JP7202661B2 - Metamaterial structures and refractive index sensors - Google Patents

Description

The present invention relates to metamaterial structures and refractive index sensors.

In recent years, while various bio-chemical detection technologies have been developed, a refractive index sensor using surface plasmon resonance has attracted attention. Conventional refractive index sensors using surface plasmon resonance are roughly divided into two types. One uses propagating surface plasmons on a glass prism on which a gold (Au) thin film is deposited (see, for example, Non-Patent Documents 1 or 2), and the other uses localized surface plasmons of Au colloids. (For example, see Non-Patent Document 3).

Akihito Ishida, "Biosensing and Plasmonics", Surface Technology, 2011, Vol.62, No.6, p.285-290 K. Willets and R. V. Duyne, “Localized surface plasmon resonance spectroscopy and sensing”, Annual Review of Physical Chemistry, 2007, Vol. 54, No. 1, p.267-297 J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species”, Chemical Reviews, 2008, Vol. 108, No. 2, p.462-493

However, as described in Non-Patent Documents 1 and 2, a refractive index sensor using propagating surface plasmon on a glass prism on which a gold (Au) thin film is formed is capable of precise measurement, but the device is The problem was that it was complicated and expensive. In addition, a refractive index sensor using localized surface plasmons of Au colloids, such as described in Non-Patent Document 3, is inexpensive and can be easily measured, but compared to refractive index sensors using propagating surface plasmons, Therefore, there is a problem that the detection accuracy is inferior.

The present invention has been made in view of such problems, and an object of the present invention is to provide a metamaterial structure and a refractive index sensor capable of inexpensive and precise measurement.

As a result of intensive research to achieve the above object, the present inventors have focused on the fact that metamaterials exhibit a sensitive response to changes in the surrounding refractive index and that the shape can be designed with a high degree of freedom. By optimizing the mode distribution of surface plasmons using As a result, it is possible to provide a refractive index sensor that enables inexpensive and precise measurement.

That is, the metamaterial structure according to the present invention is a metamaterial structure used in a refractive index sensor that detects the refractive index of a surrounding medium, and has a structure with a wavelength equal to or less than the wavelength of incident light. and has a fine structure composed of unit structures arranged two-dimensionally with rotational symmetry, and optical coupling between the incident light and the surface plasmon mode depending on the fine structure is strengthened under resonance conditions, whereby wavelength-selective characterized in that it is configured to exhibit a high photoresponse.

A refractive index sensor according to the present invention is characterized by having the metamaterial structure according to the present invention.

The metamaterial structure according to the present invention can adjust the selected wavelength of photoresponse by the dimensions of the microstructure. In addition, the metamaterial structure according to the present invention sensitively changes the selected wavelength depending on the surrounding medium. Therefore, the metamaterial structure and the refractive index sensor according to the present invention can detect the refractive index of the surrounding medium from the change in the selected wavelength or the change in reflectance at a specific wavelength due to the medium surrounding the metamaterial structure. can be done. A metamaterial structure and a refractive index sensor according to the present invention have a simpler structure and can be manufactured at a lower cost than conventional refractive index sensors using propagating surface plasmons.

A conventional refractive index sensor that utilizes surface plasmons is configured by randomly arranging gold thin films or gold fine particles, so that the enhancement of surface plasmons has not been optimized. In addition, optimization was also difficult because it was created in a bottom-up manner. On the other hand, the metamaterial structure and the refractive index sensor according to the present invention can arbitrarily design the shape of the metamaterial structure based on the concept of metamaterials, so that plasmon enhancement can be optimized. Therefore, the sensitivity can be made higher than that of the conventional one, and precise measurement can be performed. A refractive index sensor according to the present invention preferably has a refractive index sensitivity of 300 to 1000 nm/RIU.

The metamaterial structure according to the present invention can be made polarization-independent because the fine structure is composed of unit structures that are rotationally symmetrically arranged two-dimensionally. For the metamaterial structure according to the present invention, any technique may be used for shape processing of the microstructure. For example, semiconductor microfabrication technology or nanoimprint, which can mass-produce nanostructures at low cost, may be used. can.

In the metamaterial structure according to the invention, the incident light can be of any wavelength depending on what is to be detected. The metamaterial structure according to the present invention can be miniaturized by reducing the size of the microstructure. In that case, for example, the wavelength of the incident light may be 2500 nm or less.

In the metamaterial structure according to the present invention, the microstructure is formed by arranging a plurality of unit structures regularly in the vertical and/or horizontal direction along a two-dimensional arrangement plane of the unit structures. may be

In the metamaterial structure according to the invention, said microstructures may consist of any material depending on the wavelength of the incident light, for example gold, silver, copper, aluminum or transition metal nitrides. good too. Examples of transition metal nitrides include TiN, ZrN, HfN, and TaN.

