Abstract:
A thermal emission source capable of switching the intensity of light at a high response speed similarly to a photoelectric conversion element. A thermal emission source includes: a two-dimensional photonic crystal including a slab in which an n-layer made of an n-type semiconductor, a quantum well structure layer having a quantum well structure, and a p-layer made of a p-type semiconductor are stacked in the mentioned order in the thickness direction, wherein modified refractive index areas (air holes) whose refractive index differs from the refractive indices of the n-layer, the p-layer and the quantum well structure layer are cyclically arranged in the slab so as to resonate with a specific wavelength of light corresponding to a transition energy between the subbands in a quantum well in the quantum well structure layer; and a p-type electrode and an n-type electrode for applying, to the slab, a voltage which is negative on the side of the p-layer and positive on the side of the n-layer.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a thermal emission source as well as a two-dimensional photonic crystal for use in the same thermal emission source. A thermal emission source is a device in which electromagnetic waves radiated by thermal emission are used as the emission source. This device can also be regarded as a heat-to-light converter which receives heat as the input and generates light (electromagnetic waves) as the output. If this input of heat is imparted in the form of electromagnetic waves (infrared radiation), the device can be regarded as a wavelength converter. It can also be regarded as a device for generating a thermal emission by receiving electric energy as the input instead of heat. The term “thermal emission source” as used in the present application covers any of these devices. 
       BACKGROUND ART 
       [0002]    Thermal emission sources have the advantage that an emission can be obtained by merely imparting heat to an object. For example, in a gas sensor for analyzing the components in an exhaust gas from an engine, the thermal emission source can be suitably used as an emission source for converting waste heat produced by the engine into infrared radiation for the sensing. The application areas of the thermal emission source are not limited to gas sensors; it can also be used as the emission source in various sensors which utilize infrared radiation, such as a sensor for detecting an intrusion of a person, automobile, animal or the like. 
         [0003]    Electromagnetic waves emitted from an object which has been given heat have a spectrum which covers a wide range of wavelengths. For example, electromagnetic waves generated by heating an object to a temperature of tens to hundreds of degrees Celsius have a wavelength range of a few μm to several tens of μm. However, since the aforementioned infrared sensor normally utilizes only a specific wavelength of infrared radiation, using such a thermal emission source causes the measurement target to be irradiated with unnecessary wavelengths of infrared radiation other than the specific wavelength, which produces adverse effects, such as the heating of the measurement target. In the case of generating the thermal emission by supplying electric energy, using an emission source which generates a broadband emission causes the problem of an increase in the power consumption. 
         [0004]    To solve such problems, a thermal emission source is proposed in Non Patent Literature 1, which includes a slab having a quantum well structure (which will be described later) and modified refractive index areas cyclically arranged in the slab, with the refractive index of those areas differing from that of the slab. A quantum well structure is the structure of an object in which an energy potential in the form of a well (“quantum well”) is created by stacking a plurality of kinds of semiconductor layers with a thickness of approximately a few nm to a dozen nm, with each layer having a different magnitude of energy band gap. The cyclic arrangement of the modified refractive index areas in the slab creates a cyclic distribution of the refractive index. Such an object consisting of a slab in which a cyclic distribution of refractive index is formed is called the “two-dimensional photonic crystal”. It is commonly known that a two-dimensional photonic crystal can act as a resonator for light having a specific wavelength corresponding to the lattice constant of the refractive index distribution by enabling the formation of standing waves of that specific wavelength of light. 
         [0005]    In the thermal emission source disclosed in Non Patent Literature 1, when the supply of heat from the heat source is initiated, an energy transition occurs between the energy bands formed in the quantum well (for distinction from the energy bands of the semiconductor, those bands are called “subbands”), whereby a range of wavelengths of light within a specific wavelength band are generated. Among the wavelengths of light belonging to this wavelength band, the light having a specific wavelength determined by the lattice constant of the photonic crystal resonates with the photonic crystal. In this manner, the thermal emission source of Non Patent Literature 1 can generate light having a wavelength spectrum which has a narrow peak at that specific wavelength. 
       CITATION LIST 
     Non Patent Literature 
       [0000]    
       
         Non Patent Literature 1: De Zoysa Menaka et al., “Conversion of broadband to narrowband thermal emission through energy recycling”, Nature Photonics, (United Kingdom of Britain), Nature Publishing Group, Jul. 8, 2012, Vol. 6, pp. 535-539 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    A measurement with a sensor which uses infrared radiation may be adversely affected by various kinds of noise, such as the noise caused by fluctuations in the ambient temperature or a low-frequency noise of equal to or less than 100 Hz which occurs due to the alternating-current power of the power source or other factors. Accordingly, in this type of sensor, a noise removal process which utilizes intensity modulation is performed in which infrared radiation whose intensity changes at a frequency of approximately 1,000-100,000 Hz (1-100 kHz) is generated from the emission source while noise components are removed in the detector by detecting only the component of infrared radiation whose intensity changes at that frequency. 
