JP2008175615A - Surface plasmon resonance sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface plasmon resonance sensor capable of increasing a change in emitted light intensity with respect to a change in refractive index and improving a sensitivity in an optical waveguide type configuration. A metal thin film 3 that causes surface plasmon resonance at a wavelength used for measurement is formed on the surface of an optical waveguide substrate 12 having a cladding 2 that covers 1, and a surface plasmon resonance phenomenon occurs for a substance to be measured 10 that contacts the metal thin film 3. Like that. The optical waveguide is formed by an ion exchange method, and a portion having the highest refractive index is buried at a predetermined depth from the surface by extending the ion exchange time for applying an electric field by a predetermined length. The distance d separated from the metal thin film 3 by a predetermined distance is, for example, 4 μm or more. Since the core 1 is not in contact with the metal thin film 3, a portion where the electric field intensity of light propagating through the core 1 is large can be moved away from the metal thin film 3, and the influence of absorption by the metal thin film 3 can be reduced.
[Selection] Figure 2

Description

  The present invention relates to a surface plasmon resonance sensor that detects surface plasmon resonance at an interface. More specifically, the present invention relates to a surface plasmon resonance for a substance to be measured that forms a metal thin film on the surface of an optical waveguide substrate and contacts the metal thin film. The present invention relates to improvement of sensitivity characteristics in an optical waveguide type configuration that causes a phenomenon.

  Regarding the measurement using the refractive index and the refractive index change, a state where light incident from the outside is combined with electrons in the metal to generate a strong electric field under a specific condition at the interface, that is, surface plasmon resonance (SPR: Surface). There is a sensor that uses a change in Plasmon Resonance. Hereinafter, this surface plasmon resonance sensor is abbreviated as SPR sensor.

  The SPR sensor utilizes the fact that the surface plasmon excitation condition has a high sensitivity to the surrounding refractive index, and dynamically measures the physical property value of the substance to be measured without requiring labeling treatment. Can be applied in the fields of biotechnology and chemistry.

  When light is irradiated on a metal surface such as gold or silver, collective movement (plasmon) of free electrons in the metal is excited. A state called surface plasmon polariton propagating along the interface is generated by combining the dense wave due to collective vibration of electrons in the metal and the light in the dielectric at the interface between the metal and the dielectric. The condition for generating surface plasmon polaritons is determined by the refractive index of the metal and the dielectric in contact with the metal, and is very strongly influenced by changes in the refractive index. The dispersion of the surface plasmon cannot be directly excited because it does not intersect with that of light propagating freely in the dielectric. In order to excite the surface plasmon by external light, the wave number is controlled using a diffraction grating or a high refractive index prism and coupled.

  A well-known method for exciting surface plasmons is a method called Kretschmann arrangement. As shown in FIG. 1, the Kretschmann arrangement is such that the light irradiated onto the prism 11 having a high refractive index is transmitted through the metal thin film 3 and is surfaced at the interface with the opposite dielectric (the substance to be measured 10). The plasmon is excited. In this case, at the wavelength and incident angle at which the surface plasmon is excited, the external light uses energy to excite the surface plasmon, but the reflected light disappears, but the light is totally reflected under other conditions. The principle of the SPR sensor is to measure the refractive index change in the vicinity of the surface of the metal thin film (3) from the intensity change of the emitted light.

  As an SPR sensor, for example, as seen in Patent Document 1, there is an optical waveguide type SPR sensor that uses coupling by an evanescent component of an optical waveguide instead of a prism. Since light propagates through space in a prism, it is necessary to collect light with a lens or to adjust the position with high accuracy. However, a waveguide type SPR sensor can be connected to a light source or a light receiving unit through an optical fiber, so that a lens or the like can be used. These parts are not required, the apparatus can be miniaturized, and the optical fiber is fixed, so that complicated optical adjustment is not required during measurement.