The refractive index sensor according to the present invention may have any configuration as long as it can detect the medium surrounding the metamaterial structure according to the refractive index. For example, the refractive index sensor according to the present invention may have an optical fiber, and the metamaterial structure may be provided at the tip of the optical fiber. In this case, it is possible to measure in situ instead of invasive measurement in which a sample is cut out. Therefore, precise measurement is possible even for a measurement target that is dependent on the environment, such as a living body. Also, by using an optical fiber that is thinner than an injection needle, it is possible to perform measurement with less damage to the living body. In addition, since the measurement range is determined by the diameter of the optical fiber, it is possible to measure the physical quantity in a minute area by using a thin optical fiber. Also, spatial mapping of the measurement results can be performed by moving the optical fiber.

Moreover, the refractive index sensor according to the present invention may have a glass plate, and the metamaterial structure may be provided on the surface of the glass plate. In this case, by placing a substance such as a liquid on the metamaterial structure on the surface of the glass plate, the component in the substance can be detected. Therefore, for example, a blood test can be performed by dropping blood onto the metamaterial structure.

Further, the refractive index sensor according to the present invention may have a detection effect attached to the surface of the microstructure, and the detection effect may be capable of interacting with a detection target. In this case, the detection agent and the detection target can be anything as long as the detection agent can interact with the detection target and a measurable change in refractive index is observed before and after the interaction. may be The interaction between the detectable agent and the detectable entity can be any interaction, eg binding of the detectable entity to the detectable agent. Combinations of the detection agent and the detection target include, for example, detection DNA and complementary DNA, detection RNA and complementary RNA, biotin and avidin or streptavidin, antigens and antibodies, various proteins and substances acting on the proteins, These include various amino acids and substances acting on those amino acids, ligands and receptors. When the detection agent is composed of detection DNA or detection RNA, it can form a complementary strand with the DNA or RNA to be detected, and by measuring the change in refractive index due to the formation of the complementary strand, the desired DNA or RNA to be detected can be detected. In this case, the microstructure is preferably made of a material capable of binding detection DNA or detection RNA.

The metamaterial structure according to the present invention can be used not only for refractive index sensors, but also for chemical sensors, biosensors, in situ analyzers, medical devices integrated with catheters and endoscopes, and high refractive index and chemical/biological materials. It may be applied to a resolution spatial mapper. Here, the chemical sensor is a sensor intended for gases, organic chemicals, etc., and is applied to, for example, taste, food, chemicals, alcohol checks, and the like. A biosensor is a sensor that targets living organisms, living organisms, etc., and is applied to, for example, a toxicity sensor, a blood sugar sensor, a DNA sensor, a medical sensor, and the like. In these cases, by making the measurement results spatially mappable, for example, biosensors can identify the shape, size, and type of cells and DNA. concentration distribution analysis, etc. can be performed. In conventional spot measurement, which cannot perform spatial mapping, for example, when the object to be measured is a mixed substance, only one point can be measured, so it is not possible to know whether it is mixed or not, and what kind of spatial distribution and shape they have. I don't know if there is.

ADVANTAGE OF THE INVENTION According to this invention, the metamaterial structure and refractive index sensor which can perform an inexpensive and precise measurement can be provided.

