measurement or surface plasmon measurement without exchanging prisms, a refractive index measurement device, a surface plasmon sensor, and a refractive index measurement. It aims to provide a method.

  In order to solve the above-described problems and achieve the object, a refractive index measuring device according to the present invention includes a rod-shaped optical element having an arc portion and a linearly-shaped portion in a longitudinally orthogonal cross section, and a linear shape of the rod-shaped optical element. A light source unit that emits light condensed in a line shape and an image sensor that captures an intensity distribution of reflected light from the rod-shaped optical element, and the rod-shaped optical element has a refractive index distribution in the longitudinal direction. It is characterized by that.

  In the refractive index measuring apparatus according to the present invention, the rod-shaped optical element has a refractive index that increases from one side to the other side in the longitudinal direction, and the radius of the arc portion corresponding to the refractive index from one side to the other in the longitudinal direction. Is preferably small.

  In the refractive index measuring apparatus according to the present invention, it is preferable that the rod-shaped optical element has a measurement surface on a surface constituting the linear shape portion.

  In the refractive index measuring apparatus according to the present invention, it is preferable that the rod-shaped optical element has a metal thin film on a surface constituting the linear shape portion.

  A surface plasmon sensor according to the present invention includes any one of the above-described refractive index measurement devices, and has a measurement surface on an upper surface of a metal thin film.

  The refractive index measurement method according to the present invention irradiates light from a light source on an interface between a specimen and a prism, detects light reflected by the interface with a photoelectric sensor, and measures the refractive index of the specimen from a critical angle. In this method, the refractive index of the test object is obtained at a plurality of portions of the prism, and the average value is used as the refractive index of the test object.

  The refractive index measuring device, the surface plasmon sensor, and the refractive index measuring method according to the present invention have an effect that a wide range of refractive index can be measured without replacing the prism.

It is a perspective view which shows the structure of the refractive index measuring apparatus which concerns on 1st Embodiment of this invention. It is a side view which shows the structure of the refractive index measuring apparatus which concerns on 1st Embodiment of this invention. It is a top view corresponding to the detection surface of the photodetector which concerns on 1st Embodiment of this invention. It is a top view corresponding to the detection surface of the photodetector which concerns on 1st Embodiment of this invention, Comprising: It is a top view which shows the example of the image detected with a photodetector. It is a top view corresponding to the detection surface of the photodetector which concerns on 1st Embodiment of this invention, Comprising: It is a top view which shows the example of the image detected with the photodetector. It is a graph which shows the relationship between the incident angle about the measurement image shown in FIG. 5, and reflected light intensity. 6 is a graph showing the relationship between the incident angle and dI / dθ for the measurement image shown in FIG. 5. It is a perspective view which shows the structure of the prism which concerns on 2nd Embodiment of this invention. It is a perspective view which shows the entrance plane of the light ray which injects into the prism which concerns on a comparative example. It is a top view which shows the wave front by the light ray which injected into the prism which concerns on a comparative example. It is a top view which shows the wave front by the light ray which injected into the prism which concerns on 2nd Embodiment of this invention.

  Hereinafter, embodiments of a refractive index measuring device, a surface plasmon sensor, and a refractive index measuring method according to the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by the following embodiment.

<First Embodiment>
FIG. 1 is a perspective view showing a configuration of a refractive index measuring apparatus 100 according to the first embodiment. FIG. 2 is a side view showing the configuration of the refractive index measuring apparatus 100 according to the first embodiment.

  The refractive index measuring apparatus 100 includes a prism 101, a light source 102 (light source unit), and a photodetector (imaging device) 103. A cylindrical lens 104 is disposed between the prism 101 and the light source 102, and a cylindrical lens 105 is disposed between the prism 101 and the photodetector 103.

The prism 101 is a rod-shaped optical element composed of a flat upper surface 101a and a curved lower surface 101b. The upper surface 101a and the lower surface 101b of the prism 101 respectively constitute an arc portion and a linear shape portion in the longitudinal cross section. Here, the longitudinal direction of the prism 101 is a direction perpendicular to the paper surface in FIG. The prism 101 has a refractive index distribution such that the refractive index increases from one to the other in the longitudinal direction (axial direction).
An upper surface 101a of the prism 101 (a plane constituting the linear shape portion) is a measurement surface for refractive index measurement, and a test object is applied thereto.

