Non-linear optical device

Abstract: A non-linear optical device for transmitting a light or carrying out optical signal processing by using a non-linear optical material having a biasing electric field applied perpendicularly to a semiconductor laminated to form a quantum well structure, has the following characteristics that: (a) a well layer of the quantum well structure has substantially same thickness as a Bore radius of an excitor, (b) the magnitude of the biasing electric field is selected such that a distance between centers of gravity of an electron and a hole which constitute the excitor is from one severalth of to substnatially equal to the effective Bore radius, (c) the thickness and the potential of a barrier layer of the quantum well structure are selected such that the increase in the width of the absorption peak of the excitor which is caused by the ionization due to the tunneling effect of the excitor under the biasing electric field is smaller than the width of the absorption peak under no electric field, (d) the wavelength of the light to activate the device is selected such that the energy ℎω of the photon is substantially equal to or slightly smaller than Eex/2, where Eex is the energy generated by the excitor of the quantum well structure, and (e) at least one of the lights which activate the device is a polarized light whose electric field vector has a component normal to the layer of the quantum well structure.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a non-linear optical device which modulates or switches a light and carries out an operation by the light by making use of an optical non-linearity of a material. Such a device serves as a basic component in transmitting or processing information by using the light. Related Background Art A non-linear optical device (NOD) has been reported by G.I. Stegeman et al, in the Journal of Lightwave Technology, 6,953 (1988). A typical example of the NOD introduced in the above article is shown in Fig. 1, in which two optical wave guides a and b are coupled in a coupling area c shown by hatched lines. When light inputs P1 and P2 are applied to input terminals (left side in Fig. 1) of the optical wave guides a and b , light outputs P3 and P4 are produced at output terminals (right side). A useful function may be attained by using a non-linear optical material (NOM) in the coupling area c . Namely, a ratio of P1 and P2 may be varied in accordance with intensities of P1 and P2. This makes use of the fact that the coupling length is determined by a refractive index of the area c and the refractive index, which depends on the light intensity, of the optical nonlinearity. By making use of this, an optical AND gate or an optical transistor is fabricated. The NOM used in the coupling area c may be a dielectric material such as LiNbO₃ or a semiconductor material such as GaAs, but the latter is superior from the standpoints of integration with a light emitting device and machining work. In the present invention, the semiconductor material is used. The present invention is therefore explained by comparing a prior art device which also uses the semiconductor material. In the prior art device, the nonlinearity in a vicinity of a band end of a GaAs or GaAs/AlGaAs multiple quantum well structure (MQWS) was utilized. Namely, a light energy ℎω of the light which activates the NOD is selected to be ℎω ≈ Eex, where Eex is an energy needed for the creation of an exciton in the semiconductor, in order to attain a large nonlinearity. However, this raises the following problems. (1) The light pulse is distorted in the vicinity of the band end because of a large dispersion of the (linear) refractive index. As a result, it is not possible to apply a very short pulse at a high repetition frequency to activate the NOD, and a very high speed operation which should be an advantage of the optical information processing is not attained. (2) Because of large absorption, the light intensity is significantly attenuated each time the light passes through each NOD. As a result, frequent amplification in the course of transmission is required. Further, because of the absorption χ (n) /α (where χ (n) is an n-order non-linear susceptibility, and α is an absorption coefficient), which is a figure of merit for the NOM, does not increase even if χ (n) is large, because α is large. It has been known that the absorption can be reduced by setting ℎω to be slightly smaller than Eex, but even in such a case, the absorption accompanied by photon absorption which extends up to several tens mV below Eex and the two-photon absorption in the range of ℎω ≧ Eex/2 are not avoidable. SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-linear optical device which has low attenuation of light and permits high speed signal processing, as well as a method of operating such a device and an apparatus comprising such a device. The above object of the present invention is attained by a method, an apparatus, and a device as defined in claims 1, 5, and 12, respectively. The inventors of the present invention found that a large optical non-linearity is attained at ℎω = E ex 2 when an appropriate electric field is applied to an