Tunable negative-index photonic crystals using colloidal magnetic fluids

The model of using colloidal magnetic fluid to build tunable negative-index photonic crystal is established. The effective permittivity εe and permeability μe of the two-dimensional photonic crystal are investigated in detail. For transverse magnetic polarization, both εe and μe exhibit a Lorentz-type anomalous dispersion, leading to a region where εe and μe are simultaneously negative. Then, considering a practical case, in which the thickness of photonic crystal is finite, the band structures for odd modes are calculated by the plane wave expansion method and the finite-difference time-domain method. The results suggest that reducing the external magnetic field strength or slab thickness will weaken the periodic modulation strength of the photonic crystal. Simulation results prove that the negative-index can be tuned by varying the external magnetic field strength or the slab thickness. The work presented in this paper gives a guideline for realizing the flat photonic crystal lens with tunable properties at optical frequencies, which may have potential applications in tunable near-field imaging systems.     

Investigation of magnetic fluids by ultrasonic and magnetic methods

The influence of magnetic field on the acoustic and magnetic properties of magnetic liquids is discussed. By fitting the curve of Taketomi's theory to the experimental data of the anisotropy of ultrasonic attenuation, the values of quantities describing the structure of magnetic liquids have been determined. Moreover, the dependence of magnetic susceptibility on frequency has been measured. It shows that two processes of magnetization, based on the mechanisms proposed by Brown and Neel, contribute to the magnetization of the magnetic liquid.     

Protein-stabilized magnetic fluids

The adsorption of bovine serum albumin (BSA) and egg yolk phosvitin on magnetic fluid particles was investigated. Incubation mixtures were prepared by mixing an alkaline suspension of tetramethylammonium-coated magnetite cores with protein solutions at various protein/Fe₃O₄ ratios, followed by dialysis against a 5 mM TES buffer (pH 7.0), after which separation of bound and non-bound protein by high-gradient magnetophoresis was executed. Both the kinetic profiles as well as the isotherms of adsorption strongly differed for both proteins. In case of the spherical BSA, initially, abundant adsorption occurred, then it decreased and—at high protein concentrations—it slowly raised again. In contrast, with the highly phosphorylated phosvitin, binding slowly started and the extent of protein adsorption remained unchanged both as a function of time and phosvitin concentration. Competition binding studies, using binary protein mixtures composed of equal weight amounts of BSA and phosvitin, showed that binding of the latter protein is ‘unrealistically’ high. Based on the geometry of the two proteins, putative pictures on their orientation on the particle's surface in the various experimental conditions were deduced.     

Designing intelligent fluids for controlling spray applications

Recent advances in our understanding of ‘complex fluids’ have lead to the development of new chemical additives in the agricultural industry, which provide more efficient products and a higher level of public and environmental protection. Here we present one example that deals with the improved application of herbicides and pesticides during the spray treatment of plants. Using judiciously chosen dilute polymer solutions, one can simultaneously improve the spray characteristics, droplet deposition, and product retention onto the targeted plant surface. Achieving these combined benefits, requires a delicate balance between fluid hydrodynamics and the molecular interactions that control the phase behaviour of the sprayed solutions. To cite this article: V. Bergeron, C. R. Physique 4 (2003).     

Parametric resonance in magnetic fluids

A new resonance effect in the nonlinear behaviour of magnetically anisotropic objects in an alternating external magnetic field is proposed. Ferromagnetic particles with a “frozen” magnetic moment (due to a strong magnetic anisotropy), when located in an external alternating magnetic field, are able to rotate (or vibrate) and to transfer energy from the external field to the medium. The numerical solution of the appropriate parametrically driven nonlinear equation shows all types of nonlinear dynamic behaviour, including transition to chaos. The sensitivity of the proposed phenemenon could be used for an experimental analysis of the size distribution of the ferromagnetic particles in a ferrofluid or of the size of “magnetic holes”.     

Buoyancy forces in magnetic fluids

The ponderomotive force on a macroscopic body in a magnetic fluid is calculated by a hydrodynamic approach. The resulting equations are generally valid, neither small susceptibilities nor stationarity are assumed. The simple and widely-used formulaV(M-M ^bg )H is recoverd in linear order of ; magnetostrictive effects are shown to contribute in the order ³. The expressions derived here are definite and unambiguous, they do not depend on whether one starts from a theory in terms ofH, or in terms ofB: the correct evaluation of the contribution dV[-p] resolves the apparent contradiction between the force density expressions ₀ MH, orMB.     

