THERMO-OPTICAL CONVERSION ELEMENT AND THERMOELECTRIC
`CONVERSION ELEMENT
`
`DOCUMENT ID DATE PUBLISHED
`
`WO 2017141682 A1
`
`2017-08-24
`
`INVENTOR INFORMATION
`
`NAME
`
`CITY STATE ZIP CODE COUNTRY
`
`
`
`
`
`NAGASE KAZUYA N/A N/A—26158585 JP
`
`
`
`OKAMOTO KUNIYOSHIN/A N/A—76158585 JP
`
`DATE FILED
`
`2017-01-31
`
`CPC CURRENT
`
`TYPE
`
`CPCI
`CPCI
`CPCI
`CPCI
`CPCI
`CPCI
`CPCI
`
`KWIC Hits
`
`Abstract
`
`CPC
`
`HOTS §/343
`GOSS 8/412
`HOTS 5/04
`G O2 B 6/122
`HOTS 8/026
`H10 N 70/853
`MON TO/85?
`
`DATE
`
`2013-01-01
`2013-01-01
`2013-01-01
`2013-01-01
`2013-01-01
`2023-02-01
`2023-02-01
`
`A thermo-optical conversion element (2) is provided with: a substrate (10); a first semiconductorlayer
`(16) disposed on the substrate (10); a multiple quantum well layer (14) disposed onthefirst
`semiconductorlayer (16); a second semiconductorlayer (12) disposed on the multiple quantum well
`layer (14); and grid points (12A) which are periodically disposedin a first photonic crystal (PC)
`structure comprising the first semiconductorlayer (16), the multiple quantum well layer (14), and the
`second semiconductorlayer (12), and which are capable ofdiffracting light waves of a photonic band.
`The multiple quantum well layer (14) has quantum levels formed to at least the second order, andlight
`that has been energy-converted from heat by intersubbandtransition can be conductedin an arbitrary
`direction by the first PC structure. A thermo-optical conversion element and a thermoelectric
`conversion element which are capable of controlling heat propagation using photothermal conversion
`and the PC and which are easy to handle are provided.
`
`Description
`
`INVENTION-TITLE
`
`TECHNICAL-FIELD
`
`The present embodimentrelates to a heat-light conversion element and a thermoelectric conversion
`element.
`
`BACKGROUND-ART
`
`

