Surface wave assisted self-assembly of multidomain magnetic structures

An ensemble of magnetic microparticles at the liquid surface displays novel snakelike self-assembled structures induced by an alternating magnetic field. We demonstrate that these structures are directly related to surface waves in the liquid generated by the collective response of magnetic microparticles to the alternating magnetic field. The segments of the "snake" exhibit long-range antiferromagnetic ordering, while each segment is composed of ferromagnetically aligned chains of microparticles. The structures exhibit magnetic hysteretic behavior with respect to an external in-plane magnetic field and logarithmic relaxation of the remanent magnetic moment.     

Optical manipulation of microparticles using whispering-gallery modes in a silicon nitride microdisk resonator

We demonstrate optical manipulation of 1?μm sized polystyrene microparticles on silicon nitride microdisk resonator devices using whispering-gallery modes in an integrated optofluidic chip. We demonstrate multiple trapping tracks and extended trapping ranges within single wavelengths through exciting high-order modes. We observe various sets of trapping tracks and ranges through exciting various resonance modes. We switch particle traveling tracks by tuning the laser wavelength to various wavelengths. We also observe microparticles assembling along the trapping tracks.     

Formation of extended straight molecular chains by pairing of thymine molecules on the Ag–Si(111) surface

Here, we report on the assembly of thymine molecules into extended straight chains of single and multiple dimer rows on a Si(111)/Ag R30° surface. Using variable temperature scanning tunneling microscopy, we follow the nucleation process and formation of self-assembled structures. Submonolayer coverages at 120 K are disordered and exhibit a high density of thymine dimers. Upon annealing the dimers assemble into extended dimer chains along three equivalent high-symmetry surface directions. At low coverages single dimer rows are favored. At increased coverage chains with multiple dimer rows are observed, with a preference to an even multiplicity. We show that a complex cross-talk between H-bond thymine–thymine interactions and commensurization of dimer chains to the Ag/Si surface leads to this specific layout.     

Structure formation in electromagnetically driven granular media

We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, we observe the formation of a rich variety of structures: clusters, rings, chains, and networks. The dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. We find that for pure ac magnetic driving the low-frequency magnetic excitation favors compact clusters, whereas high frequency driving favors chains and netlike structures. An abrupt phase transition from chains to a network phase was observed for a high density of particles.     

Optical bistability and multistability in a parametric region

We study the optical bistability (OB) and optical multistability (OM) behaviors in a five-level $\Lambda $ -type parametric region atomic system by two-photon resonant transitions. We find that the intensity threshold of OB and switching from OB to OM or vice versa can be controlled via quantum interference between different two-photon transitions pathways.     

Sideband control of optical bistability and multistability

We present sideband control of optical bistability and multistability based on trichromatic electromagnetic-field induced transparency and quantum interference. Appearance or disappearance of the bistability and multistability, manipulation of the hysteresis loop widths, and switching between bistability and tristability are achieved simply by varying the sideband amplitudes or the relative phases of the sidebands to the central component.     

Optical bistability and multistability in a three-level Δ-type atomic system under the nonresonant condition

Under a nonresonant condition, we theoretically investigate hybrid absorptive-dispersive optical bistability and multistability behaviours in a three-level Δ-type system by using a microwave field to drive a hyperfine transition between two upper excited states inside a unidirectional ring cavity. We find that the optical bistability and multistability behaviours can be controlled by adjusting the intensity of the microwave field or the intensity of the coherent coupling field. Furthermore, our studies show an interesting phenomenon of the transition from the optical bistability to the optical multistability only by changing the negative detuning of the coupling field into the positive detuning of the coupling field.     

Transport properties of a ladder with two random dimer chains

We investigate the transport properties of a ladder with two random dimer (RD) chains. It is found that there are two extended states in the ladder with identical RD chains and a critical state regarded as an extended state in the ladder with pairing RD chains. Such a critical state is caused by the chiral symmetry. The ladder with identical RD chains can be decoupled into two isolated RD chains and the ladder with pairing RD chains can not. The analytic expressions of the extended states are presented for the ladder with identical RD chains.     

Multistability and the control of complexity

We show how multistability arises in nonlinear dynamics and discuss the properties of such a behavior. In particular, we show that most attractors are periodic in multistable systems, meaning that chaotic attractors are rare in such systems. After arguing that multistable systems have the general traits expected from a complex system, we pass to control them. Our controlling complexity ideas allow for both the stabilization and destabilization of any one of the coexisting states. The control of complexity differs from the standard control of chaos approach, an approach that makes use of the unstable periodic orbits embedded in an extended chaotic attractor. (c) 1997 American Institute of Physics.     

Hysteresis and nonlinear thermooptic waves in a semiconductor Fabry-Perot interferometer

We propose and demonstrate the existence of temperature hysteresis phenomena in a Fabry-Perot interferometer, resulting from a change in the refractive index of the interferometer material when heated by laser radiation. We also demonstrated theoretically that temperature multistability in the distributed interferometric system may cause the emergence of nonlinear thermooptic switching waves and found an approximate expression for the propagation velocity of such waves. We present the results of experimental observations of temperature multistability and thermooptic switching waves in a germanium Fabry-Perot interferometer illuminated by CO₂-laser radiation.     

