Non-volatile optical memory element utilizing optically induced magnetization reversal and operational method thereof

The present invention relates to a high-speed optical memory element. In order to increase speed of a memory element, optical pulse is recorded and read all-optically without conversion into electrical signal at very high speed. Optically-induced spin accumulation is used for recording the ferromagnetic metal embedded into optical waveguide operates as a high speed memory element. The ferromagnetic metal is sandwiched between a conductor on one side and a tunnel barrier followed by a conductor on the other side. The voltage is applied between two conductors. For data recording, the optically induced spin-polarized tunneling and spin accumulation is used. The optically induced spin-polarized tunneling occurs due to absorption of circularly polarized light. The torque of accumulated spin reverses magnetization of ferromagnetic metal. For reading Faraday rotation or non-reciprocal loss/gain in semiconductor-ferromagnetic-metal hybrid is used.

TECHNICAL FIELD OF THE INVENTION

The present invitation relates to a high-speed optical memory element(s), which forms an essential element(s) in an optical communication system and an optical application system.

DESCRIPTION OF RELATED ART

A high-speed non-volatile optical memory is very important for optical networks. High-speed data processing such as receiving, storing and resending are main functions of an optical network server. Therefore, such a high-speed optical memory is one of most important components for the optical networks.

FIG. 1shows a typical format of data used in the optical network, for example, in optical Internet. The data is sent by packages (packets). Inside the package, a time interval between pulses is very short (order of a few picoseconds). On the other hand, a time interval between packages is relatively long (order of a few hundreds of picoseconds or a few nanoseconds). The server receives packages, makes route decision and resends them to a given destination.

According to the present invention, it is possible to record, store and resent the data of such a format. In U.S. Pat. No. 5,740,117, a method of storing optical signals in a loop structure is described. In the method, a fixed amount of information within the optical loop can be stored. U.S. Pat. No. 6,647,163 discloses an optical memory device with Mach-Zender interferometer featuring semiconductor optical amplifiers.

A non-volatile high-speed optical memory element is disclosed in Japanese Patent application No. 2003-328895. In Japanese Patent Application for data recording, the effect that coercive force of ferromagnetic metal changes by heating the ferromagnetic material with an optical pulse(s) is used. In the present invention, optical induced magnetization reversal of ferromagnetic material is used for the data recording.

Current induced magnetization reversal has been proved theoretically (J. C. Slonczewski Journal of Magnetism and Magnetic Materials, Vol. 159, pp. L1–L7, 1996; J. Z. Sun Physical Review B, Vol. 62, pp. 570–578, July 2000) and experimentally (J. Z. Sun, Journal of Magnetism and Magnetic Materials, Vol. 202, pp. 157, 1999; F. J. Albert, N. C. Emley, E. B. Myers, D. C. Ralph; and R. A. Buhrman Physical Review Letters Vol. 89, pp. 226802, November 2002). In case that spin-polarized current flows from first ferromagnetic material to second ferromagnetic material, spin accumulation in the second ferromagnetic material occurs. Under torque of the accumulated spin, magnetization of the second ferromagnetic material is reversed in parallel to magnetization of the second ferromagnetic material.

In Japanese Patent application No. 2003-328895, a tunnel magneto-resistive electrode is used for data reading from an optical memory element. In the present invention, in order to read data at a high speed from a memory element, magneto-optical polarization rotation in ferromagnetic-metal-semiconductor hybrid amplifier (U.S. Pat. No. 5,598,492; J. M. Hammer, J. H. Abeles, and D. J. Channin, IEEE Photon. Technol. Lett. vol. 9, pp. 631–633, May 1997) is used. Also in the present invention, in order to read data, the dependence of optical gain in magnetization direction in ferromagnetic-metal-semiconductor hybrid amplifier (W. Zaets and K. Ando, IEEE Photon. Technol. Lett. Vol. 11, pp. 1012–1014, August 1999) is used.

SUMMARY OF INVENTION

In view of the above problem, it is an object of the present invention to speed up data recording and reading. It is another object of the present invention to provide a device capable of non-volatile storing of optical data.

