Abstract:
An non-linear optical device for generating light of an angular frequency 2ω from incident light of an angular frequency ω by means of a non-linear optical effect, comprises a substrate body, formed by a semiconductive or insulating material; 
     an embedded member embedded in the substrate body and formed by a direct-transition type semiconductive or insulating material different from the material constituting the substrate body, having a band gap Eg satisfying a relation Eg≈2nω in which n=h/2π and h is Planck constant, and having a size comparable to that of an exciton, and in this device, an asymmetric potential is provided to each of an electron and positive hole in said embedded member in such a manner that the wave functions of the electron and positive hole have mutually different centers of gravity fron each other.

Description:
This application is a continuation of application Ser. No. 07/295,882 filed Jan. 12, 1989 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a non-linear optical device for obtaining light of a shorter wavelength, utilizing a non-linear optical effect. 
     2. Related Background Art 
     There has conventionally proposed a device for generating harmonic waves in order to obtain light of a shorter wavelength. Such device is schematically shown in FIG. 1. 
     In FIG. 1, a basic wave 32 of an angular frequency ω emitted from a semiconductor laser 31 enters a non-linear optical crystal 30 composed, for example, of LiNbO 3  or KDP, thereby generating a harmonic wave 34 of an angular frequency 2ω, together with a transmitted wave 33 of the angular frequency ω. 
     Also for modulating such harmonic wave, there is already known a device as shown in FIG. 2, in which the same components as those in FIG. 1 are represented by the same numbers and will not be explained further. In said device, the non-linear optical crystal is replaced by a polar macromolecular material 35, to which a bias field is applied by a power source 38, with modulation by a modulator 39. When the bias field is turned on, the molecules of said polar macromolecular material 35 are aligned in a same direction, whereby the non-linear receptivity χ.sup.(2) is elevated to generate the harmonic wave 34. When the bias field is turned off, the orientation of the molecules is thermally disturbed to reduce χ.sup.(2), whereby the harmonic wave 34 is decreased. 
     However these devices have been associated with following drawbacks; 
     1) Efficiency of harmonic wave generation is low, due to low non-linear receptivity χ.sup.(2) ; 
     2) It is difficult to prepare such device in combination with a light-emitting device such as a semiconductor laser, on a same substrate; 
     3) In case of the device shown in FIG. 1, the non-linear optical crystal of satisfactory quality is costly to prepare; and 
     4) In case of the device shown in FIG. 2, the response is as low as several milliseconds, as it is determined by the orientation by molecular rotation by the bias field. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a non-linear optical device which is not associated with the above-mentioned drawbacks of the prior technology, is inexpensive in preparation, and has a high efficiency of harmonic wave generation. 
     Another object of the present invention is to provide a non-linear optical device capable of modulating the harmonic wave with a high response speed. 
     The above-mentioned objects can be achieved by forming, in a semiconductive or insulating substrate body, a non-linear optical device for generating light of an angular frequency 2ω from incident light of an angular frequency ω by means of a direct-transition semiconductive or insulating embedded member embedded therein, which is different from the material of said substrate body, has a band gap Eg satisfying a relation Eg≈2hω wherein h=h/2π and h is Planck constant, and is of a size approximately equal to that of the exciton, and by providing an asymmetric potential to the electron and positive hole in such a manner that the wave functions have different centers of gravity for the electron and positive hole in said embedded member. 
     More specifically, the device of the present invention is based on a phenomenon, found for the first time by the present inventor, in a so-called quantum micro hetero structure (QMHS) formed by a direct-transition semiconductive or insulating member of a size comparable to the Bohr radius of the exciton embedded in a substrate body formed by a semiconductive or insulating material, such that by applying potentials of an electron and positive hole having different centers of gravity a very large dipole moment is provided to the exciton thereby obtaining a very large non-linear receptivity χ.sup.(2) to the light satisfying a relation 2hω≈Eg. 
