Ultra-fast optical switch

Abstract

To provide an ultra high speed optical switch capable of outputting a high-speed, high-intensity signal using a simple configuration.SOLUTION: An ultra high speed optical switch disclosed herein comprises a substrate, a buffer layer formed on one surface of the substrate, and a multiple quantum well layer formed on one surface of the buffer layer. The buffer layer is made of a material obtained by adding P to GaAs, the material having a lattice constant smaller than that of the multiple quantum well layer. The multiple quantum well layer is compressively strained by the buffer layer.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an ultra-high-speed optical switch suitable for optical communication and optical information processing.

Background Art

[0002] In order to realize ultra-high-speed information processing such as exceeding 1 Tbps by optical signals, ultra-high-speed optical switches that control light by light have been developed. In such ultra-high-speed optical switches, since lights do not interact with each other, a switching operation that controls the state of light through the electronic state in a solid material such as a semiconductor is common.

[0003] In this series of processes, if the lifetime of electrons is longer than the interval at which light pulses are irradiated, a pattern effect occurs in which the information of the previous signal is superimposed on the next signal, causing distortion of the signal waveform. In order to prevent such distortion of the signal waveform, it is necessary to shorten the lifetime of electrons. On the other hand, in order to reliably perform signal processing, a larger signal intensity is required.

[0004] That is, in order to realize an ultra-high-speed optical switch, a larger signal intensity and ultra-high-speed relaxation of electrons are required, but it has been extremely difficult to realize these two conditions simultaneously. Therefore, ultra-high-speed optical switches utilizing the relaxation of the electron phase have been proposed. For example, a photon echo, which is one of the third-order nonlinear optical processes, or an optical switch using a four-wave mixing signal is known (see, for example, Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0006] To realize ultrafast optical switches, research has traditionally focused on achieving ultrafast speeds by utilizing two-photon absorption processes without the need for electrons. However, in this case, the low signal intensity necessitates the irradiation of higher-intensity optical pulses or the use of waveguide structures to increase the interaction length. Such structures present numerous challenges from the perspectives of energy conservation and signal waveform deformation.

[0007] On the other hand, from the perspective of increasing signal intensity, it is preferable to utilize the light absorption of semiconductors, especially excitons (weakly bound electron-hole states). However, since these excitons have a relatively long lifetime even at room temperature, it has been difficult to achieve an ultrafast response that allows for repeated operation within extremely short periods (less than 1 picosecond). To overcome this, methods have been proposed, such as using a micro-resonator to rapidly relax stimulated emission, or adding impurities to forcibly relax the excitons. However, these methods have drawbacks, such as reduced signal intensity, more complex structures, and increased manufacturing costs.

[0008] Furthermore, as mentioned above, the low signal intensity was a problem when using four-wave mixing. Moreover, the exciton binding energy, an indicator of exciton stability in semiconductor crystals, is extremely small compared to the thermal energy at room temperature in ordinary semiconductors. Therefore, it has been believed that excitons only exhibit exciton properties in the low-temperature region, and that excitons easily dissociate at around room temperature.

[0009] To overcome this, one might consider using semiconductors with high ionicity (e.g., GaN, ZnO), but since the light absorption energy exists in a high-energy band with a wavelength range far shorter than the communication wavelength band, it is difficult to apply this to devices in actual communication. In this situation, the use of interference phenomena between exciton levels (exciton quantum beats) has been proposed as a way to realize faster and stronger signals, but this also has the challenge of being limited to low-temperature environments.

[0010] This invention has been made in view of the above-mentioned problems, and aims to provide an ultra-high-speed optical switch that can output high-speed and high-intensity signals with a simple configuration. [Means for solving the problem]

[0011] The inventors epitaxially grew a GaAsP buffer layer on a GaAs substrate, and then epitaxially grew a multiple quantum well layer on top of it, thereby applying compressive strain to the multiple quantum well layer. When the pump-probe signal of such a sample was measured, a component that relaxes rapidly was observed. With this configuration, an ultrafast optical switch that can output a fast and high-intensity signal can be realized with a simple configuration.

[0012] One embodiment of the ultrafast optical switch proposes the following means: (1) An ultrafast optical switch according to embodiment 1 of the present invention comprises a substrate, a buffer layer formed on one surface of the substrate, and a multiple quantum well layer formed on one surface of the buffer layer, wherein the buffer layer is composed of GaAs with added P, having a smaller lattice constant than the multiple quantum well layer, and the multiple quantum well layer is a layer subjected to compressive strain by the buffer layer.

