Upconversion structure

Abstract

To provide an upconversion structure capable of suitably emitting upconverted low-energy-density light.SOLUTION: An upconversion structure provided herein comprises a plurality of upconversion units arranged in an array, each upconversion unit comprising a condensing lens and a fine sphere for upconverting and emitting light passing through the condensing lens.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an upconversion structure. [Background technology]

[0002] Patent Document 1 discloses an upconversion structure that upconverts and emits light irradiated from a light source. The upconversion structure of Patent Document 1 includes a focusing lens that focuses a laser output from an excitation light source, and a microspherical crystal that upconverts and emits light transmitted through the focusing lens. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 2908681 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned upconversion structure upconverts and emits light of a long wavelength contained in a laser having a relatively high energy density, but has not been studied for upconversion and emission of light of a long wavelength contained in light having a relatively low energy density, such as sunlight or infrared light.

[0005] An object of the present invention is to provide an upconversion structure capable of suitably upconverting and emitting light with a low energy density. [Means for solving the problem]

[0006] An upconversion structure according to a first aspect of the present invention includes a plurality of upconversion units arranged in an array, each of which has a focusing lens and a microsphere that upconverts and emits light transmitted through the focusing lens.

[0007] An upconversion structure according to a second aspect of the present invention is the upconversion structure according to the first aspect, further comprising a substrate supporting the plurality of upconversion units, the substrate having a plurality of recesses for accommodating the microspheres.

[0008] An upconversion structure according to a third aspect of the present invention is the upconversion structure according to the second aspect, wherein the shape of the opening of the recess in plan view is a regular hexagon, a square, or a rectangle.

[0009] An upconversion structure according to a fourth aspect of the present invention is the upconversion structure according to the second or third aspect, wherein the substrate is made of a material that reflects light upconverted and emitted by the microspheres.

[0010] An upconversion structure according to a fifth aspect of the present invention is the upconversion structure according to the second or third aspect, wherein the substrate is made of a material that transmits light upconverted and emitted by the microspheres.

[0011] An upconversion structure according to a sixth aspect of the present invention is the upconversion structure according to any one of the first aspect to the fifth aspect, wherein the microspheres are configured to absorb light having a wavelength of 700 nm to 1500 nm.

[0012] An upconversion structure according to a seventh aspect of the present invention is an upconversion structure according to any one of the first to sixth aspects, wherein the microspheres contain crystals containing rare earth ions, alkali metal ions, alkaline earth metal ions, and fluorine ions. Effect of the Invention

[0013] According to the upconversion structure of the present invention, light with a low energy density can be suitably upconverted and emitted. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a plan view of an upconversion structure according to an embodiment. [Diagram 2] 2 is a cross-sectional view taken along line D2-D2 in FIG. 1; [Diagram 3] FIG. 1 is a perspective view of a test upconversion structure. [Figure 4] 4 is a bottom view of a light source illuminating the test upconversion structure of FIG. 3; FIG. [Figure 5A] Figure 1. Schematic diagram of the microsphere manufacturing apparatus. [Figure 5B] Bright-field image of the cross-section of the microsphere in Figure 1 observed with a transmission electron microscope. [Figure 5C] Electron diffraction pattern of the microsphere in Figure 1. [Figure 6] Schematic diagram of the measurement device used to measure the properties of microspheres. [Figure 7] 1 is a graph showing the relationship between the wavelength and emission intensity of light upconverted and emitted by a microsphere. [Figure 8] 1 is a graph showing the relationship between the excitation energy and emission energy of light for upconversion emission by a microsphere. [Figure 9] 1 is a graph showing the relationship between the wavelength and emission intensity of light upconverted and emitted by a microsphere. [Figure 10] 1 is a graph showing the relationship between the wavelength and emission intensity of light upconverted and emitted by a bulk crystal. [Figure 11] 13 is a graph showing the relationship between excitation light intensity and emission intensity when microspheres are used. [Figure 12] 1 is a graph showing the relationship between excitation light intensity and emission intensity when a bulk crystal is used. [Figure 13] A unified graph of the graphs in Figures 11 and 12. [Figure 14] FIG. 13 is a plan view of a modified upconversion structure. [Figure 15] FIG. 13 is a cross-sectional view showing an arrangement state of an up-conversion structure according to another modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an up-conversion structure according to an embodiment of the present invention will be described with reference to the drawings.

