Patent Application
20050152024
The present invention relates to a nonvolatile medium in electro-optic applications. More specifically, it relates to nonvolatile display and nonvolatile electro-optic modulators.
Microelectronic devices can be classified to volatile and nonvolatile devices based on their power characteristics. In volatile devices, the device's states are supported by the electrical power, and the device behaves as expected as long as the circuit receives power. In contrast, in nonvolatile devices, the device's states are stable with or without the applied power, and therefore when the power is off, the device stays in their states without any changes.
The major difference between a volatile and a nonvolatile device is the fundamental designed states of the device. If the device states are stable without any power source, the device is nonvolatile. If the device states require power to maintain, the device is volatile. Nonvolatility is much more desirable than volatility due to the lower power consumption, and the ability to remember and retain information without external power sources.
An example of volatility and nonvolatility is memory devices. A DRAM (dynamic random access memory) is a volatile memory device because the DRAM states are represented by a collection of charges, stored in a capacitor. Because of the inherent leakage of the capacitor charge, the DRAM state where the capacitor is charged is not stable without power. Thus by designing the electron charges as the memory state, the DRAM memory cell is inherently a volatile device. A RRAM (resistive random access memory) is a nonvolatile memory, employing a class of memory materials that have electrical resistance characteristics changeable by external influences. The RRAM memory is represented by the multistable states of high resistance and low resistance, where the applied power is only needed to switch the states and not to maintain them. The examples of such memory materials are perovskite materials exhibiting magnetoresistive effect or high temperature superconducting effect, disclosed in U.S. Pat. No. 6,204,139 of Liu et al., and U.S. Pat. No. 6,473,332 of Ignatiev et al., hereby incorporated by reference.
Another example of volatile device is electro-optic systems for high speed optical data transfer and processing, using electric fields to control the propagation of light through their optical materials. Common electro-optic systems are currently based on devices fabricated in bulk LiNbO3 crystals which have proven maturity and long term stability. The design and selection of current electro-optic media such as LiNbO3 lead to the inevitable feature of volatility, since the current electro-optic media require the presence of electric field to maintain their optical states.
The present invention addresses the nonvolatility of the electro-optic properties in the field of light transmission. The first step in designing nonvolatile electro-optic device is to identify multistable states and multistable medium for optical applications.
The present invention discloses a nonvolatile solid state electro-optic medium which is a perovskite material having magnetoresistive effect under the influence of an electric field.
Perovskite materials having magnetoresistive effect under the influence of an electric field display nonvolatile changes in electrical resistance and reactant when subjected to an electric field. This effect has been used in the design and construction of nonvolatile RRAM. As with other known perovskite materials, this is expected to be accompanied by nonvolatile changes in electro-optic properties related to dispersion and absorption of electromagnetic radiation. The nonvolatile optical properties of these materials is exploited in the present invention for the construction of nonvolatile display and nonvolatile solid state electro-optic modulators such as waveguide switch or phase or amplitude modulators.
The first embodiment of the present invention nonvolatile electro-optic medium is a nonvolatile display cell. Since the absorption property of the perovskite material changes nonvolatily under the influence of an electric field, using the perovskite material as a display medium will allow the construction of a nonvolatile display. The applied electric field is only needed to switch state to change the absorption property of the perovskite medium and therefore change the lightness of the display cell. This reduces power consumption, and display flickering, especially for the displays with un-frequent updates.
The second embodiment of the present invention nonvolatile electro-optic medium is a nonvolatile electro-optic modulator. Since the index of refraction of the perovskite material changes nonvolatily under the influence of an electric field, using the perovskite material as an electro-optic medium will allow the construction of nonvolatile phase modulators, amplitude modulators, frequency modulators or optical switches. By design, the applied electric field is only needed to switch state to change the dispersion property of the perovskite medium.
Nonvolatility is a desired feature of the device properties mainly due to the ability to maintain the state or the information without the need for power. A nonvolatile memory device, such as a hard drive or an EEPROM, can retain the information even in the absence of power. In contrast, a volatile memory device, such as DRAM, loses the information without power. A nonvolatile device thus can go to sleep when the power is off, and when the power is restored, wakes up and is ready at the same state before the power interruption. Therefore the absence of power only delays the nonvolatile device, not terminates it.
