Light condensing device and method for manufacturing light condensing device

Abstract

This light condensing device comprises: an array in which a plurality of concave curved mirrors are two-dimensionally arranged; a dielectric layer that has a main surface above the upper ends of the plurality of concave curved mirrors while covering the plurality of concave curved mirrors; and a metal nanostructure that is provided directly above each of the plurality of concave curved mirrors with a part thereof embedded in the dielectric layer and the remainder exposed from the main surface. At least one metal nanostructure is provided directly above each of the plurality of concave curved mirrors.

Description

Light-collecting device and method for manufacturing the same

[0001] The present disclosure relates to light-collecting devices and methods for manufacturing the same.

[0002] Patent Literature 1 discloses a light absorption device having a support substrate, a light-reflecting layer, a dielectric layer, and multiple metal nanostructures. The light-reflecting layer and the dielectric layer function as a Fabry-Perot resonator when irradiated with light. The multiple metal nanostructures generate localized surface plasmon resonance when irradiated with light. Strong coupling is then induced between the localized surface plasmon and the Fabry-Perot resonator, enabling light to be collected over a wide wavelength range. The multiple metal nanostructures disclosed in Patent Literature 1 are partially embedded in the dielectric layer, with the remaining portions exposed to the outside of the dielectric layer and in contact with a substance (solid, liquid, or gas) above the dielectric layer, allowing the collected light to be used to promote interaction with other substances.

[0003] The light absorption device (photoelectrode) of Patent Document 1 can promote water oxidation when water is present on the dielectric layer and the plurality of metal nanostructures. Furthermore, it can promote photoelectric conversion when a hole transport material is present on the dielectric layer and the plurality of metal nanostructures. In other words, this photoelectrode can be applied to plasmonic solar cells and photocatalytic systems (water splitting, ammonia synthesis, carbon dioxide fixation, organic matter oxidation-reduction reactions, etc.).

[0004] Japanese Patent No. 7194944

[0005] When using photoelectrodes to generate hydrogen, oxygen, ammonia, fix carbon dioxide, and the like, the challenge is how to achieve multi-electron reactions using sunlight, a light source with a low photon density. Because electrons are generated by photons, the photon flux density, i.e., the intensity of light, must be increased to induce multi-electron reactions. The light absorption device with a planar resonator disclosed in Patent Document 1 is not suitable for inducing multi-electron reactions because it does not adequately collect light and is unable to sufficiently increase the local concentration of light.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a light-harvesting device suitable for the progression of a multi-electron reaction and a method for manufacturing the light-harvesting device.

[0007] The light-collecting device according to the present disclosure is characterized by comprising an array having a plurality of concave curved mirrors, a dielectric layer covering the plurality of concave curved mirrors and having a main surface above the upper ends of the plurality of concave curved mirrors, and metal nanostructures provided directly above each of the plurality of concave curved mirrors, with a portion embedded in the dielectric layer and the remainder exposed from the main surface.

[0008] Other features of the present disclosure are set forth below.

[0009] It is possible to provide a light-harvesting device suitable for the progression of a multi-electron reaction and a method for manufacturing the light-harvesting device.

[0010] 1 is an XZ cross-sectional view of a focusing device according to an embodiment; FIG. 2 is a perspective view showing an example of the configuration of a concave curved mirror; FIG. 3 is a plan view showing an example of the relative positions of a concave curved mirror and metallic nanostructures; FIG. 4 is an example of an optical path of incident light; FIG. 5 is a diagram showing a unit structure adopted in a simulation; FIG. 6 is a diagram showing the relationship between the number of metallic nanostructures provided directly above one concave curved mirror and the reflection spectrum; FIG. 7 is a diagram showing the distribution of electric field intensity enhanced by the focusing device; FIG. 8 is a cross-sectional view of a focusing device according to a comparative example; FIG. 9 is a diagram showing the relationship between the number of metallic nanostructures and the reflection spectrum; FIG. 10 is a diagram showing the distribution of electric field intensity enhanced by the focusing device according to the comparative example; FIG. 11 is a diagram defining the dimensions of the concave curved mirror; FIG. 12 is a diagram showing the relationship between the dimensions of the concave curved mirror and the electric field enhancement effect; FIG. 13 is a diagram defining the angle of incidence of light onto the focusing device; FIG. 14 is a diagram showing the incidence angle dependence of the electric field enhancement effect; FIG. 15 is a diagram showing an example arrangement of metallic nanostructures; FIG. 16 is a diagram showing the relationship between the number of metallic nanostructures and the electric field enhancement effect; FIG. 17 is a model diagram used in a second evaluation axis; FIG. 18 is a diagram showing the relationship between the radius of curvature of the concave curved mirror and the effective integrated electric field intensity; and FIG. 19 is a diagram showing the relationship between the thickness of the dielectric layer and the effective integrated electric field intensity. 20 is a diagram showing the incidence angle dependence of the effective integrated electric field strength. FIG. 21 is a table summarizing the data of the graph in FIG. 20. FIG. 22 is a plan view showing an example in which a reflective layer is formed using two types of concave curved mirrors, large and small. FIG. 23 is a plan view showing an example in which three types of concave curved mirrors of different sizes are formed. FIG. 24 is a diagram showing an example of a method for manufacturing a light-collecting device.

[0011] A light-collecting device and a method for manufacturing the light-collecting device according to an embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0012] 1 is a partial cross-sectional view of a light-collecting device 10 according to an embodiment. According to one example, the length (thickness) of the light-collecting device 10 in the Z direction is several hundred nanometers or less, and the lengths in the X and Y directions are on the scale of 1 meter or more or several meters. The light-collecting device 10 includes a support substrate 12. The support substrate 12 is provided to support and maintain the structure thereon. The material of the support substrate 12 is not particularly limited, but it can be, for example, glass, resin, glass with resin thereon, ceramics, or a laminated structure thereof.