[0016] <1. Overall configuration of the upconversion structure> FIG. 1 is a plan view of the upconversion structure 10 according to the present embodiment. FIG. 2 is a cross-sectional view taken along the line D2-D2 in FIG. 1. The upconversion structure 10 is configured to be able to upconvert and emit light of a long wavelength contained in light with a relatively low energy density, such as sunlight and infrared rays. The upconversion structure 10 of the present embodiment is installed, for example, on a non-light-concentrating solar cell module 60 (see FIG. 2). The solar cell module 60 absorbs light with a wavelength equal to or greater than the bandgap energy. The solar cell module 60 has a front surface 61 and a back surface 62, which is the surface opposite to the front surface 61. The front surface 61 is a surface on which sunlight is incident. The back surface 62 is, for example, a surface facing the roof (not shown) of a building via the upconversion structure 10. In the present embodiment, the upconversion structure 10 is disposed so as to face the back surface 62 of the solar cell module 60, in other words, the upconversion structure 10 is disposed between the solar cell module 60 and the roof. One or more upconversion structures 10 may be arranged for one solar cell module 60, or one or more upconversion structures 10 may be arranged for a plurality of solar cell modules 60 arranged in an array. The upconversion structure 10 is also applicable to, for example, infrared sensing or infrared imaging.

[0017] As shown in FIG. 1, the upconversion structure 10 includes a plurality of upconversion units 20 arranged in an array, and a substrate 50 supporting the plurality of upconversion units 20 .

[0018] The number of up-conversion units 20 included in one up-conversion structure 10 can be arbitrarily selected as long as it is 2 or more. In the present embodiment, one up-conversion unit 20 includes 20 up-conversion units 20. One up-conversion structure 10 may include 2 to 19 up-conversion units 20, or 21 or more up-conversion units 20.

[0019] The up-conversion unit 20 includes a condenser lens 30 and a microsphere 40. The condenser lens 30 condenses light of a wavelength not absorbed by the solar cell module 60 in the sunlight, that is, light of a wavelength less than the bandgap energy in the sunlight (hereinafter referred to as "transmitted light"), onto the microsphere 40. The type of the condenser lens 30 can be selected arbitrarily as long as it can condense the transmitted light onto the microsphere 40. In this embodiment, the condenser lens 30 is a refractive type ball lens. The condenser lens may be a hemispherical lens, a hyperhemispherical lens, a convex lens, or a Fresnel lens. The diameter L of the condenser lens 30 can be selected arbitrarily. In this embodiment, the diameter L of the condenser lens 30 is 2 mm. The solar cell module 60 absorbs light of a wavelength less than 700 nm in the sunlight, for example. Therefore, the transmitted light includes light of a wavelength of 700 nm or more. The condenser lens 30 is not limited to a refractive type lens. For example, when the upconversion structure 10 is disposed so as to face the rear surface 62 of the solar cell module 60 as in this embodiment, the condenser lens 30 may be a reflecting lens.

[0020] The microsphere 40 converts the transmitted light into light with a shorter wavelength by absorbing light with a wavelength of, for example, 700 nm to 1500 nm and emitting up-conversion light. The form of up-conversion light emitted by the microsphere 40 is an energy transfer type or a self-absorption type. In the present embodiment, the form of up-conversion light emitted by the microsphere 40 is an energy transfer type. The diameter of the microsphere 40 can be selected arbitrarily. From the viewpoint of suitably performing up-conversion light emission of the transmitted light, the diameter of the microsphere 40 is preferably 100 μm or less. The material constituting the microsphere 40 can be selected arbitrarily as long as it is a material capable of performing up-conversion light emission of the transmitted light. From the viewpoint of suitably performing up-conversion light emission of the transmitted light, the material constituting the microsphere 40 is preferably transparent crystallized glass containing microcrystals in glass. The microcrystals contained in the glass are more preferably halides. It is more preferable that the microcrystals contained in the glass contain rare earth ions, alkali metal ions, alkaline earth metal ions, and fluorine ions. The microcrystals contained in the glass are, for example, Na(Y,Er)F4 crystals. The upper limit of the size of the microcrystals is preferably 50 nm or less, more preferably 20 nm or less. The lower limit of the size of the microcrystals is preferably 1 nm or more, more preferably 4 nm or more.