appropriate quantum well structure (QWS), and they named it the GDM effect, which was reported by the inventors in Physical Review Letters, No. 61, p613 (1988) and Physical Review, No. B37, p8527 (1988). In the present invention, the GDM effect is applied to the NOD. The QWS is used as the NOM for the NOD, and an appropriate electric field (biasing electric field) is applied to the QWS to create the GDM effect. The light used is selected to meet ℎω ≈ Eex/2. Because of the GDM effect, a much larger non-linearity than that obtainable by use of a bulk semiconductor crystal is attained. Further, since the photon energy is far below the band end, the problems encountered in the prior art device are almost solved. If ℎω is set to be slightly smaller than Eex/2, the two-photon absorption according to item (2) of the problems described above is eliminated, and all of the problems encountered in the prior art device are solved. According to the study of the inventors, in order to attain the GDM effect, the thickness of the QWS is selected to be substantially equal to the effective Bohr radius of the exciton, and the magnitude of the biasing electric field is selected such that the distance between the centers of gravity of the electron and the hole which constitute the exciton is substantially equal to or a fraction of the effective Bohr radius. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a prior art non-linear optical device, Fig. 2 shows a conceptual view of a non-linear optical device of the present invention, Fig. 3 shows a perspective view of one embodiment of the present invention; Fig. 4 shows a sectional view of the device shown in Fig. 3, Fig. 5 shows a conceptual view for illustrating a dimension of an excitor relative to an effective Bore radius, Figs. 6 and 7 show changes in non-linear susceptibility to various biasing electric fields, and Figs. 8A - 8E other non-linear optical devices to which the present invention is applicable and input/ output characteristics thereof. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 conceptually shows the present invention in a most general form. An input light and a control light coact in a coupling area (which corresponds to the area c in Fig. 1) by the non-linear optical effect, and a resulting modulated input light is produced as an output light. Fig. 3 shows a perspective view of one embodiment of the present invention which embodies the configuration shown in Fig. 2. Fig. 4 shows a sectional view, taken along a line AA′ of the device shown in Fig. 3. For the convenience of explanation, Aℓ x Ga 1-x As is used as the semiconductor material, although any semiconductor material which can form the quantum well structure may be used. A ridge-type wave guide is used as the optical wave guide although any wave guide which is formed by the semiconductor may be used. In order to fabricate the device, Aℓ x Ga 1-x As is grown on an n-type GaAs substrate 4 by a molecular beam epitaxy (MBE) method to sequentially form lower cladding layers 3′ (n-type) and 3 (non-doped), an MQWS 1, and upper cladding layers 2 (non-doped) and 2′ (p-type). These layers are etched from the top thereof by a reactive ion beam to form the structure shown in Fig. 3, in which two ridge-type wave guides G₁ and G₂ having recessed stripes are coupled near the center. The proportions x of Aℓ in the respective layers may be 0.1 ∼ 1.0 in the cladding layers, 0 ∼ 0.1 in the well layer of the MQWS 1, and 0.4 ∼ 1.0 in the barrier layer. The thickness of the respective layers may be 1 ∼ 5 µm for the cladding layer 3′, 0.2 ∼ 2 µm for the cladding layer 3, 0.2 ∼ 2 µm for the cladding layer 2, 0.1 ∼ 5 µm for the cladding layer 2′, 0.1 ∼ 2 µm for the MQWS 1, and 0.2 ∼ 0.9 µm for the distance from the bottom of the ridge to the top of the MQWS 1. The widths of the respective ridges G₁ and G₂ are preferably 1 ∼ 4 µm. The width of the coupling area may be 2 ∼ 8 µm and the length thereof may be several hundreds µm ∼ several mm. The design of the wave guide structure may follow that of the normal NOD (which uses the semiconductor in the coupling area). Accordingly, the explanation thereof is omitted and only the characteristic portions of the present invention are explained in detail. One feature of the present invention resides in the structure which makes use of the GDM effect. As shown, in order to apply the electric field to the MQWS 1, the MQWS 1 and the adjacent cladding layers 2 and 3 are non-doped while the outer cladding layers 2′ and 3′ are doped with impurities. In the coupling area, an electrode 10 is attached onto the cladding layer 2′. An electrode is also attached to the substrate, and a voltage is externally applied from a power supply 11. The magnitude F bias of the biasing electric field should be selected such that the distance between the centers of gravity of the electron e and the hole h which constitute the exciton is between one fraction of and substantially equal to the effective Bohr radius R, as shown in Fig. 5, in which W denotes the well layers of the MQWS 1, and B₁ and B₂ denote the barrier layers of the MQWS 1. The cladding layers 2′ and 3′ may be doped in the same polarity, but it is better to dope them