(a) A side view showing a refractive index sensor according to an embodiment of the present invention, (b) a plan view showing a metamaterial structure according to an embodiment of the present invention. (a) reflection spectrum when the refractive index n around the metamaterial structure of the refractive index sensor of the embodiment of the present invention is 1.0 to 1.5, (b) reflection peak wavelength (Peak wavelength) Fig. 3 is a graph showing the Refractive index dependence; When light is vertically incident on the metamaterial structure of the embodiment of the present invention, which is designed to have (a) polarized light in the X direction and (b) polarized light inclined at 45° from the X direction at a resonant wavelength of 1250 nm, 4 is an electric field intensity distribution map of the upper part of the metamaterial structure; FIG. 1 is a scanning electron microscope (SEM) image of a metamaterial structure according to an embodiment of the invention; (a) Reflection spectra of the refractive index sensor of the embodiment of the present invention, measured in four types of media: air (Air), pure water (DIW), isopropanol (IPA), and glycerin (Glycerin), (b) ) is a graph showing the dependence of the peak wavelength on the refractive index. 4 is a graph showing the relationship between the concentration of the IPA aqueous solution and the peak wavelength of the reflection spectrum of the refractive index sensor of the embodiment of the present invention, measured in the IPA aqueous solution. (a) Side view showing the experimental procedure of a DNA detection experiment, (b) Reflection spectra before and after complementary DNA binding in a dry state, (c) Complementary in a wet state, of a refractive index sensor according to an embodiment of the present invention. Reflection spectra before and after DNA binding. (a) modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) ( It is a top view which shows the modification which reversed the arrangement|positioning shown to c), (e) another modification, and (f) the modification which reversed the arrangement|positioning shown to (e). (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) 3) A plan view showing a modified example in which the arrangement shown in (c) is reversed, (e) another modified example, and (f) a modified example in which the arrangement shown in (e) is reversed. (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) 3) A plan view showing a modified example in which the arrangement shown in (c) is reversed, (e) another modified example, and (f) a modified example in which the arrangement shown in (e) is reversed. (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) ) Modified example in which the arrangement shown in (c) is reversed, (e) Other modified example, (f) Modified example in which the arrangement shown in (e) is reversed, (g) Other modified example, (h) ( It is a top view which shows the modification which reversed arrangement|positioning shown to g). (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) 3) A plan view showing a modified example in which the arrangement shown in (c) is reversed, (e) another modified example, and (f) a modified example in which the arrangement shown in (e) is reversed. (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) ) Modified example in which the arrangement shown in (c) is reversed, (e) Other modified example, (f) Modified example in which the arrangement shown in (e) is reversed, (g) Other modified example, (h) ( It is a top view which shows the modification which reversed arrangement|positioning shown to g). (a) another modified example of the microstructure of the metamaterial structure of the embodiment of the present invention, (b) a modified example in which the arrangement shown in (a) is reversed, (c) another modified example, (d) ) It is a plan view showing a modification in which the arrangement shown in (c) is reversed. (a) the microstructure used in the simulation, (b) the reflection spectrum when the refractive index n around the metamaterial structure is 1.0 to 1.5, of the metamaterial structure of the embodiment of the present invention. (c) is a graph showing the dependence of the reflection peak wavelength on the ambient refractive index; (a) the microstructure used in the simulation, (b) the reflection spectrum when the refractive index n around the metamaterial structure is 1.0 to 1.5, of the metamaterial structure of the embodiment of the present invention. (c) is a graph showing the dependence of the reflection peak wavelength on the ambient refractive index; (a) the microstructure used in the simulation, (b) the reflection spectrum when the refractive index n around the metamaterial structure is 1.0 to 1.5, of the metamaterial structure of the embodiment of the present invention. (c) is a graph showing the dependence of the reflection peak wavelength on the ambient refractive index; FIG. 4 is a perspective view showing a modified example of the refractive index sensor according to the embodiment of the present invention, which has a metamaterial structure on the tip surface of the optical fiber; In the refractive index sensor of the embodiment of the present invention, (a) the connection state between the metamaterial structure, the light source and the optical detector, (b) a modification of the connection state, and (c) the connection state and (d) a perspective view showing another modification of the connection state. 19 is a graph showing the optical response of the refractive index sensor shown in FIG. 18; 19a) to 19f) are cross-sectional views showing a method of manufacturing the refractive index sensor shown in FIG. (a) Electron micrograph showing the metamaterial structure on the tip surface of the optical fiber, (b) the central portion of the tip surface of the optical fiber shown in (a), manufactured by the manufacturing method of the refractive index sensor shown in FIG. 2 is an enlarged electron micrograph (top view), and (c) an enlarged electron micrograph (perspective view) of the center of the tip surface of the optical fiber shown in (a). 23 shows reflection spectra of the refractive index sensor shown in FIG. 22 in air and ethanol. FIG. 5 is a plan view showing a modification of the metamaterial structure according to the embodiment of the present invention when the dimensional parameters are reduced; 25 shows the transmittance and reflectance spectra of the metamaterial structure shown in FIG. 24 when the surrounding refractive index n is between 1.0 and 1.2.

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below based on the drawings.
1 to 25 show metamaterial structures and refractive index sensors according to embodiments of the present invention.

(Sensor configuration and principle)
As shown in FIG. 1 , the refractive index sensor 10 has a metamaterial structure 11 , a transparent substrate 12 , a light source 13 , a half mirror 14 and a spectroscope 15 . As shown in FIG. 1, the metamaterial structure 11 has a fine structure 11a composed of a plurality of cut wires and is provided on a transparent substrate 12. As shown in FIG. As shown in FIG. 1(b), the fine structure 11a is a structure having a wavelength equal to or less than the wavelength of the incident light, and is composed of unit structures arranged rotationally symmetrically two-dimensionally. The metamaterial structure 11 is configured to exhibit a wavelength-selective optical response by increasing optical coupling between incident light and a surface plasmon mode dependent on the microstructure 11a under resonance conditions. The metamaterial structure 11 has a rotationally symmetric unit structure and is independent of polarization.

The fine structure 11a is made of gold (Au), which has a high reflectance in the near-infrared region. In addition to gold, the fine structure 11a may be made of silver, copper, aluminum, or transition metal nitrides such as TiN, ZrN, HfN, and TaN that have a high carrier concentration and exhibit plasmon characteristics. . In a specific example shown in FIG. 1(b), the fine structure 11a has a structure in which two rectangles formed of cut wires and arranged in parallel are arranged in four-fold symmetry.

As shown in FIG. 1( a ), the light source 13 is provided so that the incident light is incident on the metamaterial structure 11 from the transparent substrate 12 side via the half mirror 14 . The half mirror 14 is provided so as to transmit the incident light from the light source 13, reflect the reflected light from the metamaterial structure 11, and bend it by 90 degrees. A bifurcated optical fiber may be used instead of the half mirror 14 . The spectroscope 15 is provided so as to receive reflected light from the metamaterial structure 11 reflected by the half mirror 14 and detect its spectrum. A combination of a variable wavelength light source and a photodetector may be used instead of the spectroscope 15 .