[0021] The substrate 50 is plate-shaped having a front surface 51 and a back surface 52. The front surface 51 faces a back surface 62 of the solar cell module 60. The substrate 50 has a plurality of recesses 53 recessed from the front surface 51 toward the back surface 52. The recesses 53 are holes that do not penetrate the substrate 50, and support the upconversion units 20, respectively. The shape of the opening of the recesses 53 in plan view can be selected arbitrarily. In this embodiment, the shape of the opening of the recesses 53 in plan view is a regular hexagon. Therefore, many recesses 53 can be periodically formed in the substrate 50.

[0022] The recess 53 has a shape that tapers from the front surface 51 toward the back surface 52. More specifically, the recess 53 has a shape of a regular hexagonal pyramid with a regular hexagonal opening formed on the front surface 51 as a bottom surface in a plan view. A bottom 53A of the recess 53 is located approximately at the center in the thickness direction of the substrate 50. The microsphere 40 is disposed near the bottom 53A of the recess 53, in other words, near the apex of the regular hexagonal pyramid. Therefore, the microsphere 40 is entirely contained in the recess 53. The condenser lens 30 is disposed so as to be in contact with the inner surface of the recess 53 and so as to be partially exposed from the recess 53.

[0023] The material constituting the substrate 50 can be selected arbitrarily. When the upconversion structure 10 is disposed so as to face the rear surface 62 of the solar cell module 60 as in this embodiment, the substrate 50 is preferably made of a material (hereinafter referred to as a "reflective material") capable of reflecting the light emitted by the microspheres 40 through upconversion toward the solar cell module 60. The reflective material is, for example, a metal or a semiconductor. The metal is, for example, stainless steel, silver, or aluminum. The semiconductor is, for example, silicon. The substrate 50 may be formed, for example, by forming a resin into a thin film.

[0024] <2. Function of the upconversion structure> The transmitted light of the sunlight that is not absorbed by the solar cell module 60 is collected by the condenser lens 30 onto the microsphere 40. The microsphere 40 converts the transmitted light into light of a short wavelength by upconversion emission. The short wavelength light converted by the upconversion emission is emitted in all directions of the microsphere, and most of it is reflected by the substrate 50 toward the solar cell module 60. The solar cell module 60 absorbs the short wavelength light converted by the upconversion emission. In addition, the energy density of the excitation light in the microsphere 40 becomes higher as the area becomes narrower for the same amount of energy. Therefore, by combining the upconversion unit 20 with a large condenser lens 30 and a small microsphere 40, the energy density is improved.

[0025] <3. Light-concentrating effect of the condenser lens> FIG. 3 is a test upconversion structure 100 for the light-collecting effect of the condenser lens 30. FIG. 4 is a bottom view of the light source 200. The inventor of the present application conducted a test to analyze the ratio of light incident on the microsphere 140 to light irradiated from the light source 200 by simulation using the test upconversion structure 100. The software LightTools (manufactured by Synopsys, Inc., USA) was used for the simulation. The test upconversion structure 100 includes a condenser lens 130, a microsphere 140, and a substrate 150. The condenser lens 130 is a spherical lens with a radius of 0.5 mm. The microsphere 140 has the same configuration as the microsphere 140 of the embodiment. The substrate 150 is a cube. The length of one side of the substrate 150 is 4 mm. The shape of the opening of the recess 153 of the substrate 150 in a plan view is a square with a length of one side of 2.41 mm. Therefore, the area of ​​the opening of the recess 153 in a plan view is 5.83 mm 2 The light source 200 is a laser diode. The irradiation port 210 of the light source 200 is a square with each side measuring 1 mm.

[0026] As a result of this test, it was found that 91.5% of the light emitted from the light source 200 was incident on the microsphere 140. For this reason, it is considered that by arranging a plurality of condenser lenses 30 in an array and arranging a microsphere 40 at the focal position as in the embodiment, a large amount of transmitted light can be upconverted and emitted.

