Abstract:
A wavelength variable light source is provided with: a light emitting layer containing therein light emitting substance having a carbon-carbon inter-atomic bond; a pair of electrodes disposed on both sides of the light emitting layer while holding the light emitting layer therebetween; a pair of main reflectors disposed on both sides of the light emitting layer so as to hold the light emitting layer therebetween, thereby to constitute an optical resonator with respect to light emitted from the light emitting layer; and refractive index modulating means disposed on the optical path of the optical resonator, wherein the refractive index modulating means can reversibly vary the length of the optical path of the optical resonator so as to control the wavelength of the light emitted from the light source.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a light source capable of varying a light emitting wavelength in a light source including an organic electroluminescence (organic EL) element or the like.  
           [0003]    2. Related Art  
           [0004]    An organic EL element using an amorphous thin film made of organic substance has been actively developed in recent years. In a light source for allowing the organic substance of the organic EL element or the like to emit light, it has been difficult to modulate a light emitting wavelength because the width of a light emitting spectrum has been great. For example, in a light source disclosed in Japanese Patent Publication Laid-open No. 08-315983, a light emitting layer includes an organic light emitting material having a light emitting peak wavelength ranging from 450 nm to 570 nm and a half-value breadth ranging from 100 nm to 200 nm.  
           [0005]    Consequently, it has been difficult to use the light source for allowing the organic substance of the organic EL element or the like to emit the light as a light source for performing optical communications at a high speed by a frequency modulating system. Furthermore, since a response speed during the flickering of the light source has ranged from several micro seconds to several hundred micro seconds, it has been difficult to perform the optical communications at the high speed even by a pulse width modulation system or a pulse code modulation system.  
         SUMMARY OF THE INVENTION  
         [0006]    A first object of the present invention is to provide a wavelength variable light source which has a narrow width of a light emitting spectrum and can readily vary a light emitting wavelength in the light source for allowing organic substance of an organic EL element or the like to emit light.  
           [0007]    A second object of the present invention is to provide a wavelength variable light source which can widen an angle of view and limit the same for protection of an privacy.  
           [0008]    The wavelength variable light source according to the present invention comprises: a light emitting layer containing therein light emitting substance having a carbon-carbon inter-atomic bond; a pair of electrodes disposed on both sides of the light emitting layer while holding the light emitting layer therebetween; a pair of main reflectors disposed on both sides of the light emitting layer while holding the light emitting layer therebetween, so as to constitute an optical resonator with respect to light emitted from the light emitting layer; and refractive index modulating means disposed on the optical path of the optical resonator, wherein the refractive index modulating means can reversibly vary the length of the optical path of the optical resonator so as to control the wavelength and/or directivity of the light emitted from the light source.  
           [0009]    Examples of the refractive index modulating means according to the present invention include means consisting of a piezoelectric member held between the pair of electrodes. With the application of a voltage to the piezoelectric member by the pair of electrodes, the thickness of the piezoelectric member is varied by a piezoelectric effect, so that its refractive index can be varied. The above-described piezoelectric member is provided on the optical path of the optical resonator, thereby varying the length of the optical path of the optical resonator, so as to control the variations of the length of the wavelength of the light emitted from the light source.  
           [0010]    Either one of the pair of electrodes holding the piezoelectric member therebetween may be constituted of either one of the pair of electrodes holding the light emitting layer therebetween.  
           [0011]    Besides the above-described piezoelectric member, means having a refractive index which is varied according to a stimulus such as a voltage, a current, an electromagnetic wave, an elastic wave or heat can be used as the refractive index modulating means according to the present invention. Specifically, there can be used substance exhibiting an electrooptic effect, an acoustooptic effect, a magnetooptic effect, a thermooptic effect or a nonlinear optical effect.  
           [0012]    According to the present invention, the pair of main reflectors may be constituted of the pair of electrodes holding the light emitting layer therebetween. That is to say, in the case where the electrode is formed of a metallic thin film, this metallic thin film can be used as the main reflector.  
           [0013]    Since the pair of main reflectors constitute the optical resonator according to the present invention, it is preferable that the length of the optical path between the pair of main reflectors should be substantially equal to a multiple of a natural number of a half of a predetermined peak wavelength (i.e., a designed peak wavelength) of the light emitted from the light source. Specifically, for example, it is preferable that the length of the optical path between the pair of main reflectors should range from 99/200 to 101/200 of a multiple of a natural number of a predetermined peak wavelength of the light emitted from the light source.  
           [0014]    According to the present invention, it is preferable that the length of the optical path from the center of a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector should be substantially equal to a multiple of a natural number of a half of a predetermined peak wavelength of the light emitted from the light source. Specifically, for example, it is preferable that the length of the optical path from the center of a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector should range from 99/200 to 101/200 of a multiple of a natural number of a predetermined peak wavelength of the light emitted from the light source. With this setting, it is possible to further enhance a wavelength selecting effect.  
           [0015]    The region in the light emitting layer, in which the light emission occurs most strongly, may be located in the vicinity of the end face of the light emitting layer or at the center of the light emitting layer. Otherwise, it may be located inside of the layer adjacent to the light emitting layer. Therefore, it is preferable that the length of the optical path between the end face of the light emitting layer and the main reflector should be set as follows:  
           [0016]    Namely, it is preferable that the length of the optical path from an end face nearer a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector nearer the end face should be substantially equal to or slightly smaller than a multiple of a natural number of a half of a predetermined peak wavelength of the light emitted from the light source. Specifically, for example, it is preferable that the length of the optical path from an end face nearer a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector nearer the end face should range from 101/200 to 88/200 of a multiple of a natural number of a predetermined peak wavelength of the light emitted from the light source.  
           [0017]    It is preferable that the length of the optical path from an end face nearer a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector more remote from the end face should be substantially equal to or slightly greater than a multiple of a natural number of a half of a predetermined peak wavelength of the light emitted from the light source. Specifically, for example, it is preferable that the length of the optical path from an end face nearer a region in the light emitting layer, in which the light emission occurs most strongly, to the main reflector more remote from the end face should range from 99/200 to 112/200 of a multiple of a natural number of a predetermined peak wavelength of the light emitted from the light source.  
           [0018]    According to the present invention, an auxiliary electrode having light transmittance may be disposed outside of at least either one of the pair of electrodes holding the light emitting layer therebetween. Such an auxiliary electrode is disposed in such a manner as to be brought into contact with the electrode made of the metallic thin film, thereby constituting a composite electrode consisting of the electrode made of the metallic thin film and the auxiliary electrode. With the auxiliary electrode having the light transmittance, the thickness of the electrode made of the metallic thin film can be reduced, thereby achieving both characteristics of conductivity and light transmittance. Consequently, the light can be emitted on the side on which the above-described auxiliary electrode is disposed.  
           [0019]    It is preferable that the length of the optical path of the auxiliary electrode should be substantially equal to a multiple of a natural number of a half of a predetermined peak wavelength of the light emitted from the light source. Specifically, for example, it is preferable that the length of the optical path of the auxiliary electrode should range from 99/200 to 101/200 of a multiple of a natural number of a predetermined peak wavelength of the light emitted from the light source.  
           [0020]    Although the light emitting layer according to the present invention is, for example, an organic EL light emitting layer containing therein organic EL light emitting substance, the present invention is not limited to this. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a schematic cross-sectional view showing a wavelength variable light source in a preferred embodiment according to the present invention;  
         [0022]    [0022]FIG. 2 is a schematic cross-sectional view showing a wavelength variable light source in another preferred embodiment according to the present invention;  
         [0023]    [0023]FIG. 3 is a schematic cross-sectional view showing a wavelength variable light source in a further preferred embodiment according to the present invention; and  
         [0024]    [0024]FIG. 4 is a schematic diagram illustrating a high-speed optical communication system by the use of the wavelength variable light source according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     EXAMPLE 1  
       [0025]    [0025]FIG. 1 is a schematic cross-sectional view showing a wavelength variable light source in a preferred embodiment according to the present invention. A thin film (having a thickness of 200 nm) made of gold serving as a first electrode  2  is formed on a substrate  1  made of glass by sputtering method. On the first electrode  2 , a piezoelectric thin film  3  (having a thickness of 50 nm) having light transmittance is formed as refractive index modulating means by reactive sputtering method with an electron cyclotron resonance (abbreviated as “ECR”) plasma in an atmosphere of argon and nitrogen. The piezoelectric thin film  3  is made of aluminum nitride (AlN). On the piezoelectric thin film  3 , a transparent conductive thin film made of In 2 O—SnO 2  (ITO) (having a thickness of 50 nm) is formed as a second electrode  4  by sputtering method.  
         [0026]    On the second electrode  4 , a hole injecting layer  5  (having a thickness of 9 nm) made of copper phthalocyanine (CuPc) expressed by the following chemical formula is formed:  
                         