         [0008]    Normally, the intensity modulation is achieved using rectangular waves generated by switching infrared light between two different intensity levels (“high” and “low” or “ON” and “OFF”). If a photoelectric conversion element which directly converts electric power into light (such as a laser diode) is used as the emission source, the infrared radiation having a rectangular waveform with the output of the infrared light repeatedly changed between the “ON” and “OFF” levels can be easily generated by the simple operation of periodically turning the electric power on and off. By comparison, in the case of conventional thermal emission sources, the switching of the intensity of the output light requires either the switching of the injection and blockage of the input heat or the switching of the heating and cooling operations, so that the response speed of the switching is much lower than the photoelectric conversion element. Actually, thermal emission sources are only capable of intensity modulation at a low frequency of approximately 1-100 Hz. Therefore, it is difficult to directly use a thermal emission source in a sensor which utilizes intensity modulation. To this end, an external apparatus for mechanically turning the optical path on and off must be additionally provided. 
         [0009]    The problem to be solved by the present invention is to provide a thermal emission source capable of switching the intensity of light at a high response speed similarly to a photoelectric conversion element. 
       Solution to Problem 
       [0010]    The thermal emission source according to the present invention developed for solving the previously described problem includes: 
         [0011]    a) a two-dimensional photonic crystal including a slab in which an n-layer made of an n-type semiconductor, a quantum well structure layer having a quantum well structure, and a p-layer made of a p-type semiconductor are stacked in the mentioned order in the thickness direction, wherein modified refractive index areas whose refractive index differs from the refractive indices of the n-layer, the p-layer and the quantum well structure layer are cyclically arranged in the slab so as to resonate with a specific wavelength of light corresponding to a transition energy between the subbands in a quantum well in the quantum well structure layer; and 
         [0012]    b) electrodes for applying, to the slab, a voltage which is negative on the side of the p-layer and positive on the side of the n-layer. 
         [0013]    An operation of the thermal emission source according to the present invention is described using  FIGS. 1A-2B . Quantum wells are categorized into two groups depending on the type of semiconductor which forms the quantum well structure: the n-type quantum well, which confines electrons in the conduction band, and the p-type well, which confines holes in the valence band. 
         [0014]    Initially, the case where the quantum well structure layer is an n-type quantum well structure layer having n-type quantum wells is described using  FIGS. 1A and 1B . In the slab composed of the n-layer, n-type quantum well structure layer and p-layer stacked in the mentioned order in the thickness direction, an energy state as shown in  FIG. 1A  is formed when no voltage is applied. In this energy state, the conduction band and valence band are formed above and below the Fermi level (for electrons) which is common to all layers, with electrons held in the conduction band in the n-layer and holes in the valence band in the p-layer. In the quantum well structure layer, quantum wells are formed in the conduction band, with electrons confined in those quantum wells. A plurality of quantized energy levels (subbands) are formed in each quantum well. When the two-dimensional photonic crystal is heated, the electrons in the quantum wells are excited from one subband to another by the thermal energy and then return to the ground state (“intersubband transition”), generating light having a wavelength corresponding to the energy difference between the ground state and excited state. The intersubband transition occurs with a certain bandwidth. Within this band, a component of light having the specific wavelength corresponding to the lattice constant of the modified refractive index areas forms standing waves in the two-dimensional photonic crystal and resonates with the same crystal. Consequently, this specific wavelength of light is emitted from the thermal emission source to the outside. 
         [0015]    Next, when a voltage which is negative on the p-layer and positive on the n-layer is applied through the electrodes to the slab, the energy level of the valence band in the p-layer as viewed from the energy bands in the n-layer becomes higher, as shown in  FIG. 1B , and some of the electrons which have been held in the quantum wells in the quantum well structure layer move into the n-layer. Consequently, the amount of electrons in the quantum wells decreases, which also means a decrease in the amount of electrons contributing to the intersubband transition which occurs when the two-dimensional photonic crystal is heated. This leads to a decrease in the intensity of the light generated by the intersubband transition and consequently lowers the intensity of the specific wavelength of light resonating with the two-dimensional photonic crystal and being emitted to the outside. 
         [0016]    In this manner, in the thermal emission source according to the present invention, the intensity of light changes with the switching of the applied voltage; the intensity is at the “high” level when the voltage is not applied, while it turns to the “low” level when the voltage is applied. Such a voltage-switching operation can be performed at a much higher rate than the rate of temperature change achieved by the switching of the injection and blockage of heat or the switching of the heating and cooling operations. Furthermore, the change in the number of electrons in the quantum wells almost immediately follows the voltage-switching operation. Accordingly, the switching of the intensity of light in the thermal emission source according to the present invention can be performed at a high response speed as in the photoelectric conversion element. 
         [0017]    In the case where the quantum well structure layer is a p-type quantum well structure layer having p-type quantum wells, as shown in  FIGS. 2A and 2B , the quantum wells are formed in the valence band, with holes confined in those quantum wells. Heating this two-dimensional photonic crystal causes an intersubband transition of the holes in the quantum wells. The subsequent processes occur in the same manner as in the n-type quantum well structure layer; i.e., the intersubband transition produces an emission of light having a certain wavelength bandwidth, and the specific aforementioned wavelength of light within this band resonates in the two-dimensional photonic crystal to be eventually emitted from the thermal emission source to the outside. 
         [0018]    Then, when a voltage which is negative on the p-layer and positive on the n-layer is applied through the electrodes to the slab, the energy level of the conduction band in the p-layer as viewed from the energy bands in the p-layer becomes lower, as shown in  FIG. 2B , and some of the holes which have been held in the quantum wells in the quantum well structure layer move into the p-layer. Consequently, the amount of holes in the quantum wells decreases, which also means a decrease in the amount of holes contributing to the intersubband transition in the heating process. This leads to a decrease in the intensity of the light generated by the intersubband transition and consequently lowers the intensity of the specific wavelength of light resonating with the two-dimensional photonic crystal and being emitted to the outside. 
