THERMAL RADIATION LIGHT SOURCE

Abstract

The present invention provides a thermal radiation light source that allows a wider range of material choices than those of conventional techniques, so that light having a desired peak wavelength can easily be obtained. A thermal radiation light source 10 includes a thermo-optical converter made of an optical structure in which a refractive index distribution is formed in a member 11 made of an intrinsic semiconductor so as to resonate with light of a shorter wavelength than a wavelength corresponding to a bandgap of the intrinsic semiconductor. When heat is externally supplied to the thermo-optical converter, light having a spectrum in a band of shorter wavelengths than a cutoff wavelength is produced by interband absorption in the intrinsic semiconductor, and light of a resonant wavelength .sub.r in the wavelength band, the light causing resonance in the optical structure, is selectively intensified and emitted as thermal radiation light. In the present invention, an intrinsic semiconductor that provides a wide range of material choices is used, so that a thermal radiation light source that produces narrow-band light having a desired peak wavelength can easily be obtained.

Claims

1. A power generation device comprising: a thermo-optical converter having an optical structure in which a refractive index distribution is formed in a member made of an intrinsic semiconductor configured to resonate with light of a shorter wavelength than a wavelength corresponding to a bandgap of the intrinsic semiconductor; and a solar cell configured to perform photoelectric conversion by receiving light produced by the thermo-optical converter and using the light having a wavelength band including a resonant wavelength that causes resonance in the optical structure.

2. The power generation device according to claim 1, wherein the optical structure has asymmetry in a direction in which thermal radiation light is emitted from the thermo-optical converter.

3. The power generation device according to claim 1, wherein the optical structure has such a structure that, on a surface of a base made of material having a refractive index lower than that of the intrinsic semiconductor, members made of the intrinsic semiconductor are two-dimensionally arranged.

4. The power generation device according to claim 1, wherein the optical structure is a two-dimensional photonic crystal structure formed by periodically providing, in a plate member made of an intrinsic semiconductor, different refractive index areas having refractive index different from the plate member, wherein the different refractive index areas have an asymmetric shape in a direction perpendicular to the plate member.

5. The power generation device according to claim 4, wherein the different refractive index areas are formed so as to open in a surface of the plate member and not to open in the other surface of the plate member.

6. The power generation device according to claim 1, wherein the optical structure is a three-dimensional photonic crystal structure having a three-dimensional periodic refractive index distribution.

7. The power generation device according to claim 1, wherein the intrinsic semiconductor is Si, and the resonant wavelength is 1000 nm or shorter.

8. The power generation device according to claim 1, wherein the intrinsic semiconductor is 3C-SiC, and the resonant wavelength is 750 nm or shorter.

9. The power generation device according to claim 3, wherein the base includes a plate heater having a three-layer structure including a layer including Ti, a layer including Pt and a layer including Ti in this order from a side close to the base.

10. A power generation method comprising: emitting, to an outside of a thermo-optical converter having an optical structure in which a refractive index distribution is formed in a member made of an intrinsic semiconductor configured to resonate with light of a shorter wavelength than a wavelength corresponding to a bandgap of the intrinsic semiconductor, a light with a resonant wavelength that causes resonance in the optical structure by supplying heat; receiving the light emitted from the thermo-optical converter by a solar cell; and performing, by the solar cell, photoelectric conversion using a light having a wavelength band including the resonant wavelength.

11. The power generation method according to claim 10, wherein the optical structure has asymmetry in a direction in which thermal radiation light is emitted from the thermo-optical converter.

12. The power generation method according to claim 10, wherein the optical structure has such a structure that, on a surface of a base made of material having a refractive index lower than that of the intrinsic semiconductor, members made of the intrinsic semiconductor are two-dimensionally arranged.

13. The power generation method according to claim 10, wherein the optical structure is a two-dimensional photonic crystal structure formed by periodically providing, in a plate member made of an intrinsic semiconductor, different refractive index areas having refractive index different from the plate member, wherein the different refractive index areas have an asymmetric shape in a direction perpendicular to the plate member.

14. The power generation method according to claim 13, wherein the different refractive index areas are formed so as to open in a surface of the plate member and not to open in the other surface of the plate member.

