JPH06123931A - Optical information storage device - Google Patents

Abstract

PURPOSE:To enable the recording and reading out of information with wavelength multiplexing even if the layer thicknesses of quantum wells are not exactly controlled by differentiating the spectra of the transmitted light or absorbed light of the reading out light with which the plural quantum well layers varying in the layer thicknesses are irradiated with photon every. CONSTITUTION:A storage element 1 is irradiated with the reading out light from a reading out light source 11 and the transmitted light thereof is subjected to wavelength dispersion by a spectroscope 12 in the case of reading of the information out of the storage element 1. The transmitted light subjected to the wavelength dispersion is sent to a photodetector 13 and is converted to the electric signal corresponding to the light intensity of every wavelength. This electric signal is, therefore, the signal corresponding to the spectra of the transmitted light. This electric signal is inputted to a differentiating circuit 15 and is differentiated. This differentiation corresponds to the differentiation of the spectra of the transmitted light with photon energy and, therefore, the signal having a peak in the position corresponding to the energy of the writing light is obtd. When the information is recorded on the storage element 1 by irradiating the element with the writing light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information storage device capable of optically writing and reading information, and more particularly to an optical information storage device capable of writing and reading information by wavelength multiplexing.

[0002]

2. Description of the Related Art As a conventional optical information storage device capable of writing / reading information by wavelength multiplexing, Japanese Patent Application No. 63-17 is available.
Some are published in 7131. In the optical information storage device disclosed in this publication, a quantum well formed of a direct transition type semiconductor has a single sharp exciton absorption line spectrum based on optical absorption transition, and this exciton absorption line spectrum Was observed at different positions depending on the layer thickness of the quantum well, and furthermore, when the quantum well was irradiated with wavelength light corresponding to the exciton absorption line spectrum, the light absorption rate decreased and absorption saturation occurred. It is a thing.

That is, the quantum wells having different layer thicknesses resonate individually with the writing lights having different wavelengths to cause absorption saturation. In the quantum well where absorption saturation occurs, the absorptance of light having a wavelength at which it resonates extremely decreases, and that state is maintained for a while. Therefore, even if the quantum well that has undergone absorption saturation is irradiated with light having a wavelength at which it resonates, almost no light absorption occurs. On the other hand, in a quantum well in which absorption saturation does not occur, the light absorptance does not decrease, and therefore, when the wavelength light with which it resonates is irradiated, the light absorption of the wavelength light naturally occurs.

Therefore, whether or not absorption saturation has occurred is determined by irradiating the quantum well with read light containing a wavelength light at which the quantum well resonates, analyzing the spectrum of the transmitted light or the absorbed light, and measuring the wavelength light. It can be known by examining the absorption amount (change in absorption rate) of. The quantum well can be used as an optical memory by associating the state in which absorption saturation occurs with 1 (or 0) and the state without absorption saturation in 0 (or 1). Moreover,
If the quantum wells have different widths (layer thicknesses), they will resonate individually with light of different wavelengths to cause absorption saturation. Therefore, if quantum wells with different layer thicknesses are stacked, such quantum wells With respect to the well structure, individual information can be recorded and read out in each quantum well by wavelength multiplexing.

[0005]

The optical information storage device disclosed in the above publication is premised on that the quantum well has a single sharp exciton absorption line spectrum based on the optical absorption transition. However, the absorption spectrum of a quantum well includes a continuous absorption spectrum in addition to a single sharp exciton absorption line spectrum. Therefore, in order to record and read information in the quantum well structure by wavelength multiplexing as described above. For this reason, the exciton absorption line spectrum and the continuous absorption spectrum other than this must be accurately separated.

In order for the exciton absorption line spectrum and the continuous absorption spectrum to be accurately separated, it is necessary that the layer thickness of the quantum well be accurately controlled at the atomic layer level during crystal growth. However, as described above, it is difficult to accurately control the layer thickness of the quantum well at the atomic layer level. The absorption spectrum of a single quantum well having a non-uniform layer thickness is usually a continuous spectrum having an exciton absorption peak, and its absorption spectrum is shown in FIG.
It becomes as shown in (A).

