All-optical Memory Research Articles

Self-conjugation heteroassociative memories using thin static nonlinearly recorded holograms

The problem of all-optical holographic associative memory is elaborated with account of recording nonlinearity of thin static off-axis holograms. It is shown that the recording nonlinearity results in reconstruction of angularly separated diffraction orders, in one of which (minus 2nd) the complex congugate associative response is reconstructed as such a hologram is read out by incomplete version of the stored pattern. Being angularly separated from the readout beam, the phase conjugate response occurs be error-corrected. Moreover, recording nonlinearity provides unique feasibilities for associative coupling and reconstruction of heteroassociations beteween complex patternts accumulated at one carrier without interference. This feasibility obliged just to natural recording nonlinearity (`nonidealness') of registration medium leads to limit generalization of the concept of holographic associative memory.

Multistabilities and symmetry-broken one-color and two-color states in closely coupled single-mode lasers.

We theoretically investigate the dynamics of two mutually coupled, identical single-mode semi-conductor lasers. For small separation and large coupling between the lasers, symmetry-broken one-color states are shown to be stable. In this case the light outputs of the lasers have significantly different intensities while at the same time the lasers are locked to a single common frequency. For intermediate coupling we observe stable symmetry-broken two-color states, where both lasers lase simultaneously at two optical frequencies which are separated by up to 150 GHz. Using a five-dimensional model, we identify the bifurcation structure which is responsible for the appearance of symmetric and symmetry-broken one-color and two-color states. Several of these states give rise to multistabilities and therefore allow for the design of all-optical memory elements on the basis of two coupled single-mode lasers. The switching performance of selected designs of optical memory elements is studied numerically.

Enhanced optical bistability with film-coupled plasmonic nanocubes

Colloidally synthesized nanocubes strongly coupled to conducting films hold great promise for enhancing different nonlinear optical processes. They exhibit a robust and sensitive scattering response that can be easily controlled by their geometrical and material parameters. Strong local field enhancement is generated at the gap regions between the nanocubes and the metallic film. We show that strong optical bistability and all-optical switching behavior can be obtained by loading these nanogaps with Kerr nonlinear materials. Relatively low input intensities are required to obtain these nonlinear effects. The proposed design can lead to efficient, low-power, and ultrafast all-optical memories and scattering nanoswitches.

All-optical tunable delay-line memory based on a semiconductor cavity-soliton laser

Cavity soliton lasers in a vertical-cavity surface-emitting laser with a saturable absorber can spontaneously move provided that a proper ratio of the carrier lifetimes in the amplifier and in the absorber is considered. Use is made of this fact to propose a scheme based on which a tunable all-optical delay line can be demonstrated. This easy-to-realize delay device exhibits high delay range, a very good delay resolution, and a wide extent of available data bandwidth. Also it is shown that the ratio of delay time to pulse duration (fractional delay) can be as large as 2300.

Optical correlation approach to all-optical holographic associative memory

The problem of all-optical holographic associative memory is discussed within the framework of optical correlation approach. Analysis of the models of ghost-image holograms and nonlenearly recorded off-axis holograms read out in associative regime shows a wide set of useful abilities of such holograms to not only reconstruct the missing part of the stored data, but also detect small changes ("errors") in the stored pattern, reconstruct a whole error-corrected pattern from its incomplete version, as well as perform high-efficient heteroassociative reconstruction based on non-interference mechanism of coupling of partial patterns. It is shown that visually estimated quality of reconstructed associative response can approach quality of reconstruction provided by common off-axis hologram. We time this overview to the 50th anniversary of the holographic associative memory.

Tunable Plasmonic and Hyperbolic Metamaterials Based on Enhanced Nonlinear Response

We present here tunable and reconfigurable designs of linear and nonlinear plasmonic and hyperbolic metamaterials. Rich scattering features of multilayered composite nanoparticles are demonstrated, which include complex and exotic scattering signatures combining multiple dipolar Fano resonances and electromagnetic induced transparency (EIT) features. These dipole-dipole multi-Fano scattering responses can be further tuned through altering the plasmonic properties of the concentric layers or the permittivity of the core, for instance, by the presence of nonlinearities. Strong third-order nonlinear effects, such as optical bistability, may also be induced in the scattering response of nonlinear nanoparticles due to the highly enhanced and confined fields inside their core. Nonlinear hyperbolic metamaterial designs are also explored, which can realize tunable positive-to-negative refraction at the same frequency, as a function of the input intensity. Negative Goos-Hänchen shift is demonstrated based only on the hyperbolic dispersion properties of these layered metamaterials without the usual need of negative index metamaterials. The Goos-Hänchen shift may be tuned from positive-to-negative values, when the structure is illuminated with different frequencies. A plethora of applications are envisioned based on the proposed tunable metamaterials, such as ultrafast reconfigurable imaging devices, tunable sensors, novel nanotag designs, and efficient all-optical switches and memories.

