Bistable Loop Research Articles

Optical bistability in metal-insulator-metal plasmonic waveguide with nanodisk resonator containing Kerr nonlinear medium

We numerically investigate the optical bistability effect in the metal-insulator-metal waveguide with a nanodisk resonator containing a Kerr nonlinear medium. It is found that the increase of the refractive index, which is induced by enhancing the incident intensity, can cause a redshift for the resonance wavelength. The local resonant field excited in the nanodisk cavity can significantly increase the Kerr nonlinear effect. In addition, an obvious bistability loop is observed in the proposed structure. This nonlinear structure can find important applications for all-optical switching in highly integrated optical circuits.

Optical bistability of subwavelength metallic film coated by Kerr dielectric gratings

We studied the optical bistability of subwavelength metallic film coated by Kerr dielectric gratings numerically. It is found that the effect of optical bistability in the structure is explained by the excitations of the coupled surface plasmon polaritons at the coated metallic films. The effect of thickness of the metallic films on bistability loop is analyzed in detail, and the authors attribute the change of the bistability loop to the lifetime of the coupled surface plasmon polaritons.

Ultralow threshold green lasing and optical bistability in ZBNA (ZrF4–BaF2–NaF–AlF3) microspheres

Upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate, Er3+ doped glass, ZBNA (ZrF4–BaF2–NaF–AlF3), when pumped at 978 nm via a tapered optical fiber is demonstrated. An ultralow, green lasing threshold of ∼3 μW for 550 nm emissions is measured. This is one order of magnitude lower than that previously reported for ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF) microspheres. Optical bistability effects in ZBNA microspheres are reported and the bistable mechanism is discussed and attributed to shifts of the whispering gallery modes due to thermal expansion of the sphere, where heating is achieved by optical pumping around 978 nm. The effect of the bistability on the upconversion lasing is examined and we report multiple bistability loops within the microspheres.

Characteristics of intrinsic optical bistability and parameter optimization in Tm3+/Yb3+ codoped laser crystal

Intrinsic optical bistability （IOB） in Tm3+/Yb3+ codoped laser crystal under 650 nm laser excitation is theoretically studied. Based on the theory of nonlinear coupled rate equations, we numerically analyze IOB evolutions of the populations at Tm3+/Yb3+ energy levels, and the influence of system parameters on IOB and the dynamic operation of bistable hysteresis loop. The simulation result shows the fluorescence IOB in visible-infrared spectra of Tm3+/Yb3+ codoped laser crystal under photon avalanche pumping scheme. It is found that IOB phenomena can be largely enhanced by optimizing the Tm3+/Yb3+ concentration ratio, choosing low phonon-energy crystal host and using crystal cooling technology, and also by tuning the pump laser to match the resonant frequency of Tm3+ excited-state absorption transition. Through tuning the pump laser wavelength and adjusting the rate of change in pump intensity, the controllable IOB switching can be achieved.

One-dimensional polariton solitons and soliton waveguiding in microcavities

We extend our recent results [O.A. Egorov et al. Phys. Rev. Lett. 102, 153904 (2009)] on half-light–half-matter polariton solitons in planar semiconductor microcavities operating in the strong coupling regime. We initiate discussion on the structure of the solitons in the momentum space and its link to the instability of the upper branch of the polariton bistability loop. Numerical results showing the soliton excitation by a seed pulse are presented.

A Model for Optical Bistability in a Hybrid Semiconductor Photonic Crystal Structure

A new model for optical bistability in a photonic crystal structure closed to a semiconductor is presented. It is found that increase in the strength of self-interaction can destroy this nonlinear phenomenon whereas a small variation of this interaction from low initial value can control the area of the bistable loop.

Bistable Switches for Synaptic Plasticity

A persistent decrease in synaptic efficacy, called long-term depression (LTD), of the parallel fiber-Purkinje cell synapse is thought to underlie some forms of learning and memory in the cerebellum. Simulation studies predicted that mitogen-activated protein kinase and protein kinase C would mutually activate each other and make a bistable positive feedback loop, thereby providing a molecular basis for LTD. This claim stimulated experimenters to successfully demonstrate the feedback loop and its pivotal role in cerebellar LTD.