         [0019]    The thermal emission source according to the present invention may include an i-layer made of an insulating material between the quantum well structure layer and the p-layer in the case where the quantum well structure layer has an n-type quantum well, or between the quantum well structure layer and the n-layer in the case where the quantum well structure layer has a p-type quantum well. As will be described hereinafter, the i-layer, due to its thickness, reduces the electrical capacitance C of the thermal emission source and thereby contributes to an increase in the operation speed (frequency). 
         [0020]    In the thermal emission source according to the present invention, the upper limit of the operation speed is determined by the delay time required for the externally applied voltage to actually act on the quantum well; specifically, the upper limit of the frequency is given by the reciprocal of the product of the electric resistance R and capacitance C of the two-dimensional photonic crystal, 1/(2πRC). This upper limit of the frequency corresponds to the frequency where the ON/OFF ratio of the intensity of light is −3 dB. Therefore, the two-dimensional photonic crystal should preferably have an electric resistance R and capacitance C with which 1/(2πRC) becomes equal to or higher than the desired frequency. The electric resistance R can be controlled by the densities of the holes and electrons added in the p-layer and n-layer, respectively. The electric capacitance C can be controlled by the thickness of each layer. However, in order to avoid affecting the number of holes and electrons, it is preferable to provide the aforementioned i-layer and control the capacitance by the thickness of this layer. It should be noted that adding an excessive amount of electrons and holes may cause thermal emission within unnecessary wavelength bands. The electric resistance R and capacitance C can also be adjusted by the area S of the two-dimensional photonic crystal. The smaller the area S is, the lower the electric resistance R and capacitance C become, with the consequent increase in the upper limit of the frequency. By appropriately setting the electric resistance R and capacitance C, a thermal emission source having an operation frequency of 1-100 kHz, which is suitable as the emission source for the previously described sensor, can be realized. It is also possible to realize a thermal emission source having an operation frequency of 10-50 MHz, which can be used in a signal source for free-space communication which is unaffected by the condition of the atmosphere. 
         [0021]    In the thermal emission source according to the present invention, it is preferable that, among the p-layer and the n-layer, the layer having a larger effective mass of the carrier (the hole in the p-layer or the electron in the n-layer) has a higher level of carrier density than the layer having a smaller effective mass of the carrier. Under the same carrier density, a carrier having a larger effective mass normally shows a higher resistance. Therefore, the effect of increasing the operation frequency by decreasing the electric resistance R of the two-dimensional photonic crystal will be more noticeably achieved by modifying the density of the carrier having a larger effective mass. Another advantage exists in that modifying the density of the carrier having a larger effective mass is less likely to cause unnecessary thermal emissions. 
         [0022]    In the thermal emission source according to the present invention, it is preferable to use, as the electrodes, a metallic p-electrode which is in contact with the p-layer and forms an ohmic junction with the p-type semiconductor constituting the p-layer as well as a metallic n-electrode which is in contact with the n-layer and forms an ohmic junction with the n-type semiconductor constituting the n-layer. An ohmic junction is a junction at which a conduction state similar to the electric conduction which obeys Ohm&#39;s law is created without causing a rectifying action between the semiconductor and the metal, as opposed to the Schottky junction which causes a rectifying action. 
         [0023]    In the thermal emission source according to the present invention, it is preferable to provide the electrodes on at least one of the surfaces of the slab in such a manner that the electrodes surround an arrangement region where the modified refractive index areas are cyclically arranged. This allows the light to be extracted from the arrangement region without being blocked by the electrodes. 
         [0024]    The thermal emission source according to the present invention may further include a heating device for heating the two-dimensional photonic crystal by electric power. Examples of this heating device include a device which passes an electric current through the aforementioned electrodes to the n-layer and/or the p-layer, a device which performs a similar heating process using a dedicated electrode separately provided for the heating device around the aforementioned electrodes, or a mechanism which passes an electric current through a separately provided high-resistance wire for the heating. It is also possible to use the technique of increasing the temperature of the emission source by placing it in contact with a separate heat source instead of providing a heating device in the emission source itself. 
         [0025]    The two-dimensional photonic crystal according to the present invention is a device for use in a thermal emission source, including: 
         [0026]    a slab in which an n-layer made of an n-type semiconductor, a quantum well structure layer having a quantum well structure, and a p-layer made of a p-type semiconductor are stacked in the mentioned order in the thickness direction; and 
         [0027]    modified refractive index areas whose refractive index differs from the refractive indices of the n-layer, the p-layer and the quantum well structure layer, the modified refractive index areas cyclically arranged in the slab so as to resonate with a specific wavelength of light corresponding to a transition energy between the subbands in a quantum well in the quantum well structure layer. 
       Advantageous Effects of the Invention 
       [0028]    According to the present invention, a thermal emission source capable of switching the intensity of light at a high response speed similarly to a photoelectric conversion element can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]      FIGS. 1A and 1B  are diagrams for explaining the energy state of the electrons in the thermal emission source according to the present invention in the case where the quantum well structure layer has n-type quantum wells. 
