Non-linear optical device with quantum micro hetero structure

Abstract: 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 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 constitut­ ing the substrate body, having a band gap Eg satisfying a relation Eg ≈ 2ℎω in which ℎ = 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 from each other.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a non-linear optical device for obtaining light of shorter wavelength, utilizing non-linear optical effect. Related Background Art There has been conventionally proposed a device for generating harmonic waves in order to obtain light of 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 LiNbO₃ 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 same components as those in Fig. 1 are represented by 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 electric field is applied by means of a power source 38, being modulated by means of a modulator 39. When the bias electric field is turned on, the molecules of said polar macromolecular material 35 are aligned in a same direction, whereby the non-linear receptivity χ (2) is elevated to generate the harmonic wave 34. When the bias electric field is turned off, the orientation of the molecules is thermally disturbed thereby reducing the non-linear receptivity χ (2) , whereby the harmonic wave 34 is decreased. However these devices have been associated with following problems; 1) Efficiency of harmonic wave generation is low, due to low non-linear receptivity χ (2) ; 2) It is difficult to prepare such devices 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 slow as several milliseconds, as it is determined by the orientation by molecular rotation due to the bias electric 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 problems 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. According to the invention these objects are achieved by 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 non-linear optical effect, comprising: a substrate body material formed by a semiconductive or insulating material; an embedded member embedded in said substrate body material and formed by a direct-transition semiconductive or insulating material different from the material constituting said substrate body material, wherein said embedded member has a band gap Eg which is smaller than the band gap of said substrate body material and which satisfies a relation Eg ≈ 2ℎω in which ℎ = h/2π and h is Planck constant, and wherein at least one of the dimensions of said embedded member has a size comparable to the Bohr radius of an exciton generated by the absorption of said incident light in said embedded member; and a pair of electrodes for applying a bias electric field across the substrate body material in which said embedded member is embedded; and a power source for supplying an electric field between said electrodes. According to the invention these objects are further achieved by a non-linear optical device for generating light of an angular wave 2ω from incident light of an angular frequency ω by means of a non-linear optical effect, comprising: a substrate body material formed by a semiconductive or insulating material; a plurality of embedded members embedded in said substrate body material and formed by direct-transition compound semiconductor different from the material constituting said substrate body material, wherein said embedded members have a band gap Eg which is smaller than the band gap of said substrate body material and which satisfies a relation Eg ≈ 2ℎω in which ℎ = h/2π and h is Planck constant, and wherein at least one of the dimensions of said embedded members has a size approximately equal to the Bohr radius of an exciton generated by the absorption of said incident light in said embedded members, and have a composition of said compound semiconductor gradually varying in a direction along which the embedded members have a size comparable to the Bohr radius of said exciton according to the position of each of said members in said substrate body material. The advantages of the invention will become apparent and obvious to those skilled in the pertinent art upon referring to the following description provided in connection with the accompanying drawings, of which: 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 invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 is a schematic cross-sectional view showing a first embodiment of the non-linear optical device of the present invention, showing a substrate body semiconductor 1 composed of non-doped A1As 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 quantum micro hetero structures. The embedded semiconductor 2 has a thickness of 12 nm (120 Å) which is approximately equal to the Bohr radius of an exciton in GaAs. The article by J. Khurdin published in Appl. Phys. Lett. 51 (1987), pages 2100 to 2102, discloses a device with a similar structure. However, the thickness of the layer embedded in this structure is not comparable to and is in fact smaller than the Bohr radius of an exciton generated in this layer by absorption of light. 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 A1As, are provided in order to prevent the migration of 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, showing an energy level 71 at the bottom of a conduction band; an energy level 72 at the top of a valence electron band; a wave function 81 of an electron; and a wave function 82 of a 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 (2) . For example the value of the non-linear receptivity χ (2) of the quantum well structure 3 composed of GaAs/A1GaAs is 400 times as large as that of LiNbO₃, 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 4 as shown in Fig. 3 (satisfying a relation 2ℎω ≅ Eg wherein Eg is the band gap of the embedded semiconductor 2, ℎ = h/2π and h is Planck constant) passes through said quantum well structure 3, a part of said basic wave 10 is transmitted as a 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 the non-linear receptivity χ (2) . In the foregoing embodiment the device is formed of GaAs and A1As, but it is also possible to employ A1 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 selected 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 12. The same applies to materials other than A1 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 composable of a compound semiconductor, can be monolithically formed on a same substrate with a light-emitting device such as semiconductor laser 4 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 levels 71, 72 in such embodiment. In this embodiment the substrate body semiconductor 1 is composed of A1As, 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 centers of gravity, so that the harmonic wave 12 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 "type II staggered superlattice" 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 same components as those in Fig. 3 are represented by 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, of a quantum dot (QD) structure 13, which is composed of ZnSe particles 22 of a size of about 5 nm (50 Å), embedded in a glass 21 as a substrate body. Also in the present embodiment, the harmonic wave 12 can be generated with a high efficiency by a bias electric field which is applied by means of the power source 50. Fig. 7 is a schematic cross-sectional view of a third embodiment of the non-linear optical device of the present invention, wherein same components as those in Fig. 3 are represented by same numbers and will not be explained further. The present embodiment is different from the first embodiment due to the presence of a modulator 53 for modulating the bias electric field which is applied by means of 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 the non-linear receptivity χ (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 RC 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 structure 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 same components as those in Fig. 8 are represented by same numbers and will not be explained further. The present embodiment is different from the first embodiment due to the presence of a filter 54 for selectively separating the harmonic wave 12, 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' only, and it is rendered possible to prevent the influence of the transmitted wave 11 in the utilization of said harmonic wave 12'. Also in the present embodiment, the quantum well structure 3 may be replaced by the quantum dot structure 13 as shown in Fig. 6. Also the composition of the embedded semiconductor 2 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 3 and quantum dot structure 13 explained above, a quantum line (QL) structure may be adopted as the quantum micro hetero structure. Such quantum dot structure 13 or quantum line structure provide a non-linear receptivity χ (2) larger than that obtainable with the quantum well structure 3. Also the materials constituting the device are not limited to those described in the foregoing embodiments, but may be composed, for example, of II-VI group semiconductor. The present invention includes all these modifications within the scope 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 12 with a high efficiency; ii) being monolithically formed in combination with a light-emitting device; and iii) enabling inexpensive preparation. Also the present invention, when applied to a non-linear light modulator, enables modulation with a high response speed.