Nonvolatility is a design issue, achievable when the multiple states of the device are stable states without the need of external power. The design of nonvolatility occurs in the very beginning of the device concept, and once the concept is formed, little can be done to change the volatility or nonvolatility feature of the designed device. For example, by using a collection of electron charges to represent a memory state, this design is volatile since the charge accumulation rapidly disperses in the absence of power. Thus the volatility feature of this device is almost impossible to change. In contrast, by using multistable states of resistance in a perovskite material to represent different memory states, this design is nonvolatile since once the perovskite material is set into a resistance state, it remains there until an external influence (in this case an external electric field) moves the perovskite material into another stable resistance state. And therefore the nonvolatility of this design is assured.
This invention discloses the nonvolatile design concept and devices for electro-optic transmission using perovskite material having magnetoresistive effect under the influence of an electric field as the transmission medium. Materials having perovskite structure such as magnetoresistive (MR) materials, giant magnetoresistive (GMR) materials, colossal magnetoresistive (CMR) materials, or high temperature superconductivity (HTSC) materials can store information by the their stable magnetoresistance state, which can be changed by an external magnetic or electric field, and the information can be read by magnetoresistive sensing of such state. HTSC materials such as PbZrxTi1-xO3, YBCO (Yttrium Barium Copper Oxide, YBa2Cu3O7 and its variants), have their main use as a superconductor, but since their conductivity can be affected by an electrical current or a magnetic field, these HTSC materials can also be used as variable resistors in nonvolatile memory cells.
Typical perovskite materials having magnetoresistive effect are the manganite perovskite materials of the Re1-xAexMnO3 structure (Re: rare earth elements, Ae: alkaline earth elements) such as Pr0.7Ca0.3MnO3 (PCMO), La0.7Ca0.3MnO3 (LCMO), Nd0.7Sr0.3MnO3 (NSMO). The rare earth elements are La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The alkaline earth metals are Be, Mg, Ca, Sr, Ba, and Ra. Suitable perovskite materials for the present invention include magnetoresistive materials and HTSC materials such as PrCaMnO (PCMO), LaCaMnO (LCMO), LaSrMnO (LSMO), LaBaMnO (LBMO), LaPbMnO (LPMO), NdCaMnO (NCMO), NdSrMnO (NSMO), NdPbMnO (NPMO), LaPrCaMnO (LPCMO), and GdBaCoO (GBCO).
The nonvolatile resistance changes in the perovskite materials is a result of a wide diversity of stable ground states, occurring by an active number of degrees of freedom such as spin, charge, lattice and orbital. The ground state is then determined by the interactions of the competing relevant degrees of freedom. The resistance change of perovskite materials can be achieved not only by a magnetic or an electric field, but also by synchrotron x-ray illumination at low temperature. This change is accompanied by significant change in the lattice structure of the perovskite material.
Optical conductivity of perovskite materials such as PCMO has been studied, and the results indicate that the optical conductivity varies significantly with changes in compositions, temperatures and applied magnetic field. From the available data, the dependency of the optical properties such as optical conductivity and optical dispersion of the perovskite materials on the electrical field is expected, and similar to the nonvolatility of the resistance change of the perovskite materials, the changes in optical dispersion and absorption of electromagnetic radiation are also expected to be nonvolatile. This is the basic for the design of the present invention nonvolatile medium for electro-optic devices.
The present invention thus discloses a nonvolatile solid state electro-optic medium which is a perovskite material having magnetoresistive effect under the influence of an electric field with applications in nonvolatile displays and nonvolatile electro-optic modulators.
The first embodiment of the present invention is an electro-optic light transmission. The word “light” used in the present context is to be understood in the broad sense and not limited to the visible spectrum, but to mean an electromagnetic radiation, preferably with frequency ranging from microwave (gigahertz) to beyond x-ray. The electro-optic light transmission comprises an electro-optic medium made of perovskite material exhibited magnetoresistance under the influence of an electric field, and a pair of electrodes to establish an electric field to control the optical properties of the electro-optic medium. Various designs can be achieved depending on the relative location of the electrodes with respect to the light source.
