All-optical Switching Processing Research Articles

All-optical diffraction and ultrafast switching in a terahertz-driven quantized graphene system

Over the past few years, we have witnessed a substantial evolution in graphene optoelectronics. The tunability of the electronic structure of graphene has led to the next generation of carbon-based nano-photonic devices. Motivated by the unique optical properties of graphene, in the present work, we show that the landau-quantized graphene can be regarded as a platform for implementing a 2D laser-induced grating with high diffraction efficiency in the far-field regime. We consider a 2D doped graphene monolayer with a four-level system in Λ-configuration driven by a terahertz field. The electromagnetically induced Fraunhofer diffraction patterns are simulated for a weak probe field passing through the graphene nano-sheet. The optimum conditions for the maximum diffraction efficiency are investigated. Then, it is shown that the probe beam propagation can be coherently controlled via applying a terahertz field. The possibility of ultrafast all-optical switching of the probe beam is investigated. The proposed system may find applications in integrated nano-photonic devices for optical communications, infrared spectroscopy, astronomy, optical sensing, and also the realization of all-optical switching processes.

Realistic micromagnetic description of all-optical ultrafast switching processes in ferrimagnetic alloys

Both helicity-independent and helicity-dependent all-optical switching processes driven by single ultrashort laser pulse have been experimentally demonstrated in ferrimagnetic alloys as GdFeCo. Although the switching has been previously reproduced by atomistic simulations, the lack of a robust micromagnetic framework for ferrimagnets limits the predictions to small nanosystems, whereas the experiments are usually performed with lasers and samples of tens of micrometers. Here we develop a micromagnetic model based on the extended Landau-Lifshitz-Bloch equation, which is firstly validated by directly reproducing atomistic results for small samples and uniform laser heating. After that, the model is used to study ultrafast single shot all-optical switching in ferrimagnetic alloys under realistic conditions. We find that the helicity-independent switching under a linearly polarized laser pulse is a pure thermal phenomenon, in which the size of inverted area directly correlates with the maximum electron temperature in the sample. On the other hand, the analysis of the helicity-dependent processes under circular polarized pulses in ferrimagnetic alloys with different composition indicates qualitative differences between the results predicted by the magnetic circular dichroism and the ones from inverse Faraday effect. Based on these predictions, we propose experiments that would allow one to resolve the controversy over the physical phenomenon that underlies these helicity-dependent all optical processes.

Numerical study of all-optical switching of ferromagnetic thin film using temporally controlled optothermal and optomagnetic pulses

All-optical switching (AOS) allows the ultrafast control of magnetization states in a thin film and has the potential to increase the magnetic data recording speed by a few orders of magnitudes. The mechanism of AOS in ferrimagnetic materials has been successfully explained by using the magnetic circular dichroism and the angular momentum transfer between sublattices, but the mechanism for the observed AOS in the ferromagnetic materials is still under debate. One hypothesis explains the AOS in the ferromagnetic film by hypothesizing the existence of a prolonged magnetic field excited by the inverse Faraday effect. Theoretical calculation suggests that the switching result is sensitive to the magnetic field during the cooling down process, particularly across the Curie temperature. However, the optomagnetic excitation originates from the same pulse and decays faster than the optothermal response, which typically makes the induced magnetic field fade away before the material reaches the Curie temperature. Here, we numerically study a dual-pulse AOS scheme that temporally and spatially decouples the ultrafast optothermal and optomagnetic responses and allows the timing control of the induced magnetic field to facilitate the AOS. This dual-pulse scheme may guide to experimentally confirm the hypothesis of the prolonged magnetic field for the ferromagnetic AOS process.

Dynamics of all-optically switched magnetic domains in Co/Gd heterostructures with Dzyaloshinskii-Moriya interaction

Given the development of hybrid spintronic-photonic devices and chiral magnetic structures, a combined interest in all-optical switching (AOS) of magnetization and current-induced domain wall motion in synthetic ferrimagnetic structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In this study, we report a study on single-pulse all-optical toggle switching and asymmetric bubble expansion in specially engineered Co/Gd-based multilayer structures. In the absence of any external magnetic fields, we look into the AOS properties and the potential role of the DMI on the AOS process as well as the stability of optically written micro-magnetic domains. Particularly, interesting dynamics are observed in moon-shaped structures written by two successive laser pulses. The stability of domains resulting from an interplay of the dipolar interaction and domain-wall energy are compared to simple analytical models and micromagnetic simulations.