 
         [0027]    On the hole injecting layer  5 , a hole transporting layer  6  (having a thickness of 30 nm) made of 4,4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (NPB) expressed by the following chemical formula is formed:  
                         
 
         [0028]    On the hole transporting layer  6 , a light emitting layer  7  (having a thickness of 150 nm) made of aluminumtris(8-hydroxyquinoline) (Alq) expressed by the following chemical formula is formed:  
                         
 
         [0029]    On the light emitting layer  7 , an electron injecting layer  10  (having a thickness of 1 nm) made of lithium fluoride (LiF) is formed. On the electron injecting layer  10 , a layer (having a thickness of 10 nm) as a third electrode  11  which has light semi-transmittance and is made of an magnesium alloy (Mg:In) containing 20% by mass of indium is formed. On the third electrode  11 , an ITO thin film (having a thickness of 136 nm) is formed by a sputtering method as an auxiliary electrode  12  having light transmittance. The third electrode  11  and the auxiliary electrode  12  constitute a composite electrode.  
         [0030]    Each of the hole injecting layer  5 , the hole transporting layer  6 , the light emitting layer  7 , the electron injecting layer  10 , and the third electrode  11  are formed by a vacuum evaporation method.  
         [0031]    Here, the simplified molecular formula of CuPc is expressed by C 32 H 16 N 8 Cu, which has a mol mass of 576.08 g/mol. The simplified molecular formula of NPB is expressed by C 44 H 32 N 2 , which has a mol mass of 588.75 g/mol, a melting point of 277° C., a glass transition temperature of 96° C. , an ionization potential of 5.4 eV and an energy gap between HOMO and LUMO of 3.1 eV. The simplified molecular formula of Alq is expressed by C 27 H 18 N 3 O 3 Al, which has a mol mass of 459.4318 g/mol, a thermal decomposition temperature of 412° C., a glass transition temperature of 175° C., an ionization potential of 5.7 eV and an energy gap between HOMO and LUMO of 2.7 eV without any melting point.  
         [0032]    Table 1 shows below the material, refractive index, actual thickness and optical thickness of each of the above-described layers.  
               TABLE 1                                                                                
 
         [0033]    In Table 1, reference number  2   a  designates the position of the end face of the first electrode  2 ;  7   a , the position of the end face of the light emitting layer  7 ; and  11   a , the position of the end face of the third electrode  11 .  
         [0034]    In the present example, the first electrode  2  and the third electrode  11  are disposed on both sides of the light emitting layer  7  while holding the light emitting layer  7  therebetween, and further, they constitute main reflectors, respectively. These main reflectors further constitute a Fabry-Përot type optical resonator. Moreover, with the application of a voltage to the piezoelectric thin film  3  from the first electrode  2  and the second electrode  4 , the thickness of the piezoelectric thin film  3  can be varied, and further, the refractive index of the piezoelectric thin film  3  can be varied. Consequently, the length of the optical path of the optical resonator constituted of the first electrode  2  and the third electrode  11  can be varied by varying the refractive index of the piezoelectric thin film  3 . Thus, the wavelength peak of the light emitted from the optical resonator can be varied by varying the length of the optical path of the optical resonator.  
         [0035]    Table 2 shows the lengths of the optical paths between the main reflectors in the present example: namely, the length of the optical path between the end face  2   a  of the first electrode and the end face  11   a  of the third electrode, the length of the optical path between the end face  2   a  of the first electrode and the end face  7   a  of the light emitting layer, the length of the optical path between the end face  2   a  of the first electrode and the center of a region in the light emitting layer, in which light emission occurs most strongly (hereinafter referred to as “a light emitting region”), the length of the optical path between the end face  7   a  of the light emitting layer and the end face  11   a  of the third electrode, and the length of the optical path between the light emitting region and the end face  11   a  of the third electrode; and the length of the optical path within the auxiliary electrode  12 . Incidentally, in the present specification, a designed peak wavelength signifies a light emitting peak wavelength from the light source in the case where no voltage is applied to the piezoelectric thin film  3 . Moreover, in the present example, the light emitting region in the light emitting layer is located at a position inward by 2.7 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
                                           TABLE 2                           Example 1       Light Emitting Region: Inward by 2.7 nm from End       Face 7a of Light Emitting Layer       Designed Peak Wavelength: 525 nm                Length   Length           of the   of the           Optical   Optical Path/           Path   Designed Peak           (nm)   Wavelength                    From the End Face 2a of the 1st Electrode   525.5   200/200       to the End Face 11a of the 3rd Electrode       From the End Face 2a of the 1st Electrode   260.1    99/200       to the End Face 7a of the Light Emitting       Layer       From the End Face 2a of the 1st Electrode   262.8   100/200       to the Light Emitting Region       From the End Face 7a of the Light Emitting   265.4   101/200       Layer to the End Face 11a of the 3rd       Electrode       From the Light Emitting Region to the End   262.7   100/200       Face 11a of the 3rd Electrode       Within the Auxiliary Electrode 12   262.5   100/200                  
 