[0013] An array 14 having a plurality of concave curved mirrors is formed on the support substrate 12. The array 14 functions as a light reflecting layer. FIG. 1 illustrates that the array 14 includes concave curved mirrors 14a, 14b, 14c, 14d, and 14e. These concave curved mirrors reflect light of a desired wavelength and focus the light into a nanospace. The concave curved mirrors may be, for example, a metal or dielectric multilayer film. The concave curved mirrors may have any shape that can reflect sunlight incident on the light-focusing device 10 and focus the light into a predetermined nanospace. In one example, the concave curved mirror has a curved surface such as a parabolic surface, a hyperbolic surface, an elliptical surface, or a spherical surface. In another example, the concave curved mirror has a curved surface with multiple curvatures. The multiple curvatures may be provided to increase the light-focusing efficiency. As an example of a concave curved mirror with multiple curvatures, the joint between two concave curved mirrors may be curved upward. In this case, the curvature of the concave curved mirror is small at the center and large near the junction with the adjacent concave curved mirror. According to yet another example, the curvature of a single concave curved mirror can be varied continuously or in steps.

[0014] FIG. 2 is a perspective view showing an example of the configuration of an array 14 having a plurality of concave curved mirrors. In this example, the array 14 is shown to have a shape in which a plurality of unit structures each having a parabolic surface are arranged. For example, the vertical and horizontal lengths of one concave curved mirror are 10 nm to several μm, and the depth is determined according to the curvature of the concave curved surface. In other examples, other dimensions are adopted. For the sake of convenience, only nine concave curved mirrors are shown in FIG. 2, but it is also possible to arrange such concave curved mirrors in the vertical and horizontal directions, for example, 10 nm to several μm. ８ pcs to 10 ６ In this case, about 10 １６ From 10 １２In this example, there are no gaps between the concave curved mirror and the multiple concave curved mirrors surrounding it. For example, there are no gaps between the concave curved mirror 14b in FIG. 2 and the eight concave curved mirrors surrounding it. By arranging all the concave curved mirrors in this manner, the light receiving area can be made larger than when there are gaps between the concave curved mirrors. There are ridges at the connections between the concave curved mirror 14b and the four concave curved mirrors above, below, left, and right of the concave curved mirror 14b. In one example, the outer edge of one concave curved mirror surrounded by nine concave curved mirrors includes four ridges and four vertices.

[0015] Returning to the explanation of FIG. 1 , a dielectric layer 16 is formed on the array 14. The dielectric layer 16 covers the multiple concave curved mirrors. A major surface 16a of the dielectric layer 16 is located above an upper end 14T of the array 14. In other words, the entire array 14 is located closer to the support substrate 12 than the major surface 16a of the dielectric layer 16. Therefore, when the light-collecting device 10 is viewed from above, the major surface 16a of the dielectric layer 16 is directly visible, but the array 14 is visible only through the dielectric layer 16.

[0016] Metal nanostructures are partially embedded in the dielectric layer 16. FIG. 1 illustrates metal nanostructures 18a, 18b, 18c, 18d, and 18e. The metal nanostructures are partially embedded in the dielectric layer 16, with the remaining portions exposed from the main surface 16a of the dielectric layer 16. This allows the metal nanostructures to come into contact with the solid, liquid, or gas on the dielectric layer 16. In the example of FIG. 1, one metal nanostructure is provided directly above each of the multiple concave curved mirrors. Specifically, metal nanostructures 18a, 18b, 18c, 18d, and 18e are provided in a one-to-one correspondence directly above the concave curved mirrors 14a, 14b, 14c, 14d, and 14e, respectively. One advantage of providing one metal nanostructure per concave curved mirror is that the number of metal nanostructures blocking light entering the light-collecting device 10 can be minimized.

[0017] The primary surface 16a of the dielectric layer 16 is the light-receiving surface of this light-collecting device 10. If a portion of the reflective layer were exposed from the primary surface 16a, some of the incident sunlight would be irradiated onto that exposed portion, resulting in a reduced light-receiving area of ​​the primary surface 16a. In contrast, according to the configuration example of the light-collecting device 10 of FIG. 1, the primary surface 16a of the dielectric layer 16 is located above the upper end 14T of the array 14, as described above, which is suitable for maximizing the light-receiving area of ​​the primary surface 16a. In other words, since only the metal nanostructures are located above or at the same level as the primary surface 16a, the entire area without the metal nanostructures can be used as the light-receiving surface. This contributes to the electric field enhancement described below.

[0018] FIG. 3 is a plan view of a metal nanostructure and a concave curved mirror. In the example of FIG. 3, one metal nanostructure is formed in the nanospace at the center of one concave curved mirror. For example, one metal nanostructure 18a is located at the center of concave curved mirror 14a, one metal nanostructure 18b is located at the center of concave curved mirror 14b, and one metal nanostructure 18c is located at the center of concave curved mirror 14c. When sunlight incident on the light-collecting device 10 is concentrated into the central nanospace by the concave curved mirror, localized surface plasmon resonance is generated by the metal nanostructure. The resonant wavelength can be changed as desired by adjusting the size and shape of the metal nanostructure.

[0019] 4 is a diagram illustrating the optical path of incident light. Light La incident from the upper side near the left end of the concave curved mirror 14a is reflected by the concave curved mirror 14a and reaches the metallic nanostructure 18a. Light Lb incident from the upper side near the center of the concave curved mirror 14a is reflected by the concave curved mirror 14a and reaches the metallic nanostructure 18a. Light Lc incident from the upper side near the right end of the concave curved mirror 14a is reflected by the concave curved mirror 14a and reaches the metallic nanostructure 18a. All incident light is focused at a focusing point, at which the metallic nanostructure 18a is located. The multiple concave curved mirrors according to the embodiment may have various shapes that focus the light beam incident on the main surface of the dielectric layer 16 near the metallic nanostructure.

[0020] Such a light-collecting device 10 functions as a photoelectrode. When the light-collecting device 10 is used as a plasmonic photoanode, the array 14, which functions as a reflective layer, is made of, for example, Ag, and the dielectric layer 16 is made of, for example, TiO. ２ , SrTiO ３ On the other hand, when the light-collecting device 10 is used as a plasmonic photocathode, the array 14 functioning as a reflective layer is, for example, Pt, and the dielectric layer 16 is, for example, NiO, GaN, CoO, or Cu. ２ The metal nanostructure is a metal such as Au, Ag, Cu or Al.

[0021] By concentrating light into a nanospace using a concave curved mirror, it is possible to significantly enhance the electric field, i.e., increase the light density, compared to particle plasmons or planar resonator coupling systems. Because multi-electron reactions increase nonlinearly with carrier concentration, increasing the local light concentration, as in the light-concentrating device 10 of the present disclosure, can dramatically promote multi-electron reactions. Therefore, it is possible to provide a reaction field in which multi-photon processes can proceed, which was previously difficult.