     Such asymmetric potential can be realized by applying a bias electric field to the quantum micro hetero structure. The modulation of the harmonic wave can be achieved by modulating said bias electric field, with a high response speed shorter than a picosecond, since a slow process such as molecular rotation is not involved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are schematic views of conventional harmonic wave generating devices; 
     FIG. 3 is a schematic cross-sectional view of a non-linear optical device embodying the present invention; 
     FIGS. 4 and 5 are charts showing the energy bands in a part of the potential well structure in the present invention; 
     FIGS. 6, 7 and 8 are schematic cross-sectional views of other embodiments of the present inventions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. 
     FIG. 3 is a schematic cross-sectional view showing a first embodiment of the non-linear optical device of the present invention, wherein shown are a substrate body semiconductor 1 composed of non-doped AlAs, and an embedded semiconductor 2 composed of non-doped GaAs. The substrate body semiconductor 1 and the embedded semiconductor 2 are alternately laminated in plural cycles to constitute a quantum well structure (QWS) 3 which is one of the quantum micro hetero structures. The embedded semiconductor 2 has a thickness of 120 Å which is approximately equal to the Bohr radius of the exciton in GaAs. On both sides of the quantum well structure 3, there are respectively provided a p-area 5 and an n-area 6 for ohmic contact and electrodes 51, 52. Charge separating layers 7, 8, composed of non-doped AlAs, are provided in order to prevent the migration of the carriers from the areas 5, 6 to the quantum well structure 3. A bias electric field is applied between the electrodes 51, 52 by means of a power source 50. 
     The working principle of the above-explained device is as follows. FIG. 4 is a chart showing the energy bands in a part of the quantum well structure 3 shown in FIG. 3, wherein shown are an energy 71 at the bottom of the conduction band; an energy 72 at the top of the valence electron band; a wave function 81 of the electron; and a wave function 82 for the positive hole. The potential is generally inclined, as the bias electric field is applied to the quantum well structure 3 by means of the power source 50. Consequently the potentials for the electron and for the positive hole are asymmetric, and the corresponding wave functions 81, 82 have mutually different centers of gravity. As a result, the exciton defined by the bound state of the electron and the positive hole, has a large dipole moment. This signifies that the entire quantum well structure 3 has a large non-linear receptivity χ.sup.(2). For example, the value of χ.sup.(2) of the quantum well structure 3 composed of GaAs/AlGaAs is 400 times as large as that of LiNbO 3 , or 5000 times as large as that of KDP. 
     When a basic wave 10 of an angular frequency ω emitted, for example, from a semiconductor laser as shown in FIG. 1 (satisfying a relation 2hω≅Eg wherein Eg is the band gap of the embedded semiconductor, h=h/2π and h is Planck constant) passes through said quantum well structure 3, a part of said basic wave is transmitted as the transmitted wave 11 of the angular frequency ω, but the remaining part is converted into a harmonic wave 12 of an angular frequency 2ω. Said conversion is conducted very efficiently, as the efficiency of conversion is proportional to the square of χ.sup.(2). 
     In the foregoing embodiment the device is formed from GaAs and AlAs, but it is also possible to employ Al x  Ga 1-x  As for the semiconductors 1, 2 and the layers 7,8. The refractive index of the quantum well structure 3 becomes larger than that of the layers 7, 8 if the average value of x of the quantum well structure 3 is sselected smaller than that of the layers 7, 8. In this manner, the entire quantum well structure 3 constitutes a wave guide structure, thereby confining the light therein and increasing the concentration thereof, and further increasing the efficiency of generating the harmonic wave. The same principle applies to materials other than Al x  Ga 1-x  As, as long as the composition is so selected that the refractive index of the quantum well structure 3 becomes larger than that of the layers 7, 8. Also the device of the present invention, as being composed of a compound semiconductor, can be monolithically formed on a same substrate with a light-emitting device such as semiconductor laser which is similarly composed of a compound semiconductor. 