`

`Since 65% of all energy is converted to heat, technological development and research on
`thermoelectric conversion technology and heat-light conversion technology that can recycle energy
`that has been discarded as heat is being promoted.
`
`Further, as heat conduction, conduction by electrons, conduction by phonons, and conduction by
`radiation are generally known. In manyelectronic devices, the problem of heat generation is one of the
`major factors for ensuring reliability, and is mainly influenced by conduction by electrons and
`conduction by phonons.
`
`Here, conduction of heat by electrons and phonons movesfrom a hot region to a cold region, andit is
`very difficult to control the conduction.
`
`Asa previousstudyto try to control the heat, research on thermalrectifiers (thermal diodes) that easily
`transmit heat in one direction by combining materials whose thermal conductivity changes with
`temperature (Patent Documents 1, 2, 3, 4, Non-Patent Documents1, 2, 3, 4, 5), research that
`attempts to reduce heat conduction by using heat in the same way as sound using phononiccrystal
`structures of about 10 nm, anddistribution of heat Although researchto be arbitrarily controlled (Non-
`Patent Document5) has been carried out, a new technology that can control heat is strongly desired
`becausethe characteristics are poor or fabrication is difficult.
`
`Generally, when heat is treated as sound, a phononic crystal structure of the order of 10 nm is
`required as shownin Non-Patent Document5. This is due to the fact that heat is a longitudinal wave,
`unlike a transverse wavethatis handled by a photonic crystal (PC)oflight.
`
`In addition, research that attempts to control heat has been conducted, and there is research using
`spin (Non-Patent Document6). However, heat conduction via spin requires microwaveirradiation as
`an external field and is not suitable for device formation.
`
`Although it is not heat conduction, a techniqueforefficiently converting the energy of heat radiation
`from an object into arbitrary narrow-band light has been reported (Non-Patent Document7).
`
`CITATION-LIST
`
`PATENT-LITERATURE
`
`[1] Japanese Patent Laid-Open No. 61-272591 [2] JP-A 63-263393 [3] US Patent Application
`Publication No. 2010 /0167004A1 [4] International Publication No. WO2015 / 030239A1
`
`NON-PATENT-LITERATURE
`
`[1] C.W. Chang, D.Okawa, A.Majumdar, and A.Zettl, “Solid-State Thermal Rectifier’, Science VOL314,
`17 November(2006) 1121 [2] Wataru Kobayashi, Daisuke Sawaki, Tsubasa Omura, Takuro Katsufuji,
`Yutaka Moritomo, andIchiro Terasaki, “Thermal Rectification in the Vicinity of a Structural Phase
`Transition”, Appl. Phys. Exp2012, 02, 2012 [3] M.Criado-Sancho, LFdel Castillo, J.Casas-Vazquez,
`and D.Jou, “Theoretical analysis of thermalrectification in a bulk Si / nanoporous Si device”, Phys.
`Lett., A376, 1641 (2012) 1641 -1644 [4] Nakayama Ryusuke and Takeuchi Tsunehiro, “Thermal
`Rectification Effect Using Anomalous Electronic Thermal Conductivity in A-—-Cu—Fe Quasicrystals”,
`2013 Spring Meeting (152nd) Annual Meeting of the JapanInstitute of Metals, Vol.152nd, p .247
`(2013) [5] Martin Maldovan, “Sound and heat revolutions in phononics”, 14 NOVEMBER2013, VOL
`503, Nature, 209 (2013) [6] T.An, VIVasyuchka, K.Uchida, AVChumak, K.Yamaguchi, K.Harii, J. Ohe,
`MB Jungfleisch, Y. Kajiwara, H. Adachi, B. Hillebrands, S. Maekawa, and E. Saitoh “Unidirectional
`spin-wave heat conveyer’, Nature Materials, PUBLISHED ONLINE: 21 APRIL 2013, DOI: 10.1038/
`NMAT3628 [7] Menaka De Zoysa, Takashi Asano, Keita Mochizuki, Ardavan Oskooi, Takuya Inoue,
`and Susumu Noda, “Conversion of broadband to narrowband thermal emission through energy
`reigning
`
`DESCRIPTION-OF-EMBODIMENTS
`
`

`

`Next, the present embodimentwill be described with reference to the drawings.In the following, the
`same reference numerals are assigned to the same blocks or elements to avoid duplication of
`explanation and simplify the explanation. It should be noted that the drawings are schematic and
`different from the actual ones. Moreover,it is a matter of course that portions having different
`dimensional relationships and ratios are included between the drawings.
`
`The embodiments described below exemplify apparatuses and methods for embodying the technical
`idea, and do notspecify the arrangement of each componentas described below. This embodiment
`can be modified in various wayswithin the scopeofthe claims.
`
`(Performanceof thermoelectric conversion element)
`
`As a figure of merit of the thermoelectric conversion element, the relationship between Seebeck
`coefficient S, electrical conductivity o, and carrier concentration n is expressed as shownin FIG. 1A,
`and the relationship between thermal conductivity k and carrier concentration n is shownin FIG. It is
`expressed as shownin (b).
`
`The dimensionlessfigure of merit ZT of the thermoelectric conversion elementis
`
`ZT =S .sup.2 o/ k .Math. T (1)
`
`It is represented by
`
`Here, S represents the Seebeckcoefficient, o represents electrical conductivity, and k represents
`thermal conductivity.
`
`In addition, the Seebeck coefficient S in a commonly used n-type bulk material is
`
`S = 81 .sup.2 kK .sub.B .sup.2 T / 3 eh .sup.2 .Math. m .sup.* .Math. (1 / 3n) .sup.2/3 (2)
`
`Is represented. Where k .sub.B is the Boltzmann constant, T is the absolute temperature, e is the
`elementary chargeof electrons, h is the Planck constant, m .sup.* is the effective massof electrons,
`andnis the carrier concentration.
`
`The electrical conductivity o is
`
`oO = neu (3)
`
`It is represented by Whereu is the electron mobility.
`
`The thermal conductivity k is
`
`K = kK .sub.el + kK .sub.ph (4)
`
`It is represented by Here, k .sub.el represents the thermal conductivity due to electrons, and k .sub.ph
`represents the thermal conductivity due to phonons.
`
`Using the thermoelectric conversionfigure of merit, we will explain the double trade-off that becomes a
`barrier to improving thermoelectric properties.
`
`-Tradeoff between Seebeckcoefficient S and electrical conductivity o-
`
`In the thermoelectric conversion element, as shownin FIG. 1A, the Seebeck coefficient S and the
`electrical conductivity o are in a trade-off relationship with the carrier concentration n. This is clear
`from the above formulas (1) and (2). That is, the carrier concentration n is in the denominator of the
`equation (2) of the Seebeck coefficient S, while it is in the numerator of the equation (3) of the
`electrical conductivity o, and whenthe carrier concentration n is increased, the Seebeck coefficient S
`is Although it becomes small, it means that the electric conductivity o becomeslarge.
`
`