[0022] Here, we will give some examples of multi-electron reactions. One multi-electron reaction that requires a large number of electrons is the production of oxygen through the four-electron oxidation of water. The generation of oxygen through the four-electron oxidation of water can be expressed by the following formula: 2H ２ O → 4e － +4H ＋ +O ２ This reaction requires four photons / electrons. Another multi-electron reaction that requires a large number of electrons is the production of ammonia by the six-electron reduction of nitrogen. The production of ammonia by the six-electron reduction of nitrogen can be expressed by the following formula: N ２ +6H ＋ +6e － →2NH ３ This reaction requires six electrons.

[0023] On the other hand, multi-electron reactions that require fewer electrons include the two-electron reduction of water to produce hydrogen. This reaction requires only two electrons. Multi-electron reactions other than these can also be distinguished by the number of electrons required.

[0024] In order to induce a multi-electron reaction requiring a large number of electrons, the local electric field strength must be increased. Therefore, the highest electric field strength values ​​at the XZ cross section were investigated for the light-collecting device of this embodiment and the light-collecting device of the comparative example. The XZ cross section is, for example, the cross section shown in FIG. 1 . The investigation was performed by simulation. The simulation was performed under the following conditions. Electromagnetic field calculations were performed using the finite-difference time-domain (FDTD) method using Ansys Lumerical FDTD software. In this embodiment, a cube with mesh sizes of X = 1 nm, Y = 1 nm, and Z = 1 nm was used as the minimum Yee lattice unit for the simulation. Furthermore, a plane wave was incident so that the electric field was parallel to the X direction. For the unit structures shown in FIGS. 5 and 8 , the boundary conditions in the X and Y directions were periodic repetition conditions, and the Z direction was a perfect matched layer condition. The refractive index and extinction coefficient of nickel oxide are 2.1 and 0, respectively, and the optical constants of gold and silver are those listed in Reference 2 (for example, the refractive index and extinction coefficient of gold are 0.03 and 5.2, respectively, at a wavelength of 756 nm, and the refractive index and extinction coefficient of silver are 0.14 and 4.5, respectively, at a wavelength of 756 nm). (Reference 2) Johnson, P.B.; Christy, R.W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370-4379.

[0025] 5-7 are diagrams for explaining the simulation results of the electric field enhancement effect of the light-collecting device according to this embodiment. FIG. 5 shows the structure of the light-collecting device according to this embodiment. Here, attention is focused on the unit structure. FIG. 6 is a diagram showing the relationship between the number of metal nanostructures provided directly above one concave curved mirror and the reflection spectrum. The vertical axis represents the reflectance of light. The reflectance R is the ratio of the reflected light intensity I to the incident light intensity I ０ |E / E ０ |max is the electric field strength E of the incident light ０ is the maximum value of the ratio of the electric field intensity E of the light focused by the light-focusing device to the electric field intensity E of the light focused by the light-focusing device. Note that since the light intensity is proportional to the square of the electric field E, the light intensity is (|E / E ０ | max) ２ In Figure 7A, |E / E ０ Since |max is 196, this means that the electric field of the incident sunlight was enhanced by a maximum of 196 times. The word 785 nm at the bottom of the figure is the wavelength of the light that was evaluated.

[0026] Figure 7B shows the electric field distribution when one metal nanostructure is placed directly above one concave curved mirror. In this case too, it can be seen that the electric field is particularly enhanced where the metal nanostructure is located. |E / E ０ Since the value of |max is 118, it can be seen that the electric field of the incident sunlight is enhanced by a maximum of 118 times.

[0027] 7C shows the electric field distribution when no metal nanostructure is provided directly above one concave curved mirror. In this case, a very large electric field enhancement cannot be achieved, and |E / E ０ The value of |max was 15.3. Therefore, it can be seen that the electric field of the incident sunlight was enhanced by a maximum of 15.3 times. From this investigation result, it was found that the light-concentrating device 10 of this embodiment can enhance the electric field of the incident light by approximately 100 times.

[0028] 8-10 are diagrams illustrating the evaluation results of a light-collecting device according to a comparative example. FIG. 8 is a cross-sectional view of a light-collecting device according to a comparative example. In the light-collecting device according to the comparative example, the reflective layer 15 has a flat shape rather than a concave curve. FIG. 9 shows the reflection spectrum for each number of metal nanostructures provided in an area of ​​the same size as the structure in FIG. 5. FIG. 9A shows the reflection spectrum when one metal nanostructure (1 Au) is provided in the above-mentioned area. FIG. 9B shows the reflection spectrum when no metal nanostructure is provided in the same area.

[0029] Fig. 10 shows the distribution of the electric field strength enhanced in a light-collecting device according to a comparative example. Fig. 10 has two vertically arranged figures, both of which show the distribution of the electric field strength in the XZ cross section of a light-collecting device having a flat reflective layer as shown in Fig. 8. Fig. 10A shows the electric field distribution when one metal nanostructure is provided. In this figure, too, it can be seen that the electric field is particularly enhanced where the metal nanostructure is present. |E / E ０ Since the value of |max is 52.1, it can be seen that the electric field of the incident sunlight is enhanced by a maximum of 52.1 times.

[0030] 10B shows the electric field distribution when no metal nanostructures are provided. In this case, a very large electric field enhancement cannot be achieved, and |E / E ０ The value of |max was 1.96. Therefore, it can be seen that the electric field of the incident sunlight was enhanced by a maximum of 1.96 times. In this way, it was found that the light-collecting device according to the comparative example can enhance the electric field of the incident light by approximately 50 times by providing the metal nanostructure.

[0031] The results of this simulation show that a light-collecting device having a concave curved mirror can enhance the electric field by a factor of up to about 100, while a light-collecting device having a flat reflective layer can enhance the electric field by a factor of up to about 50. Therefore, it can be seen that the light-collecting device 10 according to this embodiment has an electric field enhancement effect that is about twice as high as that of the comparative example having a flat reflective layer.