When writing light having a photon energy equal to or higher than the exciton absorption peak is irradiated, the light absorption rate of the quantum well changes as shown by the solid line in FIG. 7B. That is, even if the writing light has a photon energy corresponding to the exciton peak, if the writing light has a higher energy, the writing light is absorbed and the light absorption rate is lowered. Therefore, even if the change in the light absorptance is examined, the result shown in FIG.
As shown in, the change in the absorptance appears at a position that does not correspond to the write light energy, and there is no one-to-one correspondence between the portion where the light absorptance decreases and the write light energy.

Further, the continuous absorption spectrum and the exciton absorption spectrum are about 10 meV or less in the case of III-V group semiconductors even if they are accurately separated. This means that the wavelength width that can be used for recording information is 10 meV or less in photon energy. Therefore, in the information storage device described in Japanese Patent Application No. 63-177131, the recording energy width is limited by the energy difference between the continuous absorption spectrum and the exciton absorption spectrum, and the layer thickness is accurate at the atomic layer level. For quantum well structures that are not controlled by the above, individual information is provided to each quantum well of different layer thickness by wavelength multiplexing corresponding to the presence or absence of absorption saturation as in the optical information storage device described in the above publication. There is a problem that it is impossible to record and read.

According to the present invention, even if the layer thickness of the quantum well is not accurately controlled at the atomic layer level, information recording / reading can be performed by wavelength multiplexing without being limited by the energy difference between exciton absorption and continuous absorption. The purpose is to provide a possible optical information storage device.

[0010]

In order to achieve the above-mentioned object, in an optical information storage device according to the present invention, read light irradiated to an optical information storage element having a plurality of quantum well layers having different layer thicknesses. It is characterized by differentiating the spectrum of the transmitted light or the absorbed light by the photon energy.

Now, the principle of the present invention will be described with reference to FIG. As described above, in the quantum well whose layer thickness is not accurately controlled at the atomic layer level, the writing light energy and the energy at which the change in the absorptance appears do not have a one-to-one correspondence. However, as shown in FIG.
The position of the absorption edge of the continuous absorption spectrum changes depending on the layer thickness of the quantum well. Therefore, when a plurality of quantum wells having different layer thicknesses are irradiated with the writing light having a certain wavelength, the light absorption rate is lowered in the quantum well having the exciton peak below the writing light energy. That is, when the change in the light absorptance is examined, as shown in FIG. 1B, the light absorptance changes rapidly at the position corresponding to the writing light energy, and the change curve of the light absorptance forms a step. To do.

Even when writing lights having different wavelengths are simultaneously irradiated, as shown in FIG. 1C, a step is formed in the change curve of the light absorption rate at the position corresponding to each writing light energy. . Therefore, by differentiating the change curve of the light absorptance with the photon energy, a signal having a peak at a position corresponding to the writing light energy can be obtained as shown in FIG.

As a result, a signal corresponding to the writing light energy in a one-to-one relationship can be obtained, so that for a quantum well structure whose layer thickness is not precisely controlled at the atomic layer level,
Information can be recorded and read by wavelength division multiplexing. By the way, since the change of the light absorption rate is reflected in the spectrum of the transmitted light or the absorbed light of the reading light, if the spectrum of the transmitted light or the absorbed light is differentiated by the photon energy, the change curve of the light absorption rate is differentiated. Similarly, it becomes possible to record / read information by wavelength multiplexing for a quantum well structure whose layer thickness is not accurately controlled at the atomic layer level.

[0014]

According to the present invention, the spectrum of the transmitted light or the absorbed light of the read light with which the plurality of quantum well layers having different layer thicknesses are irradiated is differentiated by the photon energy, so that the position corresponding to the write light energy can be obtained. A signal having a peak can be obtained, so that a signal corresponding to the write light energy in a one-to-one relationship can be obtained.

[0015]

Embodiments of the present invention will be described below with reference to FIGS. First, the structure of an optical information storage element used as an information storage medium in the optical information storage device according to the present invention will be described. As shown in FIG. 2, this optical information storage element 1 has a semiconductor substrate 1a.
Laminated portion 1b having a plurality of quantum well layers having different layer thicknesses thereon
Is formed.