Iron and Cobalt-based magnetic fluids produced by inert gas condensation

Iron and cobalt nanoparticle fluids have been prepared by inert-gas condensation into an oil/surfactant mixture. Superparamagnetic iron fluids (mean particle size=11.6±0.4 nm) and ferromagnetic cobalt fluids (mean particle size=51.6±3.4 nm) produced by this technique are promising candidates for magnetic targeting and hyperthermia applications.     

Tuning the band gap of self-assembled superparamagnetic photonic crystals in colloidal magnetic fluids using external magnetic fields

The model of tunable superparamagnetic photonic crystals self-assembled in colloidal magnetic fluids under externally applied magnetic fields is established. The mechanisms, which are in charge of the tunability of the band gaps with magnetic fields are clarified. The band structures of the triangularly-arrayed two-dimensional photonic crystals with limited heights of magnetic columns are calculated with the experimental data of structures and refractive indices in the literatures. The field-dependent properties of the first band gaps are gained for the z-odd and z-even modes, respectively. Simulation results indicate that the mid frequencies of the first band gaps of the z-odd modes can be easily tuned by the external magnetic fields, while those of the z-even modes bear relatively weak dependence on the external magnetic fields. Simultaneously, the first band gaps of both kinds of modes become wide along with the increase of the magnetic fields. The results presented in this work give a guideline for realizing the tunable photonic crystals with magnetically colloidal materials and magnetic stimuli.     

Optical phase modulators using deformable waveguides actuated by micro-electro-mechanical systems

An optical phase modulator is presented by using micro-electro-mechanical systems to actuate deformable silicon waveguides. Via mechanically stretching the waveguide length, the optical path is extended, resulting in a phase shift. The experimental results show that a phase shift of near 0.4π is achieved at 200 V for both TE- and TM-polarized waves by cascading six phase modulation units, agreeing well with the theoretical prediction. The power consumption is estimated to be smaller than 0.2 mW at 200 V, mainly resulting from the leakage current.     

Spin-glass-like magnetic ordering in Zn substituted magnetite magnetic fluids

Electron spin resonance (ESR) spectra of magnetic fluids involving polydispersed Zn(0.5)Fe(0. 5)Fe(2)O(4) (FZ5) and Zn(0.7)Fe(0. 3)Fe(2)O(4) (FZ7) nanomagnetic particles are scanned from 4.2 to 300K. The FZ7 fluid exhibits certain distinct features below 40K which are different from FZ5 fluid. These include (i) an isotropic shift in resonance field in zero-field-cooled ESR study, (ii) deviation of resonance field from sin(2)theta behavior (where theta is the angle between axis of the particle and field) in field cooled (FC) sample and (iii) abrupt increase in anisotropy field for FC sample. The results are analyzed in light of the core-shell model for nanomagnetic particles.     

Peculiar magnetic aftereffect of highly diluted frozen magnetic fluids

A very stably dispersed magnetic fluid (mother MF) and its 1000-times diluted solution were independently zero-field-cooled from room temperature to 5 K followed by application of a magnetic field of 2.86 MA/m for 300 s. After the field was removed (t=0)$(t=0)$, its residual magnetization M was measured as a function of time t for 80 000 s. After measurement, the MF sample was heated to room temperature, and the experiment was repeated after cooling to 5 K and again applying and removing the 2.86 MA/m field. We performed the same experiment several times, and obtained a different M vs t curve each time. With each cycle, the average M increased and the M vs t curve converged to a universal curve. In the initial few cycles, the value of M is very small, fluctuates and surprisingly increases with t in some time region. These characteristics are common in both the mother MF and diluted MF. We consequently propose the following physical model. When the MF is cooled, the isolated surfactant molecules in the solvent trigger the generation of magnetic colloid micelles. In other words, there occurs a phase transition from the magnetic colloids’ monodispersed phase to a micelle phase. The magnetic dipoles of the micelle's colloids make a closed magnetic flux loop. That is the origin of the anomalously small value of the residual magnetization in the early cycles. After a certain time elapses the micelles spontaneously break due to their residual stress, and a finite magnetic moment of the individual micelle develops. Consequently, M increases with t during this period.     

Microwave propagation parameters in magnetic fluids

Complex dielectric permittivity and complex magnetic permeability measurements of two magnetic fluids (as microwave propagation media), in the approximate range 0.2-5GHz were performed. The two samples consisted of magnetite nanoparticles, dispersed in kerosene and in water, respectively. Based on the dielectric and magnetic measurements, the frequency (f ) dependence of the attenuation parameter, , the phase constant, , the propagation constant, , the intrinsic impedance, Z_m, the refractive index, n , the reflection coefficient, R , the wavelength, and the skin depth, , of the investigated samples were determined.     

