Abstract:
In an optical amplifier including a metal layer having an incident/reflective surface adapted to receive incident light and output its reflective light, and a dielectric layer formed on an opposite surface of the metal layer opposing the incident/reflective surface, the incident light excites surface plasmon resonance light in the metal layer while the dielectric layer is excited, so that an extinct ion coefficient of the dielectric layer is made negative.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1 Field of the Invention 
         [0002]    The present invention relates to an optical amplifier capable of directly amplifying an optical signal in an optical communication system and its manufacturing method. 
         [0003]    2. Description of the Related Art 
         [0004]    As optical amplifiers, a prior art optical fiber amplifier and a prior art optical waveguide-type amplifier have been known (see: JP 4-46034 A and JP 4-213884 A). The prior art optical fiber amplifier and the prior art optical waveguide-type amplifier will be explained later in detail. 
         [0005]    In the prior art optical fiber amplifier and the prior art optical waveguide-type amplifier, however, the intensity of an output light signal is small. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention seeks to solve one or more of the above-described problems. 
         [0007]    According to the present invention, an optical amplifier is constructed by a metal layer having an incident/reflective surface adapted to receive incident light and output its reflective light; and a dielectric layer formed on an opposite surface of the metal layer opposing the incident/reflective surface, the incident light exciting surface plasmon resonance light in the metal layer while the dielectric layer is excited so that an extinction coefficient of the dielectric layer is made negative. 
         [0008]    Also, an incident angle of the incident light to the metal layer is a light absorption dip angle by which a reflectivity of the incident light at the incident/reflective surface of the metal layer is minimum in a total reflection region while the dielectric layer is not excited. Thus, the excited amount of surface plasmon resonance photons is maximum. 
         [0009]    Further, a thickness of the metal layer is determined so that the reflectivity of the incident light at the incident/reflective surface of the metal layer is minimum when the incident light is incident at the light absorption dip angle to the incident/reflective surface of the metal layer while the dielectric layer is not excited. Thus, the excited amount of surface plasmon resonance photons is maximum. 
         [0010]    The metal layer comprises a gold (Au) layer. 
         [0011]    The dielectric layer comprises an organic dye layer or a semiconductor layer. 
         [0012]    Further, a resonator layer is deposited on an opposite surface of the dielectric layer opposing the metal layer. The resonator layer comprises a silver (Ag) layer or a dielectric multi-layer mirror structure. 
         [0013]    Also, according to the present invention, in a method for manufacturing an optical amplifier, comprising a metal layer having an incident/reflective surface adapted to receive incident light and output its reflective light; and a dielectric layer deposited on an opposite surface of the metal layer opposing the incident/reflective surface, the incident light exciting surface plasmon resonance light in the metal layer while the dielectric layer is excited so that an extinction coefficient of the dielectric layer is made negative, a thickness of the metal layer is determined, so that the reflectivity of the incident light at the incident/reflective surface of the metal layer is minimum in a total reflection region when the incident light is incident to the incident/reflective surface of the metal layer while the dielectric layer is not excited. Then, at least one candidate of an incident angle of the incident light to the metal layer and a thickness of the dielectric layer is determined, so that the reflectivity of the incident light at the incident/reflective surface of the metal layer is minimum in the total reflection region while the determined thickness of the metal layer is maintained and the dielectric layer is not excited. Finally, a negative extinction coefficient of the dielectric layer corresponding to an excited state of the dielectric layer is determined, so that the reflectivity of the incident light at the incident angle of the candidate is beyond a predetermined value while the thickness of the dielectric layer of the candidate is maintained. 
         [0014]    Further, according to the present invention, in a method for manufacturing an optical amplifier including a metal layer having an incident/reflective surface adapted to receive incident light and output its reflective light, a dielectric layer deposited on an opposite surface of the metal layer opposing the incident/reflective surface, and a resonator layer deposited on an opposite surface of the dielectric layer opposing the metal layer, the incident light exciting surface plasmon resonance light in the metal layer while the dielectric layer is excited so that an extinction coefficient of the dielectric layer is made negative, a thickness of the resonator layer is determined in accordance with a predetermined transmittivity of the resonator layer. Then, a thickness of the dielectric layer is determined. Then, an incident angle of the incident light at the incident/reflective surface of the metal layer and a thickness of the metal layer a redetermined, so that the reflectivity of the incident light at the incident/reflective surface of the metal layer is minimum in a total reflection region when the incident light is incident to the incident/reflective surface of the metal layer while the dielectric layer is not excited. Finally, a negative extinction coefficient of the dielectric layer corresponding to an excited state of the dielectric layer is determined, so that the reflectivity of the incident light at the incident angle is beyond a predetermined value while the incident angle of the incident light at the incident/reflective surface of the metal layer and the thickness of the dielectric layer are maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments, as compared with the prior art, taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a block diagram illustrating a prior art optical fiber amplifier; 
           [0017]      FIG. 2  is a block diagram illustrating a prior art optical waveguide-type amplifier; 
           [0018]      FIG. 3  is a cross-sectional view illustrating a first embodiment of the optical amplifier according to the present invention; 
           [0019]      FIG. 4  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 3 ; 
           [0020]      FIG. 5  is an attenuated total reflection (ATR) signal spectrum diagram for explaining determination of the thickness t 2  of the Au layer at step  401  of  FIG. 4 ; 
           [0021]      FIG. 6  is an ATR signal spectrum diagram for explaining candidates (the incident angle θ, the thickness t 3  of the organic dye layer) at step  402  of  FIG. 4 ; 
           [0022]      FIGS. 7A and 7B  are ATR signal spectrum diagrams for explaining the reflectivity R max  at the selected incident angle at step  406  of  FIG. 4 ; 
           [0023]      FIGS. 8A and 8B  are ATR signal spectrum diagrams for explaining the reflectivity R max  at the selected incident angle at step  406  of  FIG. 4 ; 
           [0024]      FIG. 9  is a cross-sectional view illustrating a second embodiment of the optical amplifier according to the present invention; 
           [0025]      FIG. 10  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 9 ; 
           [0026]      FIG. 11  is an ATR signal spectrum diagram for explaining candidates (the incident angle θ, the thickness t 3a  of the GaAs layer) at step  1002  of  FIG. 10 ; 
           [0027]      FIG. 12  is an ATR signal spectrum diagram for explaining the reflectivity R max  at the selected incident angle at step  1006  of  FIG. 10 ; 
           [0028]      FIG. 13  is an enlargement of the reflection spectrum of  FIG. 12 ; 
           [0029]      FIG. 11  is a cross-sectional view illustrating a third embodiment of the optical amplifier according to the present, invention; 
           [0030]      FIG. 15  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 14 . 