Next, the action will be described.
Refractive index sensor 10 is used as follows. That is, as shown in FIG. 1A, a sample 1 whose refractive index is to be measured is dropped onto a metamaterial structure 11 on a transparent substrate 12, and the reflection spectrum of the metamaterial structure 11 is measured by a spectrometer 15. to detect. As a result, the refractive index of the sample 1 can be calculated from the reflection spectrum shift according to the refractive index change due to the dropping of the sample 1, with air (refractive index n=1.0) as a reference.

The metamaterial structure 11 can adjust the selective wavelength of photoresponse by the dimensions of the microstructure 11a. In addition, the metamaterial structure 11 sensitively changes the selected wavelength depending on the surrounding medium. Therefore, the refractive index sensor 10 can detect the refractive index of the surrounding medium from the change in the selected wavelength or the change in the reflectance at the specific wavelength due to the medium surrounding the metamaterial structure 11 . In addition, the refractive index sensor 10 has a simpler structure than a conventional refractive index sensor using propagating surface plasmon, and can be manufactured at a low cost.

Since the shape of the metamaterial structure 11 and the refractive index sensor 10 can be arbitrarily designed based on the concept of metamaterials, plasmon enhancement can be optimized. Therefore, the sensitivity can be made higher than that of the conventional one, and precise measurement can be performed. Any technique may be used for shape processing of the microstructure 11a of the metamaterial structure 11. For example, semiconductor microfabrication technology or nanoimprint, which can mass-produce nanostructures at low cost, may be used. can.

(optical design)
A metamaterial structure 11 was manufactured and the sensitivity of the refractive index was measured.
First, the optical design of the metamaterial structure 11 was performed. Rigorous Coupled-Wave Analysis method was used for optical design. In the metamaterial structure 11 used for design, a plurality of unit structures of the microstructure 11a shown in FIG. 1(b) were regularly arranged in vertical and horizontal directions. In addition, the metamaterial structure 11 has w (width of cut wire) in the figure of 90 nm, g (gap between cut wires arranged in parallel) of 150 nm, and d (gap between cut wires arranged vertically). 150 nm, l (length of cut wire) was 330 nm, Λ (period) was 1000 nm, and t (thickness) was 40 nm. In the configuration shown in FIG. 1B, in which the fine structure 11a is made of a rectangular cut wire, the aspect ratio (l/w) is 3 to 4 and the surface having a unit structure is used to increase the refractive index sensitivity. is preferably 0.08 to 0.12.

When the refractive index n around the metamaterial structure 11 is changed from 1.0 to 1.5, the reflection spectrum at each refractive index is obtained by design calculation, and the calculation result is shown in FIG. 2(a). . Note that the harmonics at the time of calculation was set to 6. Also, from FIG. 2(a), the relationship between the peak wavelength of the reflection spectrum and the refractive index is calculated and shown in FIG. 2(b). As shown in FIG. 2B, it was confirmed that the peak wavelength of the reflection spectrum changed linearly according to the refractive index change around the metamaterial structure 11 . The refractive index sensitivity of the refractive index sensor 10 obtained from FIG. 2(b) is 593 nm/refractive index unit (RIU).

The electric field intensity distribution at the upper part of the metamaterial structure 11 when light is perpendicularly incident on the metamaterial structure 11, which is designed to have a resonance wavelength of 1250 nm and polarized light in the X direction and polarized light tilted 45° from the X direction, They are shown in FIGS. 3(a) and 3(b), respectively. The directions of polarization are indicated by arrows in FIGS. 3(a) and 3(b). Moreover, the refractive index around the metamaterial structure 11 is set to 1.0. The resonance wavelength of around 1250 nm is called the "second optical window of the living body" and is known to be less absorbed and scattered by the living body.

In FIG. 3A, the electric field is amplified at both ends of the cut wire whose longitudinal direction is the X direction, and it was confirmed that the resonance characteristic is exhibited in the X direction. Further, in FIG. 3B, it was confirmed that the electric field was amplified at both ends of all the cut wires, and resonance characteristics were exhibited for any polarization direction.

(manufacturing result)
Based on the optical design, a metamaterial structure 11 was fabricated on a quartz substrate by a lift-off process. That is, first, an EB (Electron beam) resist was spin-coated on a substrate, and then a resist pattern was produced by EB lithography. A metamaterial structure 11 was manufactured by depositing Ti (thickness: 1 nm) and Au (thickness: 40 nm) thereon by EB vapor deposition, and then removing the EB resist. Other semiconductor microfabrication techniques or nanoimprinting may be used to manufacture the metamaterial structure 11 .