FIG. 3A shows a cross section for one cycle of the lamination in the lamination portion 1b. In this figure, the crystal growth direction is from the right side to the left side of the drawing. Also, FIG.
FIG. 3B shows the energy level distribution of this cross-sectional structure in correspondence with FIG. As shown in the figure, the quantum well layer 3 made of a direct transition type semiconductor is included in this cross-sectional structure.
Are formed. The quantum well layer 3 is sandwiched between the barrier layers 5a and 5b. The barrier height for holes formed by the barrier layers 5a and 5b is relatively low, and is set to a height at which holes generated by light absorption in the quantum well layer 3 can be tunneled.

On the opposite side of the barrier layer 5a from the quantum well layer 3,
A hole separation layer 7 sandwiched between auxiliary barrier layers 6a and 6b having a barrier height against electrons generated by light absorption in the quantum well layer 3 higher than that of the barrier layer 5a is formed. The hole separation layer 7 provides a stable state for holes when the holes tunneling from the quantum well layer 3 to the barrier layer 5a diffuse in the auxiliary barrier layer 6b and then fall into the hole separation layer 7. To do.

In the laminated structure shown in FIG. 3 (A), the upper end of the valence band of the hole separation layer 7 is the upper end of the valence band of the quantum well 3 as shown in FIG. 3 (B). It is located above, and similarly, the lower end of the conduction band of the hole separation layer 7 is located above the lower end of the conduction band of the quantum well 3. Then, a plurality of quantum well layers 3 having different layer thicknesses are stacked in such a stacking cycle to form a stacked portion 1b.

In such a laminated structure, as shown in FIG.
As shown in (B), the electron has the lowest energy state in the quantum well layer 3 and the hole has the lowest energy state in the hole separation layer 7. Therefore, when the quantum well layer 3 is irradiated with writing light having an energy equal to or higher than the exciton peak, and light absorption occurs to generate electron-hole pairs, the electrons remain in the quantum well layer 3 but Tunnels through the barrier layer 5a, diffuses in the auxiliary barrier layer 6b, moves to the hole separation layer 7, and is stably accumulated therein.

By spatially separating holes and electrons in this way, recombination of hole-electron pairs is less likely to occur,
The duration of the light absorption rate change can be lengthened. Therefore, the information storage time can be lengthened. In addition,
In order to suppress light absorption in the hole separation layer 7, the hole separation layer 7 is preferably formed of an indirect transition type semiconductor.

In the present embodiment, the quantum well layer 3 described above is replaced with In.
The barrier layers 5a and 5b are made of GaAs (InAlAs).
It is formed of _0.6 (InGaAs) _0.4 . The auxiliary barrier layers 6a and 6b are made of AlAsSb, and the hole separation layer 7 is made of Ga.
It was formed of AsSb. The layer thickness of each layer is the barrier layer 5
a and 5b are 3.0 nm, auxiliary barrier layers 6a and 6b are 2 nm
The thickness of the quantum well layer 3 is 0.0 nm, the hole separation layer 7 is 2.05 nm, the thickness of the quantum well layer 3 is gradually changed in the range of 2.5 to 5.0 nm, and 100 kinds of quantum well layers 3 having different layer thicknesses are used. Was formed. The layer thickness of the quantum well layer 3 was changed so that the energy intervals of exciton peaks were substantially equal (about 2 meV).

For the laminated portion 1b having such a structure,
In the energy width of 200 meV, it was possible to record binary information of 0 and 1 by wavelength multiplexing every 4 meV.
Therefore, when the diameter of the light beam is 1 μm, an optical recording density of 50 (5 Gbit / cm ² ) can be achieved for each area of 1 μm diameter. By the way, in the laminated portion 1b of the optical information storage element 1 described above, of the hole-electron pairs generated by light absorption, holes tunnel and move to the hole separation layer 7, and are stably accumulated. Although it has a structure,
The barrier height for the electrons of the barrier layers 5a and 5b is set to a height at which electrons generated by light absorption in the quantum well layer 3 can tunnel so that the electrons can tunnel through the barrier layers 5a and 5b instead of holes. The laminated portion 1b having a different structure may be formed.