           [0030]      FIGS. 2A and 2B  are diagrams for explaining the energy state of the electrons in the thermal emission source according to the present invention in the case where the quantum well structure layer has p-type quantum wells. 
           [0031]      FIGS. 3A, 3B and 3C  are a perspective view, vertical sectional view, and top view of a thermal emission source as one embodiment of the present invention, respectively. 
           [0032]      FIG. 4  is a graph showing the result of a calculation of the potential of the electrons for the thermal emission source of the present embodiment in the case where no voltage is applied between the electrodes as well as in the case where a voltage is applied. The inset is a graph showing the result of a calculation of the relationship between the voltage and the absorption coefficient. 
           [0033]      FIG. 5  is a graph showing the result of a calculation of the wavelength spectrum of the emissivity of light for the thermal emission source of the present embodiment in the case where no voltage is applied between the electrodes as well as in the case where a voltage is applied. The inset is a graph demonstrating the difference between the two wavelength spectra. 
           [0034]      FIG. 6A  is a graph showing the result of a calculation of the temporal change in the emission power of the thermal emission source of the present embodiment in which the voltage applied between the electrodes is alternated at a frequency of 10 kHz, and  FIG. 6B  is a graph showing the result of a corresponding calculation performed for a conventional thermal emission source as a comparative example in which the voltage supplied to a heater for heating the emission source is alternated at a frequency of 10 kHz. 
           [0035]      FIG. 7  is a graph showing the result of a calculation in which the change in the modulation power with respect to the modulation frequency was calculated for the thermal emission source of the present embodiment. 
           [0036]      FIG. 8A  is a microscopic image of a thermal emission source fabricated in the present embodiment, and  FIG. 8B  is a magnified microscopic image of the two-dimensional photonic crystal in the same thermal emission source. 
           [0037]      FIG. 9  is a graph showing the result of a measurement in which the current-voltage characteristics of the thermal emission source fabricated in the present embodiment were investigated at a plurality of different temperatures. 
           [0038]      FIG. 10  is a graph showing the result of a measurement of the wavelength spectrum of the infrared radiation emitted from the thermal emission source fabricated in the present embodiment when no voltage was applied between the electrodes as well as when a voltage was applied, along with a measurement result obtained for a blackbody emission source with the same area under the same temperature condition. 
           [0039]      FIG. 11A  is a graph showing the wavelength spectrum of the infrared radiation measured for various values of the voltage applied between the electrodes in the thermal emission source fabricated in the present embodiment, and  FIG. 11B  is a graph showing the wavelength spectrum of the infrared radiation calculated under the conditions corresponding to the present measurement. It should be noted that the data for voltages of 8 V and 10 V in  FIG. 11B  are shown in an overlapped form. 
           [0040]      FIG. 12  is a model diagram showing the optical system for a high-speed modulation experiment in which the temporal change in the intensity of the light emitted from the thermal emission source fabricated in the present embodiment is measured while the frequency of the voltage applied between the electrodes is gradually changed within a range from 0.1 kHz to 1.5 MHz (from 100 to 1,500,000 Hz). 
           [0041]      FIG. 13  is a graph showing the result of the high-speed modulation experiment performed at a frequency of 10 kHz for the thermal emission source fabricated in the present embodiment. 
           [0042]      FIG. 14  is a graph showing the result of an experiment in which the change in the modulation power with respect to the modulation frequency was determined for the thermal emission source fabricated in the present embodiment. 
           [0043]      FIGS. 15A-15C  are graphs showing the emissivity (left-hand panel) and delay time (right-hand panel) calculated for the thermal emission source of the present embodiment under the following conditions: the hole density N p  in the p-layer and the electron density N e  in the n-layer are both 1×10 17  cm −3  ( FIG. 15A ); N p  and N e  are both 1×10 18  cm −3  ( FIG. 15B ); and N p  is 1×10 18  cm −3  while N e  is 1×10 17  cm −3  ( FIG. 15C ). 
           [0044]      FIG. 16  is a graph showing the result of a calculation in which the change in the modulation power with respect to the modulation frequency was calculated under the condition that the air-hole arrangement region was a 0.5 mm square region. 
           [0045]      FIG. 17A  is a microscopic image of a thermal emission source having a smaller air-hole diameter than the example of  FIGS. 8A and 8B ,  FIG. 17B  is a magnified microscopic image of the two-dimensional photonic crystal in the same thermal emission source, and  FIG. 17C  is a graph showing the result of a measurement of the wavelength spectrum of the infrared radiation emitted in the same thermal emission source. 
           [0046]      FIGS. 18A and 18B  are a perspective view and top view of a modified example of the thermal emission source according to the present invention, respectively. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0047]    An embodiment of the thermal emission source according to the present invention is hereinafter described using  FIGS. 3A-18B . 
       Embodiment 
     (1) Configuration of Thermal Emission Source of Present Embodiment 
       [0048]    As shown in  FIGS. 3A-3C , the thermal emission source  10  of the present embodiment includes a slab  11 , air holes (modified refractive index areas)  121 , p-type electrode  131 , n-type electrode  132  and bases  14 . The slab  11  and the air holes  121  form the two-dimensional photonic crystal  12 . 