`

`The carrier concentration n also affects the thermal conductivity k as shownin FIG.
`
`In particular, the thermal conductivity kK .sub.ph due to phononsis constant with respect to the carrier
`concentration n, but the thermal conductivity k .sub.el due to electrons increasesas the carrier
`concentration n increases.
`
`-Tradeoff between S .sup.2 o and thermal conductivity k-
`
`Further, the relationship between S .sup.2 o and the carrier concentration n is expressed as shownin
`FIG. 1A, and the relationship between the thermal conductivity k and the carrier concentration n is
`expressed as shownin FIG. Therefore, S .sup.2 o and thermal conductivity k are in a trade-off
`relationship between S .sup.2 o and thermal conductivity k with respect to the carrier concentration n,
`as shownin FIGS. 1 (a) and 1 (b). For this reason, it is generally said that the thermoelectric
`conversion elementhasthe best performanceat a carrier concentration n3 in the range of about 10
`.sup.18 to 10 .sup.19 (cm .sup.—3).
`
`In order to eliminate the trade-off between S .sup.2 o and thermal conductivity k, it is desirable to
`lowerthe thermal conductivity k. However, in thermoelectric conversion, the optimum carrier
`concentration n3 is determined from the relationship between S .sup.2 o and thermal conductivity k.
`Since it exists, the thermal conductivity k can be loweredonlyby the structure.
`
`As a structural device for reducing the thermal conductivity k, attention is focused on increasing
`phonon scattering. In other words, thermal conductivity is reduced by increasing phonon scattering
`due to the low-dimensional structure, thermal conductivity is reduced by selecting a material that is
`difficult to transmit phononsat the atomic structure level, and thermal conductivity is reduced by
`increasing phononscattering dueto finer grains. Thereis.
`
`For example, assuming a frequency of 1 THz, the phononic crystal structure needsa lattice constant
`of about 10 nm. In the case of a phononic crystal structure, since it is not a photon but a phononthatis
`handled, not the refractive index but the material density is important. That is, the wavelength A
`.sub.ph of the phonon propagating through the phononiccrystal structure is
`
`A .sub.ph = v .sub.s / f .sub.ph (5)
`
`It is represented by Where v .sub.s is the propagation speed of phonons(not the propagation speed of
`photons),
`
`f .sub.ph represents the propagation frequency of the phonon.
`
`Propagation velocity v .sub.s of phonons,
`
`v .sub.s = (E/ p) .sup.1/2 (6)
`
`It is represented by Here, E represents Young's modulus and p represents density. In the case of
`silicon, E = 185 GPa, p = 2.3 g/cm, phononpropagation velocity v .sub.s = 8400 m/s, and phonon
`wavelength A .sub.ph = 8.4 nm.
`
`It can be seen that a phononic crystal structure of the order of 10 nm is required whenheatis treated
`as sound. This is dueto the fact that the heat is a longitudinal wave, unlike the transverse wave thatis
`handled by anoptical PC. A phononic crystal structure of about 10 nm is difficult to produce.
`
`[First embodiment]
`
`(Light control method-PC-)
`
`Aschematic bird's-eye view structure of a one-dimensional (1D: One Dimensional) PC applicable to
`the heat-light conversion element according to the first embodiment is expressed as shownin FIG. 2A,
`and is two-dimensional (2D: Two Dimensional). ) A schematic bird's-eye view structure of a PC is
`
`