15. The power generation method according to claim 10, wherein the optical structure is a three-dimensional photonic crystal structure having a three-dimensional periodic refractive index distribution.

16. The power generation method according to claim 10, wherein the intrinsic semiconductor is Si, and the resonant wavelength is 1000 nm or shorter.

17. The power generation method according to claim 10, wherein the intrinsic semiconductor is 3C-SiC, and the resonant wavelength is 750 nm or shorter.

18. The power generation method according to claim 12, wherein the base includes a plate heater having a three-layer structure including a layer including Ti, a layer including Pt and a layer including Ti in this order from a side close to the base.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1A, FIG. 1B and FIG. 1C are diagrams for describing a reason that light having a particular wavelength band is produced in a thermal radiation light source according to the present invention, and FIG. 1A is a conceptual diagram of a wavelength spectrum of interband absorption in an intrinsic semiconductor, FIG. 1B is a conceptual diagram of wavelength selection according to an optical structure, and FIG. 1C is a conceptual diagram of a wavelength spectrum of light emission by the thermal radiation light source according to the present invention.

[0031] FIG. 2 is a perspective diagram illustrating a thermal radiation light source according to a first embodiment.

[0032] FIG. 3 is a plan view of the thermal radiation light source according to the first embodiment.

[0033] FIG. 4 is a graph showing an example of a wavelength spectrum obtained, by calculation, by the thermal radiation light source according to the first embodiment.

[0034] FIG. 5 is a schematic configuration diagram illustrating an example of a photovoltaic device using the thermal radiation light source according to the first embodiment. FIG. 6A and FIG. 6B are an optical photomicrograph and an electron photomicrograph respectively taken of the thermal radiation light source produced in the first embodiment.

[0035] FIG. 7 is a graph showing a result of measurement of a wavelength spectrum obtained by the thermal radiation light source produced in the first embodiment.

[0036] FIG. 8 is a perspective view of a thermal radiation light source according to a second embodiment.

[0037] FIG. 9A is a top view of the thermal radiation light source of the second embodiment, FIG. 9B is a cross-sectional view along plane A-A, and FIG. 9C is a cross-sectional view along plane B-B.

[0038] FIG. 10 is a graph showing an example of a wavelength spectrum obtained, by calculation, by the thermal radiation light source according to the second embodiment.

[0039] FIG. 11 is a perspective diagram illustrating an alteration of the thermal radiation light source according to the second embodiment.

[0040] FIG. 12 is a perspective diagram illustrating a thermal radiation light source according to a third embodiment.

[0041] FIG. 13 is a graph showing differences in absorptance among plate members having different thicknesses.

DESCRIPTION OF EMBODIMENTS

[0042] Embodiments of a thermal radiation light source according to the present invention will be described with reference to FIG. 2 to FIG. 13.

Embodiment 1

[0043] As illustrated in FIG. 2 and FIG. 3, a thermal radiation light source 10 according to a first embodiment has an optical structure in which a plurality of columnar rods 11 made of an intrinsic semiconductor is arranged on a surface of a base 13 having a lower refractive index than that of the intrinsic semiconductor. In the present embodiment, for a material of the rod 11, Si (refractive index: 3.4) is used, and for a material of the base 13, SiO.sub.2 (refractive index: 1.5) is used. A radius r of the rods 11 is 100 nm and a height h of the rods is 500 nm. In the present embodiment, the rods 11 are arranged at grid points of a square grid whose period length a is 600 nm; however, such periodic arrangement of the rods 11 is not essential to the present invention.

[0044] From among the above parameters, a resonant wavelength .sub.r in the optical structure in the present embodiment is determined by the refractive index n and the radius r of the rods 11 and the period length a of the square grid as described below.

[0045] In the thermo-optical converter, light propagates in a height direction of the rods 11 along the rods 11 having a higher refractive index than that of the surrounding areas. Then, the light is reflected by an upper end and a lower end of each rod 11, whereby standing waves are generated and a resonant state of the light is formed. The resonant wavelength .sub.r depends on the height h of the rods 11 and also depends on the radius r of the rods 11 since the effective refractive index varies when the light leaks from the rods 11. A difference in period length a of the rods 11 affects the resonant wavelength .sub.r in terms of a difference in effective refractive index, but not so largely as a difference in radius r of the rods 11 does. The rods 11 having an excessively short period length a causes the wide range of electromagnetic field overlapped distribution among the rods 11 and interaction thereby occurs, resulting in variation in the resonant wavelength depending on the emission angle of the light. On the other hand, the period length a having a longer period length than that of a light emission wavelength invokes high-order diffraction, whereby radiation in one resonant mode is generated in a plurality of directions. Therefore, it is desirable that the period length a be longer than a distance of oozing of an electromagnetic field from each rod and be shorter than the light emission wavelength.

