It is well known that speckle fields exhibit a multitude of vortex-type phase dislocations with unitary topological charge and opposite helicities, such that the average angular momentum is null. We tackle this problem the other way around: What is the minimum vortex number embedded in a carrier beam to produce a disordered pattern and what are the necessary conditions in terms of their initial distribution and topological charges? When studying this problem, we found interesting dynamical behavior of vortices in propagation through a focal region where they are forced to interact, depending on the initial conditions, that in some cases resemble the behavior of a system of particles with an effective repulsive interaction.
We present a few of our recent theoretical and experimental results related to the behavior of micron-scale particles placed into nonlinear optical potentials. The two-dimensional optical ratchet can rectify motion of Brownian particles in any direction in the plane and unstable cubic optical potential results in noise-induced particle motion. Action of optical spin-force was demonstrated in a novel geometry where it is responsible for particle orbiting.
We report on the nonlinear effects of light propagation through a fluorescent nanocolloid, where self-collimated beams are formed. The medium is constituted by a bidisperse suspension of fluorescent and nonfluorescent nanospheres of similar diameters (60nm and 62nm, respectively) in distilled water. A CW laser beam (532 nm wavelength) was focused into the nano-suspension. The threshold power and focusing conditions to create a self-collimated beam are analyzed as a function of the incident power, and a hysteresis effect is observed for the size of the output beam when the power is increasing and decreasing. We also discuss other effects associated to the presence of the fluorescent nanospheres.
A Bessel beam generated with an axicon lens is focused by a low numerical aperture objective lens to create an optical levitation trap. This configuration allows a full three dimensional trapping of solid glass spheres of few microns in diameter immerse in water. We establish a comparison with an optical levitation trap generated with a Gaussian beam under the same focusing conditions. In both cases the particles are lifted up in an axial position above the focal region that depends on the incident power. The spatial stability is investigated in both cases as a function of axial position, which in turn depends on the input optical power.
We present and discuss a set of experiments based on the application of the nonlinear properties of colloidal nanosuspensions to induce waveguides with a high‐power CW laser beam (wavelength 532nm) and its use for controlling an additional probe beam. The probe is a CW laser of a different wavelength (632nm), whose power is well below the critical value to induce nonlinear effects in the colloidal medium. We also discuss a technique for the characterization of the induced waveguides.
It has been shown that a spatial soliton can be created when a CW laser travels through a suspension of dielectric nanoparticles, provided its power is above a critical value [Opt. Lett. Vol. 7: 276 (1982)]. Recently, it was demonstrated that these soliton-like beams can be used as waveguides for controlling an additional low-power laser (probe beam) [Opt. Lett. Vol. 38: 5284 (2013)]. Here we present an experimental study of the interaction between two solitons propagating through a nanocolloid and we analyze their use to create a beam splitter for a probe beam.
In this paper we present the experimental generation of complex beams by means of a polarization holographic
technique. The interference of a reference Gaussian beam and a complex beam having opposite circular polarization
states, stored on a highly polarization sensitive material, generates polarization holograms whose diffracted beams are
high quality complex fields. The technique is tested with the generation of three different types of beams: a simple
vortex, a Bessel and a Laguerre-Gaussian beam. This suggests an alternative method for the generation of complex
beams with predetermined polarization states.
Recently, we presented an experimental realization of a deterministic optical rocking ratchet [Arzola, A. V., et al. Phys
Rev. Lett. 106: 168104 (2011)]. We obtained a systematic motion of microparticles and demonstrated that it is possible
to control their average velocity and their direction of motion in real time by properly tuning experimental parameters.
We have extended our study in order to establish the conditions for observing the crucial effect of current reversals in
deterministic conditions, phenomenon predicted more than a decade ago, but experimentally demonstrated for the first
time in our system.
We propose a simple technique for the estimation of the local inclination angle of the helical wave fronts, and thus
the direction of the transverse energy flux, in beams with embedded optical vortices. It is based on the analysis
of the evolution on propagation of the asymmetric diffraction pattern produced by a single-slit aperture.
We demonstrate optical manipulation and sorting of micrometer-sized dielectric particles using one-dimensional
periodic interference pattern created by interference of two beams in a sample space. These beams are generated
by a combined phase grating applied on the spatial light modulator which allows to set dynamically the position
and spatial period of the interference pattern. If a microparticle of fixed size is placed into such pattern, the
optical forces acting upon it vary according to the spatial period of this optical lattice. We show how to use this
property for sorting of mixtures of particles by moving either the interference pattern or the sample chamber.
The mechanism is examined both theoretically and experimentally.
We present a theoretical model and the experimental demonstration of the rocking ratchet effect in the deterministic
regime using an optical trapping device. Our system consists of a dielectric spherical particle in a 1D optical potential
created by means of an interference pattern of asymmetric fringes. In order to achieve the asymmetry of the fringes, three
light beams are interfered by pairs by controlling their relative polarization states, intensities and phases. A periodic
time-dependent external force of zero average is introduced by moving the sample with respect to the optical pattern, for
which the translation stage is driven sideways. The drag force acting on the particle due to this relative motion has the
effect of tilting the optical potential periodically in opposite directions, providing the "rocking" mechanism. We show
that an inversion of the asymmetry in the effective optical potential occurs as the size of the particle is varied, and
therefore, we can observe opposite motion of different particles within the same optical pattern. The dynamics of the
system is studied in