         [0036]    The lengths of the optical paths shown in Table 2 are values obtained by summing the optical thicknesses of the predetermined layers shown in Table 1, respectively. Here, the length of the optical path having the light emitting region at one end is obtained in consideration of 2.7 nm of the optical thickness since the light emitting region is located by 2.7 nm of the optical thickness inward of the end face 7a of the light emitting layer. In addition, the end face of the light emitting layer near the light emitting region is the end face  7   a , so that the length of the optical path having the end face  7   a  at one end is obtained.  
         [0037]    As shown in Table 2, the length of the optical path between the main reflectors (that is, the length of the optical path from the end face  2   a  of the first electrode to the end face  11   a  of the third electrode) is substantially equal to a multiple of a natural number of a half of the designed peak wavelength. Therefore, the phase of the light of the designed peak wavelength matches with that of the optical resonator, thereby strengthening the interference of the optical wave, such that the light emission having the matched designed peak wavelength is selectively strengthened. Thus, the shape of the light emitting peak of the light to be emitted from the light source is sharpened, thereby reducing the full width at half maximum (hereinafter referred to as “the half-value breadth”).  
         [0038]    Moreover, the lengths of the optical paths between the light emitting region and the main reflectors (that is, the length of the optical path from the end face  2   a  of the first electrode to the light emitting region and the length of the optical path from the light emitting region to the end face  11   a  of the third electrode) also are substantially equal to a multiple of a natural number of a half of the designed peak wavelength. Therefore, the phase of the light of the designed peak wavelength matches with that of the optical resonator, so that the light emission having the matched designed peak wavelength is more selectively strengthened.  
         [0039]    Additionally, the length of the optical path from the end face  2   a  of the first electrode to the end face  7   a  of the light emitting layer ranges from 101/200 to 88/200 of a multiple of a natural number of the designed peak wavelength. In addition, the length of the optical path from the end face  7   a  of the light emitting layer to the end face  11   a  of the third electrode ranges from 99/200 to 112/200 of a multiple of a natural number of the designed peak wavelength.  
         [0040]    The length of the optical path within the auxiliary electrode  12  is substantially equal to a multiple of a natural number of a half of the designed peak wavelength. Therefore, the phase of the light of the designed peak wavelength matches with that of the optical resonator, so that the light emission having the matched designed peak wavelength is more selectively strengthened.  
         [0041]    The first electrode and the second electrode in the light source in the present example were electrically short-circuited to set a control voltage Vc to zero, and a current was supplied with the application of a DC voltage of 14 V between the second electrode and the third electrode. In the case of the observation on the front axis of a light emitting face parallel to the normal of the light emitting face (that is, an observation angle θ was 0), the emission of green light having a light emitting peak wavelength of 525 nm and a half-value breadth of a light emitting peak of 40 nm was observed. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of green light having a light emitting peak wavelength of 493 nm and a half-value breadth of a light emitting peak of 30 nm was observed when the angle θ was 20°. Furthermore, the emission of weak blue-green light having a light emitting peak wavelength of about 450 nm was observed when the angle θ was 30°. Moreover, the light emission was hardly observed when the angle θ was 35° or more.  
         [0042]    Subsequently, while an AC voltage of 200 V as the control voltage was applied to the first electrode and the second electrode, the current was supplied with the application of a DC voltage of 14 V between the second electrode and the third electrode. In the case of the observation on the front axis of the light emitting face parallel to the normal of the light emitting face (that is, the observation angle θ was 0), the emission of green light having the light emitting peak wavelength shifted to 521 nm and a half-value breadth of a light emitting peak of 40 nm was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of green light having a light emitting peak wavelength of 489 nm and a half-value breadth of a light emitting peak of 25 nm was observed when the angle θ was 20 . Furthermore, the emission of very weak blue-green light having a light emitting peak wavelength of about 450 nm was observed when the angle θ was 30°. Moreover, no light emission was observed at all when the angle θ was 35° or more.  
         [0043]    Next, while a DC voltage of 200 V was applied between the first electrode and the second electrode, the current was supplied with the application of a DC voltage of 14 V between the second electrode and the third electrode. In the case of the observation on the front axis of the light emitting face parallel to the normal of the light emitting face (that is, the observation angle θ was 0), the shift of the light emitting peak wavelength to 515 nm was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of green light having a light emitting peak wavelength of 484 nm and a half-value breadth of a light emitting peak of 20 nm was observed when the angle θ was 20°. The light emission was hardly observed when the angle θ was 30° or more.  
       COMPARATIVE EXAMPLE 1  
       [0044]    Neither the first electrode  2  and nor the piezoelectric thin film  3  in Example 1 were formed, and a second electrode  4  was formed directly on a substrate  1 . On the second electrode  4 , layers were formed in the same manner as in Example 1, thus fabricating a light source for comparison. The constitution and thickness of each of the layers were the same as those in Example 1.  
         [0045]    A current was supplied with the application of a DC voltage of 14 V between the second electrode and a third electrode in the light source for comparison. In the case of the observation on the front axis of a light source parallel to the normal of a light emitting face, the emission of green light having a light emitting peak wavelength of 533 nm and a half-value breadth of a light emitting peak of 80 nm was observed. The light emitting intensity was as low as less than 40% in comparison with that in Example 1. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, no variation of the light emitting peak wavelength was observed when the angle θ ranged from 0° to 75°. Although it was difficult to measure a light emitting spectrum when the angle θ exceeded 75°, no variation of the emitted light color was observed even as viewed almost sideways.  
       EXAMPLE 2  
       [0046]    [0046]FIG. 2 is a schematic cross-sectional view showing a wavelength variable light source in another preferred embodiment. A thin film (having a thickness of 50 nm) made of gold serving as a first electrode  2  was formed on a (110) surface of a sapphire substrate serving as a substrate  1  by sputtering method. Subsequently, on the first electrode  2  was formed a thin film (having a thickness of 74 nm) as a piezoelectric thin film  3  made of zinc oxide (ZnO) by reactive sputtering method with an electron cyclotron resonance (abbreviated as “ECR”) plasma in an atmosphere of argon and oxygen. On the piezoelectric thin film  3 , an ITO thin film (having a thickness of 50 nm) was formed serving as a second electrode  4  by sputtering method.  
         [0047]    On the second electrode  4 , the same hole injecting layer  5  and the same hole transporting layer  6 , as those in Example 1, were formed.  
         [0048]    On the hole transporting layer  6 , a mixture light emitting layer (having a thickness of 35 nm) was formed by a vacuum evaporation method, as a light emitting layer  7  containing 92.5% by mass of Alq, 5% by mass of 5,6,11,12-tetraphenylnaphthacene (Rubrene) expressed below by a chemical formula and 2.5% by mass of 2-methyl-6-(2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) expressed by the following chemical formula.  
                         