A scanning electron microscope (SEM) image of the manufactured metamaterial structure 11 is shown in FIG. The dimensions of the fabricated metamaterial structure 11 are w (width of cut wire) of 84 nm, g (gap between parallel cut wires) of 156 nm, and d (gap between vertically arranged cut wires) of 162 nm. , l (length of cut wire) was 312 nm, Λ (period) was 1000 nm, and t (thickness) was 40 nm. Although the width (w) and length (l) were slightly smaller and the gaps (g, d) were slightly larger than those at the time of design, they could be manufactured with good accuracy. Since the roundness of the edge of each cut wire affects the Q value of resonance, it is desirable to make the edge as sharp as possible. Moreover, it is preferable that the side surface of the cut wire is substantially perpendicular to the surface of the substrate without being tapered.

(Optical measurement result)
Using the manufactured metamaterial structure 11, reflection spectra were measured by the spectroscope 15 in four types of media: air (Air), pure water (DIW), isopropanol (IPA), and glycerin (Glycerin). The measured reflection spectrum is shown in FIG. 5(a), and the relationship between the peak wavelength of the reflection spectrum and the refractive index of the surrounding medium is shown in FIG. 5(b). In FIG. 5B, the refractive index of each medium is 1.000 for air, 1.321 for DIW, 1.368 for IPA, and 1.461 for glycerin. The sensitivity of the refractive index obtained from FIG. 5(b) was 599 nm/RIU, confirming that the sensitivity is extremely high. As a result of recalculation using the dimensions of the manufactured metamaterial structure 11, the sensitivity of the refractive index was 605 nm/RIU, which was close to the measured value. This result indicates that the refractive index sensitivity of a conventional refractive index sensor using surface plasmon is 60 to 190 nm/RIU (Takashi Yoshida, "Plasmonics", NTS Co., Ltd., August 24, 2011, p. 129), the sensitivity is more than three times higher at maximum. The difference between the calculated value and the measured value is due to the measurement error associated with the signal noise component caused by the reattachment of some of the Au particles that were exfoliated during lift-off to the vicinity of the metamaterial structure 11 and disturbing the surface plasmon mode. is thought to be a factor.

(Concentration measurement of IPA)
Using the manufactured metamaterial structure 11, when changing the concentration of the IPA aqueous solution, the change in the reflection spectrum at each concentration was measured. The room temperature was 24° C., and the measurement was performed within 30 seconds after IPA and water were mixed. FIG. 6 and Table 1 show the relationship between the concentration of the IPA aqueous solution and the peak wavelength of the reflection spectrum. The minimum wavelength resolution of the spectroscope 15 used is 1.7 nm. As shown in FIG. 6 and Table 1, it was confirmed that as the IPA concentration was decreased, the reflection peak wavelength shifted to the shorter wavelength side. It was also confirmed that the reflection peak wavelength did not change from 1357.4 nm in the IPA concentration range of 4.6% to 1.2%, and changed to 1355.7 nm when the concentration reached 0.6%. From this result, it can be said that the minimum IPA concentration change of 4% could be observed.

(Detection of DNA)
Using the manufactured metamaterial structure 11, an experiment was conducted to detect the presence of DNA. As shown in FIG. 7(a), in the experiment, first, the surface of the fine structure 11a of the metamaterial structure 11 was modified with the detection DNA 21, and the detection DNA 21 was bound with the complementary DNA 22 to be detected to complement the detection target. Chain 23 was formed. Before and after binding the complementary DNA 22, the reflectance spectra were measured in a dry state and a wet state, and a change in peak wavelength (refractive index) before and after binding the complementary DNA 22 was detected. In the experiment, a gene having a non-significant base sequence was prepared and used as the detection DNA 21 and the complementary DNA 22 . Also, depending on the design of the base sequence, it is possible to produce DNA that binds only to specific proteins.

The measurement results of the reflectance spectra before and after complementary DNA 22 binding in dry and wet conditions are shown in FIGS. 7(b) and 7(c), respectively. As shown in FIG. 7(b), it was confirmed that the binding of the complementary DNA 22 caused the reflection peak wavelength to shift by 8.5 nm to the longer wavelength side in the dry state. Moreover, as shown in FIG. 7(c), it was confirmed that the wavelength shifts to the short wavelength side by 1.7 nm in the wet state. As described above, a shift in the reflection peak wavelength was observed before and after the formation of the complementary strand, and it was confirmed that the refractive index was changed. When the refractive index of the complementary DNA 22 is calculated from the measurement results, the refractive index in the dry state is 1.13 and the refractive index in the wet state is 1.30.

Note that the detection agent and the detection target are not limited to the detection DNA 21 and the complementary DNA 22, respectively, and the detection agent can interact with the detection target. can be anything as long as it is acceptable. The interaction between the detectable agent and the detectable entity can be any interaction, eg binding of the detectable entity to the detectable agent. Combinations of the agent for detection and the target to be detected include, for example, biotin and avidin or streptavidin, antigens and antibodies, various proteins and substances acting on the proteins, various amino acids and substances acting on the amino acids, ligands and receptors. and so on.