Dimensional analysis of aqueous magnetic fluids

A comparison of the synthesis and characterization of three aqueous magnetic fluids intended for biomedical applications is presented. Stable colloidal suspensions of iron oxide nanoparticles were prepared by a co-precipitation method with the magnetite cores being coated with β-cyclodextrin, tetramethylammonium hydroxide and citric acid. Rheological properties of the fluids were investigated, i.e. viscosity (capillary method) and surface tension (stalagmometric method) in correlation with their density (picnometric method). The dimensional distributions of the ferrophase particles physical diameter of these three magnetic fluids – revealed on the basis of transmission electron microscopy (TEM) data – as well as the diameter distributions of some other magnetic fluids presented in the literature, were comparatively analyzed using the box-plot statistical method. In order to extract complementary data on the magnetic diameter of an iron oxide core, magnetization measurements as well as X-ray diffraction pattern analysis were carried out. Interpretation of all the measurement data was accomplished by assessing the suitability of the three magnetic fluid samples from the viewpoint of their stability and biocompatibility. PACS 75.50.Mm; 61.46.Df; 68.37.Lp; 96.15.Pf; 75.75.+a     

Pair correlations in magnetic nanodispersed fluids

The pair distribution function of a monodisperse magnetic fluid simulated by a liquid made of dipolar hard spheres with constant magnetic moments is calculated. The anisotropy of the pair distribution function and the related structure factor of scattering in a dc uniform magnetic field are studied. The calculation is performed by diagrammatic expansion in the volume concentration of particles and the interparticle magnetic-dipole interaction intensity using a thermodynamic perturbation theory. Limitation by three-particle diagrams makes it possible to apply the results obtained to magnetic fluids with a moderate concentration. Even for low-concentration and weakly nonideal magnetic fluids, the anisotropic interparticle magnetic-dipole correlations in a magnetic field lead to the repulsion of particles in the direction normal to the field and to the formation of particle dimers along the field.     

Sound velocity measurements in magnetic fluids

Sound velocity measurements were made in a kerosene-base magnetic fluid. Both pulse-echo and c.w. resonance techniques were used for frequencies from 2 to 45 MHz. The resulting value of velocity was 6 percent lower than that obtained for the kerosene.     

Magnetic properties of undecane-based magnetic fluids

The magnetic properties of magnetic fluids based on n-undecane have been presented. The temperature dependences of the magnetic susceptibility in the absence and presence of a bias field have been analyzed. A step change in the specific heat of the medium, which is characteristic of magnetic phase transitions, has been discovered.     

Enhancing electrooptic modulators using modulation instability

As electronic operating frequencies increase toward the terahertz regime, new electrooptic modulators capable of low‐voltage high‐frequency operation must be developed to provide the necessary optical interconnects. This Letter presents a new concept that exploits modulation instability to compensate for the intrinsically weak electrooptic effect, χ⁽²⁾. Simulations demonstrate more than 50 times enhancement of electrooptic effect at millimeter wave frequencies leading to a substantial reduction in the required modulation voltage.

     

Single-shot distributed optical-fiber temperature sensing by the frequency-derived technique

We present, for the first time to our knowledge, a distributed optical-fiber temperature sensor, based on a pulsed laser, that provides distributed temperature measurement by use of a single pulse propagating in an optical fiber. The system uses the frequency-derived technique based on the optical Kerr effect. The performance of the system is investigated for the temperature range 33-150 degrees C. A linear relationship between the temperature and the derived frequency is obtained. The best temperature resolution was determined to be +/-1.2 degrees C. The best measured spatial resolution was 0.56 m.     

Optical relaxation properties of magnetic fluids under externally magnetic fields

The relaxation behavior (including the rising and falling relaxation processes) of the transmitted light after the magnetic fluid thin films under longitudinal and transverse magnetic fields is investigated, respectively. The physical mechanisms of the two different relaxation processes are discussed. The experimental data of the rising and falling relaxation processes are fitted by using two exponential functions to achieve the rising and falling response times. The relationship between the response time and the strength of applied magnetic field, the concentration of magnetic fluid is studied experimentally. The modulation depth of the transmitted light is researched quantificationally and the deepest modulation depth is obtained with Sample 3 (with volume fraction of 5.62%) in our experiments.