When considering connection with an optical fiber, a single-mode optical waveguide is more suitable for a sensor because there is no noise caused by mode fluctuations in the optical fiber. When light of a certain wavelength is incident on a single-mode waveguide and the refractive index (dielectric constant) of the material in contact with the metal thin film is changed, the emitted light intensity is the light intensity with a peak value at the surface plasmon resonance condition. There is a decline. If this gradient is used, the refractive index can be measured. The stronger this gradient, the greater the change in light intensity with respect to the change in refractive index, and the higher the sensitivity to the refractive index.
Japanese Patent No. 3576093

  By the way, the optical waveguide type SPR sensor measures the refractive index of the object to be measured by the change of the light emitted from the optical waveguide, but in order to increase the sensitivity, the output of the surface plasmon resonance occurs and the output under the other conditions is not. It is necessary to make a large difference in the light intensity.

  Although surface plasmons are generated in the vicinity of the metal thin film, since the metal thin film has a property of absorbing light, the surface plasmon having a strong electric field in the vicinity of the metal thin film loses energy at the metal thin film portion as it propagates.

  In the conventional optical waveguide type SPR sensor, the core portion of the optical waveguide is in contact with the metal, and there is a strong electric field as guided light in the vicinity of the metal thin film. Therefore, the light propagating through the optical waveguide is not in the surface plasmon resonance condition. There is a problem that the emitted light is reduced by being absorbed by the thin film. As a result, the difference in the emitted light intensity between the case where surface plasmon resonance occurs and the case where the resonance does not occur is reduced, and the change in the emitted light intensity is small and the sensitivity is not good with respect to the change in the refractive index.

  SUMMARY OF THE INVENTION The present invention solves the above-described problems, and an object of the present invention is to provide a surface plasmon resonance sensor capable of increasing a change in emitted light intensity with respect to a change in refractive index and improving sensitivity in an optical waveguide type configuration. Is to provide.

  In order to achieve the above-described object, a surface plasmon resonance sensor according to the present invention includes a core and a clad covering the core, and confines light in the core by a refractive index difference and propagates at a specific wave number. An optical waveguide-type surface plasmon that includes a substrate, forms a metal thin film that causes surface plasmon resonance at a wavelength used for measurement on the surface of the optical waveguide substrate, and causes a surface plasmon resonance phenomenon for a substance to be measured that is in contact with the metal thin film. In the resonance sensor, the core has a configuration in which a portion having the highest refractive index is arranged at a predetermined distance so as not to contact the metal thin film.

  The optical waveguide substrate is provided with an optical waveguide that is in a single mode with respect to light used for measurement.

  If the refractive index of the object to be measured is lower than the refractive index of the waveguide substrate, a high refractive index thin film having a refractive index higher than that of the cladding is formed on the metal foil film, and the measured object has a relatively low refractive index. A substance is brought into contact with the high refractive index thin film.

  A polarization maintaining optical fiber is connected to the input side of the optical waveguide substrate and connected to the light source, and a polarization maintaining optical fiber or a single mode optical fiber is connected to the output side and connected to the light receiver.

  Further, the optical waveguide in the optical waveguide substrate is formed by an ion exchange method.

  In addition, the optical waveguide in the optical waveguide substrate is formed by a method that uses the movement of ions by applying an electric field, and the portion having the highest refractive index of the core is arranged to be separated from the surface by a predetermined distance. Alternatively, the optical waveguide in the optical waveguide substrate is arranged so that the core is in contact with the surface, and a transparent resin or glass whose refractive index is adjusted to be the same as that of the clad is attached to the surface, and the core has the highest refractive index. Is arranged at a predetermined distance from the surface.

  Further, the distance between the core having the highest refractive index and the predetermined distance from the metal thin film is set to 4 μm or more.

  By adopting such a configuration, in the present invention, in the optical waveguide substrate, since the core does not contact the metal thin film, a portion having a large electric field strength can be kept away from the metal thin film for light propagating through the core. Thereby, it is possible to reduce the influence of absorption by the metal thin film under the condition in which surface plasmon resonance does not occur. On the other hand, under the surface plasmon resonance condition, even if the core is separated from the metal thin film, the surface plasmon is excited by the evanescent wave of light propagating through the optical waveguide, and a strong electric field distribution is created in the vicinity of the metal thin film, so that the absorption of light energy is achieved. Happens greatly.

