Photothermal converter

Abstract

To provide a photothermal converter with a simple structure that enables a large-area free two-dimensional, three-dimensional structure to be easily created.SOLUTION: The present invention relates to a photothermal converter that makes good use of intense resonance at a vibration frequency where a vibration frequency of a light standing wave formed by an optical cavity and a vibration frequency of a vibrator interacting with light such as phonon are matched with each other, and absorbs or emits light at the frequency of the resonance. The figure shows an example of a converter of a type using an optical cavity of an asymmetrical Fabry-Perot interference type as the optical cavity. (a) shows that although an SiO2 layer sandwiched between respective interfaces between the atmosphere and a metal layer forms the optical cavity, the resonance occurs at a vibration frequency of phonon of the SiO2 layer. (b) shows that phonon-free Si is used for an infrared range as a light transmission material inside the optical cavity, and the resonance occurs on the interface with the SiO2 layer in contact with the top thereof.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an optical-thermal converter in the infrared region that converts between light and heat, and in particular to an optical-thermal converter of the phonon-mediated type or the like that converts between heat and infrared light of a desired spectrum with high efficiency by using phonons or the like. [Background technology]

[0002] Light-to-heat converters are key elements in infrared device technology. Recently, light-to-heat converters based on lithographically patterned two-dimensional metallic nanostructures have been proposed and fabricated with wavelength ranges specific for use in infrared detectors and heaters. The fabrication of these structures requires multiple processes, including polymer coating, light or electron beam exposure, development processes, and dry etching.

[0003] Another common type of spectroscopic light-to-heat converter is the distributed Bragg reflector (DBR)-based IR device, which requires multiple depositions of films onto a flat metal surface and the total device thickness can be several micrometers.

[0004] For examples of the above-mentioned conventional light-to-heat converters using metallic nanostructures, see, for example, US Pat. Nos. 5,399,991 and 5,433,363.

[0005] The above-mentioned devices require multiple deposition processes in addition to elaborate design optimization, which require significant costs and manufacturing times. Therefore, there is a great need for a new concept of spectroscopic converter that can be provided with simpler design principles and manufacturing processes. Summary of the Invention [Problem to be solved by the invention]

[0006] To solve the above problems, the present invention provides an infrared light-to-heat converter having a simple layer structure and capable of flexibly adjusting the spectrum by utilizing the interaction between an optical standing wave formed in a cavity and other vibration systems such as phonons. [Means for solving the problem]

[0007] According to one aspect of the present invention, An optical cavity is provided on the surface or inside of which an optical standing wave is formed, an oscillator that resonates with the optical standing wave by interacting with the optical standing wave is present in the optical cavity or in an element adjacent to the optical cavity, and an optical-thermal converter is provided that absorbs incident light at the frequency of the resonance or emits light at the frequency of the resonance. Here, the oscillator may be selected from the group consisting of a phonon oscillator, a plasmon, and an electronic excitation. The optical cavity may also have the shape of a particle. The particles may be microspheres or microrods having a diameter or longitudinal length in the range of 1 to 30 μm. Moreover, the material of the optical cavity may be selected from Si, Ge, chalcogenide-based infrared-transmitting materials, and metals. Additionally, the element carrying the oscillator and adjacent to the optical cavity may be selected from the group consisting of SiO2, TiO2, Al2O3, and polyimide. Moreover, the optical cavity may have a layered optically transmissive medium material sandwiched between a pair of reflecting surfaces, the optical standing wave is formed between the pair of reflecting surfaces, and the material constituting at least one of the pair of reflecting surfaces or the optically transmissive material may have the oscillator. In addition, light may be incident on the light-to-heat converter via the reflecting surface, which is an interface with a layer of material having the oscillator, and the absorption may occur, or light may be emitted via the reflecting surface, which is an interface with a layer of material having the oscillator. The pair of reflecting surfaces may have a uniform shape in the direction of the reflecting surfaces. Also, one of the pair of reflecting surfaces may be a surface of a metal layer, or a surface of a dielectric layer having a refractive index different from that of the light-transmitting material of the optical cavity. Moreover, the other of the pair of reflecting surfaces may be an interface between the light-transmitting material of the optical cavity and the outside of the light-to-heat converter, and the light-transmitting material may have the oscillator. The light-transmitting material may be a material having the oscillator, and the one of the pair of reflecting surfaces may be a surface of a metal layer. The other of the pair of reflecting surfaces may be a surface of a dielectric layer and may have the oscillator, and the light-transmitting material of the optical cavity may not have an oscillator that resonates in the infrared range. The light transmitting material may be a material selected from the group consisting of Si, Ge, and chalcogenide-based infrared transmitting materials, and the one of the pair of reflecting surfaces may be a surface of a metal layer. Moreover, the dielectric material providing the other of the pair of reflecting surfaces may be a material selected from SiO2, TiO2, polyimide, and Al2O3. It may also have a flat or three-dimensional shape. It may also have a shape selected from the group consisting of a cylinder, a sphere, and a polyhedron. Effect of the Invention

[0008] The light-to-heat converter of the present invention has a simple layer structure, which makes it easy to increase the area, and the light absorption and emission spectrum can be flexibly adjusted by the materials used for the layers and their thicknesses, etc. Alternatively, the light-to-heat converter of the present invention can be made into particles with a core-shell structure, etc., and in this case, the same effects as those of the layer structure can be obtained. [Brief description of the drawings]