[0013] (2) Embodiment 2 of the present invention is characterized in that, in the ultrafast optical switch of Embodiment 1, the composition ratio of P contained in the buffer layer is in the range of 3 atomic% or more and 20 atomic% or less.

[0014] (3) A third aspect of the present invention is an ultrafast optical switch according to aspect 1 or 2, characterized in that the multiple quantum well layer is a GaAs / AlAs epitaxially grown film. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide an ultra-high-speed optical switch that can output high-speed and high-intensity signals with a simple configuration. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram showing the configuration of the optical control device including the ultrafast optical switch of this embodiment. [Figure 2] This is a schematic cross-sectional view showing the configuration of the ultra-high-speed optical switch of this embodiment. [Modes for carrying out the invention]

[0017] The following describes an ultra-high-speed optical switch according to one embodiment of the present invention with reference to the drawings. The embodiments described below are provided specifically to better illustrate the spirit of the invention and do not limit the present invention unless otherwise specified. In addition, the drawings used in the following description may be enlarged for convenience to make the features of the present invention easier to understand, and the dimensional ratios of each component may not be the same as in reality.

[0018] Figure 1 is a schematic diagram showing the configuration of an optical control device including an ultrafast optical switch according to this embodiment. The optical control device 1 of this embodiment includes an input light irradiator 2, a control light irradiator 3, and an ultra-high-speed optical switch 10. The input light irradiator 2, for example, directs input light L1, capable of exciting an exciton energy level and multiple higher-order exciton energy levels, onto the ultrafast optical switch 10, which acts as an exciton.

[0019] The control light irradiator 3 makes the control light L2, which can excite the exciton energy level and a plurality of higher-order exciton energy levels, enter the ultrafast optical switch 10, and utilizes the vibration structure generated by the excitation of the plurality of exciton energy levels to control the emission of the response signal light L3 from the ultrafast optical switch 10.

[0020] The ultrafast optical switch 10 serving as an exciton causes the vibration structure of the exciton state to occur due to quantum interference when at least one of the input light L1 and the control light L2 excites a plurality of exciton energy levels.

[0021] FIG. 2 is a schematic cross-sectional view showing the configuration of the ultrafast optical switch of the present embodiment. The ultrafast optical switch 10 of the present embodiment includes at least a substrate 11, a buffer layer 12 formed on one surface 11a side of this substrate, and a multiple quantum well layer 13 formed on one surface 12a side of this buffer layer 12. Further, a p-type semiconductor layer 14 is formed on top of the multiple quantum well layer 13.

[0022] The substrate 11 is composed of, for example, gallium arsenide (GaAs). GaAs is a group III-V semiconductor having a bandgap of 1.43 eV. The substrate 11 may be formed to have a thickness of, for example, about 500 μm to 200 μm. Also, when considering a device of the type that transmits light as shown in FIG. 1, the substrate can be polished to make it thinner.

[0023] The buffer layer 12 is a layer made of an n-type semiconductor (GaAsP) in which phosphorus is added to gallium arsenide, formed on one surface 11a side of the substrate by epitaxial growth. The composition ratio of P contained in the buffer layer 12 is in the range of 3 atomic % or more and 20 atomic % or less. In the present embodiment, P is contained at 5 atomic %. In the present embodiment, the buffer layer 12 is an n-type semiconductor by impurity doping, but it can also be non-doped by making the substrate a semi-insulating substrate or the like, and the semiconductor type is not limited.

[0024] The multiple quantum well layer 13 is a layer that forms a strained multiple quantum well structure by alternately forming a well layer 13a made of gallium arsenide (GaAs) with a thickness of 7.25 nm and a barrier layer 13b made of aluminum arsenide (AlAs) with a thickness of 7.25 nm by epitaxial growth. The thickness of the multiple quantum well layer 13 can generally be in the range of 5 nm to 20 nm.

[0025] Conventionally, it has been believed that a highly crystalline, high-quality optical switch can be obtained by combining materials with lattice constants as close as possible to each other for the substrate material, the buffer layer material, and the well layer and barrier layer of the multiple quantum well layer.

[0026] However, the band gap energy is closely related to the lattice constant, and a larger difference in band gap energy between the well layer and the barrier layer is desirable for effectively confining electrons and holes in the multiple quantum well layer. For this reason, the combinations of materials that can actually be deposited by epitaxial growth are limited, and conventionally, most are III-V semiconductors. Furthermore, even with III-V semiconductors, if the difference in lattice constants is large, nanostructures called quantum dots, quantum dashes, or quantum islands are formed, and a multiple quantum well layer with a uniform film structure cannot be obtained.