Effects of the control field on the optical multistability in V-type three-level atomic system

The optical multistability behaviour in a ring cavity for the V-type atomic system, driven by a coherent field and control field (coherent + dc fields), has been analysed. The presence of dc field is having a dominant effect on generating the optical multistability of the system. We show that, the effects of the quantum interference from spontaneous emission and of the relative phase between the two fields of the control field might be of use to control the threshold value and width of the hysteresis cycle, which can adjust the optical switching process when they are taken at optimal values. Also, the optical multistability is predicted for the output field as a function of the cooperative parameter in the presence of the quantum interference of the spontaneous emission.     

Optical bistability in low-dimensional semiconductor heterostructures under cw pump laser and infrared pulse signals

We have investigated theoretically the optical bistability (OB) and optical multistability in quantum-well (QW) semiconductor heterostructures. The effect of a cw pump laser field and a control infrared pulse signal on OB is discussed. It is found that the OB threshold can be controlled by varying the strength of applied fields. It is also shown that OB can be switched to optical multistability or vice versa just using an appropriate relative phase of applied fields. This investigation may open up some new possibilities for applications in all-optical switching in quantum-well structures.     

Control of the optical multistability in a three-level ladder-type quantum well system

We theoretically investigated a hybrid absorptive-dispersive optical multistability (OM) behavior in a three-level ladder-type quantum well system inside a unidirectional ring cavity. We find that the frequency detuning of the control field and the electronic cooperation parameter as well as the total decay rates can affect the optical multistability behavior dramatically, which can be used to manipulate efficiently the threshold intensity and the hysteresis loop. The effect of the intensity of the control field on the OM is also studied. Our study is much more practical than its atomic counterpart due to its flexible design and the wide adjustable parameters. Thus it may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.     

Controllable optical bistability and multistability in asymmetric double quantum wells via spontaneously generated coherence

We investigate the nonlinear optical phenomena of the optical bistability and multistability via spontaneously generated coherence in an asymmetric double quantum well structure coupled by a weak probe field and a controlling field. It is shown that the threshold and hysteresis cycle of the optical bistability can be conveniently controlled only by adjusting the intensity of the SGC or the controlling field. Moreover, switching between optical bistability and multistability can be achieved. These studies may have practical significance for the preparation of optical bistable switching device.     

Optical multistability and ground-state coherence in a three-level system

The interaction of a three-level atomic medium with two degenerate sublevels in the ground state with a coherent radiation field in a ring cavity is investigated. We pay specific attention to atomic coherence within the ground-state doublet, and introduce separate decay rates for the ground-state coherence and the population difference. If the atomic density exceeds a critical value, the system exhibits symmetric bistability. In addition, in certain regions of parameter space we find symmetry-breaking branches, yielding optical multistability. We propose physical mechanisms for the symmetric as well as for the symmetry-breaking instability.Work supported by the Swiss National Science Foundation     

Phase-dependent optical bistability and multistability in a semiconductor quantum well system

We theoretically investigate the hybrid absorptive-dispersive optical bistability and multistability in a four-level inverted-Y quantum well system inside a unidirectional ring cavity. We find that the coupling field, the pumping field as well as the cycling field can affect the optical bistability and multistability dramatically, which can be used to manipulate efficiently the threshold intensity and the hysteresis loop. The effects of the relative phase and the electronic cooperation parameter on the OB and OM are also studied. Our study is much more practical than its atomic counterpart due to its flexible design and the wide adjustable parameters. Thus, it may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.     

Energy trapping and shock disintegration in a composite granular medium

We report the first experimental observation of impulse confinement and the disintegration of shock and solitary waves in one-dimensional strongly nonlinear composite granular materials. The chains consist of alternating ensembles of beads with high and low elastic moduli (more than 2 orders of magnitude difference) of different masses. The trapped energy is contained within the "softer" sections of the composite chain and is slowly released in the form of weak, separated pulses over an extended period of time. This effect is enhanced by using a specific group assembly and precompression.     

Optical multistability in three-level atoms inside an optical ring cavity

Optical multistability in an optical ring cavity filled with a collection of three-level Lambda-type rubidium atoms has been experimentally demonstrated. The observed multistability is very sensitive to the induced atomic coherence in the system and can evolve from a normal bistable behavior with the change of the coupling field as well as the atomic number density. The underlying mechanism for the formation of such multistability is also discussed.     

Nonlinear theory of self-oscillations in a phase-conjugate resonator

We present an exact, nonlinear analysis of self-oscillations in a phase-conjugate resonator. Optical bi- and multistability, as well as period doubling to chaos are predicted.     

Magnetic separation from superparamagnetic particle suspensions

We investigate the magnetophoretic separation of magnetic microparticles from a non-dilute flow in a microfluidic channel and their subsequent field-induced aggregation under the influence of an externally applied magnetic force. This force induces dipolar interactions between the particles that aid in their separation from the flow. Existing analytical models for dilute suspensions cannot be extended to non-dilute suspensions in which interparticle magnetic interactions play an important role. We therefore conduct a parametric investigation of the mechanics of this problem in a microcapillary flow through simulations and experimental visualization. When a magnetic field is applied, the magnetic microparticles form an aggregate on the channel wall that is influenced by the competition between the holding magnetic force and the aggregate-depleting flow shear force. Microparticle collection in the aggregate increases linearly with increasing magnetic field strength and is characterized by distinct buildup and washaway phases. The collected microparticle volume fraction in an aggregate is found to depend on a single dimensional group that depends upon characteristic system parameters.