  In the surface plasmon resonance sensor according to the present invention, since the core is not in contact with the metal thin film in the optical waveguide substrate, a portion having a large electric field strength can be kept away from the metal thin film with respect to the light propagating through the core, and the absorption by the metal thin film can be prevented. The impact can be reduced. Under the surface plasmon resonance condition, even if the core is separated from the metal thin film, the surface plasmon is excited and a large amount of light energy is absorbed. Therefore, the change of the emitted light intensity can be increased with respect to the change of the refractive index, and the sensitivity can be improved even in the configuration of the optical waveguide type.

  FIG. 2 shows a preferred embodiment of the present invention. In the present embodiment, a surface plasmon resonance sensor (SPR sensor) includes a core 1 and a clad 2 covering the core 1, and confines light in the core 1 by a refractive index difference and propagates at a specific wave number. A substrate 12 is provided. A metal thin film 3 that causes surface plasmon resonance at a wavelength used for measurement is formed on the surface of the optical waveguide substrate 12, and a surface plasmon resonance phenomenon is caused for the substance to be measured 10 that is in contact with the metal thin film 3.

  The core 1 is arranged so as to be separated from the metal thin film 3 by a predetermined portion where the refractive index is the highest. It is preferable that the distance d at which the portion having the highest refractive index of the core 1 is separated from the metal thin film 3 by a predetermined distance is 4 μm or more.

  The optical waveguide substrate 12 is provided with an optical waveguide that is in a single mode with respect to light used for measurement. The optical waveguide in the optical waveguide substrate 12 is preferably formed by, for example, an ion exchange method. In other words, the optical waveguide in the optical waveguide substrate 12 is formed by a method using the movement of ions by applying an electric field, and the core 1 is arranged so that the portion having the highest refractive index is separated from the surface by a predetermined distance. Alternatively, the optical waveguide in the optical waveguide substrate 12 is disposed so that the core 1 is in contact with the surface, but a transparent resin or glass whose refractive index is adjusted to be the same as that of the cladding 2 is attached to the surface, and the refractive index of the core 1 is determined. The highest part may be arranged to be separated from the surface by a predetermined distance.

  As a method of forming an optical waveguide, an ion exchange method is a method of forming an optical waveguide (core 1) by forming a high refractive index portion by exchanging alkali ions in glass (clad 2) and ions having different ion radii. However, the high refractive index portion can be embedded inside the substrate by applying an electric field during melting. FIG. 3 is a graph showing the cross-sectional distribution of the refractive index of the optical waveguide (core) in the formation by the ion exchange method. As can be seen from the figure, as the ion exchange time for applying the electric field is increased, the portion with the highest refractive index is moved away from the surface, and the embedding depth is increased. Therefore, the embedding depth can be freely adjusted by appropriately controlling the movement of ions by applying an electric field, and the distance d from the metal thin film 3 is set to an appropriate value for the high refractive index portion of the core 1. it can.

  It is also possible to form a high refractive index thin film having a higher refractive index than that of the clad 2 on the metal thin film 3 so that the measured substance 10 having a relatively low refractive index is brought into contact with the high refractive index thin film. . That is, in the structure having only the metal thin film 3 on the optical waveguide substrate 12, the measurable refractive index becomes a value close to that of the optical waveguide substrate 12, but the material having a higher refractive index on the metal thin film 3 than the optical waveguide. By forming the film, the corresponding refractive index can be moved to the low refractive index side.

  As a measurement system, a polarization maintaining optical fiber is connected to the input side of the optical waveguide substrate 12 and connected to the light source, and a polarization maintaining optical fiber or a single mode optical fiber is connected to the output side to the light receiver. Connect.