           [0031]      FIG. 16  is an ATR signal spectrum diagram for explaining (the incident angle θ, the thickness  12  of the Au layer) at step  1503  of  FIG. 15 ; 
           [0032]      FIG. 17  is an ATR signal spectrum diagram for explaining the reflectivity R max  at the selected incident angle at step  1505  of  FIG. 15 ; 
           [0033]      FIG. 18A  is a cross-sectional view of a modification of the Au layer of  FIGS. 3 ,  9  and  14 ; and 
           [0034]      FIG. 18B  is a plan view of the modification of  FIG. 18A . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    Before the description of the preferred embodiments, prior art optical amplifiers will now be explained with reference to  FIGS. 1 and 2 . 
         [0036]    In  FIG. 1 , which illustrates a prior art optical fiber amplifier (see: JP-4-46034 A), reference numeral  101  designates a signal light receiving section for receiving signal light,  102  designates an excitation light generating section formed by a semiconductor laser for generating excitation light or pumping light,  103  designates an optical fiber coupler coupling an optical fiber  101   a  propagating the signal light to an optical fiber  102   a  propagating the excitation light, and  104  designates a rare earth metal-doped optical fiber or erbium-doped optical fiber for amplifying the signal light with the excitation light. In the rare earth metal-doped optical fiber of  FIG. 1 , the amplification of the signal light is carried out by using a transition between excitation levels of population inversion of the rare earth metal excited by the excitation light. 
         [0037]    In  FIG. 2 , which illustrates a prior art optical waveguide-type amplifier (see: JP-4-213884 A), reference numeral  201  designates a signal light receiving section for receiving signal light, and  202  designates an optical circuit board formed by an optical waveguide  202   a  in which an AlGaAs semiconductor optical amplifier  202   b  is buried. In the optical waveguide-type amplifier of  FIG. 2 , the signal light is amplified by a current injection. 
         [0038]    In the optical amplifier of  FIG. 1  and the optical waveguide-type amplifier of  FIG. 2 , an output light intensity I out  is represented by 
         [0039]    I out =I in exp (α d) 
         [0040]    where I in  is the intensity of the signal light; 
         [0041]    α is a gain coefficient (cm −1 ); and 
         [0042]    d is an interaction length (cm). 
         [0043]    In the optical amplifier of  FIG. 1 , although the interaction length d can be large, the gain coefficient α is small or about 0.01 to 0.001, so that the output light intensity I out  is small. 
         [0044]    In the optical waveguide-type amplifier of  FIG. 2 , although the gain coefficient α can be large or about 100 to 10000, the interaction length d is small, so that the output light intensity I out  is small. 
         [0045]    In  FIG. 3 , which illustrates a first embodiment of the optical amplifier according to the present invention, this optical amplifier is constructed by a common glass prism or a BK-7 prism  1  as a transparent body for visible laser rays with a refractive index n 1  of 1.535 and a vertical angle of 90°, a gold (Au) layer  2  as a metal layer deposited by an evaporating process or the like on a surface  12  of the BK-7 prism  1  opposing the arris  11  thereof, and an organic dye layer  3  made of rhodamine B, for example, as a dielectric layer deposited on a surface S 2  of the Au layer  2  opposing the incident/reflective surface S 1  thereof. 
         [0046]    The Au layer  2  is about 1 cm long and about 10 nm to 10 μm thick. If the thickness t 2  of the Au layer  2  is smaller than 10 nm, the Au layer  2  cannot sufficiently absorb evanescent photons generated therein. On the other hand, if the thickness t 2  of the Au layer  2  is larger than 10 μm, the generation of evanescent photons in the Au layer  2  is attenuated, so as not to excite SPR photons in the Au layer  2 . 
         [0047]    Note that an about 1 to 2 nm thick metal layer made of Cr or the like may be deposited on the surface of the BK-7 prism  1  to enhance the contact characteristics between the Au layer  2  and the BK-7 prism  1 . 
         [0048]    An antireflection (AR) coating layer  4  is coated on a surface  13  of the BK-7 prism  1 . In this case, the arris  11  of the BK-7 prism  1  is formed by the surface  13  as well as a surface  14 . Note that, if the incident loss by the reflectivity such as 8% of the BK-7 prism  1  is negligible, the AR coating layer  4  can be omitted. 
