JPH0357460B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention comprises an optical waveguide having an ultra-thin multilayer structure forming a quantum well structure, and utilizes changes in the refractive index of the ultra-thin multilayer structure in response to an applied electric field. Regarding optical circuit functional elements that exhibit various external optical circuit functions, in particular, ultra-thin film multilayer structures are configured to form stepped quantum well structures with multiple energy levels, and changes in refractive index due to applied electric fields are developed. It is designed to increase efficiency.

(Prior art) Conventional optical circuit functional elements of this type include (1) thin-film optical waveguides using electro-optic crystals, (2) elements that control refractive index by current injection method, but the former ( 1) has the disadvantage that the refractive index differs depending on the direction of the electro-optic crystal, and it is difficult to miniaturize because the electrodes for forming the mirror are provided on the crystal surface, while the latter (2) The disadvantage was that the response speed was slow.

Therefore, the present inventors first applied for a patent application filed in 1983-
According to the specification of No. 39900, miniaturization with a high response speed of refractive index change is achieved by utilizing the refractive index change due to an applied electric field of an optical waveguide with an ultra-thin film multilayer structure forming a quantum well structure to exhibit various external optical circuit functions. A simple optical circuit functional element was proposed. As shown in FIG. 8, this conventionally proposed optical circuit functional element has two types of semiconductor thin films having different energy levels alternately laminated on a semiconductor substrate 1 to form an ultra-thin multilayer structure.
As shown in Figure 9, the conduction band CB and the valence band V.
B. A potential structure having a multilayer quantum well MQW is formed between the substrate 1 and the substrate 1, and electrodes 5 and 4 are provided above and below via a semiconductor layer 3 of opposite polarity to the substrate 1, and an electric field EF in the direction of the arrow is applied. The applied electric field EF
Accordingly, the potential structure shown in FIG. 9 is tilted as shown in FIG. Due to the inclination of the potential structure, when there is no electric field (V = 0), the electron wave function in the conduction band and the hole wave function in the valence band located at the center of the quantum well W, as shown by the dotted line in FIG. and depending on the applied electric field (V<0), the 11th
As shown by the solid lines in the figure, they shift in opposite directions,
The refractive index changes due to the shift in the wave functions of electrons and holes. Since such a change in refractive index occurs immediately in response to voltage application, it is possible to construct an external optical circuit having various functions with a quick response using a small semiconductor device by combining various optical waveguides provided with such an ultra-thin multilayer structure. I can do it.

(Problems to be Solved by the Invention) However, in this type of optical circuit functional element conventionally proposed as described above, the multilayer quantum well structure consists of quantum wells at a single energy level; When an electric field is applied to
Electron wave function in conduction band CB and valence band VB
As shown in FIG. There is a problem in that the manner in which the refractive index changes depending on the applied electric field is monotonous and the change in the refractive index is small.

The purpose of the present invention is to solve the above-mentioned problems in conventionally proposed optical circuit functional elements of this type, increase the change in refractive index in response to an applied electric field, and provide a multilayer multilayer device with a wider range of application and superior performance than the conventionally proposed devices. The objective is to realize an optical circuit functional element using a quantum well structure.

(Means for Solving the Problems) That is, the optical circuit functional element of the present invention includes an optical waveguide having, at least in part, an ultra-thin multilayer structure forming a stepped quantum well structure having at least three energy levels. , characterized in that the refractive index of the ultra-thin multilayer structure is changed according to the applied electric field to control the light propagating through the optical waveguide, and there are two types: an asymmetric type and a symmetric type. It can form a stepped quantum well structure.

(Function) Therefore, according to the present invention, since the quantum well structure that constitutes the ultra-thin multi-layer structure constituting the optical waveguide is made into a stepped structure, the refractive index of the ultra-thin multi-layer structure becomes irregular with respect to the applied electric field. However, it can be changed much more efficiently, and it is possible to realize an optical circuit functional element that has a much wider range of applications than the previously proposed single quantum well structure.