  In the refractive index measuring apparatus 100, the parallel light emitted from the light source 102 passes through the cylindrical lens 104 and then enters the lower surface 101 b (the curved surface forming the arc portion) of the prism 101. This incident light is reflected by a condensing line 101 c that connects the centers in the width direction of the upper surface 101 a of the prism 101, exits from the lower surface 101 b, passes through the cylindrical lens 105, and enters the photodetector 103.

  Here, the cylindrical lens 104 on the light source 102 side is arranged so as to perform line illumination along the axial direction on the upper surface 101 a of the prism 101.

  In addition, by arranging the cylindrical lens 105 on the detection side, the light emitted from the prism 101 becomes parallel light and enters the photodetector 103, so that there is no shading and measurement accuracy can be improved.

  As the light source 102, for example, a semiconductor laser or an LED (Light Emitting Diode) is used. As the photodetector (imaging device), for example, a CCD (charge coupled device), a CMOS (Complementary Metal Oxide Semiconductor), or a photoelectric sensor is used.

  Further, a light diffusing plate for diffusing light from the light source may be disposed between the light source 102 and the cylindrical lens 104. By using the light diffusing plate, the intensity of light incident on the cylindrical lens 104 can be made uniform.

  When a point light source is used as the light source 102, the intensity distribution is preferably the largest at the center position of the optical axis and becomes smaller toward the periphery, and a Gaussian distribution is preferred. By using such a light intensity distribution, it is possible to suppress a decrease in refractive index measurement accuracy or occurrence of erroneous measurement.

  FIG. 3 is a plan view corresponding to the detection surface of the photodetector 103. FIG. 4 is a plan view corresponding to the detection surface of the photodetector 103, and is a plan view illustrating an example of an image detected by the photodetector 103. In the following description, an image detected by the photodetector 103 is referred to as a measurement image.

  The horizontal axis direction of the photodetector 103 (detection surface) coincides with the direction (longitudinal direction) of the light-condensing line 101c of the line illumination on the upper surface 101a of the prism 101. Since the prism 101 has a refractive index distribution in the axial direction, the horizontal axis direction of the photodetector 103 represents a change in the refractive index of the prism 101.

On the other hand, the vertical axis of the photodetector 103 (detection surface) corresponds to the incident angle of light incident from the prism 101 through the cylindrical lens 105. That is, light incident on the photodetector 103 from the condensing line 101c of the prism 101 is incident on the photodetector 103 at an angle corresponding to a refractive index that varies depending on the position in the longitudinal direction of the prism 101. Is incident on.
For example, in the example illustrated in FIG. 3, the pixel A is represented as light reflection information at the light incident angle θ <b> 1 at the position of the prism refractive index n <b> 1.

  Here, when the refractive index of the test object is ns, the light beam is totally reflected at the interface between the test object and the prism 101 when the value of n1 · sin θ1 / ns is 1 or more. The pixels to be brightened. On the other hand, when the value of n1 · sin θ1 / ns is less than 1, the light beam is refracted and enters the test object, so that the corresponding pixel becomes dark. In this way, brightness measurement can be performed on all pixels, and a measurement image illustrated in FIG. 4 is formed. Here, the boundary line between the light and dark portions is called a critical angle line, and in the example shown in FIG.

  From the measurement image, the critical angle at a specific refractive index of the prism 101 is found. The refractive index of the test object can be obtained from each critical value at each refractive index.

  Below, the method of calculating | requiring a critical angle is demonstrated using FIGS. FIG. 5 is a plan view corresponding to the detection surface of the photodetector 103 and showing an example of an image detected by the photodetector. FIG. 6 is a graph showing the relationship between the incident angle and the reflected light intensity for the measurement image shown in FIG. FIG. 7 is a graph showing the relationship between the incident angle and dI / dθ for the measurement image shown in FIG. Here, dI / dθ is a value obtained by differentiating the reflected light intensity with respect to the incident angle θ.