Microscopic three-temperature model for all-optical switching in GdFeCo

Compared with magnetic switching by an external magnetic field or by a heat-assisted manner, all-optical switching (AOS) can complete the switching process within 100 ps, which has attracted extensive attention from researchers. Among the magneto-optical materials which can realize AOS, the ferrimagnetic GdFeCo has the ability to realize single-shot AOS and possesses great potential in all-optical magnetic storage. In this paper, a microscopic three-temperature model (M3TM) is utilized to simulate the AOS process of GdFeCo, which is also demonstrated experimentally, under the excitation of a single laser pulse based on the heating effect. By using this M3TM, the AOS dynamics and the final magnetization states of GdFeCo induced by single laser pulses with different energy and pulse widths are calculated and analyzed concretely. Compared with the atomic spin model and the Landau- Lifshitz-Bloch (LLB) model, M3TM provides a more concise time-varying expression of the magnetization of GdFeCo and explicitly addresses the dissipation of angular momentum after the laser-pulse excitation, which enables faster calculations of the heat-induced magnetization dynamics in magneto-optical materials with large areas.

Prediction of Deterministic All-Optical Switching of Ferromagnetic Thin Film by Ultrafast Optothermal and Optomagnetic Couplings

All-optical switching (AOS) of magnetization induced by ultrafast laser pulses is fundamentally interesting and promises unprecedented speed for magnetic data storage that is three orders of magnitudes faster than the current techniques. For ferrimagnetic material, the AOS is attributed to magnetic circular dichroism and angular momentum transfer between sublattices. Recently, ferromagnetic material is demonstrated in AOS under multiple pulses. Since the magnetic field needed to flip the ferromagnetic magnetization within femtosecond timescale is unphysically high, some theories hypothesized that there exists a prolonged magnetic field beyond the pulse duration in the switching process. This is intuitively inconsistent with the phenomenological explanation based on the light-induced magnetic field arising from the inverse Faraday effect (IFE). Here, we numerically study the AOS process and provide new insights into the long-standing paradox of the duration of the induced magnetic field. We show that the prolonged magnetic field duration originates from the ultrafast optothermal and optomagnetic coupling. Moreover, we numerically studied both single- and multiple-pulse AOS under different coupling strength between spins and the thermal bath in the macroscopic Fockker-Planck and Landau-Lifshitz-Bloch model. This numerical model may provide a guide to find suitable ferromagnetic materials for AOS.

Erratum to: “An enhanced refractive index with suppressed absorption in a graphene nanostructure under external magnetic field”

The dispersive-absorptive optical properties of a weak probe field are investigated based on quantum coherence and interference in a Landau-quantized graphene structure. It is found that an enhanced refractive index with vanishing absorption can be obtained in this structure through proper adjusting the controlling parameters of the system. The switching between superluminal and subluminal light propagation is also discussed. Our scheme can be employed in real experiments to develop new types of nanoelectronic devices for realizing all-optical switching process and can have practical application in dispersion compensation and solid-state quantum communication.

An enhanced refractive index with suppressed absorption in a graphene nanostructure under external magnetic field

The dispersive-absorptive optical properties of a weak probe field are investigated based on quantum coherence and interference in a Landau-quantized graphene structure. It is found that an enhanced refractive index with vanishing absorption can be obtained in this structure through proper adjusting the controlling parameters of the system. The switching between superluminal and subluminal light propagation is also discussed. Our scheme can be employed in real experiments to develop new types of nanoelectronic devices for realizing all-optical switching process and can have practical application in dispersion compensation and solid-state quantum communication.

Realization of electromagnetically induced phase grating and Kerr nonlinearity in a graphene ensemble under Raman excitation

Some optical properties including the linear and nonlinear susceptibility and electromagnetically induced phase grating (EIG) in graphene under Raman excitation is studied. A single-layer graphene nanostructure driven by coherent and incoherent fields is investigated theoretically. It is revealed that by adjusting the amplitude of control and incoherent fields, the linear and nonlinear absorption as well as Kerr nonlinearity of the medium can be optimized. It is realized that the enhanced Kerr nonlinearity can occur with zero linear absorption and nonlinear amplification. Furthermore, it should be noted that EIG in graphene is studied for the first time. The results indicate that the diffraction efficiency of the phase grating is dramatically enhanced by controlling the amplitude of coherent and incoherent fields, and an efficient electromagnetically induced phase grating can be obtained. A novel result shows a considerable improvement of the intensity of higher-order diffractions and switching between different orders of grating via incoherent pumping field. Therefore, this model can be used in real experiments for the development of new types of nanoelectronic devices used for the realization of all-optical switching processes.