Two-step selection of a single R8 photoreceptor: a bistable loop betweensenselessandroughlocks in R8 fate

Patterning of sensory organs requires precise regulation of neural induction and repression. The neurocrystalline pattern of the adult Drosophila compound eye is generated by ordered selection of single founder photoreceptors (R8s) for each unit eye or ommatidium. R8 selection requires mechanisms that restrict R8 potential to a single cell from within a group of cells expressing the proneural gene atonal (ato). One model of R8 selection suggests that R8 precursors are selected from a three-cell ;R8 equivalence group' through repression of ato by the homeodomain transcription factor Rough (Ro). A second model proposes that lateral inhibition is sufficient to select a single R8 from an equipotent group of cells called the intermediate group (IG). Here, we provide new evidence that lateral inhibition, but not ro, is required for the initial selection of a single R8 precursor. We show that in ro mutants, ectopic R8s develop from R2,5 photoreceptor precursors independently of ectopic Ato and hours after normal R8s are specified. We also show that Ro directly represses the R8 specific zinc-finger transcription factor senseless (sens) in the developing R2,5 precursors to block ectopic R8 differentiation. Our results support a new model for R8 selection in which lateral inhibition establishes a transient pattern of selected R8s that is permanently reinforced by a repressive bistable loop between sens and ro. This model provides new insight into the strategies that allow successful integration of a repressive patterning signal, such as lateral inhibition, with continued developmental plasticity during retinal differentiation.

Ferroelectric Liquid Crystal Cell With Phase Separated Composite Organic Film

We researched ferroelectric liquid crystal cells (FLCs) with a phase separated composite organic film (PSCOF) fabricated by photocuring a pre-polymer NOA-65. The conventional surface-stabilized FLC cell shows the so-called bistable hysteresis loop of electrooptical effect, whereas the PSCOF-FLC cell demonstrates monostable half-V shaped electrooptical characteristics. In this study, we tried to make this origin clear and fabricated some types of PSCOF-FLC cells under different photocuring conditions. As a result, it is found that NOA-65 has a minus electron charge and then the monostable FLC medium may be realized due to the bias electric field generated by NOA-65 film.

All-Optical Response of Semiconductor Ring Laser to Dual-Optical Injections

The all-optical response of a semiconductor ring laser (SRL) to two optical injections is characterized. Once the lasing direction is locked by one optical injection, the SRL direction of operation can be switched by another optical injection into the counterpropagating direction. The switching process manifests a typical bistable hysteresis loop, with its width and switching thresholds variable by the first injection power. Extremely sharp transition has been measured which confirms the potential of the SRL for all-optical regeneration applications.

Study on intrinsic optical bistability in Tm-doped laser crystal pumped at 648nm avalanche wavelength

Intrinsic optical bistability (IOB) in Tm-doped laser crystal pumped at 648nm avalanche wavelength is predicted theoretically and studied numerically. Based on the theory of nonlinear rate equations, the analytical formula of avalanche threshold condition is deduced in the steady-state approximation. Transient response of photon avalanche, IOB and influence of system parameters on IOB are studied numerically. The numerical simulation shows that IOB of 2μm fluorescence emission in Tm-doped laser crystal can be experimentally observed. Controllable bistable hysteresis loop can be achieved by adjusting experimentally adjustable parameters.

Optical Bistability in Active Semiconductor Microring Structures

The spectral and power (I/O) bistable characteristics of active microring (MR) structures are analyzed theoretically. The theoretical formulation is based on the rate equation under the assumption of the z-independent optical power accounting for amplified spontaneous emission. The analysis is performed for MRs in both all-pass and add-drop configurations. The effect of cavity characteristics on the bistable performance are explored. Based on these the merits of the resonant nonlinearity enhancement are discussed and strategies for optimized bistable loops are outlined.