  Assume that a measurement image as shown in FIG. 5 is obtained by the measurement. First, in the measurement image, the relationship between the reflected light intensity I and the incident angle θ at a specific refractive index np of the prism 101 is obtained. The reflected light intensity I is calculated based on the brightness at each pixel detected by the photodetector 103. The relationship between the reflected light intensity I and the incident angle θ is represented by an I-θ curve shown in FIG.

Next, a dI / dθ-θ curve (FIG. 7) is created from the I-θ curve obtained in the previous step. The dI / dθ-θ curve is a curve showing the relationship between the incident angle to the photodetector 103 and the change in the reflected light intensity I, and is a curve obtained by differentiating the I-θ curve. From this dI / dθ-θ curve, an incident angle giving the maximum value of the change in the reflected light intensity I is obtained. In the example shown in FIG. 7, the value of this incident angle is the incident angle θc at which the reflected light intensity I takes the maximum value, and this angle is the critical angle.
By applying the refractive index np and the critical angle θc obtained as described above to the following equation (3), the refractive index ns of the test object can be obtained.
np · sin θc = ns (3)

The positional relationship between the refractive index of each pixel of the photodetector 103 and the prism 101 is obtained, for example, from the following steps.
(1) Step 1: A sample having a known refractive index (refractive index ns1) is placed on the upper surface 101a of the prism 101.
(2) Step 2: Obtain a measurement image of the sample.

Subsequently, attention is paid to the lowest coordinate X1 (the coordinate having the smallest value) in the vertical axis direction of the image of the photodetector 103.
(3) Step 3: Using the I-θ curve, the critical angle with respect to the coordinate in the refractive index direction of the prism 101 is obtained.
(4) Step 4: From the critical angle obtained in Step 3 and the refractive index ns1 of the sample, the refractive index of the prism 101 at the coordinate X1 can be obtained.

  Next, attention is paid to the coordinate X2 on the right side of the coordinate X1 in the horizontal axis direction (the same refractive index of the prism) of the image of the photodetector 103. As in the case of the coordinate X1, the refractive index of the prism 101 at the position X2 can be obtained from steps 3 and 4. Similarly, the refractive index of the prism is obtained for all X coordinates of the prism 101.

  Depending on the refractive index distribution of the prism 101, there may be no critical angle. In this case, the same measurement is performed using a sample having a different refractive index from the sample (refractive index ns2).

  According to the refractive index measuring apparatus and the refractive index measuring method according to the first embodiment, the critical angle when the refractive index is changed only needs to be measured once. Therefore, the refractive index can be easily measured with a wide measurement range.

(Second Embodiment)
In the refractive index measurement device according to the second embodiment, the shape of the prism 201 as a rod-shaped optical element is different from that of the prism 101 according to the first embodiment. Other configurations are the same as those of the refractive index measurement apparatus according to the first embodiment, and the refractive index measurement method is also the same. In the following description, a detailed description of the configuration other than the prism 201 and the refractive index measurement method is omitted.

  FIG. 8 is a perspective view showing the configuration of the prism 201 according to the second embodiment.

  Similar to the prism 101 of the first embodiment, the prism 201 according to the second embodiment is a rod-shaped optical element composed of a flat upper surface 201a and a curved lower surface 201b, and has a longitudinal direction (axial direction). ) Has a refractive index distribution such that the refractive index increases from one to the other. As a specific example of the refractive index distribution, the refractive index is made the smallest on the end surface 211 side on the front side in the longitudinal direction, and the refractive index becomes larger toward the end surface 212 on the far side (FIG. 8). On the other hand, the refractive index on the front end surface 211 side may be maximized, and the refractive index may be decreased toward the inner end surface 212.

  In the prism 201, similarly to the prism 101, the light from the light source 102 is condensed on the condensing line 201c at the center of the upper surface 201a.

