High Switching Contrast Research Articles

Topologically-protected optical bistability based on two-dimensional photonic crystal L6 nanocavity dimer array.

We numerically investigate the optical bistability from a two-dimensional photonic crystal L6 nanocavity dimer array structure configured under the Su-Schrieffer-Heeger model. The localized electric field in the topological edge state is highly enhanced, which gives rise to strong nonlinear phenomena such as optical bistability. In comparison, a topologically trivial nanocavity is also designed and its field strength distribution and optical bistable response are also simulated. In order to test the robustness, three types of defects and interferences are introduced in both the topologically non-trivial and trivial cavities. Benefiting from the topological feature, the proposed topological cavity exhibits superior optical bistable performance with low threshold power and high switching contrast compared to that in the trivial cavity. Our work suggests what we believe to be a novel avenue toward the insertion of optical bistable devices with high robustness into future photonic integrated circuits and photonic neural networks.

Heat- and Pressure-driven Room-temperature Polymorphic Transition Accompanied with Switchable SHG Signal in a New Chiral Hexagonal Perovskite.

Endowing room-temperature polymorphs with both long-term stability and easy interconvertibility is a big challenge due to the complexity of intermolecular interactions. Herein, we present a chiral hexagonal perovskite (R-3-hydroxy-1-methylpiperidinium)[CdCl3 ] having two room-temperature crystalline forms featuring obviously distinct second-harmonic-generation (SHG) signals with a high switching contrast of ~18 times. The two room-temperature forms could be long-term stable yet easily interconvertible through an irreversible thermal-induced phase transition and a pressure-driven backward transition, by switching hydrogen bonds via collective reorientation of ordered homochiral cations. Based on the essential role of homochiral organic cations in inducing switchable hydrogen bond linkages, this present instance provides good evidence that relatively irregular organic cations could induce more obvious inorganic chain deformations, thus endowing polymorphs with significantly different SHG signals at room temperature.

In-memory photonic dot-product engine with electrically programmable weight banks

Electronically reprogrammable photonic circuits based on phase-change chalcogenides present an avenue to resolve the von-Neumann bottleneck; however, implementation of such hybrid photonic–electronic processing has not achieved computational success. Here, we achieve this milestone by demonstrating an in-memory photonic–electronic dot-product engine, one that decouples electronic programming of phase-change materials (PCMs) and photonic computation. Specifically, we develop non-volatile electronically reprogrammable PCM memory cells with a record-high 4-bit weight encoding, the lowest energy consumption per unit modulation depth (1.7 nJ/dB) for Erase operation (crystallization), and a high switching contrast (158.5%) using non-resonant silicon-on-insulator waveguide microheater devices. This enables us to perform parallel multiplications for image processing with a superior contrast-to-noise ratio (≥87.36) that leads to an enhanced computing accuracy (standard deviation σ ≤ 0.007). An in-memory hybrid computing system is developed in hardware for convolutional processing for recognizing images from the MNIST database with inferencing accuracies of 86% and 87%.

Enhanced electrochromic switching contrast in the blue by 3,4-propylenedioxypyrrole - implementation on structural colors.

Recent advances in nanofabrication technologies have enabled new ways to produce structural colors. By combining nanofabrication methods, it is possible to integrate electrochromic materials with the nanostructures, which enable electrical tuning of the colors and thus new types of reflective displays. Previous work has shown high quality colors and high switching contrast in general. However, so far the intensity modulation has always been more limited in the blue. In this work we prepare blue structural colors and synthesize films of an electrochromic polymer (PProDOP) that is optimized for high contrast in this spectral region. A protocol for electropolymerization of PProDOP on gold surfaces is presented. The polymer films are shown to follow Lambert-Beer behavior and can provide up to 75% contrast (difference in transmittivity). On blue nanostructures, the reflectivity can be modulated with a contrast of 50%, which is a considerable improvement in comparison with previous work. The results presented here should be useful for electrochromic or other electro-optical devices operating in the blue spectral region.