The same applies to the single well structure, but
Concomitantly with the increase in the change in refractive index in ultrathin multilayer structures, the change in light absorption due to the applied electric field also increases, but since the absolute amount of light absorption is small, such increase in light absorption change can be ignored. .

(Example) The present invention will be described in detail below with reference to the drawings.

The optical circuit functional element of the present invention is almost similar to the conventionally proposed optical circuit functional element having an optical waveguide of an ultra-thin multilayer structure having a single quantum well structure, as shown in FIG.
As shown in FIG. 1, for example, an n-type semiconductor substrate 1
An optical waveguide is constructed by laminating an ultra-thin multilayer structure 2 forming a multilayer quantum well structure MQW on top, and further providing electrode layers 5 and 4 above and below via a p-type semiconductor layer 3, and applying an electric field EF in the direction of the arrow. I am prepared, but
Unlike the conventional structure shown in FIG. 8, the ultra-thin film multilayer structure 2 is constructed by repeatedly laminating at least three types of semiconductor ultra-thin films each having a different energy level. Therefore, the potential structure formed by the ultra-thin multilayer structure having such a configuration is markedly different from the conventionally proposed single quantum well structure shown in FIG. , b has a stepped quantum well structure.

FIGS. 2a and 2b show an asymmetric stepped quantum well structure as the simplest basic configuration example of the stepped quantum well structure according to the present invention. That is, between the thin films made of, for example, InP located at both ends of the figure, there are layers of, for example, GaAlInAs, which have successively lower energy levels.
An ultra-thin semiconductor film consisting of GaInAsP and GaInAsP is sandwiched between the layers to form an asymmetric step-type quantum well W in which the energy potential changes in an asymmetric step-like manner in both the conduction band CB and the valence band VB. a and b are, for example, GaAlInAs and
The only difference is that the arrangement order of the GaInAsP semiconductor ultra-thin film is reversed. In the stepped quantum well structure with such a configuration, the quantum well W in the conduction band CB
The wave function of an electron in the quantum well W in the valence band VB and the wave function of a hole in the quantum well W in the valence band VB are as follows, even in the absence of an applied electric field as described below.
Unlike each wave function in a single quantum well shown by a dotted line in FIG. 11, the wave functions are unevenly distributed at the deep bottom of each quantum well.

Each semiconductor ultra-thin film is an ultra-thin film with a film thickness corresponding to the wavelength of electron/hole waves, and the ultra-thin film groups with the configurations shown in FIGS. 2a and 2b are arranged in the order shown, e.g.
InP-GaAlInAs-GaInAsP-InP... are repeatedly stacked in this order to form a multilayer structure with a layer thickness that corresponds to the wavelength of the light wave.

On the other hand, FIGS. 3a and 3b show examples of configurations of symmetrical stepped quantum well structures. That is, between the thin films made of, for example, InP located at both ends of the figure, for example, there is a layer having the sequentially lower energy order as described above.
Ultra-thin semiconductor films made of GaAlInAs and GaInAsP are arranged to form a symmetrical step-type quantum well W in which the energy potential changes in a symmetrical step-like manner in both the conduction band CB and the valence band VB, as shown in Figure 3. In a, conduction band CB, valence band VB
Both exhibit a downward convex potential structure,
FIG. 3b shows an upward convex potential structure in both the conduction band CB and the valence band VB. Incidentally, the ultra-thin film group having such a configuration is repeatedly laminated in the order shown in the figure to form a multilayer structure, which is the same as in the case of a symmetric stepped quantum well structure.