In this case, instead of the hole separation layer 7,
A barrier height for holes generated by light absorption in the quantum well layer 3 is higher than that of the barrier layer 5a to form an electron separation layer sandwiched between auxiliary barrier layers 6a and 6b, and holes are the lowest in the quantum well layer 3. An energy state is adopted, and the electron is configured to have the lowest energy state in the electron separation layer. The upper end of the valence band of the electron separation layer is located below the upper end of the valence band of the quantum well 3, and similarly, the lower end of the conduction band of the electron separation layer is lower than the lower end of the conduction band of the quantum well 3. It is configured to be located below. According to this structure, of the hole-electron pairs generated by light absorption, holes are left in the quantum well layer 3, but the electrons tunnel through the barrier layer 5a and then diffuse in the auxiliary barrier layer 6b. Moved to the electron separation layer 7,
It is accumulated there stably.

Therefore, also in this case, the optical information storage element 1 can act in the same manner as in the above-mentioned case where holes are accumulated in the hole separation layer 7. Next, a first embodiment of an optical information storage device according to the present invention using the above-mentioned optical information storage element 1 will be described with reference to FIG. FIG. 4 is a block diagram showing the configuration of the first embodiment.

As shown in the figure, in the present embodiment, the writing light source 10 for irradiating the optical information storage element 1 with writing light.
And a reading light source 11 that emits reading light. The writing light source 10 selectively irradiates the optical information storage element 1 with wavelength light corresponding to information to be recorded. The reading light source 11 may be one that can irradiate the storage element 1 with all the wavelength light that the writing light source 10 can irradiate. Therefore, a continuous spectrum white light source or the like can be used.

Further, in the present embodiment, a spectroscope 12 for separating the transmitted light of the read light emitted from the read light source 11 to the storage element 1 is provided. Then, the transmitted light split by the spectroscope 12 is converted by the photodetector 13 into an electric signal corresponding to the light intensity for each wavelength, and the electric signal is output from the photodetector 13. As the photodetector 13, for example, photomultiplier can be used. The electric signal output from the tube photodetector 13 is input to the differentiating circuit 15.

The operation of the optical information storage device thus configured will be described. First, when information is stored in the storage element 1, the writing light source 10 emits writing light having a wavelength corresponding to the information to be recorded in the storage element 1. As a result, in the quantum well layer having an exciton peak below the writing light energy, hole-electron pairs are formed by light absorption, and the light absorption rate is reduced.

When reading this information from the storage element 1, the reading light source 11 irradiates the storage element 1 with read light. Then, the transmitted light is wavelength-dispersed by the spectroscope 12. The wavelength-dispersed transmitted light is detected by the photodetector 13
To be converted into an electric signal according to the light intensity of each wavelength and output. Therefore, this electric signal is a signal corresponding to the spectrum of the transmitted light. Then, this electric signal is input to the differentiating circuit 15 and differentiated. And this differentiation is equivalent to what differentiated the spectrum of transmitted light by photon energy.

Therefore, when the storage element 1 is irradiated with the writing light and information is recorded, the energy of the writing light is changed as described above (see FIGS. 1B, 1C and 1D). A signal having peaks at corresponding positions can be obtained.
By irradiating the storage element 1 with writing light having different wavelengths, even when a plurality of pieces of information are stored by wavelength multiplexing, it is possible to obtain signals each having a peak at a position corresponding to the energy of each writing light. That is, it is possible to read the information corresponding to the write light energy from the storage element 1 by wavelength multiplexing.

As the above-mentioned differentiating circuit, for example, the one shown in FIG. 5 can be used. In the above-described first embodiment, the spectrum of the transmitted light of the read light with which the storage element 1 is irradiated is differentiated to obtain the peak signal corresponding to the energy of each write light in a one-to-one manner. The corresponding information can be read from the storage element 1 by wavelength multiplexing, but similar results can be obtained by differentiating the spectrum of the absorbed light instead of the spectrum of the transmitted light of the read light with which the storage element 1 is irradiated. it can. Here, the spectrum of the absorbed light can be obtained as a difference between the read light spectrum before the storage element 1 is irradiated and the transmitted light spectrum after the irradiation.

By the way, in the above-mentioned first embodiment, the differentiating circuit 15 is provided to electrically differentiate the spectrum of the transmitted light, but the spectrum of the transmitted light may be optically differentiated. In this way, a second embodiment of the present invention in which the spectrum of transmitted light is optically differentiated will be described with reference to FIG. FIG. 6 is a block diagram of the second embodiment.