         [0049]    Further, a He—Ne laser source  5  and a wavelength plate  6  are provided. As a result, a visible laser ray V whose wavelength λ is 623.8 nm is emitted from the He—Ne laser source  5  and is incident via the wavelength plate  6 , the AR coating layer  4  and the BK-7 prism  1  to the Au layer  2 . In this case, in order to generate evanescent photons in the Au layer  2 , the rotational angle of the wavelength plate  6  can be adjusted, so that the visible laser ray V incident to the Au layer  2  is polarized, i.e., TM-polarized or P-polarized in parallel with the incident/reflective surface S 1  of the Au layer  2 . 
         [0050]    Note that, since the visible laser ray V is linearly-polarized, the rotational angle of the He—Ne laser source  5  can be adjusted without provision of the wavelength plate  6  to emit the above-mentioned P-polarized light. 
         [0051]    In the Au layer  2 , surface plasmon resonance (SPR) photons are excited by incident light, and, in the organic dye layer  3 , a population inversion state is realized by the excitation due to the irradiation of an additional laser ray (pumping light) or a current injection. Therefore, the extinction coefficient k 3  of the organic dye layer  3  is made negative. As a result, the output light (reflective light) intensity I out  at the incident/reflective surface S 1  of the Au layer  2  is about 10 3  to 10 5  times the incident light intensity I in  in accordance with the negative extinction coefficient k 3  of the organic dye layer  3 . This is considered a resonance state between the SPR photons and the excitation state or population inversion state of the organic dye layer  3 . 
         [0052]    The organic dye layer  3  is formed by using a spin coating technology for depositing a host material called binaphthyl-poly (9,9-dioctylfluorene) (BN-POF) into an organic material in which rhodamine B is doped. 
         [0053]    The operational principle of the optical amplifier of  FIG. 3  is to generate evanescent photons in the Au layer  2  by the visible laser ray V to excite photons on the surface S 2  of the Au layer  2 . In this case, since the visible laser ray V is P-polarized, the visible laser ray V has an electric field component in parallel with the surface of the Au layer  2  and another electric field perpendicular to the surface of the Au layer  2 , so that the respective electric fields are amplified. For example, the intensity of the electric field of a light incident to the Au layer  2  is made to be about ten times by the SPR photons generated therein. Therefore, since the intensity of the light incident to the Au layer  2  is represented by a square value of the electric field, the light incident to the Au layer  2  is amplified by about 100 (=10×10) times. 
         [0054]    Regarding the surface plasmon resonance (SPR) photons, reference is made to Heinz Raether, “Surface Plasmons on Smooth and Rough Surfaces and on Gratings”, Springer-Verlag Berlin Heidelberg New York, pp. 16-19, 1988. 
         [0055]    Note that, since the wavelength λ of the visible laser ray V of the He—Ne laser source  5  is 632.8 nm and the wavelength of SPR photons of Au is about 600 to 1000 nm, the SPR photons would be excited. 
         [0056]      FIG. 4  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 3 . 
         [0057]    First, at step  401 , an optimum thickness t 2  of the Au layer  2  is determined. That is, if the incident angle θ of the visible laser ray V at the incident/reflective surface S 1  of the Au layer  2  is an optimum incident angle θ opt  (&gt;θ c  where θ c  is a critical angle), the number of SPR photons excited on the surface S 2  of the Au layer  2  of  FIG. 3  is maximum. In other words, when θ=θ opt &gt;θ c , the reflectivity R at the incident/reflective surface S 1  of the Au layer  2  is minimum. In this case,  FIG. 5  was obtained by a simulation which calculates a reflectivity R of light reflected from the incident/reflective surface S 1  of the Au layer  2  by angularly scanning the BK-7 prism  1  with the visible laser ray V. This simulation can be carried out by the simulation software WinSpall (trademark) developed by Max Planck Institute. 
         [0058]    In  FIG. 5 , the simulation conditions are as follows:
       1) The wavelength λ of the visible laser ray V is 632.8 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.53; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.18;   the extinction coefficient k 2  is 3; and   the thickness t 2  is variable.   
           4) For the organic dye layer  3 ,
           the thickness t 3  is 0.   
               
 
         [0069]    That is, the organic dye layer  3  is assumed to be absent. 
         [0070]    In  FIG. 5 , only one ATR signal spectrum of the Au layer  2  whose thickness t 2  is 53 nm is selected from a plurality of ATR signal spectrums of the Au layer  2  whose thickness t 2  is variable. That is, the ATR signal spectrum of  FIG. 5  shows that, if the thickness t 2  of the Au layer  2  is smaller than 10 nm, the Au layer  2  cannot sufficiently absorb evanescent photons generated therein, and if the thickness t 2  of the Au layer  2  is larger than 60 nm, the generation of evanescent photons in the Au layer  2  is attenuated, so as not to excite SPR photons in the Au layer  2 , thus increasing the reflectivity at a plasmon dip. From  FIG. 5 , the ATR signal spectrum at t 2 =53 nm shows a sharp plasmon dip where the reflectivity R is 0. Therefore, the thickness t 2  of the Au layer  2  is determined to be 53±1 nm, so that the excited SPR photons are maximum. 
         [0071]    Next, at step  402 , candidates (θ, t 3 ) where θ is the incident angle and t 3  is the thickness of the organic dye layer  3  are found. This step is carried out under the condition that the organic dye layer  3  is in a non-excited state (k 3 =0).  FIG. 6  was obtained by a simulation using the above-mentioned simulation software WinSpall (trademark) which calculates a reflectivity R of light reflected from the incident/reflective surface S 1  of the Au layer  2  by angularly scanning the BK-7 prism  1  with the visible laser ray V where the thickness t 2  of the Au layer  2  is fixed at 53 nm while the thickness t 3  of the organic dye layer  3  is changed from 0 to 50 nm. In order to simplify the description, only two ATR signal spectrums at t 3 =25 nm and 50 nm are illustrated in  FIG. 6 . 