[0009] [Figure 1]Figures showing the structure and characteristics of the MD structure light-to-heat converter with a metal-SiO2 layer configuration according to the present invention. (a) A diagram showing the conceptual structure of the MD structure light-to-heat converter (Si substrate is used). (b) A diagram showing the real part ε1 and the imaginary part ε2 of the dielectric function of SiO2 used as the dielectric layer in the wavelength range of 2μm to 20μm, and a diagram showing the dependence of the absorptance (0 to 1) of the MD structure light-to-heat converter on the thickness of the SiO2 dielectric layer with shading, where the horizontal axis is the wavelength of the incident light (2μm to 20μm) and the vertical axis is the thickness of the SiO2 dielectric layer (0μm to 20μm). This light-to-heat converter absorbs or emits infrared light by resonance at a wavelength that forms a standing wave. When this resonance coincides with the phonon frequency of the dielectric layer, the intensity of the absorption or emission becomes very large. [Diagram 2] A diagram showing that the minimum thickness of the SiO2 dielectric layer that causes resonance in the MD structure light-to-heat converter shown in Figure 1 is 1.15 μm. (a) The same diagram as the diagram in Figure 1(b) showing the dependence of the absorptance (0 to 1) of the MD structure light-to-heat converter on the thickness of the SiO2 dielectric layer in shading. (b) An enlarged view of the SiO2 dielectric layer thickness of around 1.15 μm in the diagram in (a). [Diagram 3] Fig. 1 shows the characteristics of the MD light-to-heat converter with a metal-SiO2 layer configuration according to the present invention, (a) a simulation result (dashed line) and an experimental result (solid line) of the absorptance in the wavelength range of 2 μm to 20 μm, (b) a simulation result showing the incidence angle dependence of the absorptance (0 to 1) when the horizontal axis is the wavelength of the incident light (2 μm to 20 μm) and the vertical axis is the incidence angle (0 degrees to 80 degrees or more). [Figure 4] In order to explain that the optical-thermal converter with the MDD structure according to the present invention can independently select the cavity design (thickness) and the phonon medium, unlike the MD structure, the conceptual diagrams of the MD structure and the MDD structure are shown side by side. (a) A conceptual diagram showing the structure of an optical-thermal converter with the MD structure having a metal-SiO2 layer structure. (b) A conceptual diagram showing the structure of an optical-thermal converter with the MDD structure having a metal-Si-SiO2 layer structure. [Diagram 5]1 is a diagram for explaining the operation principle of the light-to-heat converter with the MDD structure according to the present invention. Here, a metal-Si-SiO2 layer structure is taken as an example. (a) A diagram showing the conceptual structure of a light-to-heat converter with the DMM structure (Si substrate is used). (b) A diagram showing the absorptance (0-1) as a function of the thickness of the Si intermediate layer, with the horizontal axis being the wavelength of incident light (2 μm-20 μm) and the vertical axis being the thickness of the Si phonon medium layer (0-4 μm). The main operation mechanism of the MDD structure light-to-heat converter with the metal-Si-SiO2 layer structure is the hybridization of the optical cavity (standing wave) made of the Si intermediate layer and the optical phonons of the SiO2 placed on the Si cavity. Since the separation of the optical cavity layer and the phonon medium layer increases the degree of freedom in the selection of the material of the optical cavity layer, the thickness of the cavity can be made thinner than that of the metal-SiO2 layer structure (MD structure) by using a high refractive index material such as Si for the optical cavity layer. [Figure 6] 1A, 1B, and 1C are diagrams for explaining the optimization of the thickness of the SiO2 layer, which is the top layer of the MDD-structure light-to-heat converter having a metal-Si-SiO2 layer configuration according to the present invention. (a), (b), and (c) are diagrams showing the absorptance (0-1) when the horizontal axis is the wavelength of the incident light and the vertical axis is the thickness of the Si layer (0-4 μm) for the SiO2 layer thicknesses dSiO2=0.1 μm, 0.4 μm, and 0.8 μm, respectively. If the SiO2 layer thickness is too small, the effect of the enhanced phonons of SiO2 is weakened and the absorptance intensity d cannot be maximized. Conversely, if the SiO2 layer thickness is too large, undesired peaks appear and the intensity of the spectral background becomes large. Therefore, it is necessary to appropriately select the thickness dSiO2 of the SiO2 layer (in this example, dSiO2=0.4 μm shown in (b)). [Figure 7]1 shows the characteristics of a DMM-structure light-to-heat converter with a metal-Si-SiO2 layer configuration according to the present invention. (a) Simulation results (dashed line) and experimental results (solid line) of light absorptance in the wavelength range of 2 μm to 20 μm. Here, the thickness of the Si layer as the middle layer is dSi=0.7 μm, and the thickness of the SiO2 dielectric layer as the top layer is dSiO2=0.4 μm. (b) Simulation results showing the incidence angle dependence of absorptance (0 to 1) with shading, where the horizontal axis is wavelength (2 μm to 20 μm) and the vertical axis is the incidence angle of the incident light (0 degrees to 80 degrees or more). [Figure 8] Illustrative diagrams for explaining limitations that appear when using SiO2 phonons in the MD and MDD structure light-to-heat converters according to the present invention. (a) A diagram showing the conceptual structure of an MD structure light-to-heat converter using SiO2 for the dielectric layer. (b) A diagram showing the conceptual structure of an MDD structure light-to-heat converter using SiO2 for the top dielectric layer. (c) A diagram showing the results of a simulation in which the wavelength dependence of the absorptance of the MDD structure light-to-heat converter shown in (b) is shown by shading, with the horizontal axis representing the wavelength and the vertical axis representing the thickness of the Si layer that is the intermediate layer. Here, the thickness dSiO2 of the topmost SiO2 dielectric layer is 0.4 μm. (d) A diagram showing the real part ε1 and imaginary part ε2 of the dielectric function of SiO2 in the wavelength range of 2 μm to 20 μm. [Figure 9] 1A, 1B, and 1C are diagrams for explaining that the optical absorption and emission spectrum can be adjusted over a wide range by selecting the material of the phonon medium used in the intermediate layer in the DMM-structure optical-thermal converter according to the present invention. (a), (b), and (c) are diagrams showing the real part ε1 and the imaginary part ε2 of the dielectric function of SiO2, polyimide, and Al2O3 in the wavelength ranges of 2 μm to 20 μm, 2 μm to 14 μm, and 2 μm to 24 μm, respectively. Compared to SiO2 and polyimide, which have dielectric functions that result in a limited number of phonon enhancement points (wavelength range) in a narrow band, Al2O3 can be used in optical-thermal converters with a wide band. [Figure 10]1 shows the structure and characteristics of a metal-Si-polyimide layered light-to-heat converter, which uses polyimide as the top dielectric layer in a light-to-heat converter with a DMM structure according to the present invention. (a) A schematic structure of a metal-Si-polyimide layered light-to-heat converter, (b) A real part ε1 and an imaginary part ε2 of the dielectric function of polyimide used as a phonon medium in the wavelength range of 2 μm to 14 μm. (c) A shading graph showing the thickness dependence of absorptance (0 to 1) when the horizontal axis is the wavelength of incident light (2 μm to 14 μm) and the vertical axis is the thickness of the Si layer (0.5 μm to 4 μm). [Figure 11] Figure 10(a) shows the conceptual structure of the MDD-structured light-to-heat converter with metal-Si-polyimide layer configuration. By adjusting the size of the optical cavity (thickness dSi of the Si intermediate layer) in the MDD-structured light-to-heat converter, the absorptivity (emissivity) can be made to show a spectrum close to a single spectrum at a peak wavelength selected from several wavelengths. By adjusting the thickness dSi, the resonance wavelength of this light-to-heat converter can be adjusted to each of the various peaks of polyimide shown in Figure 10(b). (a), (b), and (c) are the absorption spectra (wavelength ranges 5.0μm-7.0μm, 6.0μm-7.5μm, and 5.5μm-9.5μm, respectively) when the thickness dSi of the Si intermediate layer is 2.05μm, 3.25μm, and 1.5μm, respectively, and the resonance wavelengths are 5.8μm, 6.6μm, and 7.37μm, respectively. [Figure 12]1 shows the structure and characteristics of an MDD-structured light-to-heat converter with a metal-Si-polyimide layer configuration. This light-to-heat converter aims for an almost single spectrum of light absorption / emission characteristics with a peak at a wavelength of 7.37 μm. Since there are multiple but limited phonon enhancement points (wavelengths), the absorption / emission spectrum can be close to a single spectrum with a peak at a specific enhancement point among them. This makes it possible to realize a light-to-heat converter with an absorption / emission spectrum close to a single spectrum using a broadband phonon enhancement medium. (a) A diagram showing the conceptual structure (substrate not shown) of an MDD-structured light-to-heat converter with a metal-Si-polyimide layer configuration together with the thickness of each layer. (b) A diagram showing the simulation results of the absorptance of polyimide, which is the material of the phonon medium layer arranged on the top layer, at wavelengths of 2 μm to 14 μm. Here, the thickness dSi of the intermediate Si layer is 1.5 μm. The resonant wavelength was 7.37 μm. (c) A graph showing the simulation results of the incidence angle dependence of absorptance (0 to 1) with the horizontal axis representing the wavelength of incident light (2 μm to 14 μm) and the vertical axis representing the incidence angle (0 degrees to 80 degrees or more). [Figure 13] 1 shows the structure and characteristics of a broadband metal-Al2O3 layer optical-thermal converter, which uses an Al2O3 layer as a dielectric layer in a DM optical-thermal converter according to the present invention. (a) A schematic structure of an optical-thermal converter with an MD structure (substrate is Si) according to the present invention, which uses an Al2O3 layer as the dielectric layer arranged on the top layer. (b) A diagram showing the dielectric function of Al2O3 in the wavelength range of 2 μm to 24 μm. (c) A diagram showing the simulation result of the dependence of absorptance on thickness dAl2O3, where the horizontal axis is the wavelength of incident light (2 μm to 24 μm) and the vertical axis is the thickness dAl2O3 of the Al2O3 dielectric layer (3 μm to 9 μm), in shading. When dAl2O3 is 4.8 μm, the background is minimized at wavelengths of 10 μm or more. [Figure 14]1 shows the structure and characteristics of a wideband light-to-heat converter with a metal-Al2O3 layer structure, which is a structure in which an Al2O3 layer is used as the dielectric layer in a light-to-heat converter with an MD structure according to the present invention. (a) A diagram showing the conceptual structure of a light-to-heat converter with an MD structure (substrate is made of Si) according to the present invention, which uses an Al2O3 layer as the dielectric layer arranged on the top layer. (b) A diagram showing the simulation result (dashed line) and the experimental result (solid line) of the absorptivity in the wavelength range of incident light from 2 μm to 24 μm. It can be seen that the two are in good agreement. (c) A diagram showing the simulation result of the incidence angle dependency of absorptivity in shading, where the horizontal axis is the wavelength of incident light (2 μm to 24 μm) and the vertical axis is the incidence angle of incident light (0 degrees to 80 degrees). [Figure 15] FIG. 1 shows that the light absorption wavelength can be changed in the mid-infrared range in a light-to-heat converter with a metal-Si-Al2O3 layer structure, which is an embodiment of an MDD structure obtained by adding a Si intermediate layer to the light-to-heat converter with an MD structure according to the present invention and adjusting the thickness of the intermediate layer. (a) A diagram showing the conceptual structure of a light-to-heat converter with an MDD structure (substrate is made of Si) according to the present invention, using an Al2O3 layer as the top layer. (b) A diagram showing the dependence of the absorptance on the thickness of the Si intermediate layer when the horizontal axis is the wavelength of incident light (5 μm to 24 μm) and the vertical axis is the thickness of the Si layer (1.0 μm to 4.0 μm), which is the intermediate layer, as shades of gray. The white dashed line shows the change in the absorptance depending on the wavelength when the thickness of the Si layer is 3.05 μm. In addition, m shown in the figure is the odd mode m used in the conditional equation for standing wave resonance shown in FIG. 1(b) and takes values ​​of m=1, 3, 5, 7, . . . [Figure 16] (a) A diagram showing the conceptual structure of an optical-thermal converter with an MDD structure according to the present invention, using an Al2O3 layer as the top layer (substrate not shown). (b) A diagram showing the simulation results (dashed line) and experimental results (solid line) of the absorption spectrum of the optical-thermal converter shown in (a) in the wavelength range of 5 μm to 24 μm. Again, good agreement is observed. (c) A diagram showing the simulation results of the incidence angle dependence of the absorptivity of the optical-thermal converter shown in (a) in the wavelength range of 5 μm to 24 μm, using shading. [Figure 17]1 illustrates the effect of a metal layer on the spectral background in an optical-to-thermal converter according to the present invention, (a) showing simulation results for the MD structure and (b) for the MDD structure. [Figure 18] 1A and 1B show examples of structures of an optical-to-thermal converter with a whispering gallery mode cavity according to the present invention, (a) being a microsphere shape and (b) being a microrod structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] In the present application, a light-to-heat converter is provided that is structurally simple, highly efficient, and capable of tunable spectrum by utilizing the strong coupling between phonon modes and light modes in an optical cavity or on the light-reflecting wall (light-reflecting surface) of the optical cavity. As is obvious from its operating principle, this light-to-heat converter can be constructed and used as a light absorber that converts a specific wavelength component in irradiated light into heat, or conversely, can be constructed and used as a light emitter that emits light of a specific wavelength when heated. Although it may be necessary to adjust the size and details of the structure depending on which direction of conversion is aimed for, the essential structure and operating principle of both are the same. Absorption or emission spectral characteristics that can be made nearly single spectrum can be achieved by selecting a dielectric material that has a phonon mode in the desired wavelength range and an optical cavity structure that has a resonant wavelength close to the phonon mode. The cavity structure may be, for example, one of the following forms [1] and [2]. [1] A monolayer of a dielectric material placed on a flat metal surface, which has a phonon mode at the target wavelength of light to be absorbed or emitted (in other words, a phonon oscillator at the same frequency as the target frequency of light to be absorbed or emitted). [2] A single layer of dielectric material with a phonon mode of the desired wavelength (in other words, with phonon oscillators at the same frequency as the target frequency of the absorbed or emitted light) is placed on a flat silicon (Si) layer, which is then placed on a flat metal surface.