High Contrast All-Optical Dual Wavelength Switching of Femtosecond Pulses in Soft Glass Dual-Core Optical Fiber

All-optical switching of 75 fs pulses centered at 1560 nm, driven by 270 fs, 1030 nm pulses in a dual-core optical fiber exhibiting high index contrast is presented. The fiber is made of a thermally matched pair of lead silicate and borosilicate glasses used as core and cladding material, respectively. The novel switching approach is based on nonlinear balancing of dual-core asymmetry, by control pulse intensity induced group velocity reduction of the fast fiber channel. Due to the fiber core made of soft glass with high nonlinearity high switching contrast exceeding 20 dB is reached by using control pulses of only few nanojoule energy. The optimum fiber length is 14 mm, which closely match the calculated coupling length at the signal wavelength. The results express significant progress in comparison to similar studies based on self-switching of solitonic pulses in dual-core fibers and represent high application potential.

On-Chip Programmable Nonlinear Optical Signal Processor and Its Applications

We introduce an integrated device based on microring resonator (MRR) assisted Mach-Zehnder interferometer (MZI) preceded by a tunable MZ coupler for nonlinear optical signal processing. The novel structure of this device provides both high programmability and switching contrast. These desirable features suggest its potential use as a multi-purpose optical processor providing different high-performance functionalities. We experimentally demonstrate two functionalities using the same device fabricated on a silicon-on-insulator substrate. One functionality is an all-optical thresholder enabling 40× signal contrast improvement, and the other functionality is clock-less pulse carving technique that converts long-pulse signals to short-pulse signals. With these functionalities, we discuss system-level applications of our device in optical interconnects and photonic neural networks.

Site‐Selective Occupancy of Eu2+ toward High Luminescence Switching Contrast in BaMgSiO4‐Based Photochromic Materials

AbstractLuminescent inorganic photochromics have great potential applications in 3D optical memory due to their excellent thermal stability and chemical resistance. Nevertheless, constructing inorganic photochromics with a high luminescence switching contrast remains challenging for optical information storage. Understanding inherent relationships between microenvironment changes, photophysical material properties, and rational control of their luminescence properties are crucial to address these issues. This paper proposes an effective strategy to significantly improve the luminescence switching contrast by more than 300% by site‐selective occupancy engineering on BaMgSiO4:Eu2+ photochromic materials. An effective energy transfer process is established between luminescent and photochromic units, which may provide new insights for designing high‐performance optical storage systems. The fabricated flexible films exhibit excellent waterproof, flexibility, and stretchability characteristics, accompanied by reversible color switching and larger color contrast. Optical data storage can be not only visually encoded and decoded on the flexible films by 405 and 532 nm LED excitation, respectively. Writing, erasing, and reading strategies for flexible films are particularly suitable for complex application environments in optical storage and rewritable flexible devices.

A Nonlinear Optical Switchable Sulfate of Ultrawide Bandgap

Nonlinear optical (NLO) switchable materials have attracted intense attention because of their promising applications in optoelectronic devices. However, previous studies are mainly limited to mole...

All-optical supercontinuum switching

Efficient all-optical switching is a challenging task as photons are bosons and cannot immediately interact with each other. Consequently, one has to resort to nonlinear optical interactions, with the Kerr gate being the classical example. However, the latter requires strong pulses to switch weaker ones. Numerous approaches have been investigated to overcome the resulting lack of fan-out capability of all-optical switches, most of which relied on types of resonant enhancement of light-matter interaction. Here we experimentally demonstrate a novel approach that utilizes switching between different portions of soliton fission induced supercontinua, exploiting an optical event horizon. This concept enables a high switching efficiency and contrast in a dissipation free setting. Our approach enables fan-out, does not require critical biasing, and is at least partially cascadable. Controlling complex soliton dynamics paves the way towards building all-optical logic gates with advanced functionalities.