In this embodiment, the fact that the refractive index corresponds to the derivative of the absorption spectrum ω is used. for that purpose,
The reading light emitted from the reading light source 11 is divided into two by a beam splitter or the like, one reading light is applied to the information storage unit of the storage element 1, and the other reading light does not record the information of the storage element 1. Irradiate the reference part. Then, the respective transmitted lights are overlapped again by using a beam splitter or the like, interfere with each other, and enter the spectroscope 12. Then,
Light intensity proportional to the phase difference Δφ between the two is input to the spectroscope 12 and wavelength-dispersed. Then, the wavelength-dispersed light intensity is sent to the photodetector 13, where it is converted into an electric signal corresponding to the light intensity for each wavelength and output.

In this case, since the phase difference Δφ is proportional to the difference in refractive index between the information storage section and the reference section of the storage element 1, the refractive index change Δn (ω) due to the irradiation of the writing light is determined by the spectroscope 12. It will be wavelength-dispersed. Then, the wavelength-dispersed refractive index change Δn (ω) is sent to the photodetector 13, and the refractive index change Δn (ω) for each wavelength is output from the photodetector as an optical signal or an electrical signal. Become.

However, the following Kramers-Kronig relational expression

[0035]

[Equation 1]

From the change in refractive index Δn (ω)

[0037]

[Equation 2]

And absorption rate change Δα (ω)

[0039]

[Equation 3]

Between and

[0041]

[Equation 4]

The following relationship holds. Therefore, the electric signal output from the photodetector 13 according to the spectrum of the change in the refractive index substantially corresponds to the derivative of the change in the absorption spectrum. As a result, when the storage element 1 is irradiated with the writing light and information is recorded, a signal having a peak at a position corresponding to the energy of the writing light is generated as in the case of the first embodiment described above. It is possible to obtain the information, and it is possible to read the information according to the writing light energy from the storage element 1 by wavelength multiplexing.

[0043]

As described above, according to the present invention,
A signal having a peak at a position corresponding to the write light energy can be obtained by differentiating the spectrum of the transmitted light or the absorbed light of the read light with which the quantum well layers having different layer thicknesses are irradiated, by the photon energy. As a result, a signal corresponding to the writing light energy on a one-to-one basis is obtained. Therefore, even if the quantum well layer thickness is not accurately controlled at the atomic layer level, information can be recorded and recorded by wavelength multiplexing.
A readable optical information storage device can be provided.

[Brief description of drawings]

FIG. 1 is a graph showing changes in optical absorptance of a plurality of quantum wells having different layer thicknesses.

FIG. 2 is a sectional view of an optical information storage element used in the present invention.

3A and 3B are a cross-sectional structural view and a diagram showing energy level distribution of a laminated portion of an optical information storage element used in the present invention.

FIG. 4 is a block diagram of a first embodiment of the present invention.

FIG. 5 is a diagram showing an example of a differentiating circuit.

FIG. 6 is a block diagram of a second embodiment of the present invention.

FIG. 7 is a graph showing a change in light absorption rate of a single quantum well by irradiation with writing light.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical information storage element 1a ... Semiconductor substrate laminated | stacked part 1b ... Laminated part 3 ... Quantum well layer 5a, 5b ... Barrier layer 6a, 6b ... Auxiliary barrier layer 7 ... Hole separation layer 10 ... Writing light source 11 ... Read light source 12 … Spectroscope 13… Photodetector 15… Differentiation circuit

Claims (3)

[Claims] 1. An optical information storage element having a plurality of quantum well layers having different layer thicknesses, and read light to the optical information storage element for reading information stored in the optical information storage element. Reading light irradiating means for irradiating, spectroscopic means for obtaining a spectrum of transmitted light or absorbed light of the reading light applied to the optical information storage element from the reading light irradiating means, and a spectrum obtained by the spectroscopic means for photon An optical information storage device comprising a spectrum differentiating means for differentiating with energy. 2. The optical information storage device according to claim 1, wherein the spectrum differentiating means is an electrical differentiating circuit that electrically differentiates a spectrum. 3. The optical information storage device according to claim 1, wherein the spectrum differentiating means is an optical differentiating means for optically differentiating a spectrum.