Therefore, when an electric field in the direction of the arrow y is applied to the ultra-thin film multilayer structure forming the asymmetric stepped quantum well structure shown in FIGS. Similarly, the electron wave function in the conduction band CB and the hole wave function in the valence band VB shift in opposite directions depending on the applied electric field, but in the asymmetric stepped quantum well structure, as shown in Fig. 4. As shown in Figures a and b, both the electron wave function and the hole wave function are significantly shifted to a state where they are more unevenly distributed at the deeper bottom of the quantum well W, compared to the single quantum well structure shown in Figure 11. As a result, the change in refractive index in response to the applied electric field is significantly increased. Moreover, even though the directions of electric fields applied to the same asymmetric stepped quantum well structure are opposite to each other in Figure 4 a and b, both the electron wave function and the hole wave function are significantly different in both cases. It is deviated,
Therefore, the direction of change in refractive index in response to an applied electric field can be either positive or negative, which is a much different effect than in a single quantum well structure.

On the other hand, when an electric field in the direction of the arrow y is applied to the ultra-thin film multilayer structure forming the symmetric stepped quantum well structure shown in Figure 3a and b, for example, the conduction pair CB and the valence electron pair are When both VB and VB have a downwardly convex potential structure, the hole wave function in the valence electron pair VB tends to shift toward the deeper end of the bottom of the quantum well W, as shown in Figure 5. , conduction band
The electron wave function in the CB is trapped at the deep bottom in the center of the quantum well W and is difficult to shift. Therefore, the transition between energy levels with different numbers is different from that in a single quantum well structure. Although the refractive index increases significantly, it does not increase as much as the above-mentioned asymmetric stepped quantum structure, and the refractive index changes in a manner different from that of any quantum well structure.

If the conduction band CB and valence band VB both have an upwardly convex potential structure as shown in Figure 3b, The same tendency is shown only if the mode of the corresponding shift is reversed, and even if the direction of electric field application in the symmetric stepped quantum well structure shown in Fig. 5 is reversed, the positive shift in the valence band VB A refractive index change similar to that described above with respect to FIG. 5 can be obtained by simply shifting the hole wave function deeper into the other end of the bottom of the quantum well W.

The optical circuit functional device according to the present invention includes an optical waveguide having an ultra-thin multilayer structure forming a stepped quantum well structure. Other than the increased size, it exhibits the same effect as the conventionally proposed optical circuit functional element using a single quantum well structure, so it has various functions with the same configuration as the conventionally proposed optical circuit functional element. Optical circuits can be realized much more efficiently.

For example, as shown in FIG. 6, an optical waveguide 6 having an ultra-thin multilayer structure forming a stepped quantum well structure
If a branch optical waveguide 7 is provided that shares a part of the ultra-thin multilayer structure, and an electric field is appropriately applied to the shared part of the ultra-thin multilayer structure by an electrode 5 (shown in shading), no problem can be achieved. Efficiently realizes an optical switch circuit that exhibits the function of completely reflecting the incident light wave Lin, which travels straight to the output end Oa when an electric field is applied, due to the difference in refractive index that occurs at the interface of the electric field application region, and guiding it to the output end Ob of the branch optical waveguide 7. can do.

Further, as shown in FIG. 7, part of the ultra-thin multi-layer structure of a pair of optical waveguides 6a and 6b, both of which have an ultra-thin multi-layer structure forming a stepped quantum well structure, are arranged in parallel and close to each other, Electrodes 5a, 5b are formed on the ultra-thin multilayer structure of these close parallel parts.
By providing these and applying voltages Va and Vb, respectively, it is possible to appropriately control the leakage movement of light waves between the ultra-thin multilayer structures in adjacent parallel portions, and efficiently realize a directional optical coupling circuit.