`

`represented as shownin FIG. 2B, and a schematic bird's-eye structure of a three-dimensional (3D) PC
`is represented as shownin FIG. 2C.
`
`PC is a crystal structure oflight having a periodic refractive index distribution. The lattice constant a of
`the PC lattice point is .sub.expressedbyaninterval obtained by dividing the light wavelength A by the
`refractive index n .sub.r of the material, that is, a= A/n.sub.r.
`
`1DPC can be applied to, for example, a distributed Bragg reflection (DBR)layer, and can reflect only
`light of a specific wavelength.
`
`2DPC and 3DPCcanbe applied to, for example, an optical waveguide, and can propagateonlylight of
`a specific wavelength or branch light by wavelength. 2DPC and 3DPCcanbe applied to, for example,
`a diffraction / reflection technique, can confine specific light, and can diffractlight in the direction
`perpendicular to the surface.
`
`In the heat-light conversion element according to the first embodiment, the operation principle of heat-
`light conversion is expressed as shownin FIG. That is, FIG. 3 showsthat in the Al .sub.0.3 Ga
`.sub.0.7 As / GaAs / Al .sub.0.3 Ga .sub.0.7 As-based PC structure, electrons existing in the ground
`level of a quantum well (QW)are thermally converted into secondary subbands.It is an energy band
`diagram explaining a mode to be excited.
`
`The electrons in the groundlevel of QW are excited to the secondary subband by heat. When the
`electrons return to the groundlevel, infrared light corresponding to intersubband energyis emitted.
`
`In the thermo-optic conversion element according to the first embodiment, an example of the emission
`spectrum of the device is schematically represented as shownin FIG. The curve AMis an example of
`a radiation spectrum of a device having a multi-quantum well (MQW) + PCstructure, and BB is an
`example of a black body radiation spectrum. Only a specific wavelengthis diffracted and emitted in the
`direction perpendicular to the plane dueto the influence of PC. The narrow bandis due to a
`componentin which thelight propagating in the PC by light emission between the subbandsis locked
`to a specific wavelength, although there is a place dueto the diffraction effect of the PC. 4 is a drawing
`created based on FIG. 3 disclosed in Non-Patent Document7. FIG.
`
`Aschematic bird's-eye view structure of the heat-light conversion element 2 accordingto thefirst
`embodiment is expressed as shownin FIG. Further, an enlarged view of the area Ain FIG. 5A is
`expressed as shownin FIG. Further, in the thermo-optic conversion element 2 accordingto thefirst
`embodiment, in the AlGaAs / GaAs / AlGaAs PCstructure, electrons existing in the groundlevel of the
`quantum well (QW)dueto heat are converted into the secondary subband. An energy band diagram
`for explaining the state of excitation and the state of light emission between the secondary subbands
`is schematically represented as shownin FIG.
`
`As shownin FIGS. 5A and 5B, the thermal-light conversion element 2 according to thefirst
`embodimentincludes a substrate 10, a first semiconductor layer 16 disposed on the substrate 10, 1st
`PC structure which consists of MQW layer14 arrange | positioned on 1 semiconductorlayer 16, 2nd
`semiconductor layer 12 arrange | positioned on MQW layer 14, 1st semiconductorlayer 16, MQW
`layer 14, and 2nd semiconductor layer 12 And grating points 12A that are periodically arranged and
`capable ofdiffracting a photonic bandlight wave.
`
`Here, for example, as shownin FIG. 5C, the MQWlayer 14 is in contact with the first material layer 28
`and thefirst material layer 28, and is disposed betweenthefirst material layers 28. It is formed by a
`multi-layered structure having a quantum well (QW)structure formed by two material layers 30.
`
`In the second material layer 30, quantum levels up to the second order are formed, and energyis
`converted from heatto light by utilizing the intersubband-transition (ISB-T). Is done. Thelight is formed
`on the sameplane asthe laminated surface ofthe first material layer 28 and the second material layer
`30, and has a first PC structure comprising the first semiconductorlayer 16, the MQW layer 14, the
`second semiconductorlayer 12, and the lattice points 12A. Can conductin any direction.
`
`