[0032] Next, the dimensions of the concave curved mirror will be considered with reference to Figures 11 and 12. Here, it is assumed that the curved shape of the concave curved mirror 14a can be approximated by a portion of a circle with a radius of curvature r. The larger the radius of curvature r, the larger the concave curved mirror 14a. The distance from the metallic nanostructure 18a to the bottom of the upper surface of the concave curved mirror 14a is assumed to be t. t can also be considered the maximum thickness of the dielectric layer 16. The width of the concave curved mirror is assumed to be w. In the light-collecting device defined in this way, the optical path length of light incident on the main surface of the dielectric layer 16 to the metallic nanostructure 18a is approximately constant regardless of the position of incidence of the incident light.

[0033] The light-collecting device according to this embodiment is a light-collecting antenna with phase focusing capabilities. In such a light-collecting device, when the wavelength of the light wave matches the optical path length, the light wave is trapped, inducing an electric field enhancement, and resonance occurs. As the dimension r or t of the concave curved mirror increases, the optical path length also increases, increasing the "number of waves" that are trapped. As the size of the concave curved mirror increases, periodic peaks of electric field enhancement are obtained. This is called a higher-order resonant mode.

[0034] FIG. 12A shows the change in the maximum electric field enhancement with variation in the length of the radius of curvature r. The horizontal axis shows r fluctuations of 0% for the reference structure. r in the reference structure is 480 nm, and t is 240 nm. At 0 on the horizontal axis, the maximum electric field enhancement for light with a wavelength λ of 785 nm is approximately 120 times. The data in FIG. 12A was obtained by increasing or decreasing r by -20%, -15%, -10%, -5%, +5%, +10%, +15%, and +20% while maintaining the relationship t = 0.5r. Periodic peaks in the electric field enhancement were observed at r = 0% and r = +20%. For example, by ensuring that the variation in r from the reference structure for all concave curved mirrors formed in the light-collecting device is within ±10%, |E / E ０ The value of |max can be made larger than that of a light-collecting device having a flat reflective layer as shown in Figure 8, and the degree of electric field enhancement can be stabilized. In another example, the degree of electric field enhancement can be further stabilized by limiting this ±10% to ±5%. Note that the wavelengths λ evaluated at -20%, -15%, -10%, -5%, +5%, +10%, +15%, and +20% on the horizontal axis are 796 nm, 808 nm, 717 nm, 749 nm, 770 nm, 714 nm, 760 nm, and 800 nm, respectively.

[0035] FIG. 12B shows the transition of the maximum electric field enhancement when the value of the radius of curvature r is fixed at 480 nm and t is made a variable. Here, the reference structure is the one where the t fluctuation on the horizontal axis is 0%, and this reference structure is r = 480 nm and t = 240 nm. At the horizontal axis position of 0, the maximum electric field enhancement for light with a wavelength λ of 785 nm is approximately 120 times. FIG. 12B also shows the value of electric field enhancement when t is increased or decreased while the value of r is fixed. In this figure, periodic peaks of electric field enhancement are observed, one of which is at t = -5%. For example, by limiting the variation of t from the reference structure to ±20% for all concave curved mirrors formed in the light-collecting device, |E / E ０The value of |max can be made larger than that of a light-collecting device having a flat reflective layer as shown in Figure 8, and the degree of electric field enhancement can be stabilized. In another example, the degree of electric field enhancement can be further stabilized by limiting this ±20% to ±10%. Furthermore, the degree of electric field enhancement can be further stabilized by limiting it to ±5%.

[0036] A maximum value of the electric field enhancement was observed in both Figures 12A and 12B. As the size of the structure increased, a maximum value due to a higher-order resonant mode was observed. Forming all concave curved mirrors of the light-collecting device with dimensions (r, t) that provide such a maximum value of the electric field enhancement contributes to improving the electric field enhancement effect. According to one example, r is preferably 50 to 10,000 nm, more preferably 100 to 5,000 nm, in order to efficiently collect visible to near-infrared light. According to another example, r can be in a range between any two values ​​of 50 and 10,000 nm. According to one example, t is preferably 10 to 5,000 nm, more preferably 50 to 2,000 nm, in order to efficiently resonate with visible to near-infrared light. According to another example, t can be in a range between any two values ​​of 10 to 5,000 nm. According to one example, it is preferred that all concave curved mirrors in a focusing device be of uniform size, but if variations are unavoidable, the dimensional variations can be up to ±10% or ±5% at 3σ.

[0037] If a substantially uniform reaction is intended across the entire light-collecting device, it is undesirable for the electric field strength to vary significantly depending on the location on the light-collecting device. Therefore, for example, by limiting the variation in r across the entire light-collecting device to approximately ±10%, more preferably to approximately ±5%, a substantially uniform reaction can be achieved across the entire surface of the light-collecting device. For example, by limiting the variation in t across the entire light-collecting device to approximately ±10%, more preferably to approximately ±5%, a substantially uniform reaction can be achieved across the entire surface of the light-collecting device. It is also possible to limit both the variations in the values ​​of t and r to, for example, approximately ±10% at 3σ, more preferably approximately ±5%.

[0038] Next, with reference to Figures 13 and 14, we will consider the effect of the angle of incidence of light on the light-collecting device on the electric field enhancement. For example, in a photochemical reaction caused by sunlight, the angle of incidence of light on the light-collecting device changes over time. If the electric field enhancement effect rapidly decreases with fluctuations in the angle of incidence, it may become necessary to introduce a sun tracking device or the like to maintain a state in which sunlight is incident almost perpendicularly (or at a predetermined angle) on the light-receiving surface. Needless to say, the introduction of a sun tracking device will increase costs. To avoid this, it is necessary to maintain a high electric field enhancement effect even with a certain degree of change in the angle of incidence.

[0039] Fig. 13A is a diagram defining the angle of incidence in the light-collecting device according to this embodiment. The angle of incidence θ is the angle between the normal to the incident surface and vector k in the direction of propagation of incident light. Fig. 13B is a diagram defining the angle of incidence in a light-collecting device according to a comparative example. The comparative example is a light-collecting device having a flat reflective layer. In the comparative example, the angle of incidence θ is also the angle between the normal to the incident surface and vector k in the direction of propagation of incident light.