[0011] The wavelength of the resonance and its spectral broadening can be adjusted by selecting dielectric materials and phonon oscillators with appropriate resonance wavelengths and spectral broadening.

[0012] More generally, in either of the above forms [1] and [2], the dielectric layer operates as an optical cavity, forming a standing wave of light of a specific wavelength (frequency) in the optical cavity. In the optical-thermal converter of the form [1], this standing wave of light resonates with the phonon oscillator of the dielectric constituting the optical cavity, and energy transfer occurs between the standing wave of light and the phonons of the dielectric. As a result, the components of the light incident on the optical-thermal converter that are close to the frequency of the standing wave of the light are converted into heat in the converter, and the heat of the converter is converted into the light of the corresponding component and radiated. Such conversion between light and heat occurs more efficiently the closer the frequency of the standing wave of light is to the frequency of the phonon oscillator, and is maximized when the two are equal. Here, the light is incident on the optical-thermal converter, and the light is emitted from it through the dielectric surface opposite to the metal surface. The interaction between electromagnetic waves and phonons is well known to those skilled in the art, so a detailed explanation is omitted.

[0013] In the form [1], resonance occurs between the optical standing wave and the vibration of the phonon oscillator inside the optical cavity. That is, the material inside the optical cavity has a phonon oscillator, which is one of the resonating systems, and the resonance occurs inside the optical cavity. However, the location of the phonon oscillator does not necessarily have to be in the material in which the optical standing wave reciprocates inside the cavity, and it may be in the dielectric material that constitutes at least a part of the wall surface that reflects light in the optical cavity. In the form [2], as explained above, a phonon oscillator of a desired frequency (or a frequency close to that) is present in a layer (phonon medium layer) of a dielectric material on the wall surface of the optical cavity. In the form [2], when the standing wave light in the optical cavity is reflected on the wall surface when it reciprocates between multiple wall surfaces, resonance occurs between the standing wave in the dielectric material and the vibration of the phonon oscillator of the dielectric material on the wall surface or in the area near the wall surface where the light penetrates. As will be explained below, the thickness of the phonon medium layer will affect the absorption and emission spectra, but a thickness of 10 nm or more will enable operation as an optical-to-thermal converter, although it may or may not provide optimal performance.

[0014] In the form [2], the layer structure is somewhat more complicated than in the form [1], but there is an advantage in that the medium constituting the optical cavity for forming a standing wave of a desired frequency and the material having a plasmon oscillator vibrating at a desired frequency can be selected independently. In addition, in the form [2], since the material constituting the optical cavity is not required to have a plasmon oscillator of a specific frequency, the material can be selected to have no plasmon (more precisely, to have only a plasmon oscillator of a frequency much lower than the frequency at which absorption or emission is required, such as the terahertz band). In other words, it is sufficient to transmit infrared light with low loss, and in order to make the light-to-heat converter thin, it is preferable to use a high refractive index material for the optical cavity layer. By selecting such a low-loss material, the optical cavity becomes an optical resonant system with a large Q, and thus an optical standing wave with low loss and a steep spectrum can be provided, thereby realizing light absorption and emission with high efficiency and steep spectrum. [2] As the optical cavity material, although there is no intention to limit it, for example, infrared-transmitting materials such as Si, Ge, and chalcogenide can be preferably used. Alternatively, depending on the wavelength range of light to be absorbed or emitted, there may be cases where it is difficult to realize an optical cavity using a light-transmitting material, such as when there is no suitable material for the optical cavity, when it is difficult to obtain or process the material, or when an optical cavity can be created but is not suitable for the usage environment. In such cases, an optical cavity having a structure of a gap surrounded by a reflecting wall may be adopted.

[0015] Here, to explain the forms [1] and [2] more generally, the optical standing wave formed in the optical cavity resonates with an oscillator such as a phonon oscillator possessed by the material forming the optical cavity in the form [1], and resonates with an oscillator such as a phonon oscillator possessed by the material of the element such as a layer provided on the surface of the optical cavity in the form [2]. In this way, energy transfer occurs between the light and the oscillator, and conversion between light and heat is performed. The layer containing the oscillator is called a dielectric layer above, but in terms of its function and taste in the present invention, it can also be called an oscillator layer.

[0016] In the following, the oscillator is assumed to be a phonon oscillator, but any oscillator that interacts with light can be used, including, but not limited to, plasmons (surface plasmons) and electronic excitations.

[0017] Furthermore, in the above description, the structure of the optical cavity is an asymmetric Fabry-Perot interferometer structure in which light is repeatedly reflected between a pair of reflecting surfaces, but the type of the optical cavity is not limited as long as some element made of a material having an oscillator that interacts with the standing wave formed in the optical cavity can be arranged inside or around it. Another non-limiting example of an optical cavity that can be used is a whispering gallery mode (WGM) cavity that utilizes the resonance of WGM. In this specification, the optical-thermal converter is basically described using an optical cavity with an asymmetric Fabry-Perot interferometer structure, but an optical-thermal converter using a WGM cavity will be described near the end of the specification with reference to the drawings. Note that the whispering gallery mode is well known to those skilled in the art, so a detailed description will not be given here, but if necessary, please refer to Non-Patent Document 2 and the documents cited therein.

[0018] In addition, Patent Document 1 describes a radiation structure in which a resonator layer made of an insulator is sandwiched between a plasmonic reflection layer and a laminated distributed reflection layer, and shows a structure in which light (infrared rays) is emitted from the distributed reflection layer side by heating the plasmonic reflection layer side with a heater or the like. In addition, in paragraph

[0021] , it is described that the plasmonic reflection layer may be one that exhibits metallic properties, and further, in the latter half of paragraph

[0022] , it is described that SiO2 exhibits metallic properties in the vicinity of 8 to 9 μm, which is the absorption wavelength of its optical phonons, and therefore can be used as a plasmonic reflection layer material in the vicinity of said wavelengths. This may give the impression that it is similar to the present invention, but in fact, this description in Patent Document 1 does not disclose a technical idea similar to the present invention. The invention disclosed in Patent Document 1 is to absorb and radiate light by utilizing the absorption phenomenon caused by Joule loss due to the vibration of electrons on a metal surface, and the absorbed energy (frequency) is the frequency of the photonic mode in the optical cavity (in Patent Document 1, a structure consisting of an insulator sandwiched between a plasmonic reflection layer and a laminated distributed reflection layer), and is determined regardless of the frequency of plasmons or phonons. In contrast, in the present invention, light absorption and radiation occurs at a frequency where both the photonic mode of the optical cavity and phonons match, and it should be noted that the operating principle is completely different from that of Patent Document 1.

[0019] In many of the descriptions in this specification, the structure of the optical-thermal converter is such that one of the reflective walls of the optical cavity (the lower reflective layer in the figure) is made of a metal. However, naturally, based on the principles of optical cavities, the present invention is not limited to this, and any material that reflects the light in the optical cavity with sufficient reflectivity to form an optical standing wave in the optical cavity, such as one that does not exhibit excessive optical loss at the desired wavelength and has a refractive index different from that of the light-transmitting material in the optical cavity, may be used.

[0020] Alternatively, instead of providing a lower reflective layer made of another solid material such as metal, such a reflective layer can be omitted and the material of the optical cavity can be directly exposed to the atmosphere or vacuum. Of course, in many applications, it is necessary to support the optical-to-thermal converter on a substrate or the like so as to ensure mechanical strength or to maintain a specific position or shape, and the possibility of omitting the reflective layer should be considered.