The optical properties of a weak probe field in a graphene ensemble under Raman excitation

We investigate the coherent manipulation of certain optical properties in graphene under Raman excitation by using a density-matrix approach. It is shown that the absorption and dispersion, the group velocity, and the transmission coefficient of the probe field can be efficiently controlled through proper adjustment of the intensity and frequency detuning of the control field. In addition, the optical bistability (OB) behavior is explored for the proposed system. It is found that the bistable threshold intensity and related hysteresis loop of OB can be controlled by the parameters of the system. The results obtained may be used in real experiments for the development of new types of nanoelectronic devices used for the realization of all-optical switching processes.

The effect of heteroatoms and end groups of polymethines on the all-optical switching processing application: a CC2 calculation

The structure–properties relationship of heterocyclic polymethines on the all-optical switching processing (AOSP) is studied by CC2 method. In particular, the excitation energies of the first and second excited states (E ge and E ge′ ) and the corresponding ratio (R ee′ = E ge′ /E ge, energy window width) are evaluated. The comparable energy gap between HOMO and HOMO − 1 (π type) and between LUMO and LUMO + 1 (π* type) is the key in keeping R ee′ large. Moreover, only a significant increase of such energy gap can result in the increase in R ee′ because the energy gap is a poor description of excitation energy. The results show that the heavy heteroatoms decrease the E ge and E ge′ as expected compared to light atoms such as carbon. For five-membered heterocyclic polymethines and their derivatives (two heteroatoms in total), the R ee′ is stable around 1.7 reflecting the nature of this system: The additional heteroatoms have little effects on the width of energy window. In the case of six-membered heterocyclic polymethines and their derivatives, the decrease in excitation energy is more effective compared to the former due to the relative large effective length of the end group, which can thus help avoiding the symmetry breaking. The effect of heteroatoms on R ee′ is clear that it helps keeping the energy window open compared to carbon atom. The heavier the heteroatom is, the higher R ee′ value will be when it is on the ortho position. Consequently, at least one heteroatom is necessary on the design of polymethines for AOSP application.

Controllable optical bistability and multistability in a graphene structure under external magnetic field

We investigate the behavior of optical bistability (OB) and optical multistability (OM) based on quantum coherence in a Landau-quantized graphene structure. Such a tunable four-level system is driven coherently by two coherent fields and an incoherent pumping field inside the unidirectional ring cavity. The influence of system parameters on the threshold of the onset of OB and OM is studied. It is found that one can efficiently control the OB/OM threshold intensity and the hysteresis loop by using the system parameters. The results obtained may be used in real experiments for the development of new types of nanoelectronic devices for realizing an all-optical switching process.

Enhanced third harmonic generation by organic materials on high-Q plasmonic photonic crystals.

The enhanced optical nonlinearity enabled by localized plasmonic fields has been well studied for all-optical switching processing (AOSP) devices for future optical communication systems. In this work, plasmonic photonic crystals with a nonlinear polycarbonate/polymethine blend cladding layer are designed to enhance the third harmonic generation (THG) at the telecom wavelengths (~1550 nm). Due to the presence of he two-dimensional (2-D) gold nano-patch arrays with improved Q-factor and high local fields, more than 20 × of enhanced THG signals in the hybrid organic-plasmonic nanostructure are experimentally observed. The enhanced THG in the hybrid organic-plasmonic materials suggested that such extraordinary nonlinear effects can be used for AOSP devices and wavelength conversion.

Energy-resolved magnetic domain imaging in TbCo alloys by valence band photoemission magnetic circular dichroism

We report magnetic domain imaging of a terbium cobalt (TbCo) alloy thin film with a high perpendicular magnetic anisotropy in one- and two-photon photoemission electron microscopy (PEEM). Both photoemission processes deliver a clear magnetic circular dichroism (MCD) whose strength is strongly energy dependent. Comparing the energy dependence of the MCD signal in one- and two-photon photoemission, we conclude that the magnetic contrast is mainly an initial state effect. Our results ultimately show that MCD contrast can be obtained in PEEM in valence band photoemission from a material supporting all-optical magnetization switching. This opens the way for the investigation of the all-optical switching process with simultaneous ultrahigh temporal and spatial resolution.

All‐optical magnetization switching using phase shaped ultrashort laser pulses

AbstractSwitching the magnetization without any external applied magnetic field by using ultrashort circularly polarized laser pulses is a fascinating and challenging topic of modern magnetism. Up to now there is still no consensus about the microscopic mechanism of this so called all‐optical switching (AOS) and several different models are suggested. In particular the importance of heat and helicity driven effects is still under discussion. In our experiment we varied the input of heating during the AOS process by utilizing pulse trains, created with a phase pulse shaper, to switch the magnetization. We show that AOS is possible with pulse trains having a temporal delay between the single pulses of more than a picosecond. The minimum switching fluence for pulse trains is found to be higher compared to switching with unshaped pulses. We mainly attribute this behavior to heat loss effects. Further we find that the minimum switching fluence depends on the number of pulses within a pulse train. We discuss our observations in terms of the currently debated theoretical models of AOS.