         [0072]    In  FIG. 6 , the simulation conditions are as follows:
       1) The wavelength λ of the visible laser ray V is 632.8 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the thickness t 2  is 53 nm;   the refractive index n 2  is 0.18; and   the extinction coefficient k 2  is 3.   
           4) For the organic dye layer  3 ,
           the thickness t 3  is variable;   the refractive index n 3  is 1.4; and   the extinction coefficient k 3  is 0.   
               
 
         [0085]    As shown in  FIG. 6 , since the organic dye layer  3  has no absorption loss (k 3 =0), when the thickness t 3  of the organic dye layer  3  is increased, the plasmon dip angle is shifted toward a higher angle where the depth of the plasmon dip is at a point of R=0, so that SPR photons can be excited regardless of the thickness t 3  of the non-excited organic dye layer  3 . In order to simplify the description, it is assumed that only the following two candidates (θ, t 3 ) are found: 
         [0086]    (θ, t 3 )=(49.5°, 25 nm) 
         [0087]    (θ, t 3 )=(59.5°, 50 nm) 
         [0088]    If at least one candidate (θ, t 3 ) is found, the flow proceeds from step  403  to step  404  which selects one candidate (θ, t 3 ). Otherwise, the flow proceeds from step  403  to step  411  which indicates an error. 
         [0089]    That is, step  403  determines whether there are still non-selected candidates among the candidates (θ, t 3 ) found at step  402 . If there is at least one non-selected candidate (θ, t 3 ), the flow proceeds to step  404  which selects one candidate (θ, t 3 ) from the non-selected candidates, and then, steps  405  through  409  determine the extinction coefficient k 3  of the organic dye layer  3 , i.e., determines (θ, t 3 , k 3 ). 
         [0090]    Note that the extinction coefficient k 3  of the organic dye layer  3  is defined by the intensity of its population inversion state which can be realized by irradiating the organic dye layer  3  with an additional pumping laser ray or injecting a current into the organic dye layer  3 . That is, the extinction coefficient k 3  of the organic dye layer  3  is defined by 
         [0000]        k   3 =η 1   ·I    
         [0091]    where I is the energy of the pumping laser ray or the injected current; and 
         [0092]    η 1  is a constant. 
         [0093]    At step  405 , the extinction coefficient k 3  of the organic dye layer  3  is initialized at −k min  where k min  is a positive value such as 0.001. 
         [0094]    Next, at step  406 , a reflectivity R max  at the selected incident, angle θ is obtained. For example, an ATR signal spectrum (θ, t 3 , k 3 ) using the above-mentioned simulation software WinSpall (trademark) is calculated, and the reflectivity R max  at the selected incident angle θ is obtained from this ATR signal spectrum. In this case, the reflectivity R max  at the selected incident angle θ can be obtained by directly observing the reflectivity R max . 
         [0095]    Next, at step  407 , it is determined whether or not R max &gt;R th  where R th  is a predetermined value such as 6E4 is satisfied. As a result, only when R max &gt;R th , does the flow proceed to step  410  which completes the flowchart of  FIG. 4 . On the other hand, when R max ≦R th , the flow proceeds to steps  408  and  409  which renew the extinction coefficient k 3  by decreasing the extinction coefficient k 3  by Δk such as 0.001, thus repeating the flow at steps  406  and  407 . In this case, the extinction coefficient k 3  is guarded by step  409 . That is, when k 3 ≧−k max  where k max  is a definite value such as 1.0, the flow proceeds from step  409  to steps  406  and  407 . On the other hand, the flow proceeds from step  409  to step  403 . 
         [0096]    Thus, the extinction coefficient k 3  of the organic dye layer  3  is changed from k min  to k max , the extinction coefficient k 3  is determined at the time when the reflectivity R max  reaches R th . 
         [0097]    Note that if it is determined at step  103  that there is no non-selected candidate (θ, t 3 ), the flow proceeds to step  411 , which means it is impossible to determine the extinction coefficient k 3  of the organic dye layer  3 . 
         [0098]    For example, when (θ, t 3 )=(49.5°, 25 nm) is selected at step  404  and k 3 −0.05 is obtained at step  408 , an ATR signal spectrum as illustrated in  FIG. 7A  is obtained. In this case, the reflectivity R max  at the selected incident angle θ(=49.5°) is small, so that the flow proceeds via steps  408  and  409  to step  406 . 
         [0099]    Next, when (θ, t 3 )=(49.5°, 25 nm) is maintained and k 3 =−0.077 is obtained at step  408 , an ATR signal spectrum as illustrated in  FIG. 7B  is obtained. Even in this case, the reflectivity R max  at the selected incident angle θ (=49.5°) is small, so that the flow proceeds via steps  408  and  409  to step  406 . 
         [0100]    Next, when (θ, t 3 )=(59.5°, 50 nm) is selected at step  404  and k 3 =−0.05 is obtained at step  408 , an ATR signal spectrum as illustrated in  FIG. 8A  is obtained. Even in this case, the reflectivity R max  at the selected incident angle θ (=59.5°) is small, so that the flow proceeds via steps  408  and  409  to step  406 . 
         [0101]    Finally, when (θ, t 3 )=(59.5°, 50 nm) is maintained and k 3 =−0.077 is obtained at step  408 , an ATR signal spectrum as illustrated in  FIG. 8B  is obtained. In this case, the reflectivity R max  at the selected incident angle θ (=59.5°) is large, so that the flow proceeds to step  410 . 
         [0102]    Thus, (θ, t 3 , k 3 )=(59.5° , 50 nm, −0.077) is finally determined. 