Optical bistability in fiber ring resonator containing an EDFA

In this paper, we study on optical bistability behaviors in a double-coupler ring resonator, in which an erbium-doped fiber amplifier (EDFA) is inserted into the ring for counteracting the loss by the gain of EDFA. The two groups of transmitted and reflected optical bistability loops under different parameters are investigated symmetrically. According to our optimum design of this new optical bistable device, the switching power can be less than 60mW.

De-stabilization of the positive vago-vagal reflex in bulimia nervosa

Bulimia nervosa is characterized by consuming large amounts of food over a defined period with a loss of control over the eating. This is followed by a compensatory behavior directed at eliminating the consumed calories, usually vomiting. Current treatments include antidepressants and/or behavioral therapies. Consensus exists that these treatments are not very effective and are associated with high relapse rates. We review evidence from literature and present original data to evaluate the hypothesis that bulimia involves alterations in vago-vagal function. Evidence in support of this include (1) laboratory studies consistently illustrate deficits in meal size, meal termination, and satiety in bulimia; (2) basic science studies indicate that meal size and satiation are under vagal influences; (3) anatomical, behavioral and physiological data suggest that achieving satiety and the initiation of emesis involve common neural substrates; (4) abnormal vagal and vago-vagal reflexive functions extend to non-eating activational stimuli; and (5) studies from our laboratory modulating vagal activation have shown significant effects on binge/vomit frequencies and suggest a return of normal satiation. We propose a model for the pathophysiology of bulimia based upon de-stabilization of a bi-stable positive vago-vagal feedback loop. This model is not meant to be complete, but rather to stimulate anatomical, psychobiological, and translational neuroscience experiments aimed at elucidating the pathophysiology of bulimia and developing novel treatment strategies.

Adult and larval photoreceptors use different mechanisms to specify the same Rhodopsin fates.

Although development of the adult Drosophila compound eye is very well understood, little is known about development of photoreceptors (PRs) in the simple larval eye. We show here that the larval eye is composed of 12 PRs, four of which express blue-sensitive rhodopsin5 (rh5) while the other eight contain green-sensitive rh6. This is similar to the 30:70 ratio of adult blue and green R8 cells. However, the stochastic choice of adult color PRs and the bistable loop of the warts and melted tumor suppressor genes that unambiguously specify rh5 and rh6 in R8 PRs are not involved in specification of larval PRs. Instead, primary PR precursors signal via EGFR to surrounding tissue to develop as secondary precursors, which will become Rh6-expressing PRs. EGFR signaling is required for the survival of the Rh6 subtype. Primary precursors give rise to the Rh5 subtype. Furthermore, the combinatorial action of the transcription factors Spalt, Seven-up, and Orthodenticle specifies the two PR subtypes. Therefore, even though the larval PRs and adult R8 PRs express the same rhodopsins (rh5 and rh6), they use very distinct mechanisms for their specification.

Genetic Screens forCaenorhabditis elegansMutants Defective in Left/Right Asymmetric Neuronal Fate Specification

We describe here the results of genetic screens for Caenorhabditis elegans mutants in which a single neuronal fate decision is inappropriately executed. In wild-type animals, the two morphologically bilaterally symmetric gustatory neurons ASE left (ASEL) and ASE right (ASER) undergo a left/right asymmetric diversification in cell fate, manifested by the differential expression of a class of putative chemoreceptors and neuropeptides. Using single cell-specific gfp reporters and screening through a total of almost 120,000 haploid genomes, we isolated 161 mutants that define at least six different classes of mutant phenotypes in which ASEL/R fate is disrupted. Each mutant phenotypic class encompasses one to nine different complementation groups. Besides many alleles of 10 previously described genes, we have identified at least 16 novel "lsy" genes ("laterally symmetric"). Among mutations in known genes, we retrieved four alleles of the miRNA lsy-6 and a gain-of-function mutation in the 3'-UTR of a target of lsy-6, the cog-1 homeobox gene. Using newly found temperature-sensitive alleles of cog-1, we determined that a bistable feedback loop controlling ASEL vs. ASER fate, of which cog-1 is a component, is only transiently required to initiate but not to maintain ASEL and ASER fate. Taken together, our mutant screens identified a broad catalog of genes whose molecular characterization is expected to provide more insight into the complex genetic architecture of a left/right asymmetric neuronal cell fate decision.