`

`Thelattice points 12A are periodically arranged in the first PC structure, can diffract light having a
`wavelength correspondingto the intersubband energy of the quantum level of the second material
`layer 30, and can be confinedin the first PC structure.
`
`Thelattice points 12A may be arrangedin anyof a squarelattice, a rectangularlattice, a face-centered
`rectangularlattice, or a triangularlattice. FIG. 5A shows an examplein whichthe lattice points 12A are
`arrangedin a triangular lattice.
`
`Further, the lattice point 12A may have any shape of a polygon, a circle, an ellipse, or an oval. FIG. 5A
`shows an example in which the lattice point 12A has a circular shape.
`
`In FIG. 5A, the substrate 10 is made of, for example, a GaAs substrate having a thickness of about
`650 um, andsilicon (Si) is doped to about 1 x 10 .sup.17 cm .sup.-3 .
`
`Thefirst semiconductor layer 16 is formed by thinning the GaAs substrate from about 650 um to about
`0.6 um. Therefore, it is doped n-type.
`
`The MQW layer 14 and the second semiconductorlayer 12 are sequentially formed onthefirst
`semiconductor layer 16.
`
`The second semiconductor layer 12 has, for example, a thickness of about 0.8 um and may be doped
`p-type.
`
`In FIG. 5A, the thickness D of the three-layer structure including the first semiconductor layer 16, the
`MQW layer 14, and the second semiconductorlayer 12 is about 1.9 mm, for example, and thelattice
`constant of PC is about 6 for example. The length L1 of the PC in which .5 um andthelattice points
`12A are arrangedin a triangularlattice is about 2.4 mm. The size of the PC area is about 2.4 mm x
`about 2.4 mm.
`
`Thefirst material layer 28 has, for example, a thickness of about 13 nm and is formed of an n .sub.0.3
`doped Al .sub.0.3 Ga .sub.0.7 As layer. The second material layer 30 has, for example, a thickness of
`about 6.8 nm andis formed of a non-doped GaAslayer.
`
`The MQWlayer 14 includes 63 periods of a QW structure of Al .sub.0.3 Ga .sub.0.7 As / GaAs / Al
`.sub.0.3 Ga .sub.0.7 As, and the total thicknessis, for example, about 1.25 um.
`
`The wavelength of the infrared light emitted by the intersubbandtransition (ISB-T)is, for example,
`about 9.7 um.
`
`The semiconductor material constituting the second semiconductor layer 12, the MQW layer 14, or the
`first semiconductorlayer16 is silicon (Si), GaAs, GaN, InP, SiGe / Si system, AlGaAs / GaAs system,
`AlGaN / GaN. Any oneof the system, GalnAsP / InP system, InGaAs / GaAs system, GalnNAs / GaAs
`system, GaAllnAs/ InP system, AlGalnP / GaAs system, or GalnN / GaN system is applicable.
`
`Originally, in the case of black bodyradiation, thermal energy is convertedinto light having a wide
`range of wavelengths. However, since the thermo-optic conversion element 2 accordingto thefirst
`embodimenthas a PCstructure, it can be guided by this PC structure. The wavelength is determined.
`For this reason, no transition emission occurs other than the transition between subbands. Therefore,
`thermal energy other than the energy correspondingto the transition between subbandsis not
`converted into light and is not emitted, but remains in the PC with heat.
`
`Also, thermal energy smaller than the energy correspondingto intersubband energy does not
`contribute to heat-light conversion.
`
`In the heat-light conversion element 2 according to the first embodiment, the light is confined in the
`area of the PC without being diffracted in the direction perpendicularto the plane, orthe light is
`radiated, and the light is movedin an arbitrary direction and returned to heat. A schematic bird's-eye
`view structure for explaining how to designto return to the direction is represented as shownin FIG.
`
`