[0021] In addition, in the description of the present specification, each layer constituting the light-heat converter of the present invention is described as having a flat shape, but this is merely a description of a representative three-dimensional shape of these layers for the sake of simplicity, and although it is natural considering the above-mentioned principle, these are not limited to flat layers. As another example, by making the light irradiation surface shape of each layer of the light-heat converter match the surface shape of a non-flat object and making the distance between the two constant, it is possible to uniformly irradiate infrared rays onto the surface of a non-flat object and heat the surface almost uniformly. In addition, in order to provide a light-heat converter having two-dimensional non-directional light absorption characteristics and light emission characteristics, each layer of the light-heat converter may be configured, for example, as a cylinder, so that the outer surface of the cylinder receives incident light or irradiates light from it. By using a light-heat converter configured in such a shape, it is possible to uniformly heat the inner surface of a cylindrical object, for example, by heating the inside of a narrow hole or cavity, or to detect the radiation from there. In addition, the three-dimensional shape of the light-to-heat converter, i.e., each layer constituting the converter, can be determined with a high degree of freedom according to the shape of the object, such as a sphere or polyhedron, the direction of the incident light, etc. Here, no specific shape or structure is required in the surface direction of these layers, and they simply need to spread uniformly in that surface direction, so formation and processing into a three-dimensional shape other than a flat surface can be easily realized.

[0022] Here, as for the three-dimensional shape of these layers, as long as the shape changes gradually compared to the wavelength of the light to be absorbed and radiated, the behavior of the layers may be considered to be substantially equivalent to that of flat layers as far as the present invention is concerned. Specifically, if the radius of curvature of the curve of each layer is greater than about 10 times the wavelength of the light to be absorbed and radiated, the behavior of the layers in terms of absorption and radiation is equivalent to that of flat layers. In addition, in the present invention, the surface of each layer may be of a uniform shape in the sense that it is not necessary to provide unevenness or holes, but it is not essential that the surface shape is completely uniform, and it is sufficient as long as it does not interfere with the operation of the light-to-heat converter of the present invention, such as by losing the function of the cavity. For example, as shown in FIG. 1 of Patent Document 1, when a periodic structure is present along the surface of a layer, such a structure may affect the absorption and radiation spectrum, but when the period of such a structure is greater than about 10 times the wavelength of the light to be absorbed and radiated, the effect can be ignored.

[0023] Here, a general supplementary explanation of the WGM cavity mentioned above will be given. This optical cavity has the shape of a micro-sized sphere (microsphere) or microrod with a diameter or length of about 1 to 30 μm, and is an optical cavity that utilizes a surface wave, which is a kind of standing wave formed on the surface. By forming a core-shell structure using this type of optical cavity as a core and using a dielectric having a phonon oscillator with the vibration frequency of the light to be absorbed and radiated as the shell, it is possible to provide an optical-thermal converter according to the present invention, as in the case of an optical cavity of an asymmetric Fabry-Perot interferometer structure. The material of the core can be the same as that of the optical cavity of the form [2] in the asymmetric Fabry-Perot interferometer structure, and metal can also be used. The optical-thermal converter having this core-shell structure can be said to correspond to the above-mentioned form [2] (MDD structure to be described later) in that the optical cavity and the oscillator layer are separated, but an optical-thermal converter of the form [1] (MD structure to be described later) using a WGM cavity can also be created. Specifically, the WGM cavity itself has an oscillator, and resonance caused by the interaction between light and the oscillator within the cavity causes energy transfer between the two.

[0024] In addition, in the description of this specification, it is assumed that phonons interact with the optical standing wave formed in the optical cavity, but other physical phenomena that interact with light and cause an interaction may be used instead of phonons. For example, infrared plasmons appear when a dopant is added to silicon or the like, and it is easy to configure an optical-thermal converter that uses the resonance between such plasmons and optical standing waves to absorb and radiate light at a specific wavelength.

[0025] In addition, in the description of this specification, the light-to-heat converter is formed on a Si substrate, but this is not particularly limited, and any material can be used as long as it does not adversely affect the operation of the light-to-heat converter. Depending on the thickness and spread of the light-to-heat converter, the material used, and the form of use, the converter may be completely independent, or may be configured as a converter that is sufficient with only partial support rather than full support. Therefore, the presence or absence of a substrate, which part of the light-to-heat converter is supported, etc. should be appropriately determined depending on the structure of the light-to-heat converter and the circumstances of the equipment used together.

[0026] In addition, although the present specification has been described with reference to an example of a configuration having only essential elements as a light-to-heat converter, additional components or partial changes from the conceptual structure exemplified may be required for actual use. For example, depending on the usage environment, the light-to-heat converter may be covered with a protective film or the like to prevent breakdown or change in characteristics of the light-to-heat converter due to corrosion or the like, or a means may be adopted to prevent deformation due to high temperatures or large temperature changes or deterioration of adhesion between layers. In addition, when the light-to-heat converter of the present invention is used as a sensor, it is necessary to provide an additional element for converting the temperature change caused by the conversion of light of a specific wavelength into heat into an electric signal, and when used as a heater, it is necessary to provide an electric heater or a temperature control mechanism for applying heat to the light-to-heat converter. Naturally, these and other various modifications and additions are also included in the technical scope of the present invention. EXAMPLES

[0027] The present invention will be described in more detail below with reference to experimental and simulation results. Of course, it is not intended to limit the present invention to a specific configuration in which an experiment or simulation was performed. Furthermore, it should be noted that the following description of the configuration, operation, and other various features of the light-to-heat converter and the like is not based on the specific specific structure, material, etc. referred to therein, but is a general description that is valid without the limitation of specific materials, structures, etc., unless otherwise specified or clear from the context. In addition, the following description will be given with reference to the results of experiments and simulations of the spectrum of the absorption of light, mainly in the case of absorption, among the absorption and emission of light. However, as is well known to those skilled in the art, the spectrum of light, absorption, and emission are fundamentally the same, so it should be noted that the discussion based on the absorption of light also applies to the emission of light in exactly the same way.

[0028] The structure and operation of the optical-thermal converter according to the present invention will be described in more detail below. In explaining the operation, the results of measuring the characteristics of the optical converter that was actually created and the results of computer simulation of the electromagnetic field will be referred to. For the simulation, rigorous coupled-wave analysis (RCWA) was used, and the absorption, reflection and transmission spectra were designed and simulated.