Femtojoule-Scale All-Optical Latching and Modulation via Cavity Nonlinear Optics

We experimentally characterize Hopf bifurcation phenomena at femtojoule energy scales in a multiatom cavity quantum electrodynamical (cavity QED) system and demonstrate how such behaviors can be exploited in the design of all-optical memory and modulation devices. The data are analyzed by using a semiclassical model that explicitly treats heterogeneous coupling of atoms to the cavity mode. Our results highlight the interest of cavity QED systems for ultralow power photonic signal processing as well as for fundamental studies of mesoscopic nonlinear dynamics.

Ultrafast tristable spin memory of a coherent polariton gas

Non-linear interactions in coherent gases are not only at the origin of bright and dark solitons and superfluids; they also give rise to phenomena such as multistability, which hold great promise for the development of advanced photonic and spintronic devices. In particular, spinor multistability in strongly coupled semiconductor microcavities shows that the spin of hundreds of exciton-polaritons can be coherently controlled, opening the route to spin-optronic devices such as ultrafast spin memories, gates or even neuronal communication schemes. Here we demonstrate that switching between the stable spin states of a driven polariton gas can be controlled by ultrafast optical pulses. Although such a long-lived spin memory necessarily relies on strong and anisotropic spinor interactions within the coherent polariton gas, we also highlight the crucial role of non-linear losses and formation of a non-radiative particle reservoir for ultrafast spin switching.

Nonvolatile polarization control of a bistable VCSEL

We report experimental evidence of nonvolatile all-optical memory operation using the two linear polarization states emitted from a GaAs oxide-confined VCSEL. The two polarization states coexist in a large range of pumping currents and substrate temperatures, and they can be controlled all-optically by exposing the device to polarization selective feedback, to crossed polarization reinjection orby injecting external light pulses. The active polarization state is recovered after powering off and on the VCSEL, while memory is lost if the substrate temperature is varied.

Implementation of 1-Bit Random Access Memory Cell in All-Optical Domain with Non-linear Material

This paper demonstrates an all-optical 1-bit Random Access Memory (RAM) with massive use of nonlinear material. All-optical switching mechanism is exploiting here to realize the all-optical 1-bit RAM. The all-optical switch by a composite slab of linear medium (LM) and non-linear medium (NLM) is the building block of our proposed 1-bit RAM circuit. An all-optical clocked D flip flop is the main storing element of the RAM. These circuits are simple and all-optical in nature. It can also gear up to the highest capability of optical performance in high-speed all-optical data storing, comput- ing and communicating system.

High-speed all-optical long-term memory using SOA MZIs: Simulation and experiment

We propose and demonstrate a novel all-optical memory to store high-speed optical data for long term. The key elements of the memory are Mach–Zehnder Interferometers (MZIs) incorporating semiconductor optical amplifiers (SOAs), acting as AND gate and regenerator in a loop configuration. The simulations show that the memory can be operated up to 80Gb/s. In addition, the memory was demonstrated at 21.3Gb/s.

Phase-Switched All-Optical Flip-Flops Using Two-Input Bistable Resonators

We show that switching between the two stable states of an optical flip-flop, made using a two-input Kerr resonator, can be realized through pure phase modulation. The proposed switching mechanism is compatible with cross-phase modulation induced by set and reset pulses for realization of an ultrafast, passive, all-optical memory element.

Analytical approach of using squeezed state formation of light for developing a highly noise reduced all-optical 1-bit memory cell

We report a new and novel all-optical method of developing highly noise reduced 1-bit memory cell by the use of optical logic gates with squeezed state of light. This system simultaneously offers a high speed of operation. The formation of squeezed state of light by means of optical parametric amplification (OPA) is a well known optical process. The proposed system can extend a promising application of storing and retrieving a squeezed optical data bit in a dynamic memory cell.

Bistability and All-Optical Memory in Dual-Mode Diode Lasers With Time-Delayed Optical Feedback

A proposal for an all-optical memory based on a bistability of single-mode states in a dual-mode diode laser with time-delayed optical feedback is presented. The system is modeled using a multimode extension of the Lang-Kobayashi equations with injected optical pulses. We uncover the bifurcation structure by deriving analytical expressions for the boundaries of the bistable region and demonstrate how the delay time in the external cavity determines an optimal pulse duration for efficient switching of the memory element. We also show the relevant role played by gain saturation and by the dual-mode solutions of the Lang-Kobayashi equations for the existence of the bistable regions. Our results demonstrate that feedback induced bistability can lead to significant performance improvements when compared to memory elements based on the injection locking bistability in dual-mode devices.