         [0103]    In  FIGS. 7A ,  7 B,  8 A and  8 B, note that the simulation conditions are as follows:
       1) The wavelength λ of the visible laser ray V is 632.8 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.18;   the extinction coefficient k 2  is 3; and   the thickness t 2  is 53 nm.   
           4) For the organic dye layer  3 ,
           the refractive index n 3  is 1.4.   
               
 
         [0114]    As stated above, the extinction coefficient k 3  of the organic dye layer  3  can be changed in accordance with the excitation intensity thereof. In this case, in the optical amplifier using the organic dye layer  3 , since the gain coefficient α can be larger than 10  4  cm −1 , the extinction coefficient k 3  of the organic dye layer  3  can be easily changed from 0 to 0.1 (negative value) in accordance with the excitation intensity of the organic dye layer  3 . 
         [0115]    In  FIG. 9 , which illustrates a second embodiment of the optical amplifier according to the present invention, the organic dye layer  3  of  FIG. 3  is replaced by a semiconductor layer such as a GaAs layer  3   a , and the He—Ne laser source  5  of  FIG. 3  is replaced by an infrared laser source  5   a  for generating a 900 nm infrared laser ray IR in response to the bandgap of GaAs. 
         [0116]    In the Au layer  2 , surface plasmon resonance (SPR) photons are excited by incident light, and, in the GaAs layer  3   a , a population inversion state is realized by the excitation due to the irradiation of an additional laser ray (pumping light) or a current injection. Therefore, the extinction coefficient k 3a  of the GaAs layer  3   a  is made negative. As a result, the output light (reflective light) intensity I out  at the incident/reflective surface S 1  of the Au layer  2  is about 10 12  times the incident light intensity I in  in accordance with the negative extinction coefficient k 3a  of the GaAs layer  3   a . This is considered a resonance state between the SPR photons and the excitation state or population inversion state of the GaAs layer  3   a . Also, this output light has a small beam divergence angle. 
         [0117]    The operational principle of the optical amplifier of  FIG. 9  is the same as that of the operational amplifier of  FIG. 3 . 
         [0118]    Note that, since the wavelength λ of the infrared laser ray IR of the infrared laser source  5   a  is 900 nm and the wavelength of SPR photons of Au is about 600 to 1000 nm, the SPR photons would be excited. 
         [0119]      FIG. 10  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 9 . 
         [0120]    First, at step  1001 , an optimum thickness t 2  of the Au layer  2  is determined. That is, if the incident angle θ of the infrared laser ray IR at the incident/reflective surface S 1  of the Au layer  2  is an optimum incident angle θ op:  (&gt;θ, where θ, is a critical angle), the number of SPR photons excited on the surface S 2  of the Au layer  2  of  FIG. 9  is maximum. In other words, when θ=θ opt &gt;θ c , the reflectivity R at the incident/reflective surface S 1  of the Au layer  2  is minimum. In this case, an ATR signal spectrum (not shown) was obtained by a simulation which calculates a reflectivity R of light reflected from the incident/reflective surface S 1  of the An layer  2  by angularly scanning the BK-7 prism  1  with the infrared laser ray IR. This simulation can be carried out by the simulation software WinSpall (trademark) developed by Max Planck Laboratory. 
         [0121]    In this case, the simulation conditions are as follows:
       1) The wavelength λ of the infrared laser ray IR is 900 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.22;   the extinction coefficient k 2  is 6; and   the thickness t 2  is variable.   
           4) For the GaAs layer  3   a,  
           the thickness t 3a  is 0.   
               
 
         [0132]    That is, the GaAs layer  3   a  is assumed to be absent. 
         [0133]    As a result, the ATR signal spectrum at t 2 =39 nm shows a sharp plasmon dip where the reflectivity R is 0. Therefore, the thickness t 2  of the Au layer  2  is determined to be 39±1 nm, so that the excited SPR photons is maximum. 
         [0134]    Next, at step  1002 , candidates (θ, t 3a ) where θ is the incident angle and t 3a  is the thickness of the GaAs layer  3   a  are found. This step is carried out under the condition that the GaAs layer  3   a  is in a non-excited state (k 3a =0).  FIG. 11  was obtained by a simulation using the above-mentioned simulation software WinSpall (trademark) which calculates a reflectivity R of light reflected from the incident/reflective surface S 1  of the Au layer  2  by angularly scanning the BK-7 prism  1  with the infrared laser ray IR where the thickness t 2  of the Au layer  2  is fixed at 39 nm while the thickness t 3a  of the GaAs layer  3   a  is changed from 0 to 50 nm. In order to simplify the description, only one ATR signal spectrum at t 3 =10 nm is illustrated in  FIG. 11 . 
         [0135]    In  FIG. 11 , the simulation conditions are as follows:
       1) The wavelength λ of the infrared laser ray IR is 900 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the thickness t 2  is 39 nm;   the refractive index n 2  is 0.22; and   the extinction coefficient k 2  is 6.   
           4) For the GaAs layer  3   a,  
           the thickness t 3a  is variable;   the refractive index n 3a  is 3.57; and   the extinction coefficient k 3a  is 0.00013.   
               
 
         [0148]    As shown in  FIG. 11 , since the GaAs layer  3   a  has a little absorption loss (k 3 =0.00013), when the thickness t 3a  of the GaAs layer  3   a  is 10 nm, a sharp plasmon dip is observed at a point of R=0. In order to simplify the description, it is assumed that only the following one candidate (θ, t 3a ) is found: 
         [0149]    (θ, t 3a )=(41.5°, 10 nm) 
         [0150]    If at least one candidate (θ, t 3a ) is found, the flow proceeds from step  1003  to step  1004  which selects one candidate (θ, t 3a ). Otherwise, the flow proceeds from step  1003  to step  1011  which indicates an error. 