[0046] In the present embodiment, the radius r of the rods 11 is set to 100 nm and the height h of the rods is set to 500 nm, whereby the wavelength .sub.r of a resonant mode generated in each rod becomes 950 nm. Furthermore, the period length a is set to 600 nm, which is shorter than the resonant wavelength .sub.r and sufficiently larger than the rod radius, whereby emission angle dependency is suppressed while the light emission intensity is maintained.

[0047] A principle of heat to light conversion by the thermal radiation light source 10 according to the first embodiment will be described. When the thermal radiation light source 10 is heated to a temperature of around 1400 K, energy absorption caused by interband absorption in Si, which is an intrinsic semiconductor on the wavelength shorter than a cutoff wavelength .sub.g for Si1700 nm (on the high energy side that is higher than 0.73 eV, which is photon energy corresponding to the cutoff wavelength .sub.g) occurs, and light emission corresponding to the energy occurs on the wavelength shorter than the cutoff wavelength .sub.g (high energy side). A spectrum of the light generated as above, as indicated in FIG. 1A, continues on the wavelength shorter than .sub.g, and in such continuous wavelength band, a wavelength spectrum with around a resonant wavelength .sub.r950 nm as a maximal value (peak top) can be obtained by the optical structure according to the present embodiment.

[0048] An example of a wavelength spectrum that can be obtained, by calculation, by the thermal radiation light source 10 according to the first embodiment is indicated in FIG. 4. In this example, the calculation is performed for a case where the thermal radiation light source 10 is heated to 1400 K (1127 C.). As indicated in the figure, the wavelength spectrum has a single peak with a wavelength of approximately 950 nm as a peak top. The cutoff wavelength .sub.g of Si is approximately 1700 nm (the corresponding photon energy is approximately 0.73 eV), and almost no light emission occurs on the wavelength longer than the cutoff wavelength .sub.g. Also, on the low wavelength side relative to the peak, the spectrum of blackbody radiation becomes smaller as the wavelength is shorter, and the wavelength spectrum of the thermal radiation light source 10 becomes smaller accordingly as the wavelength is shorter.

[0049] As described above, the thermal radiation light source 10 according to the present embodiment can selectively emit only wavelengths that are around the resonant wavelength .sub.r=950 nm. A silicon solar cell cannot perform photoelectric conversion of light of wavelengths exceeding approximately 1000 nm. Therefore, as illustrated in FIG. 5, a photovoltaic device 19 can be constructed by the thermal radiation light source 10, a collective lens 19A that collects sunlight to the thermal radiation light source 10, and a silicon solar cell 19B that receives thermal radiation light from the thermal radiation light source 10. Consequently, photoelectric conversion can be performed in the silicon solar cell 19B after conversion of sunlight having a wide wavelength spectrum including light of wavelengths exceeding approximately 1000 nm to light having a peak of a wavelength spectrum at a wavelength of 1000 nm or shorter (950 nm in the present embodiment) by the thermal radiation light source 10, enabling enhancement in efficiency of the photoelectric conversion.

[0050] Next, a result of measurement using an actually-produced thermal radiation light source 10 will be described with reference to FIG. 6A, FIG. 6B and FIG. 7. FIG. 6A is an optical photomicrograph taken of the produced thermal radiation light source 10, and FIG. 6B is a magnified electron photomicrograph taken of the rods 11 and the base 13 of the thermal radiation light source 10. In the produced thermal radiation light source 10, columnar rods 11 made of Si are arranged in a square grid in a square area 400 m on a side on a surface of a plate base 13 made of SiO.sub.2 and has a thickness of approximately 3 m. A radius r of the rods 11 is 100 nm, a height h is 450 nm, and a periodic length a of the square grid is 500 nm. Also, a plate heater 15 having a three-layer structure constructed of a layer of Ti, a layer of Pt and a layer of Ti in this order from the side close to the base 13 is provided, so as to be contact with a lower surface of the base 13 (surface on the opposite side of the surface on which the rods are provided). When current was flown to the heater 15, the thermal radiation light source 10 was heated to a temperature of approximately 500 K, and the heat was converted to light according to the above-described principle, whereby light emission was obtained. When a spectrum of the obtained light emission was measured, as shown in FIG. 7, a wavelength spectrum having a peak at a wavelength of approximately 900 nm was obtained.