[0040] FIG. 14 shows the incidence angle dependence of the maximum electric field enhancement. The data (broken line) labeled "parabolic" relates to the focusing device of FIG. 13A, and the data (broken line) labeled "flat" relates to the focusing device with a flat reflective layer of FIG. 13B. In the small angle variation range of the incident angle from 0 to 30°, the focusing device with a concave curved mirror achieves a better electric field enhancement effect than the focusing device with a flat reflective layer. In the incident angle range from 40° to 60°, the difference between the electric field enhancement effect of the concave curved mirror and the electric field enhancement effect of the flat reflective layer is small. Thus, the focusing device with a concave curved mirror maintains a good electric field enhancement effect in the small angle variation range, and can maintain an electric field enhancement effect equivalent to that of the comparative example even when the incident angle increases to approximately 60°.

[0041] Next, the number and arrangement of metallic nanostructures provided directly above one concave curved mirror will be explained with reference to Figures 15 and 16. Figure 15 is a plan view of one concave curved mirror and multiple metallic nanostructures provided thereon. Figure 15 shows four examples of periodically arranged metallic nanostructures.

[0042] FIG. 15A shows an example of metallic nanostructures arranged to form a two-dimensional hexagonal lattice. A metallic nanostructure 20a is formed at the center of a concave curved mirror 14a, with six metallic nanostructures 20b, 20c, 20d, 20e, 20f, and 20g surrounding it. According to one example, the diameter d of the metallic nanostructure is 30 nm, the spacing g between the six metallic nanostructures surrounding the central metallic nanostructure is 20 nm, the X-axis offset L between metallic nanostructures 20g and 20f is d / 2 + g / 2, i.e., 25 nm, and the Y-axis offset L is √3 × L. According to another example, other dimensions can be employed. According to one example, the X-axis and Y-axis lengths of the metallic nanostructures are preferably 2 to 5,000 nm, more preferably 10 to 1,000 nm, for efficient resonance with visible to near-infrared light. According to another example, the X-axis and Y-axis lengths of the metallic nanostructures can be within a range between 2 and 5,000 nm. According to one example, the thickness of the metallic nanostructure is preferably 2 to 1000 nm, more preferably 10 to 200 nm, in order to efficiently resonate with visible to near-infrared light and provide electrons and holes for photochemical reactions. According to another example, the thickness of the metallic nanostructure can be in the range between any two values ​​of 2 to 1000 nm.

[0043] 15A, seven metal nanostructures are provided for one concave curved mirror, but by adding metal nanostructures to the outside of these seven metal nanostructures so as to form a two-dimensional hexagonal lattice, the number of metal nanostructures can be increased to 19, 37, ... 91. By arranging multiple metal nanostructures so as to form a two-dimensional hexagonal lattice, it is possible to efficiently arrange many metal nanostructures in a small light-collecting area.

[0044] 15B is a diagram showing an example of metal nanostructures arranged to form a two-dimensional tetragonal lattice. A metal nanostructure 22a is formed in the center of the concave curved mirror 14a, and eight metal nanostructures 22b are formed surrounding it. While nine metal nanostructures are shown in FIG. 15B, the number of metal nanostructures can be increased to form a two-dimensional tetragonal lattice.

[0045] Fig. 15C is a diagram showing an example of a staggered arrangement of metal nanostructures. A plurality of metal nanostructures 24a are arranged in a staggered arrangement. Fig. 15D is a diagram showing an example of an arrangement of metal nanostructures to form a face-centered rectangular two-dimensional lattice. Four metal nanostructures 26b are arranged to surround a metal nanostructure 26a, forming a rectangle. The metal nanostructure 26a is provided at the face center position.

[0046] In all of the examples in Fig. 15, the metallic nanostructures are periodically arranged. A periodic arrangement different from that in Fig. 15 may also be adopted. By periodically arranging the metallic nanostructures, it is possible to concentrate many metallic nanostructures in a narrow area.

[0047] Figure 16 shows the relationship between the number of metal nanostructures arranged in a two-dimensional hexagonal lattice and the electric field enhancement effect. The horizontal axis represents the number of metal nanostructures provided directly above one concave curved mirror, and the vertical axis represents the maximum electric field enhancement. This simulation showed that the greater the number of metal nanostructures, in the range from 1 to 19, the greater the electric field enhancement effect, but that once the number of metal nanostructures exceeds 19, the more the electric field enhancement effect gradually decreases. The decrease is due to the distribution of the electric field among multiple metal nanostructures.

[0048] In this way, multiple metal nanostructures can be provided directly above each of multiple concave curved mirrors. The number of metal nanostructures cannot be excessively increased because too many metal nanostructures would block the light incident on the incident surface. On the other hand, since the metal nanostructures are the components that exhibit localized plasmons, the more metal nanostructures there are, the better. The number of metal nanostructures can be determined by balancing these two factors. In the example of Figure 16, the number of metal nanostructures that can maximize the electric field enhancement effect was 19, but a different arrangement could result in a number different from 19.

[0049] 5 to 10, the electric field strength E of the incident light ０We conducted simulations to investigate the "maximum value" of the ratio of the electric field strength E of the light focused by the light-focusing device to the electric field strength E of the light focused by the light-focusing device. Since investigating the maximum value is equivalent to investigating the electric field in a very narrow region defined by the mesh size, it is meaningful for promoting multi-electron reactions requiring a large number of electrons. Examples of multi-electron reactions requiring a large number of electrons include the generation of oxygen through the four-electron oxidation of water and the generation of ammonia through the six-electron reduction of nitrogen.

[0050] On the other hand, for multi-electron reactions requiring fewer electrons, creating a state in which a relatively strong electric field exists in a relatively wide space allows multi-electron reactions to occur at many locations. Therefore, from the perspective of inducing multi-electron reactions requiring fewer electrons, it is more appropriate to integrate a space with a certain level of electric field strength rather than the aforementioned "maximum value." An example of a multi-electron reaction requiring fewer electrons is the generation of hydrogen by the two-electron reduction of water.

[0051] The evaluation axis for evaluating the maximum value of the ratio between the electric field intensity of incident sunlight and the electric field intensity of concentrated light in the light-concentrating device is called the first evaluation axis, and evaluation using the first evaluation axis is as described above. The evaluation axis for evaluating the electric field enhancement effect over a relatively large space is called the second evaluation axis. Evaluation using the second evaluation axis will be described below.

