Sonia Melle

Discrete magnetic microfluidics

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops (volume of 5–35μl) that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed (7cm∕s) by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

Magneto-optical properties of nickel nanowire arrays

We report on the magneto-optical properties of hexagonally arranged Ni nanowires embedded in anodic alumina templates. Due to the nanowire geometry, these samples show different response depending on the polarization orientation of the incident light, which leads to a high anisotropy of both their optical and magneto-optical properties. A strong increase of the magneto-optical activity is clearly observed with respect to the Ni bulk material. We associate this behavior to plasmon resonance of the Ni nanowires.

Rotational dynamics in dipolar colloidal suspensions: video microscopy experiments and simulations results

Abstract The dynamics of field-induced structures in very dilute dipolar colloidal suspensions subject to rotating magnetic fields have been experimentally studied using video microscopy. When a rotating field is imposed the chain-like aggregates rotate with the magnetic field frequency. We found that the size of the induced structures at small rotational frequencies is larger than at zero rotating frequency, i.e. when an uniaxial magnetic field is applied. At higher frequencies, the average size of the aggregates decreases with frequency following a power law with exponent −0.5 as the hydrodynamic friction forces overcome the dipolar magnetic forces, causing the chains break up. A non-thermal molecular dynamics simulations are also reported, showing good agreement with the experiments.

Chain model of a magnetorheological suspension in a rotating field

We develop an athermal chain model for magnetorheological suspensions in rotating magnetic fields. This model is based on a balance of hydrodynamic and magnetostatic forces and focuses on the mechanical stability of chains. Using a linear approximation of the chain shape, we compute the orientation and size of a critical chain in a rotating magnetic field as a function of the Mason number Mn, which is the ratio of dipolar to hydrodynamic forces between two particles in contact. The critical chain length is found to scale with the inverse square root of Mn, and its orientation relative to the instantaneous field is independent of Mn. The actual nonlinear shape of a chain in a rotating field is then computed self-consistently. Finally, the effect of local fields on the dipolar interaction force is considered, leading to predictions for the chain shape and orientation that depend rather strongly on the magnetic permeability of the particles. A principal finding is the possibility of brittle or ductile chain fracture, depending on the permeability of the particles. Single-chain simulations confirm this prediction, as do experimental measurements.

Observation of large 10-Gb/s SBS slow light delay with low distortion using an optimized gain profile

An optimum SBS gain profile is designed to achieve better slow-light performance. It consists of a nearly flat-top profile with sharp edges. Tunable delays up to 3 pulse widths for 100-ps-long input pulses, corresponding to 10 Gb/s data rates, are found while keeping an output-input pulse-width ratio below 1.8. Bit-error-rate (BER) measurements performed for a non-return-to-zero modulation format demonstrates 28 ps of delay under error-free operation.

Slow and fast light based on coherent population oscillations in erbium-doped fibres

In this paper we review the main results on slow and fast light induced by coherent population oscillations in optical fibres doped with erbium ions. We explain the physics behind this technique and we describe the experimental realization. Finally, we summarize some recent advances in this field and future goals.

Plasmon-enhanced terahertz emission in self-assembled quantum dots by femtosecond pulses

A scheme for terahertz (THz) generation from intraband transition in a self-assembled quantum dot (QD) molecule coupled to a metallic nanoparticle (MNP) is analyzed. The QD structure is described as a three-level atom-like system using the density matrix formalism. The MNP with spherical geometry is considered in the quasistatic approximation. A femtosecond laser pulse creates a coherent superposition of two subbands in the quantum dots and produces localized surface plasmons in the nanoparticle which act back upon the QD molecule via dipole-dipole interaction. As a result, coherent THz radiation with a frequency corresponding to the interlevel spacing can be obtained, which is strongly modified by the presence of the MNP. The peak value of the terahertz signal is analyzed as a function of nanoparticles size, the MNP to QD distance, and the area of the applied laser field. In addition, we theoretically demonstrate that the terahertz pulse generation can be effectively controlled by making use of a train of femtosecond laser pulses. We show that by a proper choice of the parameters characterizing the pulse train a huge enhancement of the terahertz signal is obtained.

Morphology of anisotropic chains in a magneto-rheological fluid during aggregation and disaggregation processes

We study the morphology of the chain-like aggregates formed when a external constant and uniaxial magnetic field is applied to a magneto-rheological (MR) fluid. In order to characterize the conformation of the aggregates, we study the evolution of various fractal dimensions during aggregation and disaggregation processes (i.e., when the applied field is switched on and off), using video-microscopy and image analysis. Experiments have been performed by varying the values of two external parameters: the magnetic field amplitude and particle concentration. We found that the box-counting dimension, related with how the aggregates occupy the surrounding space, depends on the ratio R(1)/R(0). During the first stage of the disaggregation process, when the particles are moving by Brownian motion inside the aggregate, Family-Vicsek scaling function is verified.

Modulation-frequency-controlled change from sub- to superluminal regime in highly doped erbium fibers

We report a change from sub- to superluminal propagation upon increasing the modulation frequency of an amplitude-modulated 1,550 nm signal when propagating through highly doped erbium fibers pumped at 980 nm. We show that the interplay between the pump absorption and the pump-power broadening of the spectral hole induced by coherent population oscillations may drastically affect the fractional advancement or delay of the signal for the considered fibers.

Competition between the modulation instability and stimulated Brillouin scattering in a broadband slow light device

We observe competition between the modulation instability (MI) and stimulated Brillouin scattering (SBS) in a 9.2 GHz broadband SBS slow light device, in which a standard 20 km long single-mode LEAF fibre is used as the SBS medium. We find that MI is dominant and depletes most of the pump power when we use an intense pump beam at ∼1.55 μm, where the LEAF fibre is anomalously dispersive. The dominance of the MI in the LEAF-fibre-based system suppresses the SBS gain, degrading the SBS slow light delay and limiting the SBS gain-bandwidth to 125 dB GHz. In a dispersion-shifted highly nonlinear fibre, the SBS slow light delay is improved due to the suppression of the MI, resulting in a gain-bandwidth product of 344 dB GHz, limited by our available pump power of 0.82 W.