in the opposite polarities as shown and to apply the reverse bias in view of the homogeneity of the electric field and a low recombination current. The supply of voltage need not necessarily be applied by the external power supply. An internal electric fied created by doping the cladding layer 2′ and 3′ in the opposite polarities may be sufficient if the well layers W are thick and the total thickness of the layers 1, 2 and 3 is relatively small. In this case, however, the thickness and the doping of the respective layers must be precisely controlled. It is important that the structure of the MQWS 1, the electric field applied thereto and the wavelength of the light used are properly selected to meet the following conditions in order to make use of the GDM effect: (a) a well layer of the quantum well structure 1 has substantially the same thickness as a Bohr radius of an exciton. (b) the magnitude F bias of the biasing electric field is selected such that a distance between centers of gravity of an electron e and a hole h which constitute the exciton is from one fraction of the substantially equal to the effective Bohr radius, (c) the thickness and the potential of a barrier layer B₁, B₂ of the quantum well structure 1 are selected such that the increase in the width of the absorption peak of the exciton, which is caused by the ionization due to the tunneling effect of the exciton under the biasing electric field, is smaller than the width of the absorption peak under no electric field, (d) the wavelength of at least one of the lights which activate the device is selected such that the energy ℎω of the photon is substantially equal to or slightly smaller than Eex/2, where Eex is the energy needed for the creation of an exciton of the quantum well structure 1, and (e) at least one of the lights which activate the device is a polarized light whose electric field vector has a component normal to the layers of the quantum well structure 1. More specifically, let us consider a MQWS being laminated by a GaAs well layer having a thickness of L Z and an Aℓ 0.45 Ga 0.55 As barrier layer having a thickness of L B . As to the condition (a) above, since the effective Bohr radius of the GaAs exciton is approximately 120 Å, L Z is set to substantially the same value (for example, 80 ∼ 200 Å). As to the condition (b), the magnitude F bias of the biasing electric field which meets the condition (b) changes with L Z . This is illustrated in Figs. 6 and 7, which show the magnitudes of χ (2) and χ (3) caused by the GDM effect when L Z is 100 Å, 120 Å, 144 Å, 173 Å 207 Å, 249 Å and 299 Å, respectively. The ordinate in Fig. 6 represents a factor which is proportional to χ (2) , and the ordinate in Fig. 7 represents a factor which is proportional to χ (3) . If the non-linearity, which is required for the operation of the NOD, is quadratic, the magnitudes F bias of the biasing electric field around the peaks in the curves of Fig. 6 are appropriate. (For example, if L Z = 100 Å, their F bias ≈ 70 kV/cm, and if L Z = 173 Å, then F bias ≈ 15 kV/cm). In other words, this is the condition for the above condition (b). Similarly, if the non-linearity is cubical, the magnitudes Fbias around the peaks of the curves of Fig. 7 are appropriate. Similar curves for other QWS 1 may be simply derived in the manner described above. As to the condition (c), if the barrier layer is too thin (for example 10 Å), or the energy of the barrier is too low (for example, x=0.1) when the biasing electric field of the magnitude F bias determined as described above is applied, the exciton is unstable and no GDM effect appears. The condition to prevent this is given by the condition (c). For example, in the MQWS 1 of Figs. 6 and 7, it has been known that it is sufficient for L B to be equal to 60 Å for any L Z . It should be noted that too large an L B is not good, because the density of the quantum well in the MQWS 1 decreases if L B is too large and, thus, the GDM effect may not be fully utilized. It is best to add a slight thickness to L B which meets the condition (c). As to the condition (d), it should be noted that Eex changes with the magnitude F bias of the biasing electric field. For example, when L Z = 120 Å, Eex = 1.446 eV if F bias = 0, and Eex = 1.438 eV if F bias = 50 kV/cm. The photon energy ℎω must comply with one-half of the latter. If it is set slightly below one-half (more specifically, Eex - 2 ℎω is substantially equal to or slightly larger than the half-value half-width of the absorption peak of the exciton, for example, ℎω = 0.717 eV), not only the normal interband absorption (linear absorption) but also the two-photon absorption are eliminated. As a result, χ (n) /α, which is the figure of merit of the NOM, significantly increases, because not only the numerator increases by the GDM effect, but also the denominator decreases. Finally, the reason why the condition (e) is necessary is that χ (n) , which increases by the GDM effect, is limited to the projections of tensor having a component normal to the layer of the QWS 1. The type of the NOD is not limited to that described above. Some other examples are shown in Figs. 8A ∼ 8E, in which conceptual structures of the