[Modification: Arrangement Variation of Metamaterial Structure]
The two-dimensional arrangement of the metamaterial structure 11 is not particularly limited as long as the microstructures 11a made up of a plurality of cut wires are two-dimensionally periodically arranged rotationally symmetrically. 8 to 14 show examples of layout variations of the metamaterial structure 11. FIG.

The metamaterial structure 11 is, for example, a fine structure 11a (see FIG. 8) in which one or a plurality of rectangular cut wires arranged in parallel are arranged in four-fold symmetry, or a lattice, a circle, or a cross. A fine structure 11a (see FIG. 9) in which cut wires are arranged in four-fold symmetry, and a fine structure 11a (see FIG. 10) in which cut wires formed in a crossed shape of a swastika shape and an I shape are arranged in four-fold symmetry. ), circular ring, rectangular frame shape, and cut wires formed by doubling them, fine structure 11a (see FIG. 11) arranged in 4-fold symmetry, cross shape, swastika shape excluding the center part A fine structure 11a (see FIG. 12) in which cut wires formed into a shape are arranged in a 4-fold symmetry, and cut wires formed in a shape excluding a part of an annular or square frame shape are arranged in a 4-fold symmetry. A fine structure 11a (see FIG. 13), a fine structure 11a in which cut wires formed in a triangular frame shape are arranged in 6-fold symmetry, and a fine structure 11a in which cut wires formed in a triangular shape are arranged in 4-fold symmetry ( 14).

In addition, as shown in FIGS. 8 to 14, the metamaterial structure 11 may be obtained by inverting (complementary) the portion composed of the cut wires of each variation and the portion without the cut wires. . In addition, the metamaterial structure 11 is not limited to the four-fold or six-fold symmetric arrangement shown in FIGS.

(Simulation of various variations of metamaterial structures)
A simulation using the Rigorous Coupled-Wave Analysis method was performed for three types of metamaterial structures 11 . Note that the harmonics at the time of calculation was set to 6. The metamaterial structure 11 used in the simulation has the fine structure 11a arranged and sized as shown in FIGS. 15(a), 16(a), and 17(a). The transparent substrate 12 is made of SiO ₂ and the microstructures 11a are made of Au. The thickness of the microstructure 11a is 40 nm.

When the refractive index n around each metamaterial structure 11 is changed from 1.0 to 1.5, the reflection spectrum at each refractive index is obtained, and the calculation results are shown in FIGS. 15(b) and 16, respectively. (b) and FIG. 17(b). 15(b), 16(b), and 17(b), the relationship between the peak wavelength of the reflection spectrum and the refractive index is determined, and the (c). As shown in FIGS. 15(c), 16(c), and 17(c), the peak wavelength of the reflection spectrum changes substantially linearly according to the change in the refractive index around the metamaterial structure 11. was confirmed. The refractive index sensitivities of the refractive index sensor 10 obtained from FIGS. 15(c), 16(c) and 17(c) were 368 nm/RIU, 458 nm/RIU and 824 nm/RIU, respectively.

[Modification: Optical fiber refractive index sensor]
As shown in FIG. 18 , the refractive index sensor 10 may have an optical fiber 31 and the metamaterial structure 11 may be provided on the tip surface 31 a of the optical fiber 31 . In this case, as shown in FIG. 19( a ), the refractive index sensor 10 is connected to the metamaterial structure 11 , the light source 13 and the optical detector 32 via an optical fiber 31 . The photodetector 32 includes the spectroscope 15 and power meter. As shown in FIG. 19A, the refractive index sensor 10 introduces incident light from the light source 13 from the rear end face of the optical fiber 31, and converts the light reflected back from the metamaterial structure 11 into light. It is read by the detector 32 .

The refractive index sensor 10 may have the configuration shown in FIGS. 19(b) to 19(d) instead of the configuration shown in FIG. 19(a). In the configuration shown in FIG. 19( b ), incident light is introduced from the rear end face of the optical fiber 31 and the light transmitted through the metamaterial structure 11 is read by the photodetector 32 . In the configuration shown in FIG. 19C, light is irradiated from the sample side, and the light that has passed through the metamaterial structure 11 and the optical fiber 31 is read by the photodetector 32 . In the configuration shown in FIG. 19( d ), light is irradiated from the sample side, and the reflected light of the light impinging on the metamaterial structure 11 is read by the photodetector 32 .

By reading changes in resonance wavelength and changes in reflectance using the configuration described above, it is possible to sense the value of the surrounding refractive index. That is, since a slight change in the refractive index around the metamaterial structure 11 shifts the resonance wavelength, by monitoring the reflected light from the metamaterial structure 11, as shown in FIG. A change ΔR in reflectivity or a change Δλ in center (resonant) wavelength can be read in response to a small change ΔN in index N.