[0029] The first type of light-to-heat converter having the cavity structure of the above-mentioned form [1], the conceptual structure of which is shown in Figure 4(a), has a metal-dielectric double-layer structure (hereinafter also referred to as MD or MD structure) in which a dielectric layer is provided on a metal layer that functions as a reflective layer (reflective surface). As described above, the material (dielectric material) of the dielectric layer is selected to transmit infrared light of the target wavelength (i.e., infrared light to be absorbed or emitted) and to have a phonon mode of that wavelength. Here, the metal layer and the dielectric layer are not required to have any structure or shape, such as a periodic nanostructure, in the surface direction, and may simply be a uniform layer with a uniform thickness that spreads in the surface direction. Of course, since an actual light-to-heat converter cannot spread infinitely in the surface direction, there will be edges unless it is made into a circular shape that is considered to be formed by joining edges of a sphere or the like, but the edges are not required to have a specific geometric shape. Also, for some other reasons (for example, when the light-to-heat converter is attached to other equipment, or when an electric circuit or other additional element is installed), the light-to-heat converter may have a non-uniform shape or structure, but as far as light absorption and radiation are concerned, deviation from the uniform structure or shape is not essential (of course, as long as it does not interfere with light-to-heat conversion, there is nothing to prevent a non-uniform structure or shape). Due to this structural feature, when manufacturing the light-to-heat converter of the present invention, it is only necessary to sequentially stack two layers (three layers in the light-to-heat converter of the second form described later) that are uniform in the plane direction and have the required thickness, so the manufacturing process is extremely simple compared to the light-to-heat converters of the prior art, which require the formation of a specific structure in the plane direction or the stacking of a large number of layers. Furthermore, in the light-to-heat converter of the present invention, the wavelength of the absorbed and emitted light is not controlled solely by the size of the layer (layer thickness, size of the nanostructure formed along the surface direction of the layer, etc.), but depends to a large extent on the selection of the phonon medium; that is, the wavelength of the absorbed and emitted light is largely determined by the frequency of the phonon oscillator (fine adjustments and the selection of one of several frequencies are controlled by the layer thickness, i.e., the resonant frequency of the cavity), so the requirements for the accuracy of the layer size are relatively relaxed.Accordingly, it becomes easier to create a light-to-heat converter having a large size in the planar direction and an arbitrary shape. This is a very useful characteristic when applying the light-to-heat converter of the present invention to a surface light source for heating, which is required to uniformly radiate infrared light of a specific wavelength suitable for heating a variety of objects of considerable size, various shapes, and each of which has its own unique infrared absorption spectrum. This characteristic also applies to the second type of light-to-heat converter (described in detail later) having a cavity structure of the above-mentioned type [2], the conceptual structure of which is shown in Figure 4(b).

[0030] In the first type of light-to-heat converter, its resonant photonic mode can be freely designed by adjusting the thickness of the SiO2 layer, which acts as a photonic cavity with an embedded SiO2 phonon oscillator. This structure can realize perfect absorption / emission by coupling the resonant photonic mode of the SiO2 dielectric cavity with the SiO2 optical phonon mode at around 9.4 μm.

[0031] However, the thickness of the SiO2 layer cavity layer for inducing standing wave resonance in the metal-SiO2 layer structure is quite thick. If this is inconvenient, instead of the first form, in order to effectively reduce the thickness of the SiO2 layer, the second form, the conceptual structure of which is shown in Figure 4(b), can be used, i.e., a three-layer structure called metal-Si-SiO2 (more generally, metal-dielectric-dielectric three-layer structure (hereinafter, also referred to as MDD or MDD structure)) formed by inserting a Si intermediate layer between the metal layer and the SiO2 layer. The reason is that in the MD structure, the optical cavity must simultaneously function as a phonon resonator having a phonon oscillator. In other words, in order to absorb or emit light of a desired wavelength, it is necessary to use a material having a phonon oscillator with a frequency corresponding to this wavelength for the optical cavity, and the material of the optical cavity is almost determined by this requirement, so that there is practically no freedom to select a material with a large refractive index to make the optical cavity layer thinner. In contrast, in the MDD structure, the optical cavity layer and the phonon oscillator layer (phonon medium layer) are separated, so by selecting a material with a high refractive index (e.g., Si) for the optical cavity material, there is the freedom to select a high refractive index material that makes this layer thin when forming a standing wave of the desired wavelength in the optical cavity layer.

[0032] In the following, the light-to-heat converters of the MD and MDD structures are further described with various examples.

[0033] In the first embodiment of the light-to-heat converter illustrated in FIG. 1, SiO2 is used as the material of the dielectric layer. NiAl with a thickness of 200 nm is used as the metal layer as a reflector. This allows the transmission of infrared light through the metal layer to be negligible. Therefore, the absorptance A of the metal layer is calculated as A=1-R, where R is the reflectance. Reflectance measurements were performed using an FTIR Nexus 670 to compare with the simulation results.

[0034] These thin films (SiO2 and NiAl layers) were prepared using a sputter deposition system (Shibaura Mechatronics Miller). The properties of these thin films were determined by X-ray diffraction, scanning electron microscopy, and ellipsometry measurements.

[0035] Figure 1(a) shows the first embodiment (MD structure) of the light-to-heat converter of the present invention. Figure 1(b) shows the real part ε1 and the imaginary part ε2 of the dielectric function of SiO2. Figure 1(b) also shows the results of a simulation of the change in absorptance when the thickness of the dielectric layer made of SiO2 in the light-to-heat converter shown in Figure 1(a) and the wavelength of the applied infrared light are changed.

[0036] The operating principle of the light-to-heat converter of the present invention is the interference phenomenon of light in the vertical direction (thickness direction) of the film (layer). That is, when light is incident vertically on a dielectric layer, the relationship between the thickness of the layer and the wavelength of the standing wave that appears there is described by the following formula.

[0037]

number

[0038] Here, d is the thickness of the insulator layer (here, a layer of SiO2), m is the odd mode (m=1,3,5,7,...), λ0 is the wavelength of the standing wave, and n(λ0) is the refractive index of the insulator layer at wavelength λ0. This light-to-heat converter resonantly emits / absorbs infrared light at the wavelength where the standing wave is formed. If this resonance coincides with the phonon frequency, the intensity of the absorption / emission is very large. The phonon frequency of SiO2 is 9.4 μm when converted to the wavelength of light. If this wavelength coincides with the standing wave wavelength of the vertical cavity mode of the insulator layer, the absorption is very large. As shown by the white dashed line in Figure 2(b), when a SiO2 layer with a thickness of 1.25 μm is used, the resonant absorption occurs at a wavelength of 9.4 μm, and the absorptance is a very high value of 1.0 (perfect absorption). For details of this absorptance, see Figure 3(a), which shows the variation of absorptance with wavelength for the first embodiment of the light-to-heat converter when a 1.15 μm thick SiO2 layer is used. As can be seen from Figure 3(a), the simulated absorptance values ​​(dashed line) and the experimental values ​​(solid line) are in good agreement over a wide wavelength range. Also, the dependence of absorptance on the angle of incidence is small, as can be seen from Figure 3(b), which shows the variation of absorptance with both wavelength and angle of incidence. Such low dependence on the angle of incidence is a suitable property for an infrared heater.

[0039] The absorption spectrum of the light-to-heat converter of the first embodiment is broad, and the thickness of the converter is also quite large. If a narrower absorption spectrum and / or thinner light-to-heat converter is required, the second embodiment described above can be adopted. Figure 4(b) shows a conceptual diagram of a three-layer structure (MDD structure) in which NiAl is used for the metal layer and an intermediate layer (Si layer) is provided between the metal layer and the insulating layer made of SiO2 as the light-to-heat converter of the second embodiment. This Si layer can be fabricated by sputtering, for example. For comparison, Figure 4(a) shows a conceptual diagram of the corresponding light-to-heat converter of the first embodiment. In the structure of the light-to-heat converter of the second embodiment shown here, a metal layer is provided as a reflector at the bottom of the converter. By adding an intermediate layer made of Si to the MD structure such as the metal-SiO2 layer structure of the first embodiment, the single insulating layer of the first embodiment is separated into a layer that forms a standing wave of light and a layer that provides phonons that couple with the standing wave of light. This increases the degree of freedom in the materials, sizes, etc. that meet the requirements of each layer. This can increase the tunability of the resonance wavelength, for example, and also realizes an optical cavity with a higher Q because the Si used here as the material for the intermediate layer, i.e., the optical cavity, is a low-loss material in the sense that it absorbs less in the infrared range, and this results in a somewhat better single-band feature at the resonance wavelength compared to the MD structure described above.