Theory and experimental studies of all-optical transmission switching in a twist-alignment dye-doped nematic liquid crystal

We present a theoretical model and experimental demonstration of an all-optical switching process occurring in twist alignment dye-doped nematic liquid crystal cells. A Landau-deGennes theory is applied together with the thermal buildup and diffusion equation to analyze the laser-induced temperature, the order parameter, and the birefringence changes in the liquid crystal cell. The resulting transmission switching processes are calculated using a modified Jones matrix method. Experimental observations of the switching processes in cells of varying thicknesses and their dependence on the input optical power are in good agreement with theoretical predictions. We have also studied the dynamics of the switching processes and observed microseconds switching times. These studies demonstrate the possibility of low-threshold power and microseconds speed switching applications.

Making Light Work

Windows or eyeglasses that darken in bright light are examples of nonlinear optical materials, in which the response of the medium, in this case the refractive index, changes dramatically with the light intensity. However, the response time, typically seconds, of these materials is too slow to be of practical use for electro-optic or all-optical switching processes. The combination of intense coherent radiation available from lasers together with newly developed photorefractive materials is providing nonlinear optical response times on ultrafast time scales. Such systems allow photons to be added, subtracted, mixed, and manipulated in ways impossible in our familiar low-light-intensity environment. The Reviews in this special issue of Science look at some of those developments in the field of nonlinear optics and the applications they are finding. To study the dynamics of an event too fast for our eyes to see, snapshots of the event are taken, pieced together, and then played back in slow motion. Cameras equipped with fast mechanical shutters allow us to view microsecond snippets of time. The detailed dynamics of chemical reactions or of electronic transitions in crystalline materials can be revealed in a similar manner by following processes with laser pulses on time scales of picoseconds (10−12 s) or femtoseconds (10−15 s). Steinmeyer et al. (p. [1507][1]) review the role of nonlinear optics in the generation of ultrafast laser pulses and focus on the role being played by direct laser oscillators. It is necessary in spectroscopy to tune in to the energy scale that is relevant to the particular system under study. However, lasers generally emit light at very precise and fixed wavelengths. Dunn and Ebrahimzadeh (p. [1513][2]) describe how high-quality monochromatic light can be generated with optical parametric oscillators in which the interaction between photons can be controlled. This approach achieves a flexibility in generating the output light that is not available to conventional lasers. The past decade has seen an explosion in communication technology. The continuing challenge is to transfer ever-larger amounts of information from one point to another at as high a rate as possible without losing any data in the process. One proposal to overcome the transfer rate limitation of semiconductors is to use all-optical networks. Today, thousands of kilometers of optical fibers encircle the globe. As more information is transferred down a fiber, however, nonlinearities become important and can actually limit the information capacity. As a light pulse propagates through a medium, it disperses or broadens, and repeater stations are needed periodically to prevent pulses from smearing into one another. However, with sufficiently high light intensity, the refractive index of the medium is locally increased and the light is confined to that region by total internal reflection. The light then traverses the medium without loss of intensity and is described as a spatial soliton. Stegeman and Segev (p. [1518][3]) review how such nonlinearities can be used to advantage in the generation and interaction of solitons in their role as information carriers, and Cotter et al. (p. [1523][4]) review the techniques required to implement processing of the optical signals at transfer rates in excess of 100 gigabits per second. A News story (p. [1502][5]) examines the promise of storing such torrents of data in holograms: durable optical inscriptions written in nonlinear optical materials. And two other News stories (pp. [1500][6] and [1504][7]) describe progress in other fields of optics: latticelike photonic crystals, which can trap or channel light; and adaptive optics technology, which helps telescopes undo the effects of atmospheric turbulence. [1]: /lookup/doi/10.1126/science.286.5444.1507 [2]: /lookup/doi/10.1126/science.286.5444.1513 [3]: /lookup/doi/10.1126/science.286.5444.1518 [4]: /lookup/doi/10.1126/science.286.5444.1523 [5]: /lookup/doi/10.1126/science.286.5444.1502 [6]: /lookup/doi/10.1126/science.286.5444.1500 [7]: /lookup/doi/10.1126/science.286.5444.1504