(Effects of the Invention) As is clear from the above description, according to the present invention, the ultra-thin multi-layer structure in the conventionally proposed optical circuit functional element equipped with the optical waveguide of the ultra-thin multi-layer structure forming a single quantum well structure is improved. By making the quantum well structure formed into an asymmetrical or symmetrical step-type structure, the efficiency of refractive index change in response to an applied electric field of this type of optical circuit functional element can be greatly increased, and therefore,
A special effect can be obtained in that optical circuits with various functions that can be constructed using this type of optical circuit functional element can be realized with much higher efficiency.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of the structure of the optical circuit functional element of the present invention, FIGS. 4a and b are diagrams respectively showing configuration examples of the symmetric stepped quantum well structure in the optical circuit functional element, and FIGS.
Diagrams showing examples of the behavior of the asymmetric stepped quantum well structure shown in Figures a and b, depending on the applied electric field, and Figure 5 shows the applied electric field of the symmetric stepped quantum well structure shown in Figure 3 a. FIG. 6 is a diagram showing an example of a configuration in which the optical circuit functional element of the present invention is applied to an optical switch circuit, and FIG. 7 is a diagram showing an example of the operation mode according to the optical circuit functional element of the present invention. A diagram showing a configuration example applied to an optical coupling circuit, FIG. 8 is a cross-sectional view showing the configuration of a conventional optical circuit functional element, and FIG. 9 is a diagram showing the configuration of a quantum well structure in the optical circuit functional element. , FIG. 10 is a diagram showing the configuration of the quantum well structure shown in FIG. 9 when an electric field is applied, and FIG. 11 is a diagram showing the configuration of the quantum well structure shown in FIG.
FIG. 3 is a diagram illustrating how the illustrated quantum well structure behaves in response to an applied electric field. 1...Semiconductor substrate, 2...Ultra-thin film multilayer structure, 3
... Semiconductor layer, 4, 5, 5a, 5b ... Electrode,
6, 6a, 6b... optical waveguide, 7... branch optical waveguide, CB... conduction band, VB... valence band, W...
...Quantum well.

Claims (1)

[Scope of Claims] 1. An optical waveguide having at least a part of an ultra-thin multi-layer structure forming a stepped quantum well structure having at least three energy levels, the refractive index of the ultra-thin multi-layer structure depending on an applied electric field. An optical circuit functional element characterized in that it is configured to be able to control light propagating through the optical waveguide by changing it accordingly. 2. In the functional device according to claim 1, the ultra-thin film multilayer has an asymmetric stepped quantum well structure formed by asymmetrically arranging and repeatedly stacking at least three types of semiconductor ultra-thin films each having a different energy level. An optical circuit functional element characterized by having a structure. 3. In the functional device according to claim 1, the ultra-thin film multilayer has a symmetric stepped quantum well structure formed by symmetrically arranging and repeatedly stacking at least three types of semiconductor ultra-thin films each having a different energy level. An optical circuit functional element characterized by having a structure. 4. The functional device according to claim 3, characterized in that the ultra-thin film multilayer structure is configured to form the symmetric stepped quantum well structure in which the energy levels of the conduction band are symmetrically and sequentially different. Optical circuit functional element. 5. The functional device according to claim 3, characterized in that the ultra-thin multilayer structure is configured to form the symmetric stepped quantum well structure in which the energy levels of the valence band are symmetrically and sequentially different. Optical circuit functional element. 6. An optical circuit in the functional element according to claim 2, characterized in that an electric field in a direction opposite to the change in energy level in the asymmetric stepped quantum well structure is applied to the ultra-thin multilayer structure. Functional element. 7. An optical circuit functional element according to claim 2, characterized in that an electric field in the same direction as the change in energy level in the asymmetric stepped quantum well structure is applied to the ultra-thin multilayer structure. . 8. The functional device according to any one of claims 1 to 7, wherein the optical waveguide is provided with a branched optical waveguide that shares a part of the ultra-thin film multilayer structure, The light propagating through the optical waveguide is guided to the branch optical waveguide by applying an electric field to a portion of the shared portion to change the refractive index of the ultra-thin multilayer structure of the portion. Optical circuit functional element. 9. The functional elements according to any one of claims 1 to 7 are arranged in pairs with portions of each of the ultra-thin multilayer structures parallel to each other and close to each other, and the functional elements of each of the ultra-thin multilayer structures are By applying an electric field to the part to change the refractive index of the ultra-thin film multilayer structure of the part, the light propagating through one of the optical waveguides is changed between the parts of the ultra-thin film multilayer structure. An optical circuit functional element characterized by being guided to an optical waveguide.