[0027] <4. Proportion of transmitted light that is incident on the microsphere> The area SA of the condenser lens 30 in the plan view shown in FIG. 1 is calculated using the diameter L of the condenser lens 30 according to the following formula (1). SA = π (0.5L) 2 (1)

[0028] Furthermore, when the opening of the recess 53 of the substrate 50 is a regular hexagon, the area SB of the opening of the recess 53 is calculated using the diameter L of the condenser lens 30 according to the formula for the circumscribing circle of a regular hexagon as follows: SB=√3 / 2·L 2 (2)

[0029] Therefore, the ratio RA of the light incident on the condenser lens 30 to the transmitted light irradiated on the recess 53 of the substrate 50 is calculated by the following formula (3). RA = π (0.5L) 2 / √3 / 2·L 2 100 (3) That is, the ratio RA is 90.7%. From the results of the test on the focusing effect of the focusing lens described above, it is considered that 91.5% of the transmitted light irradiated to the focusing lens 30 is incident on the microsphere. Therefore, it can be understood that the ratio RB of the transmitted light irradiated to the recess 53 of the substrate 50 that is incident on the microsphere 40 is (0.907×0.915)×100, that is, the ratio RB is 83.0%.

[0030] <5. Manufacturing method of microspheres> An example of a method for producing the microspheres 40 will now be described with reference to FIG. 5A. The microspheres 40 are manufactured by a manufacturing apparatus 300. The manufacturing apparatus 300 includes a laser diode 310, a reflecting mirror 320, and a lens 330. The laser diode 310 irradiates, for example, a 980 nm laser toward the reflecting mirror 320. The reflecting mirror 320 reflects the laser irradiated by the laser diode 310 toward the lens 330. The lens 330 focuses the laser reflected by the reflecting mirror 320 and irradiates the laser on the tip of a glass fiber 400 made of the material that constitutes the microspheres 40. The tip of the glass fiber 400 melts, forming the microspheres 40 at the tip of the glass fiber 400.

[0031] More specifically, 5.8 mol% ErF3 (Kojundo Chemical, 99.9%), 5.8 mol% YF3 (Kojundo Chemical, 99.9%), 11.6 mol% NaF (Kojundo Chemical, 99.9%), 30 mol% ZnO (Kojundo Chemical, 99.9%), and 56.8 mol% B2O3 (Kojundo Chemical, 99.9%) were mixed to obtain 5.8 mol% ErF3, 5.8 mol% YF3, 11.6 mol% NaF, 30 mol% ZnO, and 56.8 mol% B2O3, placed in a platinum crucible, and melted in an electric furnace (Motoyama, Superburn) at 950° C. A quartz glass rod was placed in the melt and pulled up to produce a glass fiber 400 with a diameter of 60 μm. The tip of the glass fiber 400 was irradiated with a 980 nm laser at an output of 5 W using a manufacturing device 300 to form a sphere, thereby manufacturing the microsphere 40. FIG. 5B is a bright-field image of the cross section of the microsphere 40 observed with a transmission electron microscope (JEOL, JEM-2100F). FIG. 5C is an electron diffraction pattern of the microsphere 40. As shown in FIG. 5B, the dark contrast in the bright-field image indicates crystals, and it can be seen that microcrystals with a particle size of 20 nm or less have precipitated. As shown in FIG. 5C, it was confirmed from the electron diffraction pattern that this is attributed to the diffraction of cubic NaYF4 nanocrystals.

[0032] 6. Characteristics of microspheres 6 shows a measuring device 500 for measuring the characteristics of the microsphere 40. The inventors of the present application used the measuring device 500 to carry out tests for measuring the characteristics of the upconversion emission of the microsphere 40 of the embodiment and the characteristics of the upconversion emission of a plate-shaped bulk crystal as a comparison subject.

[0033] The measuring device 500 includes a laser diode 510, a first reflecting mirror 521, a second reflecting mirror 522, a third reflecting mirror 523, an optical lens 530, a measuring unit 540, and a backlight 550. The laser diode 510 irradiates, for example, a 1540 nm laser toward the first reflecting mirror 521. The first reflecting mirror 521 reflects the laser irradiated by the laser diode 510 toward the second reflecting mirror 522. The second reflecting mirror 522 reflects the laser reflected by the first reflecting mirror 521 toward the third reflecting mirror 523. The third reflecting mirror 523 reflects the laser reflected by the second reflecting mirror 522 toward the optical lens 530. The optical lens 530 focuses the laser toward the measuring unit 540. The measuring unit 540 includes a stage 541, a filter 542, and an optical power meter 543. The microspheres 40 or bulk crystals are placed on the stage 541. The stage 541 transmits light emitted by the microspheres 40 or bulk crystals through upconversion. The filter 542 is attached to the lower surface of the stage 541 and absorbs light with a wavelength of 1050 nm or more. The optical power meter 543 is disposed below the filter 542 and measures the emission intensity of the light that is transmitted through the stage 541 out of the light emitted through upconversion. The backlight 550 is a white LED.

