JPS61209421A - Optical modulator for which superlattice layer is used - Google Patents

Abstract

PURPOSE:To make the efficient change of refractive indices possible by the lower driving voltage by adopting layer structure in which superlattice layers are used and impressing the voltage to the superlattice layers. CONSTITUTION:This optical modulator is constituted by laminating repeatedly the 1st layers 1-1, 1-n and the 2nd layers 2-1, 1-n having the different refractive indices and using the superlattice layers consisting of extra-thin films (a), (b) consisting of different compsns. for the 2nd layers 2-1, 1-n. The repetitive period T thereof is made half the wavelength and the refractive indices are changed by applying the electric field to the superlattice layers. Light reflects at the boundary faces of the respective layers having the different refractive indices when the light is made incident through an upper transparent electrode 3 on the optical modulator of such multi-layered film type. The reflected light is intensified in the phase under certain condition and is weakened in the phase under the other condition.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical modulator using a superlattice layer that modulates light by changing the refractive index of light by applying a voltage (electric field) to the superlattice layer. It is something.

[Conventional technology]

In recent years, so-called optronics technology, such as optical communication, optical measurement, and optical information, has been actively developed, and its practical application is expected in various fields.

[Problem that the invention seeks to solve]

Optical applied technology has many possibilities, but compared to the field of electronics, there are still many issues to be solved, and much remains to be achieved in future research.

An object of the present invention is to provide an optical modulator using a superlattice layer that can efficiently change the refractive index and modulate light using a voltage.

[Means for solving problems]

For this purpose, an optical modulator using a superlattice layer according to the present invention is an optical modulator using a superlattice layer in which ultrathin films having different compositions are laminated, and the superlattice layer and the superlattice layer have a different refractive index. A large number of layers with different refractive indexes are stacked at a repeating period of 1/2 of the wavelength, or a large number of layers with different refractive indexes are stacked at a repeating period of 1/2 of the wavelength, and the middle part of the large number of layers is stacked with a repeating period of 1/2 of the wavelength. A third layer is interposed, at least one of the two layers and the third layer is a superlattice layer, and an electric field is applied to the superlattice layer to change the refractive index, thereby changing the reflectance. It is characterized by this.

Another feature is that the superlattice layer is used as an optical waveguide, and an electric field is applied to the superlattice layer to change the refractive index, thereby changing the reflectance and propagation path of light within the optical waveguide.

[Effect]

In the optical modulator using the superlattice layer of the present invention, the refractive index of the superlattice layer changes efficiently depending on the applied voltage.The degree of change in the refractive index depends on the thickness of the superlattice and the applied voltage. It depends on the magnitude of the voltage, and as these values increase, the refractive index will change corresponding to the degree of increase, and the reflectance and light propagation path will also change with this change. When a third layer is interposed in the middle part of a large number of laminated layers with a repetition period of 1/2, wavelengths with high reflectance and wavelengths with low reflectance repeatedly appear in a relatively short period due to the influence of this third layer. , this peak can be shifted with a small voltage change.Furthermore, when a superlattice layer is used as an optical waveguide, the change in refractive index is detected as a change in the reflectance and propagation path of light in the optical waveguide. .

〔Example〕

Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing an example of an optical modulator using a superlattice layer according to the present invention, FIG. 2 is a diagram showing an example of the characteristics of a multilayer film optical reflector, and FIG. FIG. 4 is a diagram showing an example of the refractive index, and FIG. 4 is a diagram showing an example of the characteristics of an optical modulator using a superlattice layer.

In order to efficiently control light, it is necessary to increase the change in refractive index and to significantly counteract this change in reflectance or propagation path. FIG. 1 shows an embodiment of an optical modulator using a superlattice layer according to the present invention that meets these requirements, and 1-1.1-
n represents the first layer, 2-1 and 'l-n represent the second layer, 3 represents the upper transparent electrode, and 4 represents the lower electrode. Hereinafter, in the present invention, a superlattice layer refers to a layer formed by laminating extremely thin films having different compositions, in which at least one of the extremely thin films has a thickness comparable to the wavelength of electrons.