All-Dielectric Metasurface-Enabled Near-Infrared Switching Based on Ge2Sb2Te5 Phase-Change Material

A metasurface for use in multiscale applications in the near-infrared (NIR) range is proposed herein. A detailed study is carried out on a polarization-insensitive, single-layered, reconfigurable swastika-like metasurface for NIR switching. Using germanium antimony telluride (Ge2Sb2Te5) as a phase-change material, the metasurface works as a NIR switch in the wavelength range from 1.8 μm and 2.2 μm. The metasurface consists of a patterned Ge2Sb2Te5 thin film on a glass substrate, making it an all-dielectric metastructure. A high switching contrast ratio, i.e., ratio of maximum to minimum transmission, of 150 was obtained. Light with different polarizations had no effect on the switching behavior of the metasurface, making it a potential candidate for the development of advanced, tunable, and integrated photonic devices.

Photothermal effect in graphene-coated microsphere resonators

In this study, graphene-coated silica microsphere resonators with Q values of ∼105 are fabricated. After wet transfer of a graphene layer onto the surface of the microsphere, the deterioration of the Q value due to the graphene layer is overcome by a laser treatment. The photothermal effect in the fabricated resonators is investigated experimentally. Using a control-probe scheme, the resonance shift by the photothermal effect is demonstrated, with a high tuning slope. Optical bistability is also observed, with a low threshold and high switching contrast. The experimental results show that the graphene-coated microsphere resonator is a promising platform to investigate the interaction between graphene and light field.

All-optical control of weak-light transport and Fano-like resonance using control-probe technique in a quantum-dot-pillar microcavity system

Control of light by light is a current research topic and is important for a variety of fundamental studies and practical applications. Here, we put forward a chip-scale scheme for all-optical control of small-signal photon transport and Fano-like lineshape in a coupled quantum-dot-pillar microcavity system based on a control-probe technique. Specifically, a bichromatic input light field, which consists of a weak probe field and a tunable control field, is applied to simultaneously drive the pillar cavity mode (called the control-probe technique) and both its reflectivity from and transmittivity through the pillar cavity constitute two output channels (i.e., a reflectivity signal channel and a transmittivity signal channel). We derive full analytical expressions for the reflectivity and transmittivity coefficients via a perturbation method. Using experimentally realistic parameters, theoretical analysis shows that the output probe light of the reflectivity and transmittivity channels can be switched on or off by simply turning on or off the control light field. Hence, the present system can be utilized to realize all-optical switching of the probe light by the control light with high switching contrast ∼1 by means of these two output channels. Interestingly, the reflectivity channel and the transmittivity channel are complementary and offer versatile applications in a two-channel switching. In addition, it is shown that the strong asymmetric Fano lineshape can be generated by properly tuning the frequency of the control light field in this control-probe scheme. The present investigation may be used for developing novel all-optical switching and tunable Fano devices on-chip.

Tunable Luminescence Contrast of Na0.5Bi4.5Ti4O15:Re (Re = Sm, Pr, Er) Photochromics by Controlling the Excitation Energy of Luminescent Centers.

High luminescent switching contrast of photochromic materials is extremely important in improving the sensitivity and resolution of optical switches and high-density optical data storage devices. To date, conventional methods, such as tuning absorption and emission bands based on electron or resonance energy transfer mechanisms in well-known organic photochromic molecules or compounds, have routinely been adopted to tune luminescent switching behavior. However, these strategies and mechanisms are not effectively applied to luminescence switching in inorganic materials because their crystal structures differ strongly from those of organic materials. In this paper, we report a new method to significantly tune the luminescent switching contrast by modifying the excitation energy of luminescent centers in a newly synthesized photochromism material: Na0.5Bi4.5Ti4O15:Re (Re = Sm, Pr, Er). A significant enhancement of luminescence switching contrast was achieved when the luminescent centers were excited by low energy photons at a given irradiation wavelength, intensity, and time, compared with high excitation energy photons. The trend "the lower the excitation energy, the higher the luminescence switching contrast" is universal in different rare earth ion-doped Na0.5Bi4.5Ti4O15 ferroelectrics. The changes in the luminescent switching contrast based on excitation energy are ascribed to nonradiative energy transfer from the luminescent center to the color center by dipole-dipole interactions according to Dexter theory. This possible utilization of excitation energy at lower energy levels is usually less destructive to both information recording and the recording material itself during luminescent readout processes while achieving higher luminescence switching contrast.