         [0151]    That is, step  1003  determines whether there are still non-selected candidates among the candidates (θ, t 3a ) found at step  1002 . If there is at least one non-selected candidate (θ, t 3a ), the flow proceeds to step  1004  which selects one candidate (θ, t 3a ) from the non-selected candidates, and then, steps  1005  through  1009  determine the extinction coefficient k 3a  of the GaAs layer  3   a , i.e., determines (θ, t 3a , k 3a ). 
         [0152]    Note that the extinction coefficient k 3a  of the GaAs layer  3   a  is defined by the intensity of its population inversion state which can be realized by irradiating the GaAs layer  3   a  with an additional pumping laser ray or injecting a current into the GaAs layer  3   a . That is, the extinction coefficient k 3a  of the GaAs layer  3   a  is defined by 
         [0000]        k   3a =η 3   ·I    
         [0153]    where I is the energy of the pumping laser ray or the injected current; and 
         [0154]    η 2  is a constant. 
         [0155]    At step  1005 , the extinction coefficient k 3a  of the GaAs layer  3   a  is initialized at −k min  where k min  is a positive value such as 0.001. 
         [0156]    Next, at step  1006 , a reflectivity R max  at the selected incident angle θ is obtained. For example, an ATR signal spectrum (θ, t 3a , k 3a ) using the above-mentioned simulation software WinSpall (trademark) is calculated, and the reflectivity R max  at the selected incident angle θ is obtained from this ATR signal spectrum. In this case, the reflectivity R max  at the selected incident angle θ can be obtained by directly observing the reflectivity R max . 
         [0157]    Next, at step  1007 , it is determined whether or not R max &gt;R th  where R th  is a predetermined value such as 6E4 is satisfied. As a result, only when R max &gt;R th , does the flow proceed to step  1010  which completes the flowchart of  FIG. 10 . On the other hand, when R max ≦R th , the flow proceeds to steps  1008  and  1009  which renew the extinction coefficient k 3a  by decreasing the extinction coefficient k 3a  by Δk such as 0.001, thus repeating the flow at steps  1006  and  1007 . In this case, the extinction coefficient k 3a  is guarded by step  1009 . That is, when k 3a ≧−k max  where k max  is a definite value such as 1.0, the flow proceeds from step  1009  to steps  1006  and  1007 .  0 n the other hand, the flow proceeds from step  1009  to step  1003 . 
         [0158]    Thus, when the extinction coefficient k 3a  of the GaAs layer  3   a  is changed from k min  n to k max , the extinction coefficient k 3a  is determined at the time when the reflectivity R reaches R th . 
         [0159]    Note that, when it is determined that there is no non-selected candidate (θ, t 3a ) at step  1003 , the flow proceeds to step  1011 , which means it is impossible to determine the extinction coefficient k 3a  of the GaAs layer  3   a.    
         [0160]    For example, when (θ, t 3a )=(41.5°, 10 nm) is selected at step  1004  and k 3a =−0.64587 is obtained at step  1008 , an ATR signal spectrum as illustrated in  FIG. 12  is obtained. In this case, the reflectivity R max  at the selected incident angle θ(=41.5°) is large, so that the flow proceeds via to step  1010 . 
         [0161]    Thus, (θ, t 3a , k 3a )=(41. 5°, 10 nm, −0.64587) is finally determined. 
         [0162]    In  FIG. 12 , note that the simulation conditions are as follows:
       1) The wavelength λ of the infrared laser ray IR is 900 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.22;   the extinction coefficient k 2  is 6; and   the thickness t 2  is 39 nm.   
           4) For the GaAs layer  3   a,  
           the refractive index n 3a  is 3.57.   
               
 
         [0173]    As stated above, the extinction coefficient k 3a  of the GaAs layer  3   a  can be easily changed from 0 to −0.1 (negative value) in accordance with the excitation intensity of the GaAs layer  3   a.    
         [0174]    As illustrated in  FIG. 13 , which is an enlargement of the reflection spectrum of  FIG. 12 , this reflection spectrum is an extremely-sharp δ-function. That is, when the thickness t 3a  of the GaAs layer  3   a  is 10 nm and the plasmon dip angle θ is 41.5°, actually, 41.45°, and the extinction coefficient k 3a  of the GaAs layer  3   a  is −0.64587, the output light (reflective light) intensity I out  at the incident/reflective surface S 1  of the Au layer  2  is about 10 12  times the incident light intensity I in . Therefore, (θ, t 3a , k 3a )=(41.5°, 10 nm, −0.64587) is determined. 
         [0175]    As stated above, the above-mentioned value 10 12  of the ratio of the output light (reflective light) intensity I out  to the incident light intensity I in  is not maximum. Note that this value corresponds to an oscillation state and, therefore. is theoretically indefinite. However, as the output light (reflective light) intensity I out  is increased, the non-linear loss such as the induced scattering is increased, so that the output light (reflective light) intensity I out , i.e., the above-mentioned ratio is not indefinite. 
         [0176]    Also, the output light has a small beam divergence angle. Generally, a very long resonator structure is needed to obtain such a small beam divergence angle. However, the resonator structure of the optical amplifier of  FIG. 9  is only 100 nm long. 
         [0177]    In  FIG. 14 , which illustrates a third embodiment of the optical amplifier according to the present invention, a silver (Ag) layer  4  serving as a resonator layer is added to the elements of the optical amplifier of  FIG. 3 . In this case, the Ag layer  4  is deposited on the surface S 2  of the Au layer  2 . 