The optical modulator using the superlattice layer shown in FIG. 1 consists of a first layer 1-1, l-n and a second layer 2-1.
are repeatedly laminated, one of which is the second one.
A superlattice layer consisting of ultrathin films a and b having different compositions is used as the layer 2-1.2-n. The repetition period T is set to 1/2 of the wavelength, and an electric field is applied to the superlattice layer. In the refractive index optical modulator, when light enters from the outside through the upper transparent electrode 3, the light is transmitted through each layer with a different refractive index. Interface (first
(the interface between the layer 1-1.1-n and the second layer 2-1.2-n). The phase of the reflected light will be strengthened under certain conditions, and the phase will be weakened under other conditions.

Therefore, in the multilayer optical modulator shown in FIG. -1, second layer 2
-1 is 1/4), and the reflection spectrum is examined as shown in Figure 2. As is clear from Fig. 2, if the number n of repetitions consisting of the first layer and the second layer is about 20, then one cycle is half the wavelength, that is, the wavelength is around 8,500 people. The reflectance also increases sharply. Therefore, in a multilayer reflector having the characteristics shown in FIG. 2, if the refractive index is changed around 8200 or 8800 people, the reflectance can be greatly changed.

By the way, GaAs/^IAs and GaAs/^lGaA
The refractive index of a superlattice layer made of s or the like has the characteristic of increasing with photon energy as shown in FIG. 3, and exhibits a protrusion at the fundamental absorption edge. The slope of this refractive index shifts in the direction of the illustrated arrow when an electric field is applied. The amount of this shift varies slightly depending on various conditions, but if the electric field strength applied to the superlattice layer is E and the film thickness of the superlattice is d, then EXd
Approximately corresponds to the O value.

Therefore, we set the electric field strength E to 104 to 10'V/cm and the film thickness d.
is 10-'cm (100 people), the slope of the refractive index shown in Figure 3 is approximately 10-'V to lO-'V (10
100 meV). As a result, 1, 3 eVNl, 4 eV in Figure 3
A change in refractive index of about 0.05 can be obtained at 100 meV in the range of It will happen.

When the refractive index changes and decreases by applying an electric field to a superlattice layer, the optical path length of that layer also decreases correspondingly. This changes the resonance conditions, causing the peak to shift. Therefore, if we look at this in the vicinity of 8,200 people in Figure 2, we can see a large change in reflectance. In addition, since the optical medium is shortened by the refractive index, the optical length corresponding to 1/4 wavelength with a refractive index of 3.5 to 4.2 is 420 mm.
Assuming that there are 0 people, the actual film thickness is 1200 people to 105 people.
There will be 0 people. According to the above considerations, GaAs/^I is used in the second layer 2-1.2-n of the multilayer optical modulator shown in FIG.
FIG. 4 shows an example of reflection spectrum characteristics when constructed using an As superlattice layer, using actually measured values (solid line) and calculated values (dotted line). In this case, if the layer thickness corresponding to the optical path length of a quarter wavelength is 600, then the second layer 2-1.
2-n is GaA made of an extremely thin film of about 100 to 80 layers.
It is composed of an s/AlAs superlattice layer. FIG. 4 shows the average reflectance of such superlattice layers.

FIG. 5 is a diagram showing another embodiment of the optical modulator using the superlattice layer according to the present invention, and FIG. 6 is a diagram showing the reflection characteristics of the optical modulator shown in FIG. 5.