All-optical logic gates based on wavelength conversion in a nonlinear directional coupler

In this paper, we theoretically investigate all-optical logic gates based on wavelength conversion in a nonlinear directional coupler. The switching of a weak continuous-wave (cw) light is controlled by two pump lights, as a result, the information on the pump light transfers to the cw light by controlling different combinations of the two pump lights. Firstly we study the switching characteristic of the device and select in a way to provide the best average values of Xratio. Finally we realize logic operations C=A⊕B and D=A⊕B¯. The Xratio is higher than that of the ordinary methods and the all-optical logic gates based on wavelength conversion exhibit high switching contrast and have a better switching characteristic.

Self-stabilized quantum optical Fredkin gate

The quantum optical Fredkin gate is an indispensable resource for networkable quantum applications. Its performance in practical implementations, however, is limited fundamentally by the inherent quantum fluctuations of the pump waves. We demonstrate a method to overcome this drawback by exploiting stimulated Raman scattering in fiber-based implementations. Using a Sagnac fiber-loop switch as a specific example, we show that high switching contrast can be maintained even in the presence of significant pump fluctuations. This unique feature of self-stabilization, together with high-speed and low-loss performance of such devices, point to a viable technology for practical quantum communications.

Electrically tunable prism grating based on a liquid crystal film with a photoconductive layer

This study is the first to investigate an optically addressed, electrically tunable prism grating based on homogeneously aligned liquid crystals (LCs) with a photoconductive layer. A conductivity-gradient electrode-like grating pattern of the polymer layer results in a spatially periodic gradient of the effective electric-field drop, producing a prism grating with a spatially periodic LC gradient reorientation. The asymmetric diffraction pattern can be adjusted by varying the dc voltage. The first-order diffraction efficiency is 64% at optimal conditions. The proposed prism grating exhibits extremely low diffraction noise in the off state, a high switching contrast inthe on–off state (~1000), simplicity of fabrication, and high controllability at a low voltage range (0 to 0.4 V/μm).

Coordinative Layer-by-Layer Assembly of Electrochromic Thin Films based on Metal Ion Complexes of Terpyridine-Substituted Polyaniline Derivatives

Preparation, metal ion complexation, and coordinative assembly into organized electro-chromic films of a polyaniline derivative P1 substituted with tert-butyloxycarbonyl (boc) and terpyridine (tpy) substituent groups in alternating sequence are described. Cleavage of the boc groups after processing into thin films is also described. P1 is prepared upon Pd-catalyzed polycondensation of N-tert-butyloxycarbonyl-4,4'-dibromodiphenylamine and 4'(4-aminophenyl)-2,2':6,2''-terpyridine. The molecular weight is in the range of oligomers, the tetramer and pentamer being the most abundant species. P1 is soluble in common organic solvents. Solutions are pale yellow with blue or green fluorescence depending on the solvent. Fluorescence quantum yields up to 68% are found. P1 is able to complex divalent metal salts such as zinc(II) chloride, for example. Titration experiments indicate the formation of 2:1 tpy:metal ion complexes. Layer-by-layer (LbL) assembled films of metal ion complexes of P1 can be prepared, if negatively charged substrates are alternately dipped into solutions of metal(II) hexafluorophosphates and P1. Films of the zinc and nickel ion complex of P1 are lemon yellow in the neutral state and change color into greenish gray upon anodic oxidation, while Co-containing films are purple and change color into grayish blue upon oxidation. All color changes are reversible under ambient conditions. ATR-IR studies indicate that thermal treatment of the films at 180 °C, or acid treatment, e.g. with 5% aqueous trifluoroacetic acid solution, results in cleavage of the boc groups. P1 is transformed into the polyaniline derivative P2 with tpy substituent groups at every second N atom in the backbone. Films of metal ion complexes of P2 are also electrochromic albeit the colors differ slightly from those of P1. For example, the absorption maximum of the Zn-P2 film is at 456 nm, whereas it is at 446 nm for the corresponding Zn-P1 film. Prior to cleavage of the boc group, electrochromic switching times are 1.1 to 2.0 s for 30 to 40 nm thick films, while after the cleavage 0.5 to 1.2 s are found. The contrast is 13 to 19%, and not affected by the cleavage. Because of high stability, fast switching, and high contrast, the films might be useful as active materials in electrochromic devices.