[0052] The second evaluation axis, that is, the electric field enhancement effect in a relatively wide space, will be explained with reference to Figures 17-21. Figure 17 is a model diagram used for the second evaluation axis. The electric field intensity in the X-Z plane region, defined as a range of 600 nm centered on the metal nanostructure in the X direction and a range of 10 nm above the main surface of the dielectric layer in the Z direction, was integrated. Specifically, the average value of the electric field intensity at the four corners of this region was calculated. ＊ Δx ＊ Δz was calculated, and the amount by which the average value exceeded a threshold (set to 3 in this example) was integrated to obtain the integrated value of the electric field strength. This is defined as the "effective integrated electric field strength." By using the effective integrated electric field strength obtained by integrating a space having an electric field strength equal to or greater than a certain level, it is possible to evaluate the spatial extent of a field that has a high degree of concentration and can promote multi-electron reactions.

[0053] Figure 18 shows the effective integrated electric field intensity versus variation in the radius of curvature r defined in Figure 11. For a reference structure with a 0% variation rate, where the radius of curvature r is 480 nm and the distance t from the metal nanostructure to the bottom of the top surface of the concave curved mirror is 240 nm, the effective integrated electric field intensity was calculated for light with a wavelength λ of 521 nm. The horizontal axis represents the variation rate (%) of the radius of curvature r from 480 nm as the reference. As r varied, t was also varied to satisfy the relationship t = 0.5r. Light with wavelengths of 542 nm, 528 nm, 547 nm, 659 nm, 641 nm, 624 nm, 546 nm, and 632 nm was evaluated with variation rates of r of -20%, -15%, -10%, -5%, +5%, +10%, +15%, and +20%, respectively. As can be seen from FIG. 18, the process margin of the radius of curvature is about ±5%, and if this margin is exceeded, the effective integrated electric field strength decreases significantly.

[0054] Figure 19 shows the effective integrated electric field strength versus variation in the distance t from the metallic nanostructure to the bottom of the top surface of the concave curved mirror. In other words, it shows the relationship between the maximum thickness of the dielectric layer and the effective integrated electric field strength. At a variation rate of 0%, the effective integrated electric field strength was calculated for light with a wavelength λ of 528 nm for a reference structure with a curvature radius r of 480 nm and a maximum dielectric layer thickness t of 240 nm. The horizontal axis represents the variation rate (%) of the maximum dielectric layer thickness t from the reference value of 240 nm. r was fixed at 480 nm for all values ​​of t. Light with wavelengths of 743 nm, 733 nm, 720 nm, 659 nm, 641 nm, 568 nm, 555 nm, and 544 nm was evaluated for variations in t of -20%, -15%, -10%, -5%, +5%, +10%, +15%, and +20%, respectively. As can be seen from FIG. 19, periodic maximum values ​​were observed in the effective integrated electric field strength due to higher-order resonance modes.

[0055] If a uniform photochemical reaction is intended across the entire light-collecting device, it is undesirable for the effective integrated electric field strength to vary significantly across the light-collecting device. According to one example, it is preferable for all concave curved mirrors in the light-collecting device to be uniform in size, but if variations are unavoidable, dimensional variations of up to ±10% or ±5% over 3σ can be achieved. This allows for a generally uniform reaction across the entire surface of the light-collecting device.

[0056] FIG. 20 shows the incidence angle dependence of the effective integrated electric field strength. In FIG. 20, the "curve" line indicates the incidence angle dependence of the effective integrated electric field strength when the reflective layer is a concave curved mirror, while the "flat" line indicates the incidence angle dependence of the effective integrated electric field strength when the reflective layer is flat. FIG. 21 summarizes the data from the graph in FIG. 20 in a table. The following can be seen from FIGS. 20 and 21: Light-collecting devices with concave curved mirrors maintain good effective integrated electric field strength over an incidence angle range of 0° to 60°. Light-collecting devices with flat reflective layers, on the other hand, exhibit significant fluctuations in effective integrated electric field strength with variations in the incidence angle. This is because variations in the incidence angle cause fluctuations in the optical path length of the planar resonator, which in turn changes the resonant wavelength, impairing the light-collecting function. Therefore, light-collecting devices with concave curved mirrors are highly valuable in industrial applications because they can maintain high light-collecting function for light sources with continuously varying incidence angles, such as sunlight.

[0057] Next, making the sizes of the concave curved mirrors non-uniform will be explained with reference to Figures 22 and 23. The size of the concave curved mirror determines the optical path length. As mentioned above, when the wavelength of the light wave matches the optical path length, the light wave is confined and an electric field enhancement is induced, so changing the size of the concave curved mirror to change the optical path length is equivalent to changing the wavelength of the light that enhances the electric field. In the example of Figure 1, the multiple concave curved mirrors were all uniform in size, but in order to enhance the electric field using light of various wavelengths, the concave curved mirrors can be made to have two or more different sizes.

[0058] FIG. 22 is a plan view showing an example in which a reflective layer is formed using two types of concave curved mirrors, one large and one small. Small concave curved mirrors 14f and large concave curved mirrors 14g are arranged alternately in the vertical and horizontal directions. In one example, the two types of concave curved mirrors 14f and 14g are arranged without any gaps. In one example, one large concave curved mirror is connected to four small concave curved mirrors and four large concave curved mirrors, with ridges at the connection points. In this case, the outer edge of one concave curved mirror surrounded by eight concave curved mirrors includes eight ridges and eight vertices. On the other hand, one small concave curved mirror is connected to four large concave curved mirrors, with ridges at the connection points. In this case, the outer edge of one concave curved mirror surrounded by four concave curved mirrors includes four ridges and four vertices. The wavelength of light whose electric field is enhanced by concave curved mirror 14f is shorter than the wavelength of light whose electric field is enhanced by concave curved mirror 14g. Therefore, by concentrating light of different wavelengths in a nanospace, it is possible to significantly enhance the electric field using light of multiple wavelengths.

[0059] FIG. 23 is a plan view showing an example in which a reflective layer is formed using three differently sized concave curved mirrors. FIG. 23 illustrates a vertical arrangement of a large concave curved mirror 14h, a concave curved mirror 14i smaller than the concave curved mirror 14h, and a concave curved mirror 14j smaller than the concave curved mirror 14i. In one example, the three differently sized concave curved mirrors 14h, 14i, and 14j are arranged without any gaps. In one example, one concave curved mirror is connected to six other concave curved mirrors, with ridges at the connections. In this case, the outer edge of the concave curved mirror includes six ridges and six vertices. Thus, three differently sized concave curved mirrors are arranged without any gaps to form a single focusing device. In this case, the electric field is enhanced by the three differently sized concave curved mirrors, and by concentrating light of different wavelengths in a nanospace, significant electric field enhancement by light of multiple wavelengths is possible.