In the multilayer optical modulator shown in FIG. 5, 5 is an electrode, 6 and 8 are laminated parts, 7 is a third layer, 9 is an electrode, and 10 is an A1GaAs layer. Laminated parts 6 and 8 as shown
A third layer 7 using a superlattice is further interposed between the two layers. With such a structure, light incident from the outside through the transparent electrode 5 is transmitted to the interface between each layer of the laminated portion 6, the interface between the laminated portion 6 and the third layer 7, and the interface between the third layer 7 and the laminated portion. 8 and at the interfaces of each layer of the laminated portion 8. Therefore, the reflected light from each of these interfaces will strengthen each other's phases at different wavelengths, and weaken each other's phases at different wavelengths, and as a result, the measured value of the reflection spectrum (solid line) as shown in Figure 6 and The calculated value (point vA) is obtained. As is clear from this characteristic, the envelope shows a characteristic that depends on the laminated portion 6.8 as explained earlier in FIGS. The wavelength appears repeatedly in a relatively short period,
A very steep change in reflectance can be obtained. Therefore, the fifth
In the structure shown in the figure, it is possible to greatly change the reflectance by a small change in the electric field applied to the electrode.

In addition, in the structure shown in FIG. 5, the upper and lower laminated parts (multilayer reflective films) 6.8 are normal multilayer films that do not use a superlattice layer, and only the third layer 7 has a superlattice structure. .8 (between electrodes 5 and 9) or above and below the third layer 7 (laminated part 6)
．． 8, electrodes are not shown), or a method in which a superlattice layer is used in one or both of the upper and lower laminated parts 6.8, and the third layer 7 is a normal multilayer without using a superlattice layer. As a membrane,
Between the upper and lower parts of the laminated part 6.8 (electrodes 5.9), between the upper and lower parts of the laminated part 6 (the electrodes provided between the electrodes 5 and the third layer 7), or between the upper and lower parts of the laminated part 8 (the third layer 7) A method may be adopted in which a voltage is applied between the electrode provided between the electrode 9) and the electrode 9). Note that the above structure can be similarly applied to transmitted light.

As explained above, the optical modulator using a superlattice layer of the present invention employs a layer structure using a superlattice layer and is configured to apply a voltage to this superlattice layer, thereby achieving a low driving voltage. This makes it possible to efficiently change the refractive index. Therefore, as shown in Figures 1 and 5, one quarter
Based on a laminated structure with multiple repeating layers with a film thickness equivalent to the optical path length of the wavelength, it can be used not only as a reflector that changes the reflectance by changing the refractive index, but also as an optical waveguide that changes the reflectance and propagation path of light. It can be applied. An example thereof will be described below.

FIGS. 7 and 8 are diagrams showing other embodiments of the present invention applied to a waveguide in which light is incident in parallel from a direction along the interface of layers. In the figure, 11, 16, 18 and 23 are electrodes, 1
2.15.19 and 22 are AlGaAs layers, 13 is a separation layer, 14 and 21 are waveguides, 17 and 24 are GaAs substrates, 2
0 indicates an uneven portion, respectively.

In the application example of the waveguide shown in FIG. 7, a superlattice layer is used for the waveguide 14, and this waveguide 14 is placed close to both sides of the separation layer 13, and the light B is directed in parallel from the direction along the interface. Electrodes 11 and 16 are provided above and below this waveguide 14. In this way, the light B moves between adjacent waveguides every time it travels a predetermined distance. Since the distance differs depending on the refractive index, by controlling the voltage applied to the superlattice layer of the waveguide using electrodes 11 and 16, it is possible to output either light B or light B2 as an optical switch. It is possible to switch the selection and take it out. In addition, At
The GaAs layers 12 and 15 have a low refractive index and form layers that allow light to enter, and are used to confine light B so that it does not escape to the outside.