  Further, in the prism 201, the radius of the arc corresponding to the lower surface 201b in the longitudinal cross section is made smaller as the refractive index is larger. As shown in FIG. 8, when the refractive index increases from the front end surface 211 toward the back end surface 212, the refractive index increases from the end surface 211 side having a low refractive index corresponding to the refractive index distribution. The radius of the arc becomes smaller toward the end face 212 (FIG. 8).

FIG. 9 is a perspective view showing a light incident surface 920 incident on the prism 901 according to the comparative example. FIG. 10 is a plan view showing a wavefront caused by light rays incident on the prism 901 according to the comparative example.
The prism 901 according to the comparative example is a rod-like optical element composed of a flat upper surface 901a and a curved lower surface 901b, similar to the prism 201 of FIG. 8, and is on the end surface 911 side on the front side in the longitudinal direction. The refractive index is the smallest, and the refractive index increases toward the end surface 912 on the back side. In the prism 901, the radius of the arc corresponding to 901b in the longitudinal cross section is constant. In the prism 901, the light from the light source 102 is condensed on a condensing line 901c that connects the centers of the upper surfaces 901a in the width direction.

  In the prism 901, the wavefront 920 of the light beam is gradually shifted from the center line 901c of the prism 901 due to the optical path length difference due to the refractive index distribution in the longitudinal direction of the prism 901 (FIG. 10). Therefore, the light ray is incident on the center line 901c of the prism 901 at an angle shifted from the vertical.

FIG. 11 is a plan view showing a wavefront caused by light rays incident on the prism 201 according to the second embodiment.
In the prism 201, the end surface 212 having a large refractive index has a shape in which the width of the upper surface 201a (the radius of the lower surface 201b) is smaller, and the end surface 211 having a smaller refractive index has a larger width (the radius of the lower surface 201b). . For this reason, it becomes possible to make the advancing direction of a light beam perpendicular to the condensing line 201c at the center of the prism 201 (FIG. 11). In other words, the condensing line 201c and the wavefront 220 of incident light are parallel to each other. Therefore, the light ray is incident at an angle perpendicular to the center line 901c of the prism 901.
Therefore, the critical angle can be measured more accurately than when the prism 901 of the comparative example shown in FIGS.
In addition, about another structure, an effect | action, and an effect, it is the same as that of 1st Embodiment.

Here, modified examples of the first and second embodiments will be described.
The refractive index ns of the test object is preferably calculated by a plurality of refractive index portions of the prism 101 or the prism 201, and the average of the calculated values is preferably used as the refractive index of the test object.

  Thereby, since the refractive index of the test object is obtained from many measurements, the measurement error can be reduced and the refractive index can be measured more accurately.

  In addition, a measurement image when nothing is placed on the prism 101 or the prism 201 is referred to as Image0, and a measurement image (Image1) when the test object is disposed is set as Image1 / Image0 for each pixel of the photodetector 103. It is preferable to obtain the ratio and reconstruct the measurement image.

  Thereby, the light source unevenness on the sample surface (the upper surface 101a of the prism 101 or the upper surface 201a of the prism 201) can be corrected, so that the refractive index can be measured more accurately.

(Third embodiment)
The third embodiment is an embodiment of the surface plasmon sensor of the present invention. The surface plasmon sensor according to the third embodiment has a configuration in which a metal coat is applied on the prism 101a of the first embodiment.

  The metal coat is preferably 300 nm or less, and for example, a metal coat of 30 to 80 nm is applied. When gold or silver is used for this metal coat film, the surface plasmon resonance phenomenon occurs most efficiently. Chromium may be deposited in advance between the metal coat film and the prism 101 to improve adhesion. The prism for forming the metal coat film may be the prism 201 of the second embodiment.

  In a metal, free electrons collectively vibrate like a wave to generate a dense wave called a plasma wave. The collective vibration of electrons generated on the metal surface is called surface plasmon. Using this surface plasmon, there are a method for measuring the refractive index of a sample, a method for quantitative analysis of substances in the sample, and a method for detecting physicochemical changes in substances as the biochemical reaction progresses. Proposed. Among them, a surface plasmon sensor using an optical system called a Kretschmann arrangement is particularly well known.