         [0049]    The slab  11  is a rectangular laminate structure with a p-layer  111 , i-layer  113 , quantum well structure layer  114  and n-layer  112  arranged from top to bottom. In the present embodiment, the p-layer  111  and n-layer  112  have a thickness of 800 nm, while the i-layer  113  and quantum well structure layer  114  have a thickness of 200 nm. The material used for the p-layer  111  is p-GaAs, a p-type semiconductor prepared by doping GaAs with Be as the p-type impurity. The material used for the n-layer  112  is n-GaAs, an n-type semiconductor prepared by doping GaAs with Si as the n-type impurity. The material used for the i-layer  113  is GaAs with no dopant implanted. The hole density N p  in the p-layer  111  and the electron density N e  in the n-layer  112  are both 1×10 17  cm −3 . The quantum well structure layer  114  includes a number of GaAs layers and n-AlGaAs layers alternately arranged in a laminate form. According to this configuration, a number of n-type quantum wells are formed inside the quantum well structure layer  114 , with each GaAs layer corresponding to the inner space of the well. An energy potential with a number of quantum wells formed in this manner is called the “multiple quantum wells”, and a structure having multiple quantum wells is called the “multiple quantum well structure”. 
         [0050]    The air holes  121  are arranged in a triangular lattice pattern in the slab  11  within a rectangular region with one side length L=2 mm. The lattice constant a of this triangular lattice is 4.8 μm. Such a cyclic arrangement of the air holes  121  enables the two-dimensional photonic crystal  12  to act as a resonator for an infrared radiation having a wavelength of 9.2 μm in vacuum. The air holes  121  have a columnar shape. This column has a design radius r of approximately 0.96 μm (0.20a) and a thickness of approximately 1.3 μm. 
         [0051]    Among the layers constituting the slab  11 , the n-layer  112  has a larger area than the other layers and has, at both longitudinal ends, a portion which is not in contact with the quantum well structure layer  114 . The n-type electrode  132  is a metallic electrode made of Ge/Au/Ni/Au alloy which forms an ohmic junction with the n-layer  112 . This electrode is formed on the aforementioned portion of the n-layer  112  by vapor deposition. The p-type electrode  131  is a metallic electrode made of Au/Zn/Au alloy which forms an ohmic junction with the p-layer  111 . This electrode is formed on the surface of the p-layer  111  by vapor deposition. The provision of the p-type and n-type electrodes  131  and  132  at those locations means that a voltage will be applied between the p-layer  111  and the n-layer  112  across the quantum well structure layer  114  and the i-layer  113 . Additionally, since the p-type electrode  131  is provided in such a manner as to surround the region where the air holes  121  are cyclically arranged (arrangement region), the light can be extracted from the arrangement region without being blocked by the p-type electrode  131 . 
         [0052]    The bases  14  are two rectangular-parallelepiped members made of an electrical insulator GaAs with no dopant added. The two bases  14  are attached to the lower surface of the n-layer  112  near the longitudinal ends of the slab  11  and parallel to the shorter side of the same slab  11 . By these bases  14 , the slab  11  is supported free from contact with other objects except for the portions to which the bases  14  are attached. In the process of heating the thermal emission source  10 , the heat is transferred from an external heat source through these bases  14  to the two-dimensional photonic crystal  12  by thermal conduction. 
       (2) Result of Calculation on Characteristics of Thermal Emission Source  10  in Present Embodiment 
       [0053]    Next, the results of simulations conducted to calculate the characteristics of the thermal emission source  10  in the present embodiment are described. In the following description, the polarity of the voltage applied between the p-type electrode  131  and the n-type electrode  132  is negative on the p-layer  111  (p-type electrode  131 ) and positive on the n-layer  112  (n-type electrode  132 ) unless otherwise stated. 
         [0054]    Initially, using  FIG. 4 , the result of a calculation is described in which the potential of the electrons contributing to the thermal emission in the p-layer  111 , i-layer  113 , quantum well structure layer  114  and n-layer  112  of the thermal emission source  10  was calculated. Two cases were considered in this calculation: (a) no voltage is applied between the p-type electrode  131  and the n-type electrode  132 ; and (b) a voltage of 8 V is applied between the electrodes. The heating temperature was set at 200° C. In both cases (a) and (b), a number of quantum wells (multiple quantum wells) are formed in the quantum well structure layer  114 , as shown in the graph of  FIG. 4 . In case (a), all quantum wells have almost equal potentials. By comparison, in case (b), the quantum wells located close to the p-layer  111  have generally higher energy levels than those in (a). This means that, when the voltage is applied, no electrons exist in the quantum wells located close to the p-layer  111 , so that the amount of electrons which contribute to the thermal emission is smaller than when no voltage is applied (see  FIGS. 1A and 1B ). Accordingly, when the voltage is applied, the coefficient of optical absorption due to the intersubband transition (ISB-T) in the quantum wells becomes lower than when no voltage is applied (see the inset in  FIG. 4 ), and the emission intensity also becomes lower. 