[0060] It is possible to provide multiple concave curved mirrors of non-uniform sizes using methods other than those shown in Figures 22 and 23. That is, it is possible to combine four or more types of concave curved mirrors of different sizes, or to arrange the concave curved mirrors in a manner different from that shown in Figures 22 and 23. According to one example, a single light-collecting device can be constructed by arranging unit structures having multiple concave curved mirrors of different sizes in a two-dimensional array. In the example of Figure 22, the unit structure is a structure made up of concave curved mirrors 14f and 14g, and in the example of Figure 23, it is a structure made up of concave curved mirrors 14h, 14i, and 14j.

[0061] According to one example, the aforementioned structural fluctuation and |E / E ０ The relationship between |max (Figure 12) and the relationship between structural variation and effective integrated electric field strength (Figures 18 and 19) can be utilized to determine the size of the concave curved mirrors. For example, the sizes of multiple concave curved mirrors can be set to correspond to sizes that achieve periodic peaks (maximum values) of electric field enhancement. For example, the size of a first concave curved mirror is set to achieve a certain peak of electric field enhancement, and the size of a second concave curved mirror is set to achieve the next peak of electric field enhancement. In this way, the sizes of multiple types of concave curved mirrors can be designed to achieve a maximum value of the electric field strength resulting from any resonance mode. In other words, since the electric field is enhanced when the optical path length = n × wavelength (n = 1, 2, ...), the sizes of multiple types of concave curved mirrors can be determined to achieve a maximum value of the electric field strength resulting from the corresponding resonance mode. This improves the electric field enhancement effect for multiple wavelengths.

[0062] FIG. 24 illustrates an example of a method for manufacturing a light-collecting device according to an embodiment. The manufacturing of a light-collecting device begins with forming an array having multiple concave curved mirrors on a support substrate. FIG. 24A shows concave curved mirrors 14a, 14b, and 14c formed on a support substrate 12. Examples of methods for fabricating the concave curved surface include thermal nanoimprinting, which involves heat-molding the surface of a thermoplastic polymer film. Instead of polymer films, low-melting-point glass substrates may also be used. Alternatively, a concave curved surface can be fabricated by photo-nanoimprinting, in which a photo-curable resin is applied to a carrier film and then cured with ultraviolet light using a photo-nanoimprinting method. The concave curved mirror can be formed on the concave curved surface using well-known methods, such as sputtering, CVD, or vacuum deposition. Next, a dielectric layer is formed. FIG. 24B illustrates a dielectric layer 30. The dielectric layer 30 covers the array 14 having multiple concave curved mirrors and has its main surface above the top ends of the multiple concave curved mirrors. One example of a method for forming the dielectric layer is wet coating and baking. A dielectric layer is formed on a concave curved mirror by wet-coating a film-forming solution containing a raw material compound containing the elements constituting the dielectric onto the concave curved mirror and then heating it. Instead of the film-forming solution, a dispersion of pre-crystallized constituent compounds of the dielectric layer dispersed in a solvent may be used. Examples of wet-coating methods for performing this wet-coating process include spin coaters, gravure coaters, bar coaters, die coaters, spray coaters, and stencil printers. Another method is atomic layer deposition (ALD). A dielectric layer is obtained by vapor-phase oxidation of raw material compounds containing the elements constituting the dielectric. The wet-coating and baking method may also be combined with the ALD method. These methods are capable of forming a planar dielectric layer surface on a concave curved surface, making them suitable for forming a dielectric layer with a light-concentrating structure as shown in FIG. 2.

[0063] Next, metallic nanostructures are formed. Figure 24C shows that metallic nanostructures 18a, 18b, and 18c are formed on the dielectric layer 30 directly above each of the multiple concave curved mirrors. Here, the metallic nanostructures are preferably formed at the focal point or the nanospace where the light is focused with a positional accuracy of 100 nanometers or less, and more preferably with a positional accuracy of 50 nanometers or less. If the metallic nanostructures cannot be formed with high positional accuracy, the electric field enhancement effect of these structures will be reduced. Furthermore, high throughput is required to manufacture large-area light-collecting devices. Thus, considering industrial applications, both high throughput and high positional accuracy are required for metallic nanostructures. Forming metallic nanostructures by electron beam lithography has poor throughput, making it disadvantageous for manufacturing large-area light-collecting devices. Photolithography cannot form metallic nanostructures with high positional accuracy when the reflective layer has a curved surface. According to one example, photo-nanoimprinting can be used to form metallic nanostructures with high positional accuracy and high throughput.

[0064] Metal nanostructures can be formed, for example, by the following photo-nanoimprinting method. A metal layer is formed on a dielectric layer, for example, by sputtering or electroless plating, followed by the formation of a UV-curable resin layer. A mold with a concave pattern is then placed on the metal layer, and the resin composition layer is cured using a UV irradiation device. This results in a cured resin composition with a convex shape, inversely transferred to the concave pattern of the mold's transfer area. The remaining film of the cured resin composition and the metal layer directly below it are removed by dry etching, forming an array of isolated metal nanostructures in positions corresponding to the concave portions of the mold. Another method is the lift-off method. A sacrificial layer that is easily soluble in solvents or the like is formed on the dielectric layer, and a UV-curable resin layer is then formed on top of it. A mold with a convex pattern is then placed on the resin composition layer, and the resin composition layer is cured using a UV irradiation device. Another highly etch-resistant UV-curable resin layer, the remaining film in the concave portions of the cured resin composition and the sacrificial layer directly below it are then removed by anisotropic dry etching, and a metal layer is then formed on top of the sacrificial layer by a well-known method, such as sputtering, to obtain metal nanostructures in direct contact with the dielectric and a metal layer deposited on the sacrificial layer and resin composition. Thereafter, by removing the sacrificial layer containing the metal layer and the resin composition, an array of metallic nanostructures can be formed at positions corresponding to the convex portions of the mold. Here, in order to form metallic nanostructures with high positional accuracy, the substrate having a dielectric layer and the mold must be aligned with high precision before the fine pattern is transferred. According to one example, the substrate and the mold can be aligned with high precision by an alignment method using scattered light. In the alignment method using scattered light, an array is first formed on the substrate having a concave curved mirror and a dielectric layer formed thereon and on the mold. According to one example, a first array having a first periodic structure and a second array having a second periodic structure are formed on the substrate. The first array and the second array are also formed on the mold. Next, the substrate and the mold are laminated to provide a laminate. In the laminate, the array on the substrate and the array on the mold are maintained in a state in which they do not overlap.Next, illumination light from an illumination device is irradiated onto the stack, and light from the first array of substrates is detected as a first signal, light from the second array of substrates is detected as a second signal, light from the second array of molds is detected as a third signal, and light from the first array of molds is detected as a fourth signal. By fitting these signals, the positional deviation between the substrates and the mold is calculated. A predetermined mathematical formula can be used for the fitting. Based on the amount of positional deviation calculated in this way, the substrates and the mold are aligned by, for example, driving a movable stage.