  In the present invention, since the core 1 is not in contact with the metal thin film 3 in the optical waveguide substrate 12, a portion having a large electric field strength can be kept away from the metal thin film 3 for the light propagating through the core 1. Thereby, the influence of the absorption by the metal thin film 3 under the condition in which surface plasmon resonance does not occur can be reduced. On the other hand, under the surface plasmon resonance condition, even if the core 1 is separated from the metal thin film 3, the surface plasmon is excited by the evanescent wave of light propagating through the optical waveguide, and a strong electric field distribution is generated in the vicinity of the metal thin film 3. Energy absorption occurs greatly. Therefore, the change in the intensity of the emitted light can be increased with respect to the change in the refractive index, and high sensitivity can be obtained even in the configuration of the optical waveguide type.

  When the optical waveguide type SPR sensor according to the present invention, that is, the configuration shown in FIG. 2 was numerically calculated, the characteristics shown in FIG. 4 were obtained. This numerical calculation is based on the beam propagation method. The metal thin film 3 is made of gold having a thickness of 50 nm, and FIG. 4 shows a change in output intensity with respect to a change in refractive index using the embedding depth of the core 1 as a parameter.

  As is clear from FIG. 4, the peak is strong at 7 μm where the core 1 is embedded most deeply, but as the embedding becomes shallower, the loss other than the resonance point gradually increases. Disappear.

It is a block diagram explaining a Kretschmann arrangement. It is sectional drawing which shows the surface plasmon resonance sensor which concerns on this invention. It is a graph which shows the cross-sectional distribution of the refractive index of the optical waveguide (core) in the formation by an ion exchange method. It is a graph which shows the result of numerical calculation about the optical waveguide type SPR sensor which concerns on this invention, and shows the output intensity change with respect to refractive index change by making the embedding depth of a core into a parameter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Core 2 Clad 3 Metal thin film 10 Measured substance 11 Prism 12 Optical waveguide substrate

Claims (8)

An optical waveguide substrate having a core and a clad covering the core and confining light in the core by a refractive index difference and propagating at a specific wave number, and having a wavelength used for measurement on the surface of the optical waveguide substrate In an optical waveguide type surface plasmon resonance sensor that forms a metal thin film that causes surface plasmon resonance and causes a surface plasmon resonance phenomenon with respect to a measurement substance that contacts the metal thin film,
The surface plasmon resonance sensor according to claim 1, wherein the core is disposed at a predetermined distance so that a portion having the highest refractive index is not in contact with the metal thin film.   The surface plasmon resonance sensor according to claim 1, wherein the optical waveguide substrate is provided with an optical waveguide that is in a single mode with respect to light used for measurement.   3. A high refractive index thin film having a refractive index higher than that of the cladding is formed on the metal thin film, and a substance to be measured having a relatively low refractive index is brought into contact with the high refractive index thin film. The surface plasmon resonance sensor described in 1.   A polarization maintaining optical fiber is connected to the input side of the optical waveguide substrate and connected to the light source, and a polarization maintaining optical fiber or a single mode optical fiber is connected to the output side and connected to the light receiver. The surface plasmon resonance sensor according to any one of claims 1 to 3.   4. The surface plasmon resonance sensor according to claim 1, wherein the optical waveguide in the optical waveguide substrate is formed by an ion exchange method.   The waveguide formed using the ion exchange method on the optical waveguide substrate is formed by a method using ion movement by applying an electric field, and the portion having the highest refractive index of the core is separated from the surface by a predetermined distance. 4. The surface plasmon resonance sensor according to claim 1, wherein the surface plasmon resonance sensor is arranged.   The optical waveguide in the optical waveguide substrate is arranged so that the core is in contact with the surface, and a transparent resin or glass having a refractive index adjusted to be the same as that of the cladding is attached to the surface, and the refractive index of the core is highest. The surface plasmon resonance sensor according to any one of claims 1 to 3, wherein the high portion is arranged to be separated from the surface by a predetermined distance.   The surface plasmon resonance sensor according to claim 5, wherein the portion having the highest refractive index of the core has a predetermined interval of 4 μm or more from the metal thin film.

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