     In the foregoing embodiment, the asymmetric potential is realized by an externally applied bias electric field, but such asymmetric potential may also be realized by locally varying the composition of the embedded semiconductor. FIG. 5 shows energy bands in such embodiment. In this embodiment the substrate body semiconductor 1 is composed of AlAs, while the embedded semiconductor 2 has a gradually varying composition from (InAs) 1-x  (GaAs) x  to (GaSb) 1-y  (GaAs) y  in the direction of thickness, so that the energy level 71 at the bottom of the conduction band and the energy level 72 at the top of the valence electron band are inclined in the embedded semiconductor 2. Consequently the wave functions 81, 82 for the electrons and for the positive holes have mutually different center of gravity, so that the harmonic wave can be generated with a high efficiency as in the foregoing embodiment. Such structure allows to dispense with the power source 50 and the electrodes 51, 52 shown in FIG. 3, since the application of the bias electric field is no longer necessary. The variation in composition can be achieved, for example, by &#34;type II staggered superlattice&#34; reported by L. Esaki in IEEE J. Quantum Electronics, QE-22, P1611 (1986). 
     FIG. 6 is a schematic cross-sectional view showing a second embodiment of the non-linear optical device of the present invention, wherein the same components as those in FIG. 3 are represented by the same numbers and will not be explained further. 
     In the present embodiment, the quantum micro hetero structure is composed, instead of the quantum well structure 3 explained above, in a quantum dot (QD) structure 13, that is composed of ZnSe particles 2 of a size of about 50 Å, embedded in a glass 21 as a substrate body. Also in the present embodiment, the harmonic wave can be generated with a high efficiency, by a bias electric field applied by the power source 50. 
     FIG. 7 is a schematic cross-sectional view of a third emboidment of the non-linear optical device of the present invention, wherein the same components as those in FIG. 3 are represented by the same numbers and will not be explained further. 
     The present embodiment is different from the first embodiment in the presence of a modulator 53 for modulating the bias electric field supplied by the power source 50. Said modulator 53 may be composed, for example, of an ordinary switch. The turning on and off of the bias electric field by the modulator 53 varies χ.sup.(2). thereby modulating the harmonic wave 12, with a response speed shorter than a picosecond as explained before. In the present embodiment, the actual response speed is determined by the CR time constant in the circuit, and can be actually reduced to the order of picosecond. Also in the present embodiment, the quantum well structure 3 may be replaced by a quantum dot layer 13 as shown in FIG. 6. 
     FIG. 8 is a schematic cross-sectional view showing a fourth embodiment of the non-linear optical device of the present invention, wherein the same components as those in FIG. 8 are represented by the same numbers and will not be explained further. 
     The present embodiment is different from the first embodiment in the presence of a filter 54 for selectively separating the harmonic wave, at the exit side of the quantum well structure 3. Said filter 54 may be composed of a color glass, a multi-layer coated glass or a diffraction grating. Said filter 54 is so designed as to transmit the harmonic wave 12 of angular frequency 2ω but to intercept the transmitted wave 11 of angular frequency ω. Consequently the device emits the harmonic wave 12&#39; only, and it is rendered possible to prevent the influence of the transmitted wave 11 in the utilization of said harmonic wave 12&#39;. Also in the present embodiment, the quantum well structure 3 may be replaced by the quantum dot layer 13 as shown in FIG. 6. Also the composition of the embedded layer may be gradually varied as shown in FIG. 5. Furthermore, the use of a modulator 53 as shown in FIG. 7 allows to obtain the modulated harmonic wave only from the device. 
     In addition to the foregoing embodiments, the present invention is subject to various modifications. For example, in addition to the quantum well structure and quantum dot structure explained above, a quantum line (QL) structure may be adopted as the quantum micro hetero structure. Such quantum dot structure or quantum line structure provide a non-linear receptivity χ.sup.(2) larger than that obtainable with the quantum well structure. Also the materials constituting the device are not limited to those described in the foregoing embodiments, but may be composed, for example, of II-IV group semiconductor. The present invention includes all these modifications within the scope and spirit of the appended claims. 
     As detailedly explained in the foregoing, the non-linear optical device of the present invention, in which asymmetric potentials are given to the quantum micro hetero structure, provides the advantages of: 
     i) generating the harmonic wave with a high efficiency; 
     ii) being monolithically formed in combination with a light-emitting device; and 
     iii) enabling inexpensive preparation. Also te present invention, when applied to a non-linear light modulator, enables modulation with a high response speed.