[0040] FIG. 6 shows the results of a simulation of the influence of the thickness of the Si layer functioning as the optical cavity and the thickness of the layer of the dielectric material (phonon medium, SiO2 used here) having phonons coupled to the mode of the optical cavity on the optical absorptance. For example, the resonant wavelength can be adjusted by selecting the material used for the phonon medium layer and the thickness of the Si layer. FIG. 6 shows the absorptance when the phonon medium layer (SiO2 layer) is made thicker in the order of (a) → (b) → (c), and as can be seen, the absorptance increases as the thickness of the phonon medium layer increases. More specifically, if the SiO2 layer is too thin (FIG. 6(a)), the effect of the enhanced phonons in the SiO2 layer is weak and the absorptance cannot be maximized. On the other hand, if the SiO2 layer is too thick (FIG. 6(c)), unwanted peaks appear and the intensity of the spectral background increases. Therefore, it is desirable to properly select the optimal thickness of the SiO2 layer (here, d SiO2 =0.4μm).

[0041] As can be seen from Figures 5, 6 and 8, the size of the wavelength point (actually a wavelength range with a certain degree of spread) where the phonon enhancement of SiO2 appears is limited, so the tuning range of the resonance wavelength is small. To address the problem of the narrow tuning range of the resonance wavelength, the phonon medium can be replaced from SiO2 to another phonon medium where the phonon enhancement appears at multiple points. Figure 9 shows the dielectric functions of three types of dielectric materials that can be used for the phonon medium layer. Of course, there are many other materials that can be used for the phonon medium layer, such as TiO2, and they can be selected appropriately as needed.

[0042] Figure 9(a) shows the dielectric function of SiO2 in these three dielectric materials. This material contains a single frequency phonon oscillator in the mid-infrared. The single-band characteristic that emerges here couples with the photonic modes of the cavity, resulting in strong cavity-enhanced optical absorption (and emission) as seen in the examples described and illustrated above.

[0043] The second dielectric material, whose dielectric function is shown in Figure 9(b), is polyimide. This material is given as a non-limiting example of a material that contains many steep and strong phonon oscillators associated with molecular vibrational modes in the mid-infrared range. Since there are many steep absorption bands due to molecular vibrations, it is possible to select from these phonon modes (absorption bands) and couple them with cavity photonic modes (standing wave light) to provide strong cavity-enhanced infrared absorption in a single band.

[0044] As an example of this embodiment, FIG. 10 shows phonon-mediated absorption and emission by a metal-Si-polyimide layer structure. FIG. 10(a) is a conceptual diagram of the structure, FIG. 10(b) shows the dielectric function of the polyimide used in the phonon medium layer (see Non-Patent Document 1 for details), and FIG. 10(c) shows the simulation results of the absorptance (equivalent to emissivity) of an element based on the conceptual structure shown in FIG. 10(a) as a function of the thickness of the Si layer. FIG. 11 shows that by adjusting the size of the optical cavity in the element whose conceptual structure is shown in FIG. 10(a), the absorptance (emissivity) can be made to show a spectrum close to a single spectrum at a peak wavelength selected from several wavelengths. As shown in FIG. 11, the parameters of this converter (here, the thickness d of the intermediate Si layer, Si ) the wavelength of absorption (thermal radiation) of this converter is controlled. Also, as shown in Figure 12, the dependence of infrared absorption / radiation on the angle of incidence is relatively small. This characteristic is favorable for a thermal radiator.

[0045] A light-to-heat converter with an MD structure using Al2O3, the third dielectric material, as the phonon medium layer material, whose dielectric function is shown in Fig. 9(c), will be described with reference to Fig. 13. As shown in Fig. 9(c) and Fig. 13(b), the dielectric function of Al2O3 indicates that Al2O3 has a phonon oscillator with very broadband characteristics from 10μm to 24μm. A cavity device with an MD structure using this material as the upper phonon medium, the conceptual structure of which is shown in Fig. 13(a), gives a very broadband step function-like absorption characteristic above 10μm, as shown in Fig. 13(c). This absorption characteristic is brought about by hybridization between the cavity mode and the broadband Al2O3 phonons, and the absorptance (emissivity) is 1 at a wavelength of about 10.2μm. The broadband spectrum and relatively small incidence angle dependence of this MD structure Al2O3 device are shown in Fig. 14(b) and Fig. 14(c), respectively.

[0046] Furthermore, an optical-thermal converter with an MDD structure using Al2O3 as a phonon medium layer material will be described with reference to FIG. 15. The optical-thermal converter with an MDD structure, the conceptual structure of which is illustrated in FIG. 15(a), has strong absorption characteristics and a rather wide band, and the resonance wavelength at the center of this band is adjustable in the range of 15 μm to 20 μm. The reason for this tunability can be seen from FIG. 15(b). In the figure, the straight fundamental cavity mode with m=1 becomes a rather wide band when it is hybridized with the phonon mode at 10 μm or more. An example of the spectrum when the thickness of the Al2O3 layer is d=3.05 μm is shown in FIG. 16(b). Also, FIG. 16(a) and FIG. 16(c) show the incidence angle dependence of the thickness and absorptance of each layer in this case, respectively. Here again, the incidence angle dependence is small, which indicates that this is a suitable characteristic for a thermal radiator.

[0047] Furthermore, the material of the metal layer functioning as the reflective layer in the MD structure and the MDD structure will be described with reference to Fig. 17. Fig. 17(a) and Fig. 17(b) show the results of simulating the absorption spectrum in the MD structure and the MDD structure, respectively, when the metal layer material is Cu, Al, and NiAl. Here, Al2O3 is used as the dielectric layer material in the MD structure, and Si and Al2O3 are used as the materials of the optical cavity layer (middle layer) and the phonon medium layer (top layer) in the MDD structure, respectively. As can be seen from these results, in the light-to-heat converter of the present invention, in both the MD structure and the MDD structure, the smaller the optical loss of the metal layer material, the better the spectral background, that is, the closer the absorption rate in the relevant region is to zero.

[0048] As already explained, the optical-thermal converter according to the present invention can be constructed based on a cavity other than the above-mentioned laminated structure (asymmetric Fabry-Perot interferometer structure). Here, as an example of such a cavity, an optical-thermal converter using a whispering gallery mode (WGM) cavity will be described in detail with reference to the drawings.