         [0055]    Next, using  FIG. 5 , the result of a calculation is described in which the wavelength spectrum of the emissivity of light from the thermal emission source  10  heated to 200° C. with an external heat source was calculated for two cases: (a) no voltage is applied and (b) a voltage of 8V is applied. The emissivity is defined as the emission intensity of the thermal emission source  10  divided by the intensity of the blackbody emission at the same temperature at each wavelength. The wavelength and wavenumber in the graph of  FIG. 5  indicate those of light in vacuum. This graph shows that the peak emissivity at a wavelength of 9.2 μm in (b) is approximately one eighth of the value in (a). Although emissivity peaks are also recognized at wavelengths other than 9.2 μm, all of those peak values are sufficiently lower than the peak value at 9.2 μm in (a). Furthermore, none of the peaks except the one at 9.2 μm has a significant difference in the peak value between (a) and (b). This fact is also evident from the graph in the inset in  FIG. 5 , which shows the difference between (a) and (b). These calculation results mean that only a narrow wavelength band of infrared radiation around 9.2 μm undergoes intensity modulation by the voltage on/off operation. Accordingly, in a sensor which uses this thermal emission source  10 , it is possible to selectively obtain a response in a narrow band of infrared radiation around 9.2 μm based on a signal whose intensity has been modulated at the frequency corresponding to the voltage on/off operation among the detected optical signals. 
         [0056]    Next, using  FIGS. 6A and 6B , the result of a calculation is described which was conducted to investigate the temporal change in the emission power which occurs when the voltage applied between the electrodes is turned on and off while the thermal emission source  10  is heated. The voltage in the “ON” state was set at 8 V, and the on/off frequency (modulation frequency) was set at 10 kHz. The heating temperature was set at 200° C. Taking into account the fact that commonly used infrared detectors detect infrared radiation within a specific wavelength band without resolving it into component wavelengths, the emission power was accumulated over a wavelength band from 5 to 15 μm. As a comparative example, a similar calculation was conducted for a conventional thermal emission source under the condition that the input power supplied to the emission source to change its temperature was turned on and off at a frequency of 10 kHz. The results of these calculations demonstrated that the difference in the emission power (this difference is hereinafter called the “modulation power”) between the “ON” and “OFF” periods of the heater in the comparative example ( FIG. 6B ) was at an extremely low level of 0.066%, whereas the modulation power in the present embodiment ( FIG. 6A ) was approximately 67%. This means that the device according to the present embodiment can modulate the intensity of light at a high modulation frequency of 10 kHz and with a modulation depth of approximately 1000 times the conventional value. As already noted, the data shown in  FIG. 6A  are values obtained by accumulating the emission power over a wavelength band from 5 to 15 μm. However, taking into account the spectra in  FIG. 5 , it is possible to conclude that it is the narrow wavelength range of infrared radiation around 9.2 μm that contributes to the intensity modulation. 
         [0057]    Next, using  FIG. 7 , the result of a calculation is described in which the difference in the emission power between the “OFF” and “ON” periods of the voltage was calculated for various modulation frequencies. The modulation power at each modulation wavelength is expressed by its ratio to the modulation power at a low frequency (100 Hz) in decibels. The calculation was performed for two different forms of the region where the air holes  121  were arranged in the slab  11  (arrangement region): a square region with one side length L=2 mm and a square region with L=0.5 mm. The modulation frequency at which the value on the vertical axis in  FIG. 7  becomes −3 dB (i.e. one half of the modulation power recorded when the modulation frequency indicated by the horizontal axis was at 100 Hz) was approximately 700 kHz for L=2 mm and 10 MHz for L=0.5 mm. Accordingly, these thermal emission sources in the present embodiment can perform a definite modulating action even when the modulation frequency is set at higher levels than the conventional maximum value of 100 Hz. A comparison of the two results obtained for L=0.5 mm and L=2 mm indicates that reducing the size of the arrangement region makes higher modulation frequencies available. 
       (3) Result of Experiments for Thermal Emission Source of Present Embodiment 
       [0058]    Next, the result of an experiment performed for an actually fabricated thermal emission source (which is hereinafter called the “thermal emission source  10 A”) is presented.  FIGS. 8A and 8B  are microscopic images of the thermal emission source  10 A taken from the upper side in the perspective view shown in  FIG. 3A . Specifically,  FIG. 8A  is an image showing the entire top surface of the thermal emission source  10 A taken with an optical microscope, while  FIG. 8B  is a magnified image showing a portion of the two-dimensional photonic crystal  12  taken with an electron microscope. 
         [0059]      FIG. 9  shows the measured result of the current-voltage characteristics for the obtained thermal emission source  10 A. In this figure, a voltage which is positive on the p-type electrode  131  and negative on the n-type electrode  132  is expressed by a positive value, as is the common practice for this type of measurement performed for a PIN diode consisting of a p-type semiconductor, insulator and n-type semiconductor sequentially joined. The measurement, which was conducted at a plurality of temperatures from room temperature to 200° C., proved that the obtained thermal emission source  10 A exhibited the characteristics as a diode at any of those temperatures; i.e. it allowed electric current to pass through when the voltage was positive, while it barely allowed the passage of the current when the voltage was negative. This result demonstrates that the thermal emission source  10 A utilizes the transfer of electrons between the quantum wells and the n-type semiconductor, without allowing the passage of the electric current, when the voltage is within a range from zero to the negative bias voltage in a PIN diode. 