devices are shown on the left side. Fig. 8A shows a directive coupler, Fig. 8B shows a distributed feedback type grating, Fig. 8C shows a Mach-Zehnder interferometer, Fig. 8D shows a mode sorter, and Fig. 8E shows a prism coupler, in which Pi denotes an input light and Po denotes an output light. The curves on the right side show input/output characteristics of the respective devices. The abscissa represents a power of the input/output power and the ordinate represents the ratio of the output light power to the input light power. Of the two curves of Fig. 8A, the upper one shows one beat length and the lower one shows 1/2 beat length. In any NOD, the performance is remarkably improved if the NOM and the light are selected as shown in the embodiment. As explained above, the following advantages are offered by the present invention. (I) By using light which meets the condition ℎω ≈ Eex/2, the light pulse is hard to be distorted because the dispersion of the (linear) refractive index is small compared to the band end. Accordingly, it is possible to apply an extremely short pulse at a high repetition frequency to activate the NOD in order to attain a very high speed processing. (II) By using light which meets the condition ℎω ≈ Eex/2, there is little absorption in the NOM. Accordingly, the light intensity is not significantly attenuated each time the light passes through each NOD, and it is not necessary to frequently amplify the light in the course of transmission. When light which meets the condition ℎω < Eex 2 is used, the two-photon absorption is eliminated as well. (III) As to χ (n) , which is the figure of merit of the NOM used for the NOD, χ (n) has a much larger value than that of the bulk crystal because of the GDM effect. Further, since α is small as described in (II), the figure of merit is very large. As a result, smaller device size, the operation at lower light power and larger light modulation than those of the conventional NOD are attained. (IV) Since the response speed of the GDM effect is very high (less than 1 psec), the very high speed operation of the NOD is attained together with (I). (V) Since a semiconductor material is used as the NOM, it can be readily integrated with an electrical or light emitting device.

Claims: Method of operating a nonlinear optical device for controlling the transmission of light or for carrying out optical signal processing, wherein the device comprises a semiconductor laminated to form a quantum well structure (1) with at least a well layer (W) with a thickness substantially equal to the effective Bohr radius (R) of an exciton generated in said structure, and barrier layers (B₁, B₂) of a thickness and potential height selected such that the increase in the width of the absorption peak of said exciton by ionisation due to the tunnelling effect of said exciton under application of a biasing electric field perpendicularly to said structure is smaller than the width of the absorption peak under no electric field, comprising the steps of: applying said biasing electric field perpendicularly to said structure with a magnitude such that the distance between the centers of gravity of the electron (e) and the hole (h) constituting said exciton is from a fraction of to substantially equal to said effective Bohr radius (R); and choosing the wavelength and polarization of at least one of the light beams which activate the device such that the energy of the photon corresponding to said wavelength is substantially equal to or slightly smaller than E ex /2, where E ex is the energy needed for creation of said exciton, and the electric field vector corresponding to said polarization has a component normal to the layers of said structure (1). A method according to claim 1, wherein said biasing electric field is supplied from an external power supply through electrodes. A method according to claim 1, wherein said device has cladding layers provided above and below said quantum well structure, and said biasing electric field is supplied by an internal electric field caused by doping said cladding layers with impurities so as to impart opposite polarities to said cladding layers. A method according to claim 1, wherein said semiconductor comprises a substrate, an optical wave guide layer of a multiple quantum well structure (1) formed on said substrate, said optical wave guide layer comprising two stripe-shaped optical wave guide layers (G1, G2) having portions thereof coupled to each other at a predetermined area, a pair of cladding layers (2, 3) formed on the opposite sides of said optical wave guide layer and an electrode formed in said coupling area to apply the biasing electric field to said optical wave guide layer, and an external power supply (11) connected to said electrode. A non-linear optical apparatus for controlling transmission of light or carrying out optical signal processing, comprising: a non-linear optical device, wherein the device comprises a semiconductor laminated to form a quantum well structure (1) with at least a well layer (W) with a thickness substantially equal to the effective Bohr radius (R) of an exciton generated in said structure, and barrier layers (B₁, B₂) of a thickness and