[0051] Although an example in which Si is used for the material of the rods 11 has been described up to here, an intrinsic semiconductor other than Si such as SiC or Cu.sub.2O may be used. If 3C-SiC is used for the material of the rods 11, the cutoff wavelength .sub.g is 800 nm, which is shorter than that in the case of Si, and thus, the height and the radius of the rods 11 is set to be small compared to those in the case of Si. This obtains a thermal radiation light source that produces thermal radiation light having a wavelength spectrum with a peak on the wavelength shorter than 750 nm. A thermal radiation light source having such characteristics as above can suitably be used as a light source that emits thermal radiation light resulting from conversion of sunlight to a GaAs solar cell.

[0052] Also, although in the first embodiment, the rods 11 are arranged in a square grid, arrangement in, e.g., a triangular grid may be employed. Also, although the shape of the rods 11 is a columnar shape, a shape such as a square rod, a cone or a pyramid may be employed. Furthermore, the surrounding areas of the rods 11 may be filled with a material having a lower refractive index than that of the rods 11, such as SiO.sub.2.

Embodiment 2

[0053] As illustrated in FIG. 8 and FIG. 9, a thermal radiation light source 20 according to a second embodiment has a structure in which columnar holes 22 each extending in a direction inclined by 45 from the normal to an upper surface of a block-member 21 made of an intrinsic semiconductor are periodically formed. The columnar holes 22 are arranged in a triangular grid with a period length a in the upper surface of the block member 21. Also, directions in which the columnar holes 42 in adjacent rows with grid points in the triangular grid extend are different by 90 from each other (alternation of the cross-sections along A-A and the cross-sections along B-B in FIG. 9A) (FIG. 9A and FIG. 9C). Such configuration allows formation of a three-dimensional photonic crystal structure having a three-dimensional periodic refractive index distribution in a blocked member 21 made of an intrinsic semiconductor (optical structure in the present embodiment). In the present embodiment, a material of the block member 21 is Si, and a period length a is 680 nm.

[0054] An example of a wavelength spectrum obtained by the thermal radiation light source 20 according to the second embodiment is indicated in FIG. 10. In this example, a wavelength spectrum if the thermal radiation light source 20 is heated to 1300 K (1027 C.) was obtained by calculation. As illustrated in the figure, a wavelength spectrum with a wavelength of approximately 1300 nm as a peak top was obtained.

[0055] The material of the block member 21 is not limited to Si mentioned above and, e.g., SiC or Cu.sub.2O may be used. Also, instead of the columnar holes 22, members having a lower refractive index than that of the block member 21 may be used. Alternatively, as illustrated in FIG. 11, a three-dimensional photonic crystal in which layers having rod members 29 each made of an intrinsic semiconductor and arranged in parallel are stacked may be used, where a direction of the rod members in a layer is different by 90 from that of the rod members in a longitudinally adjacent layer (see Patent Literature 3).

Embodiment 3

[0056] As illustrated in FIG. 12, a thermal radiation light source 30 according to a third embodiment has a configuration in which holes (different-refractive index areas) 32 is periodically provided in a plate member 31, whereby a two-dimensional photonic crystal structure (optical structure in the present embodiment) is formed. For a material of the plate member 31, in the present embodiment, Si (refractive index: 3.4) is used. The holes 32 are arranged in a triangular grid. A planar shape of the individual holes 32 is a round shape. In this configuration, in the thermal radiation light source 30 according to the present embodiment, holes 32 each having a refractive index of approximately 1 are periodically arranged in the Si plate member 31 made of an intrinsic semiconductor and has the refractive index of 3.4, whereby a two-dimensional periodic refractive index distribution is formed.