         [0060]      FIG. 10  shows the result of an experiment in which the wavelength spectrum of the radiation intensity of the light from the thermal emission source  10 A was measured under the condition that the voltage applied between the p-type electrode  131  and the n-type electrode  132  was (a) 0 V or (b) 10 V (which is presented as a negative value in  FIG. 9 ), along with a measurement result obtained for a blackbody emission source with the same area under the same temperature condition. The radiation intensity of the light is represented in arbitrary unit, with the intensities adjusted so that the measurement results for (a), (b) and the blackbody emission source can be compared with each other. Similarly to the calculated result shown in  FIG. 5 , this experimental result demonstrates that the radiation intensity at the wavelength of 9.2 μm in (b) was dramatically lower than the value in (a), while the emission at the other wavelengths was either unobservable or detected with approximately equal intensities in both (a) and (b). 
         [0061]      FIG. 11A  shows the result of a measurement in which the wavelength spectrum around the wavelength of 9.2 μm was measured for a plurality of voltages applied within a range from 0 to 10 V, and  FIG. 11B  shows the result of a calculation corresponding to this measurement. The measured result is in good agreement with the calculated result at any of the applied voltages. 
         [0062]    Next, the result of a measurement is presented in which the intensity of the emission from the thermal emission source  10 A was measured while the voltage applied between the p-type electrode  131  and the n-type electrode  132  was repeatedly turned on and off at a modulation frequency equal to or higher than 10 kHz. This measurement was conducted with the experimental system  20  shown in  FIG. 12 . It should be noted that numeral “10” is used for the thermal emission source in  FIG. 12  as well as in the following description in this paragraph, because the experimental system  20  is also used for another thermal emission source  10 B (which will be described later). The experimental system  20  includes a heater  21  for supporting and heating the thermal emission source  10 , a pulse generator  22  for generating a rectangular radio-frequency voltage with a specified modulation frequency to be applied between the p-type electrode  131  and the n-type electrode  132 , a detector  23  for detecting the light emitted from the thermal emission source  10 , an optical system  24  for guiding the light from the thermal emission source  10  to the detector  23 , and a digital oscilloscope  25  for monitoring the output signal from the detector  23  by synchronizing this signal with the radio-frequency voltage generated by the pulse generator  22 . A HgCdTe (mercury cadmium telluride) detector was used as the detector  23 . The level of the radio-frequency voltage generated by the pulse generator  22  during the “ON” period was set at 10 V. 
         [0063]      FIG. 13  shows the measured result for a modulation frequency of 10 kHz. An output signal having a square waveform corresponding to the frequency of the applied voltage was obtained. 
         [0064]      FIG. 14  shows the result of a measurement in which the change in the modulation power with respect to the modulation frequency was measured using the experimental system  20 . The modulation power was equal to or higher than −3 dB within a frequency range of approximately 600 kHz or lower. This result is in good agreement with the calculate result shown in  FIG. 7 . 
         [0065]    To further increase the modulation frequency, or to achieve an even higher speed of operation, the carrier densities in the p-layer  111  and n-layer  112  as well as the area of the air-hole arrangement region were investigated. The result is hereinafter described. 
         [0066]    Initially, a description related to the carrier density is given, using  FIGS. 15A-15C .  FIG. 15A  is a graph showing the emissivity (left-hand panel, a reproduction of the graph in  FIG. 5 , exclusive of the inset) and the delay time tr (right-hand panel) in the case where the hole density N p  in the p-layer  111  and the electron density N e  in the n-layer  112  are both at the value used in the previous embodiment (1×10 17  cm −3 ). The delay time tr is the period of time required for the voltage applied between the electrodes to actually act on the quantum wells in the central portion of the photonic crystal. More specifically, in the present case, the delay time tr is defined as the period of time required for the voltage applied to the quantum well structure layer  114  to reach the value equal to (1−e −1 ) times the value which was ultimately recorded after a sufficient amount of time (where e is the base of natural logarithm; 1−e −1 ≅0.63). The voltage between the p-type electrode  131  and the n-type electrode  132  was 8 V in the left-hand panel and 10 V in the right-hand panel. In the calculation of  FIG. 15A , the change in the emissivity between the “OFF” and “ON” periods of the voltage was 0.69. The delay time tr was 0.62 μs. 
         [0067]    Next, the hole density N p  in the p-layer  111  and the electron density N e  in the n-layer  112  were both increased to 1×10 18  cm −3 . The result was as shown in  FIG. 15B , where, as compared to  FIG. 15A , the delay time tr was shortened to 0.061 μs and the change in the emissivity between the “OFF” and “ON” periods of the voltage was decreased to 0.19. This means that the device performance improved in terms of the delay time tr but deteriorated in terms of the ON/OFF ratio. Additionally, as can be seen in the left-hand panel in  FIG. 15B , unnecessary emissions at wavelengths other than the target wavelength (9.2 μm) increased. 
         [0068]    Accordingly, another calculation was performed, in which only the hole density N p  in the p-layer  111  was increased to 1×10 18  cm −3  while the electron density N e  in the n-layer  112  was maintained at the original value of 1×10 17  cm −3 . This means that the density of the holes in the p-layer  111 , which have a larger effective mass as the carrier than the electrons in the n-layer  112 , was increased. The result was as shown in  FIG. 15C , where the delay time tr was shortened from the value in  FIG. 15A  to 0.091 μs, while the change in the emissivity between the “OFF” and “ON” periods of the voltage was 0.56, which means a better performance in terms of emissivity as compared to  FIG. 15B . Unnecessary emissions at wavelengths other than the target wavelength were also lower than in  FIG. 15B . 
