Observation of Polarization Singularities and Topological Textures in Sound Waves.

Polarization singularities and topological polarization structures are generic features of inhomogeneous vector wave fields of any nature. However, their experimental studies mostly remain restricted to optical waves. Here, we report the observation of polarization singularities, topological Möbius-strip structures, and skyrmionic textures in 3D polarization fields of inhomogeneous sound waves. Our experiments are made in the ultrasonic domain using nonparaxial propagating fields generated by space-variant 2D acoustic sources. We also retrieve distributions of the 3D spin density in these fields. Our results open the avenue to investigations and applications of topological features and nontrivial 3D vector properties of structured sound waves.

Acoustic analysis of a broadband spiral source for the simultaneous generation of multiple Bessel vortices in air.

A detailed characterization and analysis of the acoustic field radiated by an active diffraction grating is presented. This is a spiral-shaped source of constant width of the spiral paths and spacing among them, built with flexible ferroelectrets, which is suitable to operate within a broad range of ultrasonic frequencies. The simultaneous airborne generation of multiple Bessel vortex beams with different topological charges along the propagation axis is thoroughly discussed in terms of the structure of the spiral source and the driving frequency, along with some technical details of the prototype. Finally, the quality of the experimentally generated Bessel vortices is discussed, including the phase structure and the amplitude of the acoustic pressure.

Steering and switching of soliton-like beams via interaction in a nanocolloid with positive polarizability.

We unveil different regimes for the interaction between two orthogonally polarized soliton-like beams in a colloidal suspension of nanoparticles with positive polarizability. The interaction is always attractive. However, it noticeably changes as a function of the angle and the power distribution between the input beams. For small angles, both interacting solitons fuse into a single entity, whose propagation direction can be continuously steered. As the interaction angle increases, the resulting self-collimated beam can be practically switched between two positions when the power imbalance between the beams is changed. For interaction angles larger than ∼10°, the result is no longer a single emerging soliton when the input power is balanced between the two beams.

Omnidirectional Transport in Fully Reconfigurable Two Dimensional Optical Ratchets.

A fully reconfigurable two-dimensional (2D) rocking ratchet system created with holographic optical micromanipulation is presented. We can generate optical potentials with the geometry of any Bravais lattice in 2D and introduce a spatial asymmetry with arbitrary orientation. Nontrivial directed transport of Brownian particles along different directions is demonstrated numerically and experimentally, including on axis, perpendicular, and oblique with respect to an unbiased ac driving. The most important aspect to define the current direction is shown to be the asymmetry and not the driving orientation, and yet we show a system in which the asymmetry orientation of each potential well does not coincide with the transport direction, suggesting an additional symmetry breaking as a result of a coupling with the lattice configuration. Our experimental device, due to its versatility, opens up a new range of possibilities in the study of nonequilibrium dynamics at the microscopic level.

3D micromanipulation at low numerical aperture with a single light beam: the focused-Bessel trap.

Full-three-dimensional (3D) manipulation of individual glass beads with radii in the range of 2-8 µm is experimentally demonstrated by using a single Bessel light beam focused through a low-numerical-aperture lens (NA=0.40). Although we have a weight-assisted trap with the beam propagating upward, we obtain a stable equilibrium position well away from the walls of the sample cell, and we are able to move the particle across the entire cell in three dimensions. A theoretical analysis for the optical field and trapping forces along the lateral and axial directions is presented for the focused-Bessel trap. This trap offers advantages for 3D manipulation, such as an extended working distance, a large field of view, and reduced aberrations.

Optical sorting of nonspherical and living microobjects in moving interference structures.

Contactless, sterile and nondestructive separation of microobjects or living cells is demanded in many areas of biology and analytical chemistry, as well as in physics or engineering. Here we demonstrate advanced sorting methods based on the optical forces exerted by travelling interference fringes with tunable periodicity controlled by a spatial light modulator. Besides the sorting of spherical particles we also demonstrate separation of algal cells of different sizes and particles of different shapes. The three presented methods offer simultaneous sorting of more objects in static suspension placed in a Petri dish or on a microscope slide.

Enhanced standing-wave acoustic levitation using high-order transverse modes in phased array ultrasonic cavities.

Airborne acoustic trapping by ultrasonic phased arrays has seen great advances in recent years, and yet the manipulation of objects with different shapes and sizes or heavy particles remains challenging. Here, we demonstrate that the manipulation capabilities of a standing-wave acoustic levitator can be extended by introducing intracavity high-order transverse (HOT) modes in the azimuthal direction, enabling the simultaneous trapping of several objects within a wide range of shapes and sizes with positional and rotational stability, including objects with sizes larger than one wavelength and weights in the scale of millinewtons. The conditions to generate different HOT modes are theoretically analyzed and experimentally implemented. We numerically calculate the pressure distributions, exhibiting good qualitative agreement with the experimental pressure distributions obtained with schlieren images. In addition, we calculate the acoustic force field for several examples of HOT modes and different particle sizes, which leads to a qualitative understanding of the experimental observations.

Quantitative characterization of the energy circulation in helical beams by means of near-field diffraction.

We present a method to measure the skew angle of the wave-fronts in an optical vortex, which is directly related with the energy flux. It is based on the analysis of the evolution on propagation of the near-field diffraction pattern generated by a single-slit, consisting of two main lobes that shift in opposite directions depending on the vortex helicity. The transverse displacement of each lobe as a function of the propagation distance allows to quantify the energy circulation. Analytical, numerical and experimental results are compared, showing good agreement. We illustrate the method for the case of Bessel beams, although we also discuss other types of helical beams, such as Laguerre-Gauss and Mathieu beams.