[0049] Figure 18 shows examples of such light-to-heat converters, where (a) is a microsphere-shaped converter, and (b) is a microrod-shaped converter. Both of these are particles with a core-shell structure, and the core part is the WGM cavity. (a) is a central cross-sectional view of the microsphere-shaped light-to-heat converter, the lower left side of (b) is a cross-sectional view taken along a plane parallel to the longitudinal axis of the microrod-shaped light-to-heat converter (XZ plane), and the upper right side is a cross-sectional view taken along a plane perpendicular to the longitudinal axis (YZ plane).

[0050] In this type of cavity, a surface wave of light, which is a kind of standing wave, is formed on the surface by the WGM operation. The size of the cavity varies depending on the material and the wavelength of the light to be absorbed and emitted, but in the microsphere shape of FIG. 18(a), the diameter is about 1 to 30 μm, and in the microrod shape of FIG. 18(b), the length in the longitudinal direction is about 1 to 30 μm. In the light-to-heat converter of the type exemplified in FIG. 18(a) and (b), a layer of a material having a phonon oscillator (or other oscillator) with the vibration frequency of the light to be absorbed and emitted is provided on the surface of the cavity, which is a particle made of a light-transmitting material (which may be a metal), to form a core-shell particle structure, which is used as a light-to-heat converter. As a result, the surface wave on the core surface of such a particle structure and the phonon oscillator in the shell that is also present on the core surface are coupled to each other, thereby absorbing and emitting light with the vibration frequency of the phonon oscillator. Note that such particles with a core-shell structure can be produced by, for example, a thermal plasma method.

[0051] Here, it is desirable to use a light-transmitting material that does not have a phonon frequency close to the frequency of the light to be absorbed and emitted as the light-transmitting material that can be used for the core, and a high refractive index material is particularly desirable. Examples of such materials include Si, Ge, and chalcogenide-based infrared-transmitting materials. Examples of metals that can be used for the core include Au and Al. The material for the shell is selected from various materials depending on the wavelength of the light to be absorbed and emitted, and examples of the materials that can be used include SiO2, TiO2, Al2O3, and other materials mentioned above.

[0052] Here, we have illustrated and explained an optical-thermal converter in which the core functions as a WGM cavity and the shell covering the core functions as an oscillator layer. However, instead of such a core-shell structure, it is also possible to realize a configuration corresponding to the above-mentioned form (MD structure) of [1], in which the WGM cavity itself has an oscillator and simultaneously exchanges energy through the interaction between the WGM cavity and an optical standing wave (surface wave) and the oscillator.

[0053] Of course, each light-to-heat converter using a WGM cavity may be used alone, but because of its small size of the order of micrometers, a large-sized light-to-heat converter can be constructed by distributing a large number of these particulate light-to-heat converters on a large substrate. A light-to-heat converter using a large number of WGM cavities can be easily made large in size compared to the light-to-heat converter using the asymmetric Fabry-Perot interferometer type cavity described above, and it is also easy to form a curved light absorption and emission surface. Furthermore, since a large number of particulate light-to-heat converters can be dispersed in a liquid, such a dispersion liquid can be used in the manufacturing process of the above-mentioned light-to-heat converter, or such a dispersion liquid can be included in the final light-to-heat converter, enabling different uses and applications from those of the light-to-heat converter using the asymmetric Fabry-Perot interferometer type cavity. [Industrial Applicability]

[0054] As described above in detail, the present invention provides a light-to-heat converter that has a simple structure but can absorb and radiate a variety of wavelengths of light by appropriately selecting the size and material of each part. The converter can be used, for example, in a planar heater that heats an object by uniformly irradiating a wide area with infrared light of a selected wavelength, other planar infrared light sources, and a sensor that measures infrared light of a selected wavelength by converting it into heat. [Prior art documents] [Patent documents]

[0055] [Patent Document 1] International Publication 2019 / 225726 [Patent Document 2] International Publication 2021 / 024909 [Non-patent literature]

[0056]

Non-licensed literature 1

Non-licensed Document 2

Claims

1. By providing an optical cavity on the surface or inside that forms an optical standing wave, An oscillator that resonates with the optical standing wave by interacting with it exists in the optical cavity or in an element adjacent to the optical cavity, A photothermal converter that absorbs incident light at the aforementioned resonance frequency or emits light at the aforementioned resonance frequency.

2. The photo-to-thermal converter according to claim 1, wherein the oscillator is selected from the group consisting of phonon oscillators, plasmons, and electronic excitations.

3. The photo-to-thermal converter according to claim 1 or 2, wherein the optical cavity has the shape of a particle.

4. The photothermal converter according to claim 3, wherein the particles are microspheres or microrods having a diameter or longitudinal length in the range of 1 to 30 μm.

5. The optical-thermal converter according to claim 1, wherein the material of the optical cavity is selected from Si, Ge, chalcogenide-based infrared-transmitting materials, and metals.

6. The element having the oscillator and adjacent to the optical cavity is SiO ２ , TiO ２ Al ２ O ３ The photo-thermal converter according to claim 1, selected from the group consisting of polyimide.

7. The optical cavity has a layered light-transmitting material sandwiched between a pair of reflective surfaces, and the optical standing wave is formed between the pair of reflective surfaces. The material constituting at least one of the pair of reflective surfaces or the light-transmitting material has the oscillator. The optical-thermal converter according to claim 1.

8. The optical-thermal converter according to claim 7, wherein light is incident on the optical-thermal converter through the reflective surface which is the interface with the layer of material having the oscillator, and absorption occurs, or light is emitted through the reflective surface which is the interface with the layer of material having the oscillator.

9. The optical-thermal converter according to claim 7, wherein the pair of reflective surfaces have a uniform shape in the direction of the reflective surfaces.

10. The photo-thermal converter according to claim 7, wherein one of the pair of reflective surfaces is the surface of a metal layer or the surface of a dielectric layer having a refractive index different from that of the light-transmitting material of the optical cavity.

11. The other of the pair of reflective surfaces is the interface between the light-transmitting material of the optical cavity and the outside of the photo-thermal converter. The light-transmitting material has the oscillator, The optical-thermal converter according to claim 7.

12. The aforementioned light-transmitting material is a material having the oscillator, One of the pair of reflective surfaces is the surface of a metal layer. The optical-thermal converter according to claim 11.

13. The other of the pair of reflective surfaces is the surface of the dielectric layer and has the oscillator, The optical-to-thermal converter according to claim 10, wherein the optical-transmitting material of the optical cavity does not have an oscillator that resonates in the infrared region.

14. The aforementioned light-transmitting material is a material selected from Si, Ge, and chalcogenide-based infrared-transmitting materials. One of the pair of reflective surfaces is the surface of a metal layer. The optical-thermal converter according to claim 13.

15. The dielectric layer that gives the other of the pair of reflective surfaces is SiO ２ , TiO ２ , polyimide and Al ２ O ３ The optical-thermal converter according to claim 14, wherein the material is selected from the above.

16. A photo-to-thermal converter according to any one of claims 7 to 15, having a flat or three-dimensional shape.

17. The optical-thermal converter according to claim 16, having a shape selected from the group consisting of cylinders, spheres, and polyhedra.

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