 
         [0049]    The simplified molecular formula of Rubrene is expressed by C 42 H 28 , which has a mol mass of 532.68 g/mol. The simplified molecular formula of DCM2 is expressed by C 23 H 21 ON 3 , which has a mol mass of 355.43 g/mol.  
         [0050]    On the light emitting layer  7 , a layer (having a thickness of 142 nm) was formed as an electron transporting layer  9  made of pure substance of Alq. On the electron transporting layer  9 , a layer (having a thickness of 6 nm) was formed as an electron injecting layer  10  made of lithium oxide (Li 2 O) Both of the above-described layers were formed by a vacuum evaporation method.  
         [0051]    On the electron injecting layer  10 , a silver layer (having a thickness of 10 nm) was formed as a third electrode  11 ; and further, on the third electrode  11 , an ITO layer (having a thickness of 168 nm) was formed as an auxiliary electrode  12 .  
         [0052]    Table 3 shows below the material, refractive index, actual thickness and optical thickness of each of the layers.  
               TABLE 3                                                                                
 
         [0053]    Similarly to Table 2, Table 4 shows below the lengths of optical paths in the present example. In the present example, a designed peak wavelength is 620 nm, and further, a light emitting region is located at a position inward by 2.9 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
                                           TABLE 4                           Example 2       Light Emitting Region: Inward by 2.9 nm from End       Face 7a of Light Emitting Layer       Designed Peak wavelength: 620 nm                Length   Length           of the   of the           Optical   Optical Path/            Path   Designed Peak           (nm)   Wavelength                    From the End Face 2a of the 1st Electrode   619.9   200/200       to the End Face 11a of the 3rd Electrode       From the End Face 2a of the 1st Electrode   307.1    99/200       to the End Face 7a of the Light Emitting       Layer       From the End Face 2a of the 1st Electrode   310   100/200       to the Light Emitting Region       From the End Face 7a of the Light Emitting   312.8   101/200       Layer to the End Face 11a of the 3rd       Electrode       From the Light Emitting Region to the End   309.9   100/200       Face 11a of the 3rd Electrode       Within the Auxiliary Electrode 12   309.1   100/200                  
 
         [0054]    As shown in Table 4, also in the present example, the length of each of the optical paths is substantially equal to a multiple of a natural number of a half of the designed peak wavelength, like in Example 1.  
         [0055]    The first electrode and the second electrode in the light source in the present example were electrically short-circuited, and a current was supplied with the application of a DC voltage of 15 V between the second electrode and the third electrode. In the case of observation on the front axis of the light source parallel to the normal of a light emitting face, the emission of red light having a light emitting peak wavelength of 620 nm and a half-value breadth of a light emitting peak of 60 nm was observed. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of yellow light having a light emitting peak wavelength of 585 nm and a half-value breadth of a light emitting peak of 50 nm was observed when the angle θ was 20°. Furthermore, the emission of very weak green light having a light emitting peak wavelength of about 540 nm was observed when the angle θ was 30°. Moreover, no light emission was observed at all when the angle θ was 35° or more.  
         [0056]    Subsequently, while an AC voltage of 200 V was applied between the first electrode and the second electrode, the current was supplied with the application of a DC voltage of 14 V between the second electrode and the third electrode. In the case of the observation on the front axis of the light source parallel to the normal of the light emitting face, the shift of the light emitting peak wavelength to 611 nm was observed according to the application of the AC voltage. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of yellow light having a light emitting peak wavelength of 574 nm and a half-value breadth of a light emitting peak of 45 nm was observed when the angle θ was 20°. Furthermore, the emission of very weak green light having a light emitting peak wavelength of about 530 nm was observed when the angle θ was 30°. Moreover, no light emission was observed at all when the angle θ was 35° or more.  
       COMPARATIVE EXAMPLE 2  
       [0057]    Neither the first electrode  2  and nor the piezoelectric thin film  3  in Example 2 were formed, and a second electrode  4  was formed directly on a substrate  1 . On the second electrode  4 , layers were formed in the same manner as in Example 2, thus fabricating a light source for comparison. Here, the constitution and thickness of each of the layers were the same as those in Example 2.  
         [0058]    A current was supplied with the application of a DC voltage of 15 V between the second electrode and a third electrode in the light source. Then, the emission of slightly orangy red light having a light emitting peak wavelength of 645 nm and a half-value breadth of a light emitting peak of 100 nm was observed. The light emitting intensity was as low as less than 40% in. comparison with that in Example 2. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, no variation of the light emitting peak wavelength was observed when the angle θ ranged from 0° to 75°. Although it was difficult to measure a light emitting spectrum when the angle θ exceeded 75°, no variation of the emitted light color was observed even as viewed almost sideways.  
       EXAMPLE 3  
       [0059]    [0059]FIG. 3 is a schematic cross-sectional view showing a wavelength variable light source in a further preferred embodiment. A thin film (having a thickness of 50 nm) made of gold as a first electrode  2  was formed on a (110) surface of a sapphire substrate as a substrate  1  by sputtering method. Subsequently, on the first electrode  2 , a thin film (having a thickness of 50 nm) made of zinc oxide (ZnO) was formed as a piezoelectric thin film  3  by the same reactive sputtering method as in Example 2. On the piezoelectric thin film  3 , an ITO thin film (having a thickness of 48 nm) was formed as a second electrode  4  by sputtering method. Thereafter, a hole transporting layer  6  (having a thickness of 20 nm) made of NPB was formed.  
         [0060]    On the hole transporting layer  6 , a mixture light emitting layer (having a thickness of 30 nm) was formed as a light emitting layer  7  containing 94% by mass of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) expressed below by a chemical formula and 6.0% by mass of iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′)picolinato (IrX) expressed by the following chemical formula.  
                         