Steering and guiding light with light in a nanosuspension.

We experimentally demonstrate guiding of a low-power probe beam (633 nm wavelength) by means of a light-induced waveguide generated by the self-focusing of a strong pump beam (532 nm wavelength) in an artificial nonlinear medium, constituted by a colloidal suspension of dielectric nanoparticles. We also demonstrate optical steering of the probe beam by controlling the direction of propagation of the pump beam. The distance over which guiding is demonstrated (5 mm) is remarkably long when compared with previous experiments.

Light with enhanced optical chirality.

Tang and Cohen [Phys. Rev. Lett.104, 163901 (2010)] recently demonstrated a scheme to enhance the chiral response of molecules, which relies on the use of circularly polarized light in a standing wave configuration. Here we show a new type of light that possesses orbital angular momentum and enhanced chiral response. In the locations where the beams show enhanced optical chirality, only the longitudinal components of the electric and magnetic fields survive, which has unexpectedly shown what we believe is a new way to yield an enhanced optical chiral response.

Experimental control of transport and current reversals in a deterministic optical rocking ratchet.

We present an experimental demonstration of a deterministic optical rocking ratchet. A periodic and asymmetric light pattern is created to interact with dielectric microparticles in water, giving rise to a ratchet potential. The sample is moved with respect to the pattern with an unbiased time-periodic rocking function, which tilts the potential in alternating opposite directions. We obtain a current of particles whose direction can be controlled in real time and show that particles of different sizes may experience opposite currents. Moreover, we observed current reversals as a function of the magnitude and period of the rocking force.

Force mapping of an extended light pattern in an inclined plane: deterministic regime.

We present a full quantitative mapping of the non-linear optical trapping force associated to an extended interference pattern of fringes as a function of the position. To map this force, we studied the dynamics of microscopic spherical beads of different sizes (8, 10 and 14.5 microns in diameter) moving through the light pattern. For this range of particle sizes, the system is overdamped due to the viscous drag and the effect of thermal noise is negligible. The novel experimental approach consists in tilting the sample cell a small angle with respect to the horizontal, thus we have a deterministic particle in an inclined plane. The combined action of the optical force and gravity gives rise to a washboard potential. We compared our experimental results with a ray optics model and found a good quantitative agreement. For each size of the microsphere we studied different spatial periods of the interference fringes.

Transverse particle dynamics in a Bessel beam.

Spatially periodic optical fields can be used to sort dielectric microscopic particles as a function of size, shape or refractive index. In this paper we elucidate through both theory and experiment the behavior of silica microspheres moving under the influence of the periodic optical field provided by a Bessel beam. We compare two different computational models, one based on Mie scattering, the other on geometrical ray optics and find good qualitative agreement, with both models predicting the existence of distinct size-dependent phases of particle behavior. We verify these predictions by providing experimental observations of the individual behavioral phases.

Dynamical analysis of an optical rocking ratchet: theory and experiment.

A thorough analysis of the dynamics in a deterministic optical rocking ratchet [introduced in A. V. Arzola et al., Phys. Rev. Lett. 106, 168104 (2011)] and a comparison with experimental results are presented. The studied system consists of a microscopic particle interacting with a periodic and asymmetric light pattern, which is driven away from equilibrium by means of an unbiased time-periodic external force. It is shown that the asymmetry of the effective optical potential depends on the relative size of the particle with respect to the spatial period, and this is analyzed as an effective mechanism for particle fractionation. The necessary conditions to obtain current reversals in the deterministic regime are discussed in detail.

Attractive-repulsive dynamics on light-responsive chiral microparticles induced by polarized tweezers.

Multifunctional colloidal micro and nano-particles with controlled architectures have very promising properties for applications in bio and nanotechnologies. Here we report on the unique dichotomous dynamical behaviour of chiral spherical microparticles, either fluid or solid, manipulated by polarized optical tweezers. The particles are created using a reactive mesogen mixed with a chiral dopant to form cholesteric liquid crystal droplets in water emulsion. The photopolymerization enables the chiral supramolecular configurations to be frozen in solid particles. Different internal architectures in the supramolecular structures, guided by the interfacial chemistry, enable optically isotropic or anisotropic spherical objects to be obtained. For particles having radial configuration of the cholesteric helices, we show that light can exert either a repulsive or attractive force depending on the handedness of its circular polarization, due to the unique selective reflection property of the cholesteric phase. On the other hand, very exotic dynamics is observed in the case of anisotropic chiral particles. Depending on the light handedness, they behave like Janus spherical particles with dissimilar optical properties, meaning that the surface of the dielectric particles is partly transparent and partly reflecting. We foresee interesting potential applications in micro and optofluidics, microphotonics and materials science.

Transfer of angular momentum to matter from acoustical vortices in free space.

An experimental demonstration of the mechanical transfer of orbital angular momentum to matter from acoustical vortices in free field is presented. Vortices with topological charges l=+/-1 and l=+/-2 were generated and a torsion pendulum was used to study the angular momentum transfer to hanging disks of several sizes. This allowed us to make a comparative study of the effective acoustical torque in terms of topological charge of the vortex, the disk radius, and its position along the main propagation axis. A theoretical discussion of the generated sound fields is also provided.