[0065] In this way, the concave mirror substrate and the mold are aligned with high precision. That is, the concave and convex structure on the mold side for forming the metallic nanostructures is located directly above the concave mirror. Then, for example, by forming an array of metallic nanostructures using the method described above, the structure shown in FIG. 24C can be obtained.

[0066] In the optical nanoimprinting method, various technological developments are underway to improve alignment accuracy. In the light-collecting device having the concave curved mirror of this embodiment, it is important to position the metal nanostructures with high positional accuracy at the light-collecting position of the concave curved mirror, so it is particularly beneficial to employ a technology that improves alignment accuracy. According to one example, the optical nanoimprinting method described above allows more than 90 out of 100 metal nanostructures to be accurately formed at the desired positions.

[0067] Next, a process is performed to partially embed the metallic nanostructures in the dielectric layer 30 and expose the remaining portions from the dielectric layer 30. This process is called a partial embedding process. The partial embedding process is not particularly limited as long as it achieves the purpose of partially embedding the metallic nanostructures in the dielectric layer and exposing the remaining portions from the dielectric layer, but it may be any one of the following five processes, for example. <First partial embedding process> As shown in FIG. 24D, an additional dielectric layer 40 is formed on the dielectric layer 30 to cover the metallic nanostructures with the dielectric layer. Next, by etching a portion of the additional dielectric layer 40, the metallic nanostructures 18a, 18b, and 18c are partially embedded in the additional dielectric layer 40 and the remaining portions are exposed from the additional dielectric layer 40 as shown in FIG. 24E. <Second partial embedding process> From the state shown in FIG. 24C, an additional dielectric layer 40 is formed on the dielectric layer 30 as shown in FIG. 24E, so that the metallic nanostructures are partially embedded in the additional dielectric layer 40 and the remaining portions are exposed from the additional dielectric layer 40. The additional dielectric layer 40 can be formed by, for example, atomic layer deposition (ALD). In this case, the state shown in FIG. 24D is not passed through. <Third Partial Embedding Process> By heating the metallic nanostructures 18a, 18b, and 18c formed on the dielectric layer 30, parts of the metallic nanostructures 18a, 18b, and 18c are embedded in the dielectric layer 30, and the remaining parts are exposed from the dielectric layer 30. <Fourth Partial Embedding Process> By forming the additional dielectric layer 40 on the dielectric layer 30 and covering the metallic nanostructures with a dielectric, and then performing a heat treatment, parts of the metallic nanostructures are embedded in the additional dielectric layer 40, and the remaining parts are exposed from the additional dielectric layer 40. <Fifth Partial Embedding Process> By mechanically pressing the metallic nanostructures formed on the dielectric layer 30 toward the support substrate 12, parts of the metallic nanostructures are embedded in the dielectric layer 30, and the remaining parts are exposed from the dielectric layer 30.

[0068] In this way, one dielectric layer may be sufficient in some cases, and two layers may be required in other cases. Therefore, the partial embedding process can be abstractly explained as embedding a part of the metallic nanostructure in the dielectric layer or the additional dielectric layer, while leaving the remaining part exposed from the dielectric layer or the additional dielectric layer.

[0069] The light-collecting device and the method for manufacturing the light-collecting device according to this embodiment can be modified in various ways without departing from the features of the present disclosure.

[0070] 10 Light-collecting device, 12 Support substrate, 14 Array, 14a, 14b, 14c, 14d, 14e Concave curved mirror, 16 Dielectric layer, 16a Main surface, 18a, 18b, 18c, 18d, 18e Metal nanostructure

Claims

1. A light-focusing device comprising: an array having a plurality of concave curved mirrors; a dielectric layer covering the plurality of concave curved mirrors and having a principal surface above the upper ends of the plurality of concave curved mirrors; and metal nanostructures disposed directly above each of the plurality of concave curved mirrors, a portion of which is embedded in the dielectric layer and a remainder of which is exposed from the principal surface.

2. The light-collecting device according to claim 1, wherein the plurality of concave curved mirrors collect a bundle of light rays incident on the main surface to the vicinity of the metal nanostructure.

3. The light-collecting device according to claim 1 or 2, wherein one of the metal nanostructures is provided directly above each of the plurality of concave curved mirrors.

4. A light-collecting device according to claim 1 or 2, wherein a plurality of said metal nanostructures are provided directly above each of said plurality of concave curved mirrors.

5. The light-collecting device according to claim 4, wherein the metallic nanostructures are periodically arranged.

6. The light-collecting device according to claim 1 or 2, wherein the plurality of concave curved mirrors are uniform in size.

7. The light-collecting device according to claim 1 or 2, wherein the sizes of the plurality of concave curved mirrors are non-uniform.

8. The light-collecting device according to claim 7, wherein the plurality of concave curved mirrors are unit structures each having a plurality of concave curved mirrors of different sizes arranged in a two-dimensional array.

9. A light-collecting device according to claim 1 or 2, wherein the optical path length of light incident on the main surface to the metal nanostructure is approximately constant regardless of the position of incidence of the incident light.

10. A method for manufacturing a light-focusing device, comprising: forming an array having a plurality of concave curved mirrors on a supporting substrate; forming a dielectric layer covering the plurality of concave curved mirrors and having a main surface above the upper ends of the plurality of concave curved mirrors; forming metal nanostructures on the dielectric layer directly above each of the plurality of concave curved mirrors by a nanoimprinting method; and embedding portions of the metal nanostructures in the dielectric layer or an additional dielectric layer, while exposing remaining portions from the dielectric layer or the additional dielectric layer.

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