         [0178]    In  FIG. 14 , it is assumed that the organic dye layer  3  is excited by an additional pumping ultraviolet laser ray having a wavelength λ of 320 nm. In this case, the Ag layer  4  has a high reflectivity to the visible laser ray V (λ=623.8 nm), while the Ag layer  4  has a low reflectivity or a high transmittance to the additional pumping ultraviolet laser ray (λ=320 nm). Therefore, the Ag layer  4  has a selective reflectivity, and therefore, the Ag layer  4  can serve as a resonator layer. 
         [0179]    The operational principle of the optical amplifier of  FIG. 14  is that the visible laser ray V is incident to the Au layer  2 , and apart of the visible laser ray V transmitted via the organic dye layer  3  to the Ag layer  4  is reflected by the Ag layer  4 . On the other hand, the organic dye layer  3  is excited by the ultraviolet laser ray transmitted from the Ag layer  4 . As a result, evanescent photons are generated in the Au layer  2  by the visible laser ray V incident thereto and reflected from the Ag layer  4  to excite photons on the surface S 2  of the Au layer  2 . In this case, since the visible laser ray V is P-polarized, the visible laser ray V has an electric field component in parallel with the surface of the Au layer  2  and another electric field perpendicular to the surface of the Au layer  2 , so that the respective electric fields are amplified. For example, the intensity of the electric field of a light incident to the Au layer  2  is about twenty times by the SPR photons generated therein. Therefore, since the intensity of the light incident to the Au layer  2  is represented by a square value of the electric field, the light incident to the Au layer  2  is amplified by about 400 (=20×20) times. 
         [0180]    Even in the optical amplifier of  FIG. 14 , since the wavelength λ of the visible laser ray V of the He—Ne laser source  5  is 632.8 nm and the wavelength of SPR photons of Au is about 600 to 1000 nm, the SPR photons would be excited. 
         [0181]      FIG. 15  is a flowchart for explaining a method for manufacturing the optical amplifier of  FIG. 14 . 
         [0182]    First, at step  1501 , the thickness t 4  of the Ag layer  4  is determined, so that the transmittivity of the Ag layer  4  to the ultraviolet laser ray (λ=320 nm) is a predetermined value such as about 70 to 80%. For example, the thickness t 4  of the Ag layer  4  is determined to be 10 nm to realize the above-mentioned transmittivity. 
         [0183]    Next, at step  1502 , the thickness t 3  of the organic dye layer  3  is determined. For example, the thickness t 3  of the organic dye layer  3  is determined to be 50 nm in view of the above-described first embodiment. That is, since the organic dye layer  3  has no absorption loss (k 3 =0), when the thickness t 3  of the organic dye layer  3  is increased, the plasmon dip angle is shifted toward a higher angle where the depth of the plasmon dip is at a point of R=0, so that SPR photons can be excited regardless of the thickness t 3  of the non-excited organic dye layer  3 . However, the plasmon dip angle should be smaller than 90°. Note that, if t 3 =200 nm, the plasmon dip angle is beyond 90°. 
         [0184]    Next, at step  1503 , (θ, t 2 ) where θ is an optimum incident angle and t 2  is the thickness of the Au layer  2  is determined. That is, if the incident angle θ of the visible laser ray V at the incident/reflective surface S 1  of the Au layer  2  is an optimum incident angle θ opt  (&gt;θ, where θ, is a critical angle), the number of SPR photons excited on the surface S 2  of the Au layer  2  of  FIG. 14  is maximum. In other words, when θ−θ opt &gt;θ c , the reflectivity R at the incident/reflective surface S 1  of the Au layer  2  is minimum. In this case,  FIG. 16  was obtained by a simulation which calculates a reflectivity R of light reflected from the incident/reflective surface S 1  of the Au layer  2  by angularly scanning the BK-7 prism  1  with the visible laser ray V. This simulation can be carried out by the above-mentioned simulation software WinSpall (trademark). From  FIG. 16 , (θ, t 2 )=(46.5°, 39 nm) is determined. 
         [0185]    In  FIG. 16 , the simulation conditions are as follows:
       1) The wavelength λ of the visible laser ray V is 632.8 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.18;   the extinction coefficient k 2  is 3; and   the thickness t 2  is variable.   
           4) For the organic dye layer  3 ,
           the refractive index n 3  is 1.4;   the extinction coefficient k 3  is 0; and   the thickness t 3  is 50 nm.   
               
 
         [0198]    Next, at steps  1504  to  1510 , the extinction coefficient k 3  of the organic dye layer  3  is determined so as to obtain a desired reflectivity R max  at the incident/reflective surface S 1  of the Au layer  2 . Even in this case, the extinction coefficient k 3  of the organic dye layer  3  is defined by the intensity of its population inversion state which can be realized by irradiating the organic dye layer  3  with an additional pumping laser ray or injecting a current into the organic dye layer  3 . That is, the extinction coefficient k 3  of the organic dye layer  3  is defined by 
         [0000]        k   3 =η 3   ·I    
         [0199]    where I is the energy of the pumping laser ray or the injected current; and 
         [0200]    η 3  is a constant. In the resonator structure of  FIG. 14 , η 3  is larger than η 1  and η 2 . 
         [0201]    Steps  1505  to  1510  will be explained below in detail. 
         [0202]    At step  1505 , the extinction coefficient k 3  of the organic dye layer  3  is initialized at −k min  where k min  is a positive value such as 0.001. 