The objects of the invention are achieved by the technology set forth below.(1) The present invention provides a high-speed optical memory element which comprises on an optical waveguide having two cladding regions, and a core region whose refractive index is higher than that of the cladding. Most of light travels in the core region while reflected by the cladding, but little light travels inside the cladding. The size of ferromagnetic metal region is small enough so that the ferromagnetic metal region is in single-domain state. The ferromagnetic-metal region is close enough to the waveguide core region or is inside core region so that light propagating in the waveguide penetrates into ferromagnetic metal region. The ferromagnetic metal is sandwiched between a first conductive region and tunnel barrier followed by a second conductive region. The data is stored in the memory element by means of two opposite direction of magnetization of ferromagnetic-metal region(2) The present invention provides a recording method for a high-speed memory element utilizing optically-induced spin-accumulation in the ferromagnetic metal. The voltage is applied between the first and second conductive regions. Circularly-polarized optical input pulse is absorbed by ferromagnetic metal. It excites spin-polarized electrons to high energy level. These electrons are tunneling through tunnel barrier into second conductive region. The electrons of both spin polarization flows from first conductive region into ferromagnetic metal. Thus, spin is accumulated in the ferromagnetic metal. The torque of accumulated spin reverses the magnetization of ferromagnetic metal.(3) The present invention provides a recording method for a high-speed memory element utilizing optically-induced spin-accumulation in the ferromagnetic metal. The voltage is applied between the first and second conductive regions. Circularly-polarized optical input pulse is absorbed by the second conductive region. It excites spin-polarized electrons to high energy level in the second conductive region. These electrons are tunneling from the second conductive region through tunnel barrier into ferromagnetic metal. The electrons of both spin polarization flow from ferromagnetic metal. Thus, spin is accumulated in the ferromagnetic metal. The torque of accumulated spin reverses the magnetization of ferromagnetic metal.(4) The present invention provides a reading method for a high-speed memory element utilizing dependence of optical gain in ferromagnetic-metal-semiconductor hybrid on direction of magnetization of ferromagnetic metal.(5) The present invention provides a reading method for a high-speed memory element utilizing dependence of a direction of polarization rotation in ferromagnetic-metal on direction of magnetization of ferromagnetic metal.(6) The present invention provides a high-speed optical memory which contains a set of high-speed memory elements (1), which is able to record and read the train of optical pulses. The delayed clock pulse is used to select corresponding memory element to read and record each pulse in the pulse train. Formation of circularly-polarized pulse from linearly-polarized clock pulse and linearly polarized input pulse provides high speed of data recording into memory element.

Thus, the present invention possesses a number of advantages or purposes, and there is no requirement that every claim directed to that invention be limited to encompass all of them.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

DATAILED DESCRIPTION OF THE INVENTION

Description of representative embodiments of the present invention is now given, referring to drawings. While the present invention is not necessarily limited to such embodiments, an appreciation of various aspects of the invention is best gained through a discussion of the various examples.

FIG. 1is a schematic diagram illustrating timing between pulses used in optical communication links. (tpulses—short interval between pulses in one package, tpackages—relatively long time interval between packages.)

FIG. 1shows a typical format of data used in optical networks, for example, in optical Internet. The data is sent by packages (or packets). Inside the package the time interval between pulses is very short (order of a few picoseconds). On the other hand, a time interval between packages is relatively long (order of a few hundreds of picoseconds or a few nanoseconds). A server receives packages, makes route decision and resends them to a given destination. According to the present invention, it is possible to record, store and resent the data in such a format.

FIG. 2shows a schematic diagram illustrating an optical memory element according to the present invention. The memory element comprises an optical rib waveguide with embedded ferromagnetic metal. The waveguide made up of a core region (6) sandwiched between two cladding regions (5) and (7). Light propagates inside the core region (6) along the rib. Light penetrates into the cladding regions (5) and interacts with the ferromagnetic metal. Data is stored in ferromagnetic metal through two opposite directions of magnetization along magnetization easy axes. Ferromagnetic metal is surrounded by a first conductive region (1) on one side and a tunnel barrier (4) following a second conductive region (2) on the other side. The voltage is applied between first and second conductive regions.

FIG. 3shows a schematic diagram illustrating a recording method for a memory element according to the present invention (in case of light absorption by ferromagnetic metal). Negative voltage is applied to the conductive region near the ferromagnetic metal. That region is a source for electrons. Positive voltage is applied to the conductive regions near the tunnel barrier. That is a drain for electrons. Circularly-polarized light of data pulse illuminates the ferromagnetic metal. Circularly-polarized light is absorbed by the ferromagnetic metal. In the ferromagnetic metal region, due to the absorption of light the spin-polarized electrons are excited. The excited spin-polarized electrons are tunneling through the tunnel barrier into the drain. Non-spin-polarized current flows from the ferromagnetic metal into the drain region. There is a spin accumulation in ferromagnetic metal. The torque of accumulated spin reverses magnetization of ferromagnetic metal. The data is stored.