Quasi-hydrodynamic simulation of the features of electrical conduction in nanosized multilayer heavily doped heterostructures

The electrical conduction in nanosized layered heterostructures in the presence of strong electric fields is studied via mathematical simulation based on the energy-balance equation and the quasi-hydrodynamic description of electron drift with allowance for the diffusion and thermal diffusion components of the current, the divergence of the thermal electron flow, and the temperature dependences of the mobility and energy-relaxation time. For the doping level in the range (5−1) × 1017 cm−3, the resulting I−V characteristics of heterostructures with a barrier height of 0.3 eV and equal widths of the narrow-and wide-band-gap components (50 nm) exhibit either a sharp peak of the differential conductivity or a bistability loop. A physical model that interprets the shape of the calculated characteristics on the basis of the cumulative effect of the electrostatic decrease in the height of heterobarriers and an increase in the electron temperature in the vicinity of the injecting heteroboundaries is proposed.

Deterministic characterization of stochastic genetic circuits.

For cellular biochemical reaction systems where the numbers of molecules is small, significant noise is associated with chemical reaction events. This molecular noise can give rise to behavior that is very different from the predictions of deterministic rate equation models. Unfortunately, there are few analytic methods for examining the qualitative behavior of stochastic systems. Here we describe such a method that extends deterministic analysis to include leading-order corrections due to the molecular noise. The method allows the steady-state behavior of the stochastic model to be easily computed, facilitates the mapping of stability phase diagrams that include stochastic effects, and reveals how model parameters affect noise susceptibility in a manner not accessible to numerical simulation. By way of illustration we consider two genetic circuits: a bistable positive-feedback loop and a negative-feedback oscillator. We find in the positive feedback circuit that translational activation leads to a far more stable system than transcriptional control. Conversely, in a negative-feedback loop triggered by a positive-feedback switch, the stochasticity of transcriptional control is harnessed to generate reproducible oscillations.

Ferroelectric Liquid Crystal Cell With Phase Separated Composite Organic Film

We try to adopt a phase separated composite organic film (PSCOF) to a ferroelectric liquid crystal (FLC) cell. It has been found that in order to realize the PSCOF structure, it is important to select an adequate photocure condition such as the concentration of pre-polymer in LC medium, temperature and the UV intensity. The conventional FLC cell shows the so-called bistable hysteresis loop of electrooptical effect, whereas the PSCOF-FLC cell possesses a monostability without threshold. This monostablity is able to realize LC displays which possess grayscale capability.

A mathematical simulation of the effect of the bistability of current characteristics in nanosized multiple-layer heavily doped heterostructures

Simulation methods based on the energy-balance equation are used to study the electrical conductivity of layered nanosized heterostructures in high electric fields. A quasi-hydrodynamic description of the electron drift is used with regard to the diffusion and thermal-diffusion components of the current, the divergence of the electron heat flux, and the temperature dependence of the electron mobility and energy relaxation time. Current-voltage characteristics are obtained for a layered heterostructure with a barrier height of 0.3 eV and with lengths of both the narrow-and wide-gap layers equal to 50 nm. Depending on the doping level in the range (5–1) × 1017 cm−3, the characteristics exhibit either a sharp peak of the differential conductivity or a bistability loop corresponding to the thermal-injection instability. A physical model is suggested that attributes the shape of the calculated current-voltage characteristics to the cumulative effect of the electrostatic lowering of the heterobarrier height and the increase in electron temperature near the injecting heteroboundaries.