[0027] On the other hand, in the ultra-high-speed optical switch 10 of this embodiment, by adding 3 atomic% to 20 atomic% or less, for example 5 atomic% of P, to the buffer layer 12, it is possible to form a highly crystalline, high-quality multiple quantum well layer 13 in which compressive strain is applied to the multiple quantum well layer 13.

[0028] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Examples]

[0029] The ultrafast optical switch of the present invention was actually fabricated as follows. First, a buffer layer was formed on a GaAs substrate with a thickness of approximately 350 μm by growing GaAsP to a thickness of approximately 100 nm. The amount of phosphorus (P) in this GaAsP was set to 5 atomic percent. In this embodiment, the GaAs substrate used was a bulk GaAs substrate on which a GaAs epitaxial film with a thickness of approximately 400 nm had been formed.

[0030] Next, a multiple quantum well layer was formed on top of this buffer layer by alternately epitaxially growing well layers made of GaAs with a thickness of 7.25 nm and barrier layers made of AlAs with a thickness of 7.25 nm. Furthermore, a p-type semiconductor layer was formed on top of this multiple quantum well layer. In this embodiment, the well layer and the barrier layer are formed to have the same thickness, but the thicknesses of the well layer and the barrier layer may be different and are not limited to this. Through the above process, the ultrafast optical switch of the embodiment was formed in which compressive strain was applied to the multiple quantum well layer by 5 atomic percent of P contained in the buffer layer.

[0031] Using the ultrafast optical switch obtained in this embodiment, pump-probe spectroscopy was performed at room temperature, exciting with pump light and measuring with probe light. A component that relaxes rapidly was observed. Since the energy of the laser light used for the measurement was approximately at the center of the energies of the two quantized excitons (heavy hole exciton and light hole exciton), it is thought that rapid relaxation occurs due to interference between exciton levels.

[0032] Although no clear oscillation structure due to quantum beats was observed in the measured signal, it is thought that the excitons generated at the moment of pulse irradiation interfered, and the coherence relaxed in a time of less than one cycle, resulting in the observation of fast relaxation.

[0033] These results confirm that an ultrafast optical switch can be realized with a simpler structure than conventional ones. In comparison with conventional systems, such fast relaxation components were not observed in the multiple quantum well layers of optical switches with a configuration similar to that of the present invention, except for a buffer layer without phosphorus (GaAs).

[0034] The key points of this experiment are that it was conducted at room temperature and that measurements were performed using pump-probe spectroscopy. To date, there have been very few examples of observing quantum beats caused by the simultaneous excitation of two types of excitons at room temperature, and this is especially rare in GaAs-based semiconductor materials with low exciton binding energies.

[0035] Furthermore, the ultrafast optical switch using quantum beats described above was based on measurement results obtained by four-wave mixing, but this reflected the phase relaxation of excitons and did not observe changes in exciton lifetimes. Therefore, it did not essentially solve the pattern effect problem.

[0036] In contrast, the present invention posits that the signal observed by pump-probe spectroscopy is a change in lifetime and is directly related to the pattern effect. Therefore, it is believed that the pattern effect can be suppressed by irradiating pulses in accordance with the interference period. Since this interference period can be controlled by changing the thickness of the quantum well, faster switching can be achieved. [Industrial applicability]

[0037] The ultrafast optical switch of the present invention can be applied to an ultrafast optical switch that controls the emission of response signal light from an exciton by irradiating it with input light and control light. Therefore, the present invention has industrial applicability. [Explanation of Symbols]

[0038] 1… Optical control device 2…Input light irradiator 3…Controlled light irradiator 10…Ultra-fast optical switch 11… Circuit board 12... Buffer layer 13…Multiple quantum well layers 13a... Well layer 13b... Barrier layer

Claims

1. The device comprises a substrate, a buffer layer formed on one surface of the substrate, and a multiple quantum well layer formed on one surface of the buffer layer. The buffer layer is composed of GaAs with added P, which has a smaller lattice constant than the multiple quantum well layer. The ultrafast optical switch is characterized in that the multiple quantum well layer is a layer subjected to compressive strain by the buffer layer.

2. The ultrafast optical switch according to claim 1, characterized in that the composition ratio of P contained in the buffer layer is in the range of 3 atomic% or more and 20 atomic% or less.

3. The ultrafast optical switch according to claim 1 or 2, characterized in that the multiple quantum well layer is a GaAs / AlAs epitaxially grown film.

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