 
         [0061]    Subsequently, on the light emitting layer  7 , a hole inhibitable electron transporting layer  8  (having a thickness of 10 nm) was formed, that is made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) expressed by the following chemical formula.  
                         
 
         [0062]    Thereafter, an electron transporting layer  9  (having a thickness of 85 nm) made of Alq was formed, and further, on the electron transporting layer  9  was formed an electron injecting layer  10  (having a thickness of 6 nm) made of Li 2 O. Both of the above-described layers were formed by a vacuum evaporation method.  
         [0063]    Subsequently, a third electrode  11  (having a thickness of 10 nm) made of Ag and an auxiliary electrode  12  (having a thickness of 117 nm) made of ITO were formed in sequence by sputtering method.  
         [0064]    The simplified molecular formula of CBP is expressed by C 36 H 24 N 2 , which has a mol mass of 484.60 g/mol. The simplified molecular formula of BCP is expressed by C 26 H 20 N 2 , which has a mol mass of 360.45 g/mol. The simplified molecular formula of IrX is expressed by C 28 H 16 N 3 O 2 Ir, which has a mol mass of 502.44 g/mol.  
         [0065]    Table 5 shows below the material, refractive index, actual thickness and optical thickness of each of the layers.  
               TABLE 5                                                                                
 
         [0066]    A designed peak wavelength of the wavelength variable light source in the present example is 470 nm. Furthermore, a light emitting region is located at a position inward by 2.3 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
         [0067]    Similarly to Table 2 in Example 1, Table 6 shows below the lengths of optical paths in the present example.  
                                           TABLE 6                           Example 3       Light Emitting Region: Inward by 2.3 nm from End       Face 7a of Light Emitting Layer       Designed Peak wavelength: 470 nm                Length   Length           of the   of the            Optical   Optical Path/            Path   Designed Peak           (nm)   Wavelength                    From the End Face 2a of the 1st Electrode   469.7   200/200       to the End Face 11a of the 3rd Electrode       From the End Face 2a of the 1st Electrode   232.6    99/200       to the End Face 7a of the Light Emitting       Layer       From the End Face 2a of the 1st Electrode   234.9   100/200       to the Light Emitting Region       From the End Face 7a of the Light Emitting   237.1   101/200       Layer to the End Face 11a of the 3rd       Electrode       From the Light Emitting Region to the End   234.8   100/200       Face 11a of the 3rd Electrode       Within the Auxiliary Electrode 12   234.0   100/200                  
 
         [0068]    As shown in Table 6, the length of each of the optical paths is substantially equal to a multiple of a natural number of a half of the designed peak wavelength.  
         [0069]    The first electrode and the second electrode in the light source in the present example were electrically short-circuited, and a current was supplied with the application of a DC voltage of 12 V between the second electrode and the third electrode. In the case of observation on the front axis of the light source parallel to the normal of a light emitting face, the emission of blue light having a light emitting peak wavelength of 470 nm and a half-value breadth of a light emitting peak of 35 nm was observed. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of blue light having a light emitting peak wavelength of 441 nm and a half-value breadth of a light emitting peak of 30 nm was observed when the angle θ was 20°. Furthermore, the emission of very weak blue light was observed when the angle θ was 30°, wherein it was difficult to measure a light emitting spectrum. Moreover, no light emission was observed at all when the angle θ was 35° or more.  
         [0070]    Subsequently, while an AC voltage of 200 V was applied between the first electrode and the second electrode, the current was supplied with the application of a DC voltage of 14 V between the second electrode and the third electrode. In the case of the observation on the front axis of the light source parallel to the normal of the light emitting face, the shift of the light emitting peak wavelength to 465 nm was observed according to the application of the AC voltage. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, the emission of blue light having a light emitting peak wavelength of 437 nm and a half-value breadth of a light emitting peak of 20 nm was observed when the angle θ was 20°. Furthermore, no light emission was observed at all when the angle θ was 30° or more.  
       COMPARATIVE EXAMPLE 3  
       [0071]    Neither the first electrode  2  and nor the piezoelectric thin film  3  in Example 3 were formed, and a second electrode  4  was formed directly on a substrate  1 . On the second electrode  4 , layers were formed in the same manner as in Example 3, thus fabricating a light source for comparison. The constitution and thickness of each of the layers were the same as those in Example 3.  
         [0072]    A current was supplied with the application of a DC voltage of 12 V between the second electrode and a third electrode in the light source. In the case of the observation on the front axis of the light source parallel to the normal of a light emitting face, the emission of considerably cyanic blue light having a light emitting peak wavelength of 480 nm and a half-value breadth of a light emitting peak of 70 nm was observed. The light emitting intensity was as low as less than 40% in comparison with that in Example 3. Even in the case where the voltage to be applied between the second electrode and the third electrode was varied, no variation of the light emitting peak wavelength was observed. In the case of the observation in a direction inclined by the angle θ from the normal of the light emitting face, no variation of the light emitting peak wavelength was observed when the angle θ ranged from 0° to 75°. Although it was difficult to measure a light emitting spectrum when the angle θ exceeded 75°, no variation of the emitted light color was observed even as viewed almost sideways.  
         [0073]    [0073]FIG. 4 is a schematic diagram illustrating a high-speed optical communication system by the use of the wavelength variable light source according to the present invention. As illustrated in FIG. 4, a plurality of wavelength variable light sources  20   a  is disposed, thereby configuring a transmitting device  20 . Furthermore, a plurality of photodiodes  21   a  are disposed at positions corresponding to the wavelength variable light sources  20   a , respectively, thereby configuring a receiving device  21 . The wavelength of a light emitting spectrum to be emitted from the wavelength variable light source  20   a  can be varied by providing the refractive index modulating means in the wavelength variable light source  20   a  with variations in voltage corresponding to an electric signal. The photodiode  21   a  receives the variation of the light emitting spectrum, thereby converting an optical signal sent from the wavelength variable light source  20   a  into an electric signal.  
         [0074]    The high-speed optical communication device illustrated in FIG. 4 is merely one example of communication devices using the wavelength variable light source according to the present invention. In other words, the wavelength variable light source according to the present invention is not limited to the device illustrated in FIG. 4.  
       EXAMPLE 4  
       [0075]    A light source having a structure shown below in Table 7 was fabricated in the same process as in Example 1.  
                                                                                     TABLE 7                                                       Length of                                       the                               Total   Total   Optical                               Length of   Length of   Path                               the   the   Between a                       Actual   Optical   Optical   Optical   Main       Ref.           Refractive   Thickness   Thickness   Paths   Paths   Reflectors       No.       Material   Index   (nm)   (nm)   (nm)   (nm)   (nm)                                 1   Substrate   Glass                                2   First Electrode   Al   —   200.0   —        2a   End Face                       (2a)    (2a)         3   Refractive Index   AIN   1.9   76.3   145.0       432.3   580.0           Modulating           Means        4   Second   ITO   1.93   75.1   145.0   287.3           Electrode        5   Hole Injecting   CuPc   1.76   9.0   15.8           Layer        6   Hole   NPB   1.76   71.9   126.5           Transporting           Layer        7a   Interface                       (7a)         7   Light Emitting   Alq   1.76   83.1   146.3       147.7           Layer       10   Electron   LiF   1.39   1.0   1.4           Injecting Layer       11a   Interface                       (11a)   (11a)       11   Third Electrode   Al   —   10.0   —       12   Auxiliary   ITO   1.93   150.3   290.0       290.0           Electrode                  
 