In the surface plasmon sensor having such a configuration, when a light beam is incident on the metal coat film at a total reflection angle or more, the amplitude increases in the sample in contact with the metal coat film as the distance from the interface between the metal coat film and the sample increases. A surface electric field (evanescent field) is generated in which is attenuated. The wave number vector k _ev of the evanescent wave is determined by the incident angle θ according to the following equation (4).
k _ev = ω · n _p · sin θ / c (4)
Here, ω is the angular frequency of incident light, c is the speed of light in vacuum, and n _p is the refractive index of the dielectric prism.

When the light beam is incident at a specific incident angle θ _sp that is equal to or greater than the total reflection angle, surface plasmons are excited at the interface between the metal coat film and the sample by the evanescent wave. Wave number k _sp of the surface plasmon, according to the following equation (5), determined by the dielectric constant of the material.
k _sp = [ω · (ε _m * ε _s / (ε _m + ε _s )) ^1/2 ] / c (5)
Here, omega is the angular frequency of the incident light, c is the velocity of light in vacuum, epsilon _m metal dielectric constant, the epsilon _m is the dielectric constant of the material in contact with the metal.

When the incident angle θ is adjusted and the wave number vector k _ev of the evanescent wave is made equal to the wave number k _{sp of the} surface plasmon, both are in a plasmon resonance state, and the energy of the incident light is transferred to the surface plasmon. The intensity of the light totally reflected at the interface with the sharply decreases. Therefore, the dielectric constant or refractive index of the medium in contact with the surface of the metal coating film is changed, the wave number k _sp of the surface plasmon is changed, the incidence angle theta _sp incident light changes to resonance it. That is, knowing the incident angle θ _sp enables quantitative analysis of a specific substance in the substance.

Since the surface plasmon sensor of the third embodiment can determine the surface plasmon resonance angle θ _sp when the refractive index of the prism 101 is changed by a single measurement, the surface using the surface plasmon resonance phenomenon having a wide measurement range. A plasmon sensor can be realized.

  As described above, the refractive index measurement device, the refractive index measurement method, and the surface plasmon sensor according to the present invention are useful for a wide range of refractive index measurement or surface plasmon measurement.

100 refractive index measuring device 101 prism (rod-shaped optical element)
101a Upper surface 101b Lower surface 101c Condensing line 102 Light source 103 Photo detector 104, 105 Cylindrical lens 201 Prism (rod-shaped optical element)
201a Upper surface 201b Lower surface 201c Condensing lines 211, 212 End face 220 Wavefront 901 Prism (rod-shaped optical element)
901a Upper surface 901b Lower surface 901c Condensing line 911, 912 End surface 920 Wavefront

Claims (6)

A rod-shaped optical element having a circular arc part and a linearly shaped part in the longitudinal cross section;
A light source unit that emits light that is condensed into a linear shape on the linear optical part of the rod-shaped optical element;
An image sensor for imaging an intensity distribution of reflected light from the rod-shaped optical element;
With
The rod-like optical element has a refractive index distribution in the longitudinal direction, and the refractive index measuring device.   The rod-shaped optical element has a refractive index that increases as it goes from one side of the longitudinal direction to the other side, and a radius of the arc portion that decreases according to the refractive index as it goes from one side of the longitudinal direction to the other side. The refractive index measuring apparatus according to 1.   The refractive index measuring device according to claim 1, wherein the rod-shaped optical element has a measurement surface on a surface constituting the linear shape portion.   The refractive index measuring device according to any one of claims 1 to 3, wherein the rod-shaped optical element has a metal thin film on a surface constituting the linearly shaped portion.   5. A surface plasmon sensor comprising the refractive index measurement device according to claim 4 and having a measurement surface on an upper surface of the metal thin film. A refractive index measurement method for irradiating light from a light source to an interface between a test object and a prism, detecting light reflected by the interface with a photoelectric sensor, and measuring a refractive index of the test object from a critical angle. ,
A refractive index measurement method, wherein a refractive index of the test object is obtained at a plurality of portions of the prism, and an average value thereof is set as a refractive index of the test object.

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