         [0203]    Next, at step  1506 , a reflectivity R max  at the selected incident angle θ is obtained. For example, an ATR signal spectrum (θ, t 3 , k 2 ) using the above-mentioned simulation software WinSpall (trademark) is calculated, and the reflectivity R max  at the selected incident angle θ is obtained from this ATR signal spectrum. In this case, the reflectivity R max  at the selected incident angle θ can be obtained by directly observing the reflectivity R max . 
         [0204]    Next, at step  1507 , it is determined whether or not R max &gt;R th  where R th  is a predetermined value such as 6E4 is satisfied. As a result, only when R max &gt;R th , does the flow proceed to step  1509  which completes the flowchart of  FIG. 15 . On the other hand, when R max ≦R th , the flow proceeds to steps  1507  and  1508  which renew the extinction coefficient k 2  by decreasing the extinction coefficient k 3  by Δk such as 0.001, thus repeating the flow at steps  1505  and  1506 . In this case, the extinction coefficient k 3  is guarded by step  1508 . That is, when k 3 ≧−k max  where k max  is a definite value such as 1.0, the flow proceeds from step  1508  to steps  1505  and  1506 . On the other hand, the flow proceeds from step  1508  to step  1510 , which indicates an error. 
         [0205]    Thus, the extinction coefficient k 3  of the organic dye layer  3  is changed from k min  to k max , the extinction coefficient k 3  is determined at the time when the reflectivity R max  reaches R th . 
         [0206]    For example, when k 3 =−0.211 is obtained at step  1507 , an ATR signal spectrum as illustrated in  FIG. 17  is obtained. In this case, the reflectivity R max  at the selected incident angle θ(=46.5°) is large, i.e., R max &gt;R th , so that the flow proceeds to step  1509 . 
         [0207]    Thus, k 3 =−0.211 is finally determined. 
         [0208]    In  FIGS. 7A ,  7 B,  8 A and  8 B, note that the simulation conditions are as follows:
       1) The wavelength λ of the visible laser ray V is 632.8 nm.   2) For the BK-7 prism  1 ,
           the refractive index n 1  is 1.535; and   the extinction coefficient, k 1  is 0.   
           3) For the Au layer  2 ,
           the refractive index n 2  is 0.18;   the extinction coefficient k 2  is 3; and   the thickness t 2  is 39 nm.   
           4) For the organic dye layer  3 ,
           the refractive index n 3  is 1.4.   
               
 
         [0219]    Thus, under the presence of the Ag layer  4  serving as a resonator layer, the extinction coefficient k 3  of the organic dye layer  3  can be easily changed from 0 to −0.1 (negative value) in accordance with the excitation intensity of the organic dye layer  3 . 
         [0220]    As illustrated in  FIG. 17 , when the extinction coefficient k 3  of the organic dye layer  3  at the plasmon dip angle θ=46.5° (actually, 46.3°) of  FIG. 16  is −0.211, the output light (reflective light) intensity I out  at the incident/reflective surface S 1  of the Au layer  2  is about 10 5  times the incident light intensity I in . 
         [0221]    Note that, instead of the Ag layer  4 , a dielectric multi-layer mirror structure such as SiO 2 /TiO 2  having a selective reflectivity can be used as a resonator layer. 
         [0222]      FIG. 18A  is a cross-sectional view illustrating a modification of the Au layer  2  of  FIGS. 3 ,  9  and  14 , and  FIG. 18B  is a plan view of the modification of  FIG. 18A . 
         [0223]    As illustrated in  FIGS. 18A and 18B , holes  21  are regularly perforated in the Au layer  2 . In this case, the holes  21  have a diameter smaller than the wavelength λ of the visible laser ray V (or the infrared laser ray IR). Therefore, when the visible laser ray V (or the infrared laser ray IR) is incident to the incident/reflective surface S 1  of the Au layer  2 , a part of the visible laser ray V (or the infrared laser ray IR) is incident into the holes  21 , so that this part is hardly radiated from the holes  21  due to the small diameter thereof, while evanescent photons are generated in the Au layer  2 . This phenomenon is known as means for generating evanescent photons using very small holes. Additionally, as indicated by arrows in  FIG. 18B , evanescent photons generated in one of the holes  21  propagate into an adjacent one of the holes  21  to enhance the intensity of the evanescent photons. As a result, SPR photons are easily excited on the photoelectric surface S 2  of the Au layer  2  by the enhanced evanescent photons. 
         [0224]    In the above-described embodiments, the additional laser source for generating pumping light can be pulsated so as to pulsate the output light (reflective light). That is, when the pumping light applied to the dielectric layer such as the organic dye layer  3  or the GaAs layer  3   a  is pulsated, to realize a population inversion state or a negative extinction coefficient state therein, the output light (reflective light) would have a sharp peak. For example, in the optical amplifier of  FIG. 9 , a pulse output light having a peak value of 1 GW can be obtained for 1 mW incident light. 
         [0225]    Also, in the above-described embodiments, since the Au layer  2  for generating SPR photons can serve as an electrode, a current injection into the dielectric layer can be easily carried out by using the Au layer  2  as such an electrode. Particularly, in the optical amplifier of  FIG. 14 , the Au/dielectric layer/Ag resonator structure resembles a structure for injecting a current into the dielectric layer. 
         [0226]    Further, when the present invention is applied to an infrared light emitting device with a high population inversion state which has a high internal quantum efficiency and a low external quantum efficiency, an output light (reflective light) can be extracted from the device at an efficiency of about 100%. 
         [0227]    Note that the above-mentioned flowcharts of  FIGS. 4 ,  10  and  15  can be stored in a read-only memory (ROM) or another nonvolatile memory or in a random access memory (RAM) or another volatile memory.