The disclosed electro-optic light transmission device is nonvolatile due to the expected nonvolatility of the optical properties of the perovskite medium, therefore the electric field applied to the electrodes is needed only for switching, and not for maintain the optical states. The power necessary for display and modulation of optical signals can be significantly reduced with this invention.
The above disclosed electro-optic light transmission device can be applied to the design of nonvolatile displays. The disclosed displays use the electro-optic medium made of perovskite materials, but otherwise similar to the construction of standard volatile liquid crystal displays (LCD) or electroluminescent (EL) devices.
For a prior art LCD display, the applied electrical potential is periodic and causes a change in the birefringence of the electro-optic medium when the potential or signal is present. This change in the birefringence varies the polarization state of light passing through the electro-optic medium, and in combination with fixed polarizers can be used to generate a visual contrast between adjacent pixels. The visual contrast of the LCD medium exists as long as the electric field is present, and the medium (all pixels) relax to the ground state when the power is removed. For a prior art EL display, the applied potential is also periodic and causes emission of light from the electro-optic medium. Similar to a LCD, this light emission is volatile, meaning that it persist as long as the power is applied.
For the present invention display, the electro-optic medium is a perovskite material that changes the optical properties when an appropriate electric field or potential is applied, similar to the prior art LCD or EL displays. However, in contrast to LCD, EL, or other electro-optic displays, these optical changes are expected to persist after the electric field or potential is removed. Hence, no additional power is required to maintain the preferred state of the electro-optic medium and the device is nonvolatile.
In a second embodiment of the present invention nonvolatile electro-optic device, the perovskite medium can be used in electro-optic modulators (such as switches, logic gates or memories).
Electro-optic modulators use electric fields to control the amplitude, phase, and polarization state of an optical beam. Electro-optic modulators can be used in communications systems to transfer information utilizing an optical frequency carrier. Since external modulators do not modulate directly the laser source, they do not cause any degrading effects on laser line width and stability. Examples of modulators include feed back systems to hold the intensity in a laser beam constant, or optical choppers to produce a pulse stream from a continuous laser beam, and stabilizer of the laser beam frequency.
Electro-optic modulators typically utilize bulk configurations and integrated optical configurations. Bulk modulators are made from large piece of electro-optic medium and are typically low insertion losses and high power. Integrated-optic modulators are typical wavelength specific because of the waveguide technology used in fabrication. One of the principal advantages of integrated electro-optic modulators compared to bulk crystals is that lower voltages and powers may be used, and faster modulation rates also may be achieved.
The electro-optic effect is the change in the index of refraction of the material under the application of an external electric field. Certain media are birefringent, meaning the index of refraction depends on the orientation of the medium, and therefore the refractive index is best described by an index ellipsoid.
The simplest electro-optic modulator is the phase modulator where the light beam experiences an index of refraction change, hence an optical path length change. The phase of the output optical beam therefore depends on the applied electric field.
Like the bulk modulator, the integrated-optic modulator also works on the principle of electro-optic effect. An integrated-optic phase modulator is constructed using a dielectric optical waveguide and the applied electric field to control the index of refraction of the waveguide.
Typical amplitude modulators are Mach-Zehnder interferometer fabricated on a perovskite substrate as shown in
The directional coupler can also serve as a optical switch, where the input light beam 70 entering the waveguide branch 74A can emerge from the branch 74B to the output 73B if no voltage is applied, and can emerge from the branch 77B to the output 73A in the presence of the electric field.
The electro-optic modulators disclosed are similar in construction to the prior art electro-optic modulator, with the exception of the perovskite medium. By using perovskite material as the electro-optic medium, the disclosed electro-optic modulators are nonvolatile, meaning keeping their optical properties when the electric field is turned off.
Thus a novel nonvolatile electro-optic device and its display and modulator applications have been disclosed by the employment of an electro-optic medium which is a perovskite material having magnetoresistive effect under the influence of an electric field. It will be appreciated that though preferred embodiments of the invention have been disclosed with regard to specific displays and modulators, further variations and modifications thereof may be made within the scope of the invention as defined in the appended claims. Further, although the invention has been described with reference to displays and modulators for use with nonvolatile light propagation applications, other applications of the inventive concepts disclosed herein will also be apparent to those skilled in the art.