potential height selected such that the increase in the width of the absorption peak of said exciton by ionisation due to the tunnelling effect of said exciton under application of a biasing electric field perpendicular to said structure is smaller than the width of the absorption peak under no electric field, said apparatus further comprising means for supplying said device with said biasing electric field perpendicularly to said structure; and means for inputting into said device light beams including at least a polarized light which has a wavelength such that the energy ℎω of the photon is substantially equal to or slightly smaller than E ex /2, wherein E ex is the energy needed for the creation of an exciton of the quantum well structure (1) and which has an electric field vector having a component normal to the layer of the quantum well structure (1), characterized in that the magnitude of said biasing electric field is such that the distance between the centers of gravity of the electron (e) and the hole (h) constituting said exciton is from a fraction of to substantially equal to said effective Bohr radius (R). A non-linear optical apparatus according to claim 5, wherein the means for supplying said biasing electric field comprises electrodes (10) for applying the biasing electric field to said semiconductor and an external power supply (11) connected to said electrodes (10). A non-linear optical apparatus according to claim 5, characterized in that said semiconductor comprises a substrate, an optical wave guide layer of a multiple quantum well structure (1) formed on said substrate and a pair of cladding layers (2, 3) formed on the opposite sides of said optical wave guide layer. A non-linear optical apparatus according to claim 7, characterized in that said optical wave guide layer comprises two stripe-shaped optical wave guide layers (G1, G2) having portions thereof coupled to each other at a predetermined area. A non-linear optical apparatus according to claim 8, wherein the means for supplying said biasing electric field comprises an electrodes formed in said coupling area to apply the biasing electric field to said optical wave guide layer, and an external power supply (11) connected to said electrode. A non-linear optical apparatus according to claim 7, characterized in that each of said cladding layers (2, 3) is doped with impurities of the opposite polarity to each other. A non-linear optical apparatus according to claim 7, characterized in that said substrate, said optical wave guide layer and said cladding layers (2,3) are made of GaAs and AlGaAs. A non-linear optical device for controlling the transmission of light or for carrying out optical signal processing, comprising a semiconductor laminated to form a quantum well structure (1) with at least a well layer (W) and barrier layers (B₁, B₂), wherein said well layer (W) of the quantum well structure (1) has a thickness substantially equal to the Bohr radius (R) of an exciton generated in said structure, the wavelength of at least one of the light beams which activate the device is chosen so that energy ℎω of the photon corresponding to said wavelength is substantially equal to or slightly smaller than E ex 2 , where E ex is the energy needed for the creation of an exciton in said quantum well structure (1), the thickness and the potential of said barrier layers (B₁, B₂) of the quantum well structure (1) are selected such that the increase in the width of the absorption peak of the exciton which is caused by the ionisation due to the tunnelling effect of the exciton under application of a biasing electric fied perpendicularly to said structure is smaller than the width of the absorption peak under no electric field, and at least one of the light beams which activate the device has a polarization whose electric field vector has a component normal to the layer of the quantum well structure (1), characterized in that the device has cladding layers (2,3) provided above and below the quantum well structure, and in that said biasing electric field is an internal biasing electric field, with a magnitude such that the distance between the centers of gravity of the electron (e) and the hole (h) constituting said exciton is from a fraction of to substantially equal to said effective Bohr radius (R), which field is supplied by doping said cladding layers with impurities so as to impart opposite polarities to said both cladding layers. A non-linear optical device according to claim 12, characterized in that said semiconductor comprises a substrate and an optical wave guide layer of a multiple quantum well structure (1) formed on said substrate. A non-linear optical device according to claim 13, characterized in that said optical wave guide layer comprises two stripe-shaped optical wave guide layers (G1, G2) having portions thereof coupled to each other at a predetermined area. A non-linear optical device according to claim 13, characterized in that said substrate, said optical wave guide layer and said cladding layers (2,3) are made of GaAs and AlGaAs.

Inventors: SHIMIZU AKIRA, SHIMIZU, AKIRA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G02F1
IPC codes: H01S3, G02F1, G02F3