In the example of application of the waveguide shown in FIG. 8, a superlattice layer is used for the waveguide 21, and uneven portions 20 are provided on the surface of the waveguide 21 at intervals where one period corresponds to 1/2 wavelength. are incident in parallel from the direction along the interface. Figure 8) shows the state in which the AlGaAs layer 19 and electrode 18 have been removed in Figure 8 (al).On the surface of such a waveguide 21, uneven portions 20 are formed at intervals where one period is 1/2 wavelength. In the structure provided, light C is reflected at the uneven portion 20, but the electrode 18 is not connected to the uneven portion 20 or the superlattice portion in between
When a voltage is applied using and 23, the refractive index changes and the optical path length changes, so the reflection characteristics at the uneven portion 20 can be changed. In FIG. 8, the uneven portion 20 may be formed on the entire surface or only on one side. If the uneven portion 20 is located on both sides of the regions ■ and ■ with the intermediate region ■ in between, the length of the intermediate region ■ may be arbitrary. You may choose to. In addition, the electrodes can be configured to apply voltage to all of these regions ■, ■, and ■, or to apply voltage only to area ■, only to area ■, only to area ■, or to areas ■ and ■. The configuration may be such that the voltage is applied.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, a layered structure using a superlattice layer is adopted and a voltage is applied to this superlattice layer, so that the refractive index can be efficiently controlled by a low driving voltage. Can be changed often. Therefore, in a reflector that reflects light with a multi-repetitive laminated structure of layers with a film thickness equivalent to the optical path length of a quarter wavelength, the reflectance can be efficiently adjusted by changing the electric field of the superlattice layer and changing the refractive index. Furthermore, when applied as an optical waveguide, the reflectance and propagation path of light within the waveguide can be efficiently changed by changing the electric field of the superlattice layer and changing the refractive index. Moreover, even if the driving voltage is low,
The refractive index can be changed efficiently. Therefore,
It can be widely applied to devices that control and modulate light by changing the refractive index.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of an optical modulator using a superlattice layer according to the present invention, FIG. 2 is a diagram showing an example of the characteristics of a multilayer film optical reflector, and FIG. Figure 4 shows an example of the refractive index, Figure 4 shows an example of the characteristics of an optical modulator using a superlattice layer, and Figure 5 shows an example of the characteristics of an optical modulator using a superlattice layer.
6 is a diagram showing another embodiment of the optical modulator using the superlattice layer according to the present invention, FIG. 6 is a diagram showing the reflection time characteristics of the optical modulator shown in FIG. 5, and FIGS. FIG. 8 is a diagram showing another embodiment of the present invention applied to a waveguide in which light is incident in parallel from the direction along the interface of layers. 1-1, l-n...first layer, 2-1.2-n...
second layer, 3... upper transparent electrode, 4... lower electrode,
5.9.11.16.18 and 23...electrodes, 6 and 8.
...Lamination part, 7...Third layer, 10.12.15.1
9 and 22-AlGaAs layer, 13... Separation layer, 14 and 21... Waveguide, 17 and 24... GaAs substrate, 2
0...Concave and convex portion patent applicant Ryuukichi Abe Patent attorney representing New Technology Development Corporation Figure 1 Figure 2 Figure 3 Mitsuko, Energy (eV) Figure 4 Figure 6 Kaiho (A)

Claims (3)

[Claims] (1) An optical modulator using a superlattice layer formed by laminating ultrathin films with different compositions, in which the superlattice layer and a layer having a different refractive index from the superlattice layer are repeated at a repetition period of half the wavelength. An optical modulator using a superlattice layer, characterized in that a large number of superlattice layers are laminated, and an electric field is applied to the superlattice layer to change the refractive index, thereby changing the reflectance. (2) An optical modulator using a superlattice layer made by laminating ultrathin films with different compositions, in which a large number of two layers with different refractive indexes are laminated at a repeating period of half the wavelength, and A third layer is interposed in the laminated intermediate portion, and the above 2
The superlattice layer is characterized in that at least one of the two layers and the third layer is a superlattice layer, and an electric field is applied to the superlattice layer to change the refractive index, thereby changing the reflectance. light modulator. (3) An optical modulator using a superlattice layer made by laminating ultrathin films with different compositions, in which the superlattice layer is used as an optical waveguide, and an electric field is applied to the superlattice layer to change the refractive index. An optical modulator using a superlattice layer that changes the reflectance and propagation path of light within an optical waveguide.