         [0069]      FIG. 16  shows the result of a calculation in which the change in the modulation power with respect to the modulation frequency was calculated under the condition that the air-hole arrangement region was a 0.5-mm square region. The conditions for the present calculation differ from those applied in the experiment shown in  FIG. 14  in that only the hole density in the p-layer  111  was increased to 1×10 18  cm −3  (the electron density N e  in the n-layer  112  was maintained at 1×10 17  cm −3 ; in other words, the carrier densities were the same as in the case of  FIG. 15C ), and in that the size of the air-hole arrangement region was decreased. The result of this calculation demonstrates that the upper limit of the frequency can be increased to approximately 70 MHz. 
         [0070]    Next, using  FIGS. 17A-17C , a thermal emission source  10 B which has a smaller radius of the air holes  121  than the thermal emission source  10 A in the previous embodiment is described. In the present example, the lattice constant a of the triangular lattice was 4.5 μm, and the radius r of the air holes  121  was 0.63 μm (0.14a). The other conditions were the same as applied to the thermal emission source  10 A.  FIGS. 17A and 17B  are microscopic images of the obtained thermal emission source  10 B, while  FIG. 17C  is a graph showing the result of a measurement in which the wavelength spectrum of the infrared radiation emitted in the thermal emission source was measured with the experimental system  20 . The graph shows wavelength spectra for two cases in which the applied voltage was 2 V (at which the emission intensity was at the highest level) and 10 V (at which the emission intensity was at the lowest level), respectively. When the applied voltage was 2 V, the full width at half maximum (FWHM) of the wavelength spectrum of the infrared radiation emitted from the thermal emission source  10 B was 7.0 cm −1  and narrower than 12.5 cm −1  which is the FWHM of the spectrum in  FIG. 11A  achieved by the thermal emission source  10 A when the applied voltage was 0 V. This means that a narrower emission spectrum was obtained by decreasing the diameter of the air holes. Furthermore, changing the voltage applied to the thermal emission source  10 B from 2 V to 10 V significantly decreased the emission intensity. This confirms that an intensity modulation by the voltage control is similarly possible as with the thermal emission source  10 A. 
         [0071]    The present invention is not limited to the previous embodiment. 
         [0072]    Although the p-layer  111 , n-layer  112  and i-layer  113  in the previous embodiment are made of semiconductors using GaAs as the main constituent, other semiconductors may also be used. The quantum well structure layer  114  may also be made of a material other than the previously mentioned one. The quantum well structure layer  114  is not limited to one having n-type quantum wells; a quantum well structure layer having p-type quantum wells may also be used, such as a laminate structure composed of a number of InP layers and p-InGaAs layers alternately arranged in a laminate form. The quantum well structure layer  114  is also not limited to one having a multiple quantum well structure as in the previous embodiment; a quantum well structure in which only a single quantum well is formed may also be used. The thickness values of the p-layer  111 , n-layer  112 , i-layer  113  and quantum well structure layer  114  are not limited to the previously mentioned ones, which are mere examples. The hole density in the p-layer  111  and the electron density in the n-layer  112  (the densities of the impurities in those layers) are not limited to the previously mentioned values, which are also mere examples. The p-layer  111  and the quantum well structure layer  114  may be directly joined together without using the i-layer  113 . 
         [0073]    In the previous embodiment, air holes  121  are used as the modified refractive index areas. Alternatively, a tangible material whose refractive index differs from that of the slab  11  may be used for the modified refractive index areas. The shape and size of the modified refractive index areas (air holes  121 ) are not limited to those mentioned in the previous embodiment; as for the shape, they may have the shape of a triangular column, rectangular column or similar shape. The lattice constant and lattice pattern of the modified refractive index areas (air holes  121 ) are also not limited to those mentioned in the previous embodiment; as for the lattice pattern, a square lattice or other lattice patterns may also be used. 
         [0074]    In the previous embodiment, the p-type electrode  131  is provided in such a manner as to surround the arrangement region where the air holes  121  are cyclically arranged.  FIGS. 18A and 18B  show another example, in which a pair of p-type electrodes  131 A are provided on both sides of the arrangement region. Such a configuration also allows the light to be extracted from the arrangement region without being blocked by the p-type electrodes  131 A. In  FIGS. 18A and 18B , the n-type electrodes  132 A are also in the form of a pair of strips similar to the p-type electrodes  131 A, although it is not essential to give similar shapes to the p-type and n-type electrodes. 
         [0075]    In the previous embodiment, thermal conduction from an external heat source is used to heat the thermal emission source. The heating method is not limited to this one. Electric power for the heating may be directly injected into the thermal emission source itself. Other heating means may also be used. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10 ,  10 A,  10 B . . . Thermal Emission Source 
           11  . . . Slab 
           111  . . . p-Layer 
           112  . . . n-Layer 
           113  . . . i-Layer 
           114  . . . Quantum Well Structure Layer 
           12  . . . Two-Dimensional Photonic Crystal 
           121  . . . Air Hole 
           131 ,  131 A . . . p-Type Electrode 
           132 ,  132 A . . . n-Type Electrode 
           14  . . . Base 
           20  . . . Experimental System 
           21  . . . Heater 
           22  . . . Pulse Generator 
           23  . . . Detector 
           24  . . . Optical System 
           25  . . . Digital Oscilloscope