         [0076]    A designed peak wavelength in the present example is 580 nm. Furthermore, a light emitting region is located at a position inward by 2.7 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
       EXAMPLE 5  
       [0077]    A light source having a structure shown below in Table 8 was fabricated in the same process as in Example 2.  
                                                                                     TABLE 8                                               Total   Total   Total                       Actual   Optical   Length of   Length of   Length of       Ref.           Refractive   Thickness   Thickness   the Optical   the Optical   the Optical       No.       Material   Index   (nm)   (nm)   Paths (nm)   Paths (nm)   Paths (nm)                                 1   Substrate   Glass                                2   First   Au   —   200.0   —   —   —   —           Electrode        2a   Interface                       (2a)    (2a)         3   Refractive   ZnO   2   95.0   190.0       362.9   720.2           Index           Modulating           Means        4   Second   ITO   1.84   75.6   139.1   172.9           Electrode        5   Hole   CuPc   1.72   8.0   13.8           Injecting           Layer        6   Hole   NPB   1.72   11.6   20.0           Transporting           Layer        7a   Interface                       (7a)         7   Light   Alq +   1.72   8.7   15.0   169.0   357.3           Emitting   Rubrene +           Layer   DCM2        9   Electron   Alq   1.72   89.5   154.0           Transporting           Layer       10   Electron   Li 2 O   1.39   6.0   8.3           Injecting           Layer       12   Auxiliary   ITO   1.84   97.8   180.0           Electrode       11a   Interface                       (11a)   (11a)       11   Third   Ag   —   10.0   —   —   —   —           Electrode                  
 
         [0078]    A designed peak wavelength in the present example is 720 nm. Furthermore, a light emitting region is located at a position inward by 2.9 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
       EXAMPLE 6  
       [0079]    Dielectric layers  21  to  24 , which were different in refractive index from each other, were laminated on a glass substrate  1 , thereby obtaining a dielectric multi-layer mirror. An ITO layer as an electrode  2  layer was formed by sputtering method.  
         [0080]    Thereafter, a light source having a structure shown below in Table 9 was fabricated in the same process as in Example 5.  
                                                                                     TABLE 9                                               Total   Total   Total                       Actual   Optical   Length of   Length of   Length of       Ref.           Refractive   Thickness   Thickness   the Optical   the Optical   the Optical       No.       Material   Index   (nm)   (nm)   Paths (nm)   Paths (nm)   Paths (nm)                                 1   Substrate   Glass                               24   Low Refractive   SiO 2     1.46   119.9   175.0           Index Layer       23   High Refractive   TiO 2     2.35   74.5   175.0           Index Layer       22   Low Refractive   SiO 2     1.46   119.9   175.0           Index Layer       21   High Refractive   TiO 2     2.35   74.5   175.0           Index Layer        2   First Electrode   ITO   1.84   97.8   180.0        2a   Interface                       (2a)    (2a)         3   Refractive Index   ZnO   2   95.0   190.0       362.9   720.2           Modulating Means        4   Second Electrode   ITO   1.84   75.6   139.1   172.9        5   Hole Injecting   CuPc   1.72   8.0   13.8           Layer        6   Hole Transporting   NPB   1.72   11.6   20.0           Layer        7a   Interface                       (7a)         7   Light Emitting   Alq + Rubre   1 .72   8.7   15.0   169.0   357.3           Layer   ne + DCM2        9   Electron   Alq   1.72   89.5   154.0           Transporting Layer       10   Electron Injecting   Li 2 O   1.39   6.0   8.3           Layer       12   Auxiliary Electrode   ITO   1.84   97.8   180.0       11a   Interface                       (11a)   (11a)       11   Third Electrode   Ag   —   10.0   —   —   —   —                  
 