The refractive index sensor 10 provided with the metamaterial structure 11 on the tip surface 31a of the optical fiber 31 is not an invasive measurement in which the sample is cut as long as the tip of the optical fiber 31 reaches, but the original location (in situ ) can be measured. Therefore, precise measurement is possible even for a measurement target that is dependent on the environment, such as a living body. In addition, by using the optical fiber 31 thinner than the injection needle, it is possible to perform measurement with less damage to the living body or the like. In addition, since the measurement range is determined by the diameter of the optical fiber 31, by using the thin optical fiber 31, it is possible to measure the physical quantity in a minute area. Further, by measuring while moving the optical fiber 31, spatial mapping of the measurement results can be performed.

(Manufacturing method of optical fiber type refractive index sensor)
A refractive index sensor 10 in which the metamaterial structure 11 was provided on the tip surface 31a of the optical fiber 31 was manufactured. FIGS. 21a) to 21f) show a method of manufacturing the metamaterial structure 11 on the tip surface 31a of the optical fiber 31. FIG. In the manufacturing method, first, the tip surface 31a of the cut optical fiber 31 is cleaned by ultrasonic cleaning using pure water or an organic solvent or by ultraviolet/ozone treatment, and then the tip surface 31a of the cut optical fiber 31 is , UV curing resin 33 is applied. Next, the tip surface 31a of the optical fiber 31 coated with the ultraviolet curable resin 33 is vertically pressed against a flat glass plane 34, and the ultraviolet curable resin 33 is cured by irradiating ultraviolet rays (see FIG. 21a). After that, it is separated from the flat glass plane 34 and the ultraviolet curable resin 33 is flattened. Note that when the metamaterial structure 11 is formed on the glass substrate instead of forming it on the tip surface 31a of the optical fiber 31, the step of FIG. 21a) is unnecessary. Further, even when the tip surface 31a of the optical fiber 31 is sufficiently smooth, the step of FIG. 21a) is unnecessary.

Next, Ti, Au, and Cr are deposited in this order on the surface of the flattened ultraviolet curing resin 33 (the glass substrate surface when forming on a glass substrate) to a thickness of 1 nm, 40 nm, and 5 nm. (see FIG. 21b)). A sputtering device or a vapor deposition device is used for the deposition. Subsequently, an ultraviolet curing resin 35 is applied onto the film formation surface (see FIG. 21c). After that, prepare a separately manufactured silicon or quartz mold (template), press the mold against the ultraviolet curing resin 35, and irradiate the ultraviolet curing resin 35 with ultraviolet rays for about 120 seconds to cure the ultraviolet curing resin 35 (see FIG. 21d)). . Note that Ti is used for the purpose of enhancing the adhesion between Au and the ultraviolet curable resin 33 (or the glass substrate), and Ti is not necessary if the adhesion is good.

Next, after removing the excess uncured UV curable resin 35 adhering to the side surface of the optical fiber 31 by rinsing it with ethanol for about 30 seconds, the optical fiber 31 is heated with a hot plate at 100° C. for about 30 seconds. dry. After that, ion milling (8 kV, 200 μA) is performed for about 45 seconds to sequentially etch Cr, Au, and Ti (see FIG. 21e)). Subsequently, Cr is wet-etched for about 10 seconds to remove Cr and the UV curable resin 35 remaining on Cr (see FIG. 21f)). Here, by removing Cr by wet etching, the UV curable resin 35 remaining on Cr is also peeled off. Thus, the metamaterial structure 11 is manufactured on the distal end surface 31 a of the optical fiber 31 .

(Manufacturing result of optical fiber type refractive index sensor)
Electron micrographs of the metamaterial structure 11 manufactured on the tip surface 31a of the optical fiber 31 are shown in FIGS. 22(a) to 22(c). FIG. 23 shows the reflection spectrum of the manufactured optical fiber type refractive index sensor 10 . As shown in FIG. 23, when the metamaterial structure 11 formed on the tip surface 31a of the optical fiber 31 is exposed to environments of air and ethanol respectively, the resonance wavelength changes due to the change in the refractive index. It was confirmed that it changed and functioned as the refractive index sensor 10 .

[Modification: Layout dimensions of metamaterial structures]
Since the metamaterial structure 11 obeys the scaling law, there are basically no physical restrictions on its dimensions. That is, if the size is reduced, the resonance wavelength shifts to the shorter wavelength side, and if the size is increased, the resonance wavelength shifts to the longer wavelength side. For example, a system configuration as shown in FIG. 1 is possible. However, in order to avoid contact between the cut wires constituting the unit structure of the metamaterial structure 11, w (width of cut wire), g (gap between cut wires arranged in parallel), d (vertical arrangement) The gap between the cut wires) and l (the length of the cut wires) should be greater than 0 nm.

Regarding the thickness t of the cut wire, taking the physical quantity of the skin depth as a guideline, for example, since the skin depth of aluminum in visible light is 13 nm, t should be 13 nm or more. . Also, the lower limits of the dimension parameters other than t are determined by the limits of manufacturing technology at that time.