FIG. 4shows a band diagram and operational principal illustrating a recording method for a memory according to the present invention (the case of light absorption by ferromagnetic metal (10)). Negative voltage is applied to the conductive region (9) near the ferromagnetic metal. That region is a source for electrons. Positive voltage is applied to the conductive regions (12) near the tunnel barrier (11). That is a drain for electrons. The tunnel barrier is high enough that without light illumination there is no current through the tunnel barrier. When circularly-polarized light (8) of data pulse illuminates the ferromagnetic metal (10), circularly-polarized light is absorbed by the ferromagnetic metal. In the ferromagnetic metal region, the absorption of light excites spin-polarized electrons (up-spin electrons). The excited spin-polarized electrons tunnel through the tunnel barrier into the drain thereby causing electron current. Since the current from the ferromagnetic metal to the drain is spin-polarized (up-spin only) and current from the source to the ferromagnetic metal is not spin-polarized (both spin-up and spin-down), there is a spin accumulation in the ferromagnetic metal (spin-down accumulation). The torque of accumulated spin reverses magnetization of the ferromagnetic metal and the data is memorized.

FIG. 5shows a schematic diagram illustrating an optically-induced spin accumulation in the ferromagnetic metal region (in the case of light absorption by ferromagnetic metal). Only spin-up current flow from the ferromagnetic-metal into the drain. Spin-up and spin-down current flows from the source into the ferromagnetic metal. Thus, spin-down electrons are accumulated in the ferromagnetic metal region.

FIG. 6shows a schematic diagram illustrating a recording method for a memory element according to the present invention (in case of light absorption by the source region). Negative voltage is applied to a conductive region near a tunnel barrier. That region is a source for electrons. The positive voltage is applied to a conductive region near the ferromagnetic metal. That is a drain for electrons. Circularly-polarized light of data pulse illuminates a source region. The circularly-polarized light is absorbed by the source region. In the source region, due to the absorption of light, the spin-polarized electrons are excited. The excited spin-polarized electrons tunnel through the tunnel barrier into the ferromagnetic metal region. Non-spin-polarized current flows from the ferromagnetic metal to the drain region. There is a spin accumulation in ferromagnetic metal. The torque of accumulated spin reverses magnetization of ferromagnetic metal. The data is stored.

FIG. 7shows a band diagram and operational principal illustrating a recording method for a memory according to the present invention (in the case of light absorption by the source region). Negative voltage is applied to the conductive region near the tunnel barrier. That region is the source for electrons. Positive voltage is applied to the conductive regions near the ferromagnetic metal. That is the drain for electrons. The tunnel barrier is high enough that, without light illumination, there is no current through the tunnel barrier. Circularly-polarized light of data pulse illuminates the source region. The circularly-polarized light is absorbed by the source region. In source region, the absorption of light excites spin-polarized electrons (up-spin electrons). The excited spin-polarized electrons tunnel through the tunnel barrier into the ferromagnetic metal thereby causing electron current. Since the current from the source to the ferromagnetic metal is spin polarized (up-spin only) and current from the ferromagnetic metal to the drain is not spin-polarized (both spin-up and spin-down), there is a spin accumulation in the ferromagnetic metal (spin-up accumulation). The torque of accumulated spin reverses magnetization of ferromagnetic metal and the data is memorized.

FIG. 8shows a schematic diagram illustrating an optically-induced spin accumulation in the ferromagnetic metal region (in the case of light absorption by the source region). Only spin-up current flow from the source into the ferromagnetic-metal. Spin-up and spin-down current flows from the ferromagnetic metal into the drain. Thus, spin-up electrons are accumulated in the ferromagnetic metal region.

FIG. 9shows a band diagram and operational principal illustrating a recording method for the memory element shown inFIG. 4, where the source and the drain are made of semiconductors. Since ferromagnetic transition metals (like Co, Fe, Ni) form Ohmic contact with p-type semiconductors, the source material is made of p-type semiconductor. There is an Ohmic contact between the ferromagnetic metal and the source. The drain is made of n-type semiconductors. The bandgap energy of semiconductor of both the drain and the source is high enough that the light is not absorbed by the source and the drain. The light is absorbed only by the ferromagnetic metal.

FIG. 10shows a band diagram and operational principal illustrating a recording method for the memory element shown inFIG. 7, where a source and a drain are made of semiconductors. The both the source and drain are made of p-type semiconductor. There is an Ohmic contact between the ferromagnetic metal and the source. The bandgap energy of the drain is high enough that the light is not absorbed by the drain. The bandgap energy of the source is equal or smaller than photon energy of light. Thus, light excites electrons in the source region from a valance band to a conductive band. Such a structure has a long life time for interband transitions in semiconductors, which leads to high efficiency of spin accumulation.

FIG. 11shows a band diagram and operational principal illustrating recording method for the memory element depicted inFIG. 4, where the source and drain are made of semiconductors and a tunnel barrier is formed by Shottky barrier in which metal and semiconductor is in contact. The source is p-type semiconductor and the drain is n-type semiconductor. Since ferromagnetic transition metals (like Co, Fe, Ni) form Ohmic contact with p-type semiconductors and Schottky contact with n-type semiconductor, there is a barrier at drain-ferromagnetic-metal boundary and there is no barrier at source-ferromagnetic-metal boundary.