         [0081]    A designed peak wavelength in the present example is 720 nm. Furthermore, a light emitting region is located at a position inward by 2.9 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
       EXAMPLE 7  
       [0082]    A substrate  1  was made of quartz glass. An electrode  2   b  was formed by depositing a silver thin film in the vacuum by an electron beam heating method.  
         [0083]    Subsequently, on the electrode  2   b , a zinc oxide (ZnO) layer was formed as a piezoelectric thin film  3   b  by laser ablation method using a KrF excimer pulse laser having a wavelength of 248 nm. At that time, the partial pressure of oxygen was set within the range of 10 −6  Pa to 10 −7  Pa, and further, the temperature of the substrate was set to 600° C. The temperature of the substrate was able to be set variably within the range of 500° C. to 700° C. As a result of an analysis of the resultant zinc oxide (ZnO) layer by X-ray diffraction, it was confirmed that the c-axes of zinc oxide were aligned in a direction perpendicular to the surface of the substrate.  
         [0084]    Subsequently, on the piezoelectric thin film  3   b , an ITO layer was formed as an auxiliary electrode  2   c  by sputtering method.  
         [0085]    Next, on the auxiliary electrode  2   c , a silver thin film was formed as a first electrode  2  by sputtering method.  
         [0086]    Thereafter, on the electrode  2 , a zinc oxide layer was formed as a piezoelectric thin film  3  by the laser ablation method in the same manner as described above.  
         [0087]    And then, on the piezoelectric thin film  3 , an ITO layer was formed as an electrode  4  by sputtering method.  
         [0088]    Thereafter, a light source having a structure shown below in Table 10 was fabricated in the same process as in Example 3. IrX for use in a light emitting layer  7  was replaced by an organic metal compound ReX containing rhenium therein, expressed by the following chemical formula.  
                                                                                           TABLE 10                                                                                                          Total   Total   Total                       Actual   Optical   Length of   Length of   Length of       Ref.           Refractive   Thickness   Thickness   the Optical   the Optical   the Optical       No.       Material   Index   (nm)   (nm)   Paths (nm)   Paths (nm)   Paths (nm)                    1   Substrate   Quartz                                       Glass       2b   Electrode 2b   Ag   —   100.0   —   —   —   —       3b   Refractive Index   ZnO   2   67.5   135.0       265.0           Modulating           Means       2c   Auxiliary   ITO   2   65.0   130.0           Electrode       2   First Electrode   Ag   —   10.0   —   —   —   —       2a   Interface                       (2a)   (2a)       3   Refractive Index   ZnO   2   67.5   135.0       263.0   530.0           Modulating           Means       4   Second   ITO   2   49.0   98.0   128.0           Electrode       6   Hole   NPB   1.83   16.4   30.0           Transporting           Layer       7a   Interface                       (7a)       7   Light Emitting   CBP +   1.83   60.3   110.4       267.0           Layer   ReX       8   Hole Inhibitable   BCP   1.83   10.0   18.3   156.6           Electron           Transporting           Layer       9   Electron   Alq   1.83   71.0   130.0           Transporting           Layer       10   Electron Injecting   Li 2 O   1.39   6.0   8.3           Layer       11a   Interface                       (11a)    (11a)        11   Third Electrode   Ag   —   10.0   —   —   —   —       12   Auxiliary   ITO   2   65.5   131.0       131.0           Electrode                  
 
         [0089]    A designed peak wavelength of a wavelength variable light source in the present example is 530 nm. Furthermore, alight emitting region is located at a position inward by 2.0 nm of the optical thickness from the end face  7   a  of the light emitting layer.  
         [0090]    (Light Emitting Characteristics)  
         [0091]    Light emitting characteristics in each of the above-described examples and comparative examples are shown below in Tables 11 to 14.  
                                                                             TABLE 11                                       Light   Half-Value                               Emitting   Breadth of           Control   Control       Peak   Light           Voltage   Voltage   θ   Wavelength   Emitting   Relative   Emitted Light           Vc/V   Vc   (°)   (nm)   Peak (nm)   Luminance   Color                                Ex. 1   0   none   0   525   40   1   Green       Ex. 1   0   none   10   517   39   0.9   Green       Ex. 1   0   none   20   493   30   0.6   Green       Ex. 1   0   none   30   450   Cannot be   0.1   Blue-Green                           Measured       Ex. 1   0   none   35   Cannot be   Cannot be   0.01   Cannot be                       Measured   Measured       Measured       Ex. 1   200   a.c.   0   521   40   1   Green       Ex. 1   200   a.c.   10   513   38   0.85   Green       Ex. 1   200   a.c.   20   489   25   0.5   Green       Ex. 1   200   a.c.   30   450   Cannot be   0.1   Blue-Green                           Measured       Ex. 1   200   a.c.   35   No Light   No Light   0   No Light Emission                       Emission   Emission       Ex. 1   200   d.c.   0   515   35   0.95   Green       Ex. 1   200   d.c.   10   507   34   0.8   Green       Ex. 1   200   d.c.   20   484   20   0.3   Green       Ex. 1   200   d.c.   30   Cannot be   Cannot be   0.05   Cannot be                       Measured   Measured       Measured       Ex. 1   200   d.c.   35   No Light   No Light   0   No Light Emission                       Emission   Emission       Comp. Ex. 1   none   none   0   533   80   1   Green       Comp. Ex. 1   none   none   45   533   80   0.6   Green       Comp. Ex. 1   none   none   75   533   80   0.2   Green       Ex. 4   0   none   0   550   25   0.8   Green       Ex. 4   0   none   10   540   30   0.9   Green       Ex. 4   0   none   20   530   40   1   Green       Ex. 4   0   none   30   520   40   1   Green       Ex. 4   0   none   35   480   18   0.7   Green       Ex. 4   0   none   40   460   15   0.5   Blue-Green       Ex. 4   0   none   45   450   Cannot be   0.1   Blue-Green                           Measured       Ex. 4   0   none   50   No Light   No Light   0   No Light Emission                       Emission   Emission       Ex. 4   100   d.c.   0   535   35   0.9   Green       Ex. 4   100   d.c.   10   530   40   1   Green       Ex. 4   100   d.c.   20   515   38   0.9   Green       Ex. 4   100   d.c.   30   470   15   0.3   Blue-Green       Ex. 4   100   d.c.   35   450   Cannot be   0.1   Blue-Green                           Measured       Ex. 4   100   d.c.   40   No Light   No Light   0   No Light Emission                       Emission   Emission       Comp. Ex.4   none   none   0   533   80   1   Green       Comp. Ex.4   none   none   45   533   80   0.6   Green       Comp. Ex.4   none   none   75   533   80   0.2   Green                  
 