Claims: A non-linear optical device for generating light (12) of an angular frequency 2ω from incident light (10) of an angular frequency ω by means of a non-linear optical effect, comprising: a substrate body material (1) formed by a semiconductive or insulating material; an embedded member (2) embedded in said substrate body material (1) and formed by a direct-transition semiconductive or insulating material different from the material constituting said substrate body material (1), wherein said embedded member (2) has a band gap Eg which is smaller than the band gap of said substrate body material and which satisfies a relation Eg ≈ 2ℎω in which ℎ = h/2π and h is Planck constant, said device being characterized in that at least one of the dimensions of said embedded member has a size comparable to the Bohr radius of an exciton generated by the absorption of said incident light (10) in said embedded member (2); and in that it comprises a pair of electrodes (51, 52) for applying a bias electric field across the substrate body material (1) in which said embedded member (2) is embedded; and a power source (50) for supplying an electric field between said electrodes (51, 52). A non-linear optical device according to claim 1, wherein the electron and the positive hole defining said exciton in said embedded member (2) are provided with an asymmetric energy band potential (71, 72) in such a manner that the wave functions (81, 82) of said electron and positive hole have mutually different centers of gravity from each other. A non-linear optical device according to claim 1 or 2, further comprising a modulator (53) for modulating the electric field supplied by said power source (50). A non-linear optical device according to claim 1 or 2, further comprising a filter (54) for transmitting the light (12) of the angular frequency 2ω only. A non-linear optical device according to claim 1 or 2, further comprising two layers (7, 8) sandwiching said substrate body material (1) and having a refractive index lower than that of said substrate body material (1). A non-linear optical device according to claim 1 or 2, wherein said embedded member (2) is formed by a plurality of layers each having a thickness approximately equal to the Bohr radius of the exciton, said plurality of layers being alternately laminated with a plurality of layers formed by said substrate body material (1). A non-linear optical device according to claim 1 or 2, wherein said embedded member (22) is formed by a plurality of dot-like particles each having a size approximately equal to the Bohr radius of the exciton, said plurality of dot-like particles being dispersed in said substrate body (21). A non-linear optical device for generating light (12) of an angular frequency 2ω from incident light (10) of an angular frequency ω by means of a non-linear optical effect, comprising: a substrate body material (1) formed by a semiconductive or insulating material; a plurality of embedded members (2) embedded in said substrate body material (1) and formed by direct-transition compound semiconductor different from the material constituting said substrate body material (1), wherein said embedded members (2) have a band gap Eg which is smaller than the band gap of said substrate body material which satisfies a relation Eg ≈ 2ℎω in which ℎ = h/2π and h is Planck constant, said device being characterized in that at least one of the dimensions of said embedded members (2) has a size approximately equal to the Bohr radius of an exciton generated by the absorption of said incident light (10) in said embedded members (2), wherein said embedded members have a composition of said compound semiconductor gradually varying in a direction along which the embedded members (2) have a size comparable to the Bohr radius of said exciton according to the position of each of said members (2) in said substrate body material (1). A non-linear optical device according to claim 8, further comprising a filter (54) for transmitting the light (12) of the angular frequency 2ω only. A non-linear optical device according to claim 8, further comprising two layers (7, 8) sandwiching said substrate body material (1) and having a refractive index lower than that of said substrate body material (1). A non-linear optical device according to claim 8, wherein said plurality of embedded members (2) is formed by a plurality of layers each having a thickness approximately equal to the Bohr radius of the exciton, said plurality of layers being alternately laminated with a plurality of layers formed by said substrate body material (1). A non-linear optical device according to claim 8, wherein said plurality of embedded member (22) is formed by a plurality of dot-like particles each having a size approximately equal to the Bohr radius of the exciton, said plurality of dot-like particles being dispersed in said substrate body material (21).

Inventors: SHIMIZU AKIRA, SHIMIZU, AKIRA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G02F1
IPC codes: G02F1, H01S5