`

`Further, the description of the display of the heat-light conversion TP is expressed as shownin FIG. In
`the structure of FIG. 6, the substrate 10 is not thinned.
`
`The display of the heat-light conversion TP represents a state where the phonon wave TW
`accompanyingheatis convertedinto the light wave PW, as shownin FIG. 6B. 180PW1 and 180PW2
`schematically show light wavesdiffracted by 180 degrees.
`
`Thelight converted from heat should bediffracted by the PC in the direction perpendicular to the plane
`and emitted in the direction perpendicular to the plane, as shownbythelight VD in the direction
`perpendicularto the plane of FIG. However, in the heat-light conversion element 2 according to the
`first embodiment, conversely,if the light is confined in the PC region without being radiated and moved
`in any direction to return to heat, the heat control element Can be realized. Moreover,if it can be
`designedto return in one direction, a thermal diode can be realized. Moreover, it becomespossible to
`make small the heat conductivity of the thermoelectric conversion element which combined the
`thermal diode by these applications.
`
`The heat-light conversion element according to the first embodiment converts the thermal energy from
`light (photons), not as sound (phonons), to change the waveproperties from longitudinal waves to
`transverse waves,thereby increasing the wavelength to 10 nm. Manufacture is also easy becauseit
`can be handled as a 10 ym orderPCstructure from an order phononiccrystal.
`
`(Thermal control element and thermal diode)
`
`If light waveguides are confined in the plane without emitting light in the direction perpendicular to the
`plane, and an optical waveguide based on PC is combined, heat can be propagatedin the light state.
`For this reason, the propagation of heatitself can be controlled. The thermal control element can be
`constructedbyfinally absorbingit at an arbitrary place and returning it to heat. Also, a thermal diode
`can be constructed by designingit to return in one direction. Moreover, it becomes possible to make
`the thermal conductivity of the thermoelectric conversion element which combined the thermal diode
`small by these applications, and it can comprise a thermal conductivity control element.
`
`—Device example using 2DPC—
`
`2DPCthermal control element for explaining a state in which thelight is confined in the region of the
`PC without being radiated in the direction perpendicular to the PC and movedin anarbitrary direction
`and returnedto the heat in the thermo-optic conversion element accordingto the first embodiment A
`schematic bird's-eye view structure of 2C is expressed as shownin FIG.
`
`In the thermal control element 2C accordingto the first embodiment, the substrate 10 is not thinned.
`Otherconfigurations are the sameas thoseofthe first embodiment.
`
`As shownin FIG. 7, the thermal control element 2 </ b> C according to the first embodiment includes a
`first PC structure including a first semiconductorlayer 16, an MQW layer 14, and a second
`semiconductor layer 12. The 2nd PC structure in which the light which propagates can propagate in a
`specific direction may be provided.
`
`Here, as shownin FIG. 7, the second PCstructure mayinclude optical waveguides 22GA and 22GB.
`
`Further, a plurality of optical waveguides 22G maybeprovided.
`
`The two-dimensional PC optical waveguide can be formedbya line defect at the lattice point 12A.
`
`In the heat-light conversion element 2 according to the first embodiment, the heat conduction can be
`controlled by moving the light emitted from the MQWlayerto an arbitrary region by the second PC
`structure and reconverting it into heat. .
`
`The second PCstructure may include a thermal control element capable of propagating light emitted
`from the quantum structure toward an arbitrary region.
`
`