         [0092]    [0092]                                                                             TABLE 12                                           Half-Value                               Light Emitting   Breadth of           Control   Control       Peak   Light           Voltage   Voltage   θ   Wavelength   Emitting   Relative   Emitted Light           Vc/V   Vc   (°)   (nm)   Peak (nm)   Luminance   Color                                Ex. 2   0   none   0   620    60   1   Red       Ex. 2   0   none   10   611    55   0.8   Red       Ex. 2   0   none   20   585    50   0.5   Yellow       Ex. 2   0   none   30   540   Cannot be   0.1   Green                           Measured       Ex. 2   0   none   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 2   200   a.c.   0   611    60   1   Yellow       Ex. 2   200   a.c.   10   602    50   0.7   Yellow       Ex. 2   200   a.c.   20   574    45   0.2   Yellow       Ex. 2   200   a.c.   30   530   Cannot be   0.1   Green                           Measured       Ex. 2   200   a.c.   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 2   100   d.c.   0   605    50   0.8   Yellow       Ex. 2   100   d.c.   10   600    45   0.6   Yellow       Ex. 2   100   d.c.   20   570   Cannot be   0.1   Yellow                           Measured       Ex. 2   100   d.c.   30   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 2   100   d.c.   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Comp. Ex.2   none   none   0   645   100   1   Red       Comp. Ex.2   none   none   45   645   100   0.6   Red       Comp. Ex.2   none   none   75   645   100   0.2   Red       Comp. Ex.5   none   none   0   645   100   1   Red       Comp. Ex.5   none   none   45   645   100   0.6   Red       Comp. Ex.5   none   none   75   645   100   0.2   Red                    
         [0093]    [0093]                                                                             TABLE 13                                       Light   Half-Value                               Emitting   Breadth of           Control   Control       Peak   Light           Voltage   Voltage   θ   Wavelength   Emitting   Relative   Emitted Light           Vc/V   Vc   (°)   (nm)   Peak (nm)   Luminance   Color                                Ex. 5   0   none   0   710   30   0.6   Red       Ex. 5   0   none   10   700   40   0.7   Red       Ex. 5   0   none   20   680   55   1   Red       Ex. 5   0   none   30   640   60   1   Red       Ex. 5   0   none   35   620   50   0.6   Red       Ex. 5   0   none   40   600   30   0.2   Red       Ex. 5   0   none   45   Cannot be   Cannot be   0.05   Red                       Measured   Measured       Ex. 5   0   none   50   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 5   100   d.c.   0   680   60   1   Red       Ex. 5   100   d.c.   10   680   60   1   Red       Ex. 5   100   d.c.   20   650   60   0.7   Red       Ex. 5   100   d.c.   30   600   40   0.2   Red       Ex. 5   100   d.c.   35   580   Cannot be   0.1   Red                           Measured       Ex. 5   100   d.c.   40   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 6   0   none   0   710   30   0.5   Red       Ex. 6   0   none   10   700   35   0.6   Red       Ex. 6   0   none   20   680   50   1   Red       Ex. 6   0   none   30   640   50   0.9   Red       Ex. 6   0   none   35   620   50   0.5   Red       Ex. 6   0   none   40   600   30   0.1   Red       Ex. 6   0   none   45   Cannot be   Cannot be   0.05   Red                       Measured   Measured       Ex. 6   0   none   50   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 6   100   d.c.   0   680   55   1   Red       Ex. 6   100   d.c.   10   680   55   0.9   Red       Ex. 6   100   d.c.   20   650   55   0.6   Red       Ex. 6   100   d.c.   30   600   35   0.1   Red       Ex. 6   100   d.c.   35   Cannot be   Cannot be   0.05   Red                       Measured   Measured       Ex. 6   100   d.c.   40   No Light   No Light   0   No Light                       Emission   Emission       Emission       Comp. Ex.6   none   none   0   645   100   1   Red       Comp. Ex.6   none   none   45   645   100   0.6   Red       Comp. Ex.6   none   none   75   645   100   0.2   Red                    
         [0094]    [0094]                                                                             TABLE 14                                       Light   Half-Value                               Emitting   Breadth of           Control   Control       Peak   Light           Voltage   Voltage   θ   Wavelength   Emitting   Relative   Emitted Light           Vc/V   Vc   (° )   (nm)   Peak (nm)   Luminance   Color                                Ex. 3   0   none   0   470   35   1   Blue       Ex. 3   0   none   10   465   35   0.8   Blue       Ex. 3   0   none   20   441   30   0.5   Blue       Ex. 3   0   none   30   Cannot be   Cannot be   0.1   Blue                       Measured   Measured       Ex. 3   0   none   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 3   200   a.c.   0   465   35   1   Blue       Ex. 3   200   a.c.   10   460   35   0.7   Blue       Ex. 3   200   a.c.   20   437   20   0.3   Blue       Ex. 3   200   a.c.   30   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 3   200   a.c.   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 3   100   d.c.   0   450   30   0.8   Blue       Ex. 3   100   d.c.   10   445   25   0.5   Blue       Ex. 3   100   d.c.   20   435   20   0.1   Blue       Ex. 3   100   d.c.   30   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 3   100   d.c.   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Comp. Ex. 3   none   none   0   480   70   1   Blue       Comp. Ex. 3   none   none   75   480   70   0.2   Blue       Comp. Ex. 7   none   none   0   460   50   1   Blue       Comp. Ex. 7   none   none   75   460   50   0.2   Blue       Ex. 7   0   none   0   500   50   0.8   Blue       Ex. 7   0   none   10   480   40   0.9   Blue       Ex. 7   0   none   20   460   30   1   Blue       Ex. 7   0   none   30   460   30   1   Blue       Ex. 7   0   none   35   440   25   0.8   Blue       Ex. 7   0   none   40   420   20   0.4   Blue       Ex. 7   0   none   45   400   Cannot be   0.1   Blue                           Measured       Ex. 7   0   none   50   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 7   100   d.c.   0   460   35   1   Blue       Ex. 7   100   d.c.   10   460   30   1   Blue       Ex. 7   100   d.c.   20   445   25   0.9   Blue       Ex. 7   100   d.c.   30   400   Cannot be   0.1   Blue                           Measured       Ex. 7   100   d.c.   35   No Light   No Light   0   No Light                       Emission   Emission       Emission       Ex. 7   100   d.c.   40   No Light   No Light   0   No Light                       Emission   Emission       Emission                    
         [0095]    As described above, according to the present invention, it is possible to reduce the light emitting spectrum width and readily vary the light emitting wavelength in the light source for allowing the organic substance in the organic EL element to emit the light. Thus, the wavelength variable light source according to the present invention can be used in the high-speed optical communications or the like.