All-optical loadable and erasable memory cell design based on inversionless lasing and electromagnetically induced transparency effects

A loadable and erasable all-optical memory cell is designed by using two coupled micro-ring resonators with electromagnetically induced transparency (EIT) and lasing without inversion (LWI). Toread out stored data, an additional phase is introduced in the upper ring resonator due to EIT. To compensate the fibre loss, use is made of LWI. The EIT is induced by inserting Λ-type three level quantum dots in the right-hand half of the upper ring and LWI is implemented by inserted Y-type four level quantum dots in the left-hand half of both rings. This optical memory cell can operate at a low light power level corresponding to several photons.

Optical Dynamic RAM for All-Optical Digital Processing

The concept of a novel solution for an n-bit all-optical dynamic random access memory for advanced functionalities in all-optical digital processing is investigated. The scheme is composed of a matrix of 1-bit memory elements, each one obtained by an all-optically controlled variable optical buffer. The buffer is implemented by a wavelength preserving semiconductor optical amplifier-based fiber loop in which cross-gain modulation nonlinear effect allows for polarization and wavelength-independent operation in the whole C-band. The cell enabling command, as well as the read and write control signals, are obtained through all-optical logic operations between the row and column selecting signals. True dynamic and asynchronous random access properties, including write, read, and refresh operations, are verified for the single memory cell in terms of extinction ratio (ER) as a function of the storage time. A 1.77-μs reading delay is demonstrated without signal degradation. Multicell performance is also investigated. For an 8 × 8 matrix structure, the ER is reduced by 3 dB. A multiregister matrix for synchronous operation can be also implemented once n-bit packets are stored in each specific cell.

Nonlinear optical generation of time-delayed entanglement

A model is presented of nth order nonlinear processes in whispering gallery mode resonators, with scattering coherently coupling degenerate counter propagating modes. It is shown that such systems generate strong squeezing and time-delayed entanglement. The model can be generally applied to any pair of nonlinear coherently coupled cavities and is of particular relevance to whispering gallery mode resonators. A feature of the entanglement is that, by tuning the coherent coupling rate the peak entanglement can be tuned to occur away from the carrier frequency. This has technological significance allowing low frequency noise sources around the carrier frequency to be avoided. All-optical time-delayed entanglement has many applications, such as an all-optical quantum memory.

All-optical method of developing a phase encoded latch by using EDFA

Optics has already proved its meaningful application in information processing and computation, where the parallelism of light is exploited to achieve the desired result. In this communication a novel concept of all-optical memory unit based on phase encoding process is proposed. The unit is simple in architecture comprising phase sifters, mirrors ( M′) and two blocks of Erbium doped fiber amplifier. The optical feedback is supported by use of beam splitters and mirrors. It is independent of intensity and therefore requires a small switching power and can work at a bit rate far above 100 Gb/s.

All-optical memory based on injection-locking bistability in photonic crystal lasers

We have demonstrated an all-optical memory by using InGaAsP/InP buried heterostructure photonic crystal (BH-PhC) lasers. We achieved distinct optical injection locking bistability in an ultra-compact active region (4 × 0.3 × 0.16 μm3) with only 25 μW pump power in the PhC waveguide, which is two orders less than previously reported optical memories based on other bistable semiconductor lasers. Dynamic memory operations were achieved with pump power of 100 μW and switching power of 22 μW and 71 μW in the PhC waveguide. Fast switching times of about 60 ps were achieved. To the authors' best knowledge, this is the first demonstration of PhC laser-based all-optical memory.

Optoelectronic and all-optical multiple memory states in vanadium dioxide

Vanadium dioxide exhibits a well-known insulator-to-metal transition during which several of its physical properties change significantly. A hysteresis loop develops for each of them as the material is heated and then cooled through the transition. In this work VO2/SiO2 samples were maintained—by heat sinking—at a selected temperature within the heating branch of the hysteresis loops for resistance and near-infrared transmittance, while brief thermal excursions of the VO2 film were caused by either voltage pulses applied to the film or laser light pulses irradiating the film. These pulses had durations from milliseconds to a few seconds and the resulting drops in resistance or transmittance were easily and repeatably measurable without appreciably affecting their new values. A sequence of equal-duration pulses (for either equal-voltage or equal-irradiation pulses) caused the resistance and infrared transmittance to continue to drop, each time by a smaller amount, and larger energy pulses were required in order to cause drops comparable with the initial one. The ability of the film to change the values of the measurands in this manner with additional pulses was maintained up to a limit defined by the outer hysteresis curve for the measurand in question. The results presented show that a plurality of memory “states” in VO2 can be established or “written” either by voltage pulses or by light pulses applied to the material, and queried or “read” by resistance or transmittance readings, or both. These states were found to remain stable for at least several hours, as long as temperature was kept constant, and are expected to persist indefinitely under this condition. In the all-optical case, if the same light beam is used for writing and reading the memory state, the device is an optical analog of a memristor.