`

`Further, the second PC structure mayinclude a thermal diode capable of propagating light emitted
`from the MQWlayerin a direction different from a region where heat is not desired to propagate.
`
`In the thermal control element 2C accordingto the first embodiment, the light converted from heatis
`confined in the PC andreturnedto the heat source side using the optical waveguide 22G configured
`by the same PC,or an arbitrary It can be propagated to the region sothatlight is again absorbed and
`returned to heat. That is, the propagation of heat can be controlled bylight.
`
`For example, as shownin FIG. 7, the light waves PW5 and PW6 converted from heat are propagated
`to the heat regions 26H and 27H throughthe optical waveguide 22GA.Here, the light waves PW7 and
`PW8propagating through the optical waveguide 22GA maypropagate in opposite directions to be
`concentrated in the heat regions 26H and 27H, respectively. The light waves PW1, PW2, PW3, and
`PW4 converted from heat may propagate through the optical waveguide 22GB and be concentratedin
`the heat region 28H.
`
`In the thermo-optic conversion element according to the first embodiment, a schematic bird's-eye view
`structure of a 2DPC thermal diode 2D for explaining a state of being designed to be confined in a PC
`region without being radiated and returnedin one direction is as follows: , As shownin FIG.
`
`In the thermal diode 2D accordingto the first embodiment, the substrate 10 is not thinned. Other
`configurations are the sameas thoseofthe first embodiment.
`
`The display of the heat-light conversion TP is the same asin FIG. 180PW1, 180PW2, and 180PW3
`schematically representlight wavesthat are diffracted by 180 degrees. 9OPW schematically shows a
`light wave diffracted by 90 degrees.As indicated by NVDin FIG. 8, light converted from heatis not
`diffracted in the direction perpendicular to the plane.
`
`In the thermal diode 2D accordingto the first embodiment, light converted from heat is confined in the
`PC and returned to the heat source (HEAT)side using the optical waveguides 22G1 and 22G2
`configured by the same PC.Or, it can be propagated to an arbitrary region so thatlight is absorbed
`again and returned to heat. That is, the propagation of heat can be controlled bylight.
`
`In the thermo-optic conversion element according to the first embodiment, the second PC structure
`(optical waveguide) hasa structure for propagating light converted by heat in a direction different from
`the direction in which heat is not desired to be transmitted. A thermal diode is provided.
`
`In the thermal light conversion element according to the first embodiment, a thermal control element or
`a thermal diode can berealized.
`
`Further, in the thermo-optic conversion element according to the first embodiment, the thermal
`conductivity can be reduced. For this reason, when the thermo-optic conversion element concerning
`this Embodimentis applied to a thermoelectric conversion element, a temperature difference can be
`enlarged and thermoelectric conversion efficiency can be improved.
`
`Whenheatis handled as phonons,thelattice constant of a phononic crystal on the order of 10 nm is
`required, but by convertingit to light, it can be produced with the lattice constant of a micron order PC,
`makingit suitable for manufacturing. Yes.
`
`One methodfor absorbinglight in order to convert light into heat is interaction with carriers in the
`semiconductor. Form an absorber such as metal(a material that is not transparent to propagating
`light), or form high-concentration regions or crystal defects using ion implantation techniques, etc. Can
`be absorbed by
`
`(Example of energy band diagram oflayer structure)
`
`In the thermo-optic conversion device accordingto the first embodiment, in the AlGaAs / GaAs/
`AlGaAsPCstructure, a structure in which the second material layer 30 is sandwiched between the
`first material layers 28 is defined as a unit structure UA. An energy band diagram ofa structure in
`
`