An example of arrangement of the metamaterial structures 11 when each dimension parameter is reduced is shown in FIG. Here, the transparent substrate 12 is made of _SiO2 , and the fine structure 11a of the metamaterial structure 11 is made of silver (Ag). The dimensions are w=40 nm, g=60 nm, d=20 nm, l=140 nm, Λ=500 nm, t=40 nm.

Calculate the transmittance spectrum and reflectance spectrum of the metamaterial structure 11 when the refractive index n around the metamaterial structure 11 shown in FIG. 24 is changed from 1.0 to 1.2. , as shown in FIG. The Rigorous Coupled-Wave Analysis method was used for the calculation. The harmonics at the time of calculation was set to 6. As shown in FIG. 25, it was confirmed that the refractive index change and the peak wavelength shift have approximately a linear relationship. From FIG. 25, the refractive index sensitivity of this metamaterial structure 11 is 750 nm/RIU. This result greatly exceeds the sensitivity of the conventional refractive index sensor using surface plasmons.

On the other hand, the upper limit of each dimensional parameter can be arbitrarily set depending on the wavelength (frequency) to which the range of application of the refractive index sensor 10 is to be expanded, since there are no manufacturing restrictions. In the above, the visible to near-infrared (wavelength of about 2000 nm or less) has been assumed. 999 μm)) can also be envisaged. In this case, if the dimensions are simply estimated by the scaling law, for example, the design wavelength of 1.24 μm is similarly expanded to 0.3 THz (wavelength of 999 μm), w=72.5 μm, g=120.8 μm, d= 120.8 μm, l=265.9 μm, Λ=805.6 μm, t=32.2 μm.

Terahertz (THz) waves are safe for living organisms, and there are many chemical biomaterials that have an absorption peak at THz (because the absorption frequency is unique to the material, it can be used to identify the material). Due to the property of THz transmission, the use of THz for security checks at airports and drug checks in international mail (drugs have a unique fingerprint spectrum at a specific frequency of THz) is under consideration. According to the metamaterial structure 11 and the refractive index sensor 10 of the embodiment of the present invention, it becomes possible to develop a bio-chemical sensor based on the refractive index sensor 10 in the terahertz region.

[Modification: Application to blood test]
The refractive index sensor 10 may have a glass plate and the metamaterial structure 11 is provided on the surface of the glass plate. Further, the surface of the microstructure 11a of the metamaterial structure 11 may be attached with a ligand such as the detection DNA 21 shown in FIG. 7(a). In this case, by dropping blood onto the metamaterial structure 11 on the surface of the glass plate and obtaining the refractive index from the refractive index change, the components in the blood can be detected. In addition to blood, by placing a substance such as other liquid on the metamaterial structure 11, a component in the substance can be detected. Further, according to the manufacturing method of FIG. 21, the metamaterial structure 11 can be manufactured on the surface of the glass plate.

REFERENCE SIGNS LIST 1 sample 10 refractive index sensor 11 metamaterial structure 11a fine structure 12 transparent substrate 13 light source 14 half mirror 15 spectroscope

21 DNA for detection
22 Complementary DNA
23 complementary strand

31 optical fiber 31a tip surface 32 photodetector 33, 35 ultraviolet curing resin 34 glass plane

Claims (10)

A metamaterial structure for use in a refractive index sensor that detects the refractive index of a surrounding medium, comprising:
A structure having a structure equal to or less than the wavelength of incident light, and having a fine structure composed of unit structures arranged periodically two-dimensionally in a rotationally symmetrical manner ,
wherein optical coupling between the incident light and the surface plasmon mode dependent on the fine structure is strengthened under resonance conditions, thereby exhibiting wavelength-selective optical response .
Characterized metamaterial structure. 2. A metamaterial structure according to claim 1, wherein said incident light has a wavelength of 2500 nm or less. 3. The metamaterial structure according to claim 1, wherein the metamaterial structure is independent of polarization. The unit structure has a structure in which two rectangles arranged in parallel are arranged in four-fold symmetry,
3. The fine structure is formed by arranging a plurality of unit structures regularly in the vertical direction and/or the horizontal direction along a two-dimensional arrangement plane of the unit structures. Metamaterial structure according to any one of. 5. A metamaterial structure according to any one of claims 1 to 4, wherein said microstructure is made of gold, silver, copper, aluminum or a transition metal nitride. A refractive index sensor comprising the metamaterial structure according to any one of claims 1 to 5. having an optical fiber,
7. The refractive index sensor according to claim 6, wherein the metamaterial structure is provided at the tip of the optical fiber. having a glass plate,
7. The refractive index sensor according to claim 6, wherein said metamaterial structure is provided on the surface of said glass plate. a sensing agent attached to the surface of the microstructure;
9. The refractive index sensor according to any one of claims 6 to 8, wherein the detection action body is capable of interacting with a detection target. The detection agent is composed of detection DNA or detection RNA, and is capable of forming a complementary strand with the detection target DNA or RNA,
10. The refractive index sensor according to claim 9, wherein said microstructure is made of a material capable of binding said detection DNA or said detection RNA.

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