A novel method of developing all-optical frequency encoded memory unit exploiting nonlinear switching character of semiconductor optical amplifier

The very fast running optical memory and optical logic gates are the basic building blocks for any optical computing data processing system. Realization of a very fast memory-cell in the optical domain is very challenging. In the last two decades many methods of implementing all-optical flip-flops have been proposed. Most of these suffer from speed limitation because of low switching response of the active devices. In our present communication the authors propose a method of developing a frequency encoded memory unit based on the switching action of semiconductor optical amplifier (SOA). Nonlinear polarization rotation characters of SOA and ‘SOA based Mach-Zehnder Interferometer’ switch, i.e. ‘SOA-MZI’ switch, are exploited for the purpose of some switching action with least switching power (<−3 dB m) and high switching contrast ratio (20 dB). Here two logic states (‘0’ state and ‘1’ state) of the memory is encoded by two different frequencies, which will remain unchanged throughout the data communication irrespective of loss of light energy due to reflection, refraction, attenuation, etc. Though the SOA based switch runs with the operational speed 100 Gb/s, still due to the presence of the other optical components in the memory unit, the overall speed of the proposed system will come down to 10 Gb/s.

FERROELECTRICS IN PLANAR GEOMETRY: FABRICATION AND PERSPECTIVES FOR INTEGRATION

ABSTRACT Single crystalline ultrathin films of (Ba,Sr)TiO3 (BST) and BiFeO3 (BFO) on MgO (001) substrate were fabricated by RF sputtering at high oxygen pressure, electric filed in planar geometry was applied by interdigital electrode system. Based on the obtained materials switchable ferroelectric and multiferroic nanostructures were fabricated by focused ion beam milling. The structure, dielectric, linear-optical and nonlinear (electro)optical properties of the films of different thicknesses in wide range of frequencies of applied voltage were studied. High efficiency of planar ferroelectric switching is shown. Nanostructuring allows to change dielectric and optical properties of ferroelectric materials and increase in this way the range of switchability of functional parameters. Fabricated structures with high switchability efficiency and low dielectric losses are very promising for fabrication of electro-optical modulators; small size combined with high switching contrast provides conditions for efficient integration in planer geometry.

All-optical switches on a silicon chip realized using photonic crystal nanocavities

We demonstrate all-optical switching in the telecommunication band, in silicon photonic crystals at high speed (∼50ps), with extremely low switching energy (a few 100fJ), and high switching contrast (∼10dB). The devices consist of ultrasmall high-quality factor nanocavities connected to input and output waveguides. Switching is induced by a nonlinear refractive-index change caused by the plasma effect of carriers generated by two-photon absorption in silicon. The high-quality factor and small mode volume led to an extraordinarily large reduction in switching energy. The estimated internal switching energy in the nanocavity is as small as a few tens of fJ, indicating that further reduction on the operating energy is possible.