`

`whichthe layers are repeatedly stacked is expressed as shownin FIG. Here, the first material layer 28
`can be formed of an Al .sub.x Ga .sub.1-x As layer, and the second material layer 30 can be formed of
`a GaAslayer. As shownin FIG. 9, in the second material layer 30, heat-light conversion is possible
`betweenthe quantum levels E .sub.1 and E .sub.2 by the intersubbandtransition (ISB-T).
`
`In the thermo-optic conversion device accordingto the first embodiment, in the AlGaAs / GaAs/
`AlGaAsPCstructure,the first material layer 28, the second material layer 30, the second material
`layer 30 are replaced with the first material layer 28. The structure in which the second material layer
`30 is sandwiched between the third material layer 32 disposed so as to be sandwiched between the
`first material layer 28 and the third material layer 32 is repeated as a unit structure UB. The energy
`band diagram of the laminated structure is expressed as shownin FIG. Here, thefirst material layer 28
`can be formed of an Al .sub.x Ga .sub.1-x Aslayer, the second material layer 30 can be formedof a
`GaAslayer, and the third material layer 32 can be formed of, for example, an Al .sub.y Ga .sub.1-y As
`layer. As shownin FIG. 10, in the second material layer 30, heat-light conversion is possible between
`quantum levels E .sub.1 and E .sub.2 by intersubbandtransition (ISB-T).
`
`FIG. 11 shows an energy band diagramof a structure in which a plurality of unit structures UA, UB,
`and UC are repeatedly stacked in the AlGaAs / GaAs / AlGaAs PCstructure in the thermo-optic
`conversion elementaccordingto the first embodiment.It is expressed in Here,the first material layer
`28 can be formed of an Al .sub.x Ga .sub.1-x As layer, the second material layer 30 can be formed of a
`GaAslayer, and the third material layer 32 can be formed of, for example, an Al .sub.y Ga .sub.1-y As
`layer. As shownin FIG. 11, in the second material layer 30, heat-light conversion is possible between
`quantum levels E .sub.1 and E .sub.2 by intersubbandtransition (ISB-T).
`
`In the direction perpendicular to the interface betweenthefirst material layer 28 and the second
`material layer 30, the second material layer 30 has a quantum structure that does not necessarily have
`the samelayer structure as the unit structures UA and UB and mainly converts heatinto light. A
`plurality of unit structures UA, UB, and UC are repeatedly stacked, and the first PC structure and the
`second PCstructure include a thermal control element capable of propagating light emitted from the
`quantum structure toward an arbitrary region. May be.
`
`In addition, in the direction perpendicular to the interface betweenthefirst material layer 28 and the
`second material layer 30, the quantum structure that mainly converts heatinto light is not necessarily
`formed in the same layer configuration as the unit structures UA and UB. A plurality of unit structures
`UA, UB, and UC are prepared and repeatedly stacked, and thefirst PC structure and the second PC
`structure allow light emitted from the quantum structure to propagatein a different direction from the
`region whereheatis not desired to propagate. An appropriate thermal diode may be provided.
`
`In the thermo-optic conversion device according to the first embodiment, in the AlGaN / GaN / AlGaN-
`based PCstructure,the first material layer has a structure in which the second material layer 36 is
`sandwiched betweenthefirst material layers 34. An energy band diagram of a structure in which a
`triangular potential well structure formed at one interface of the second material layer 36 and the
`second material layer 36 is repeatedly stacked as a unit structure UD is expressed as shownin FIG.
`Thefirst material layer 34 can be formed of an AlGaNlayer, and the second material layer 36 can be
`formed of a GaNlayer. As shownin FIG. 12A, the quantum structure formed in the second material
`layer 36 near oneinterface with the first material layer 34 has a triangular potential well structure, and
`has quantum levels E .sub.1 and E .sub.1. between .sub.2, the intersubband transition (ISB-T), can be
`heatlight conversion.
`
`In the thermoelectric conversion element accordingto the first embodiment, the first material layer 34
`is sandwiched betweenthefirst material layers 34 in the AlGaAs / GaAs / AlGaAs-based PCstructure.
`An energy band diagram ofa structure in which the triangular potential well structure formedat the
`interface between both the layer 28 and the second material layer 30 is repeatedly stacked as the unit
`structure UE is expressed as shownin FIG. Thefirst material layer 28 can be formed of an AlGaAs
`layer, and the second material layer 30 can be formed of a GaAslayer. As shownin FIG. 12B, the
`quantum structure formed in the second material layer 30 in the vicinity of the interface with the first
`
`

`

`material layer 28 has a triangular potential well structure, and includes quantum levels E .sub.1 and E
`.sub.2 . In between, heat-light conversion is possible by intersubband transition (ISB-T).
`
`[Second Embodiment]
`
`FIG. 13 shows an energy band diagram of an AlGaAs / GaAs / AlGaAs MQWstructure designed so
`that the inter-subband energy of the quantum level has an integer multiple relationship in the thermo-
`optic conversion device according to the second embodiment. Represented as shown. Alsoin the
`thermal-light conversion element according to the second embodiment, the device structure has the
`sameconfiguration as that of the first