[0057] In the present embodiment, a period length a of the holes 32 is set to 600 nm. Also, a radius of the holes 32 is set to 150 nm. Also, the plate member 31 has a thickness of 500 nm, and the holes 32 are formed to have a depth of 200 nm from one surface of the plate member 31. The holes 32 are provided so as to form openings in one surface of the plate-like member 31 and not form the openings in the other surface, which forms asymmetry in a direction perpendicular to the plate member 31. Thus, thermal radiation light can be emitted with a larger intensity from the surface of the plate member 31 through which the holes 32 extend. In the present embodiment, as a diameter of the holes 32 is larger, an average refractive index in a case of combining the plate member 31 and the holes 32 is smaller, and thus, if the period length a remains constant, as the diameter is larger, the wavelengths in air is shorter.

[0058] In the present embodiment, only light of wavelengths close to a resonant wavelength .sub.r=1600 nm on the wavelength shorter than a cutoff wavelength .sub.g1700 nm is selected and amplified and emitted to the outside.

[0059] The thickness of the plate-like member 31 can be changed within a certain range while the resonant wavelength is maintained to be similar, by adjusting, e.g., the diameter, depth or periodicity of the holes. However, excessive increase in the thickness causes energy absorption by intrinsic carriers in the intrinsic semiconductor, which may result in undesired light emission on the wavelength longer than the cutoff wavelength .sub.g. As an example, each energy absorptance of a Si plate member having a thickness of 0.5 m (500 nm), which is the same as that of the present embodiment, and Si plate members having a thickness of 10 m and 100 m, respectively, when the members are heated to 1400 K (1127 C.) was obtained by calculation. As shown in FIG. 13, on the longer wavelength side than the cutoff wavelength .sub.g for Si1700 nm (corresponding photon energy: 0.73 eV), the absorptance has a value of almost 0.7 where the thickness of the plate member is 100 m, and the absorptance has a value of 0.30 to 0.35 where the thickness is 10 m, whereas the absorptance has a value of almost zero where the thickness is 0.5 m. This means that as the thickness of the plate member is larger, undesired light emission on the long wavelength side occurs with larger intensity. Therefore, in order to suppress such light emission, it is desirable that the thickness of the plate member 31 be thin. However, if the thickness of the plate member 31 is extremely small, light emission having the resonant wavelength also decreases, and thus, it is desirable to select a proper thickness in view of this point. In the present embodiment, 0.5 m is optimum.

[0060] Such unnecessary light emission on the long wavelength side may occur in the first and second embodiments. In the first embodiment, an effective thickness of intrinsic semiconductor averaged in a direction parallel to a surface of the base 13 can be changed by changing the height or diameter of the rods 11 made of the intrinsic semiconductor, enabling adjustment so that light emission on the long wavelength side is suppressed. In the second embodiment, the periodicity of the columnar holes 22 or the rod members 29 is adjusted so that a three-dimensional photonic bandgap is formed on the wavelength longer than the resonant wavelength, whereby light on the longer wavelength cannot exist in the three-dimensional photonic crystal structure. Therefore, production of the light is thus suppressed.

[0061] Although an example in which Si is used for the material of the plate member 31 has been described up to here, an intrinsic semiconductor other than Si such as SiC or Cu.sub.2O may be used. Also, although an example in which the holes 32 are arranged in a triangular grid has been described up to here, arrangement in, e.g., a square gird may be employed. Although a planar shape of the holes 32 is a round shape, a planar shape other than a round shape such as a regular triangle shape may be employed. Furthermore, instead of the holes 32, members having a lower refractive index than that of the material of the plate member 31 such as members made of SiO.sub.2 may be provided.

REFERENCE SIGNS LIST

[0062] 10, 20, 30 . . . Thermal radiation light source [0063] 11 . . . Rod [0064] 13 . . . Base [0065] 15 . . . Heater [0066] 19 . . . Photovoltaic Device [0067] 19A . . . Collective Lens [0068] 19B . . . Silicon Solar Cell [0069] 21 . . . Block Member [0070] 22 . . . Columnar Hole [0071] 29 . . . Rod Member [0072] 31 . . . Plate Member [0073] 32 . . . Hole