FIG. 12shows a schematic diagram illustrating a method for recording pulse train in a memory device, which contains 4 memory elements according to the present invention. The memory elements are referred to as “Cell1”, “Cell2”, “Cell3” and “Cell4”. There are two input waveguide paths. One is for input of data pulses and another one is for clock pulse. Polarization of clock pulse and input pulses should be mutually orthogonal. In the figure, the polarization of input pulses is depicted as “TE” and polarization of clock pulse is depicted as “TM”. Each memory element is illuminated by the input pulses and the clock pulse. From cell to cell, the clock pulse has addition of delay, which equals to the period of input pulses. For first cell, the clock time arrives at the same time as a first one of input pulses. For a second cell, the clock time arrives at the same time as a second one of the input pulses and so on. The phase of the input pulses is shifted by a quarter of wavelength period proportional to the phase of input pulses. Intensities of input pulses and clock pulse are adjusted to be equal. Thus, at each cell, TE polarized input pulse and quarter-period phase-shifted TM-polarized clock pulse are combined to form a circular polarized pulse. The circularly polarized pulse reverses magnetization of ferromagnetic metal of a memory element.

FIGS. 13A,13B,13C and13D show a schematic diagrams illustrating a time diagram for an input pulse train and clock pulse at different cells. Polarization of input and clock pulses is mutually orthogonal. For the Cell1, a clock pulse and a first input pulse arrive at the same time. Their polarization and phase are so as to form a circularly polarized pulse. The circularly polarized pulse excites only spin-up electrons. That causes spin accumulation and magnetization reversal of the ferromagnetic metal. Thus, the data of the first input pulse is memorized at Cell1. Second, third and forth pulses are linearly polarized, so that they excite equally spin-up and spin-down electrons. They do not effect on magnetization of ferromagnetic metal of Cell1. For the Cell2, the clock pulse is delayed so the clock pulse and second input pulse arrive at the same time at this cell. Their polarization and phase are set so as to form a circularly polarized pulse. The circularly polarized pulse excites only spin-up electrons. That causes spin accumulation and magnetization reversal of ferromagnetic metal. Thus, data of the second input pulse is memorized at Cell2. First, third and forth pulses are linearly polarized, so as to excite equally spin-up and spin-down electrons. They do not effect on magnetization of ferromagnetic metal of Cell2.

FIG. 14shows a schematic diagram illustrating a reading method from a memory device, which contains 4 memory elements according to the present invention. The easy axis of ferromagnetic metal is directed perpendicularly to a waveguide of reading line. An optical gain is provided to compensate the absorption of ferromagnetic metal. As it was shown in (W. Zaets and K. Ando IEEE Photon. Technol. Lett. Vol. 11, pp. 1012–1014, August 1999), in this case the light will be absorbed or amplified depending on a direction of magnetization of ferromagnetic metal. In the example ofFIG. 14, the light will be amplified if the magnetization is directed to the left, and the light is absorbed if the magnetization is directed to the right. The clock pulse passes through each memory element and is coupled into an output waveguide path. The clock pulse has different delay for each memory cell, thus a pulse train forms at output line. The sequence of pulses in pulse train corresponds to data stored in memory cells.

FIG. 15shows a schematic diagram illustrating a reading method from a memory device, which contains 4 memory elements according to the present invention. The easy axis of ferromagnetic metal is directed to and along a waveguide of reading line. The polarization of light rotates in clockwise or counterclockwise direction, depending on the magnetization direction of ferromagnetic metal. The clock pulse passes through each memory element and is coupled into an output line. The clock pulse has different delay for each memory cell, thus a pulse train forms at an output line. At the common output line, there is one more cell (“common cell”) followed by analyzer. An axis of analyzer is perpendicular to the axis of the polarizer. Magnetization of the common cell, for example, is along a propagation direction and polarization rotation is clockwise. For memory cells, in which magnetization direction is opposite to propagation direction and polarization rotation is counterclockwise, a pulse passes through this cell, and at the common cell no polarization rotation is carried out as a whole. Thus, it will be blocked by the analyzer. For memory cells, in which magnetization direction is along propagation direction, and polarization rotation is clockwise, a pulse passes through this cell, and at the common cell non-zero clockwise polarization rotation is carried out as a whole. Thus, it will pass through the analyzer. Thus, the sequence of pulses in the pulse train after the analyzer corresponds to data stored in memory cells.

As explained above in detail, according to the present invention, it is possible to obtain the effects set forth below.1. It is possible to record and read optical data at very high speed.2. It is possible to use non-volatile memory in optical communication links.

As mentioned above, although the best forms for carrying out the present invention are explained in connection with the Embodiments, the present invention is not limited to these embodiments. For Example,

The disclosure of Japanese Patent Application No. 2004-197964 filed on Jul. 5, 2004 including specification, drawings and claims is incorporated herein by reference in its entirety.