[0034] Fig. 7 is a graph showing the relationship between the wavelength and emission intensity of light converted by up-conversion emission of a microsphere 40 with a diameter of 80 µm fabricated at the tip of a glass fiber 400 (see Fig. 5A). The intensities of the laser diode 510 are 0.9 mW, 2.3 mW, 3.5 mW, 4.4 mW, 5.0 mW, 5.7 mW, and 6.4 mW. As shown in Fig. 7, it was confirmed that the light irradiated by the laser diode 510 is converted into light with a short wavelength of about 400 nm to 700 nm and high emission intensity by the up-conversion emission of the microsphere 40.

[0035] 8 is a graph showing the relationship between the excitation energy and the emission energy of the upconversion emission. In this test, although the light emitted above the microsphere 40 out of the light emitted by the microsphere 40 through upconversion was not measured, it was confirmed that approximately 45% of the excitation energy was emitted as the upconversion emission.

[0036] Fig. 9 is a graph showing the relationship between the wavelength and emission intensity of light converted by the upconversion emission of the microsphere 40, similar to Fig. 7. The intensities of the laser diode 510 are 13.1 mW, 25.1 mW, 36.9 mW, 53.2 mW, 63.5 mW, 82.2 mW, and 90.6 mW. As shown in Fig. 9, it was confirmed that the light irradiated by the laser diode 510 is converted into light with a short wavelength of about 400 nm to 700 nm and high emission intensity by the upconversion emission of the microsphere 40.

[0037] Fig. 10 is a graph showing the relationship between the wavelength and the emission intensity of the light converted by the upconversion emission of the bulk crystal. The intensity of the laser diode 510 is 13.1 mW, 25.1 mW, 36.9 mW, 53.2 mW, 63.5 mW, 82.2 mW, and 90.6 mW. As shown in Fig. 10, the light irradiated by the laser diode 510 was converted to a short wavelength of about 400 nm to 700 nm by the upconversion emission of the bulk crystal, but it was confirmed that the emission intensity of the converted light was low compared to the case of the microsphere 40.

[0038] 11 is a graph showing the relationship between the intensity of the exciting laser irradiated by the laser diode 510 and the emission intensity of the light converted by the upconversion emission of the microsphere 40. A semiconductor laser of 1540 nm was used as the exciting light.

[0039] 12 is a graph showing the relationship between the intensity of the exciting laser irradiated by the laser diode 510 and the emission intensity of the light converted by the upconversion emission of the bulk crystal. A semiconductor laser of 1540 nm was used as the exciting light.

[0040] FIG. 13 is a graph summarizing the results of FIG. 11 and FIG. 12. In the case of the microsphere 40, even when the excitation laser intensity is low, around 10 mw, it was confirmed that the ratio of increase in emission intensity to increase in excitation laser intensity is higher than that of the bulk crystal. In addition, in the case of the microsphere 40, when the excitation laser intensity is around 100 mw, it was confirmed that the ratio of increase in emission intensity to increase in excitation laser intensity is approximately "1", that is, the excitation state is saturated. Therefore, even when sunlight is used as a light source, for example, with the microsphere 40 of this embodiment, highly efficient up-conversion can be realized by concentrating light energy to about 10 mW on the microsphere 40. Therefore, highly efficient up-conversion can be realized by combining the microsphere 40 with the concentrating lens 30 and arranging them in an array on the substrate 50. On the other hand, in the case of the bulk crystal, even when the excitation laser intensity is 100 mw or more, it was confirmed that the ratio of increase in emission intensity to increase in excitation laser intensity is "1" or more, that is, the excitation state is not saturated.

[0041] <7. Variations> The above-mentioned embodiment is an example of a form that the upconversion structure according to the present invention can take, and is not intended to limit the form. The upconversion structure according to the present invention can take a form different from the form exemplified in the embodiment. One example is a form in which a part of the configuration of the embodiment is replaced, changed, or omitted, or a form in which a new configuration is added to the embodiment. Some examples of modified embodiments are shown below.

[0042] <7-1> In the above embodiment, the shape of the opening of the recess 53 of the substrate 50 is a hexagon, but the shape of the opening of the recess 53 of the substrate 50 is not limited thereto. FIG. 14 is a plan view of an upconversion structure 10X of a modified example. The upconversion structure 10X of the modified example includes a substrate 50X. The shape of the opening of the recess 53X of the substrate 50X is a square. That is, the recess 53X is a regular pyramid. Alternatively, the shape of the opening of the recess 53X may be a triangle, a rectangle, a pentagon, a polygon having heptagons or more, a circle, or an ellipse.

[0043] In addition, when the opening of the recess 53 of the substrate 50 is a square, the area SC of the opening of the recess 53 is expressed as follows, using the diameter L of the condenser lens 30: 2 Therefore, the ratio RC of the light incident on the condenser lens 30 to the transmitted light irradiated on the recess 53X of the substrate 50X can be calculated by the following formula (4). RC = π (0.5L) 2 / L 2 100 (4)

[0044] That is, the ratio RC is 78.5%. From the results of the above-mentioned test on the focusing effect of the focusing lens, it is considered that 91.5% of the transmitted light irradiated to the focusing lens 30 is incident on the microsphere. Therefore, it can be understood that the ratio RD of the transmitted light irradiated to the recess 53X of the substrate 50X that is incident on the microsphere 40 is (0.785×0.915)×100, that is, the ratio RD is 71.8%.

[0045] <7-2> In the above embodiment, the upconversion structure 10 is disposed so as to face the rear surface 62 of the solar cell module 60, but the arrangement of the upconversion structure 10 is not limited thereto. As shown in FIG. 15, the upconversion structure 10 may be disposed so that the rear surface 52 of the substrate 50 faces the front surface 61 of the solar cell module 60. In this modification, the substrate 50 is preferably made of a material that transmits the light emitted by the microsphere 40 through upconversion. The material that transmits the light emitted through upconversion is, for example, resin or glass. In this modification, a part of the sunlight incident on the upconversion structure 10 is concentrated on the microsphere 40 by the concentrating lens 30. The microsphere 40 converts the sunlight into light with a short wavelength by absorbing light with a wavelength of, for example, 700 nm to 1500 nm and emitting the light through upconversion. The light with a short wavelength converted by the upconversion emission is emitted in all directions of the microsphere, and a part of it passes through the substrate 50 and enters the solar cell module 60. The solar cell module 60 absorbs the short wavelength light converted by the upconversion emission. Another part of the sunlight incident on the upconversion structure 10 passes directly through the substrate 50 and enters the solar cell module 60. The solar cell module 60 absorbs, for example, light of the sunlight with a wavelength of less than 700 nm.

[0046] <7-3> In the above embodiment, a plurality of recesses 53 are formed on one substrate 50. However, the substrate 50 may be divided into regions each having a recess 53, and these may be arranged in an array. [Explanation of symbols]

[0047] 10, 10X: Upconversion structure 20: Up-conversion unit 30: Condenser lens 40: Microsphere 50: Circuit board 53: Recess

Claims

1. The method includes the steps of: Each of the plurality of up-conversion units includes a condenser lens and a microsphere that up-converts and emits light transmitted through the condenser lens; the microsphere is disposed outside the focusing lens; Further comprising a substrate supporting the plurality of up-conversion units; The substrate has a plurality of recesses for receiving the microspheres. Upconversion structure.

2. The shape of the opening of the recess in a plan view is a regular hexagon, a square, or a rectangle.

2. The upconversion structure of claim 1.

3. The substrate is made of a material that reflects the light upconverted and emitted by the microspheres.

3. An upconversion structure according to claim 1 or 2.

4. The substrate is made of a material that transmits the light emitted by the microspheres through the upconversion.

3. An upconversion structure according to claim 1 or 2.

5. The microspheres are configured to absorb light having wavelengths between 700 nm and 1500 nm. An upconversion structure according to any one of claims 1 to 4.

6. The microspheres contain crystals containing rare earth ions, alkali metal ions, alkaline earth metal ions, and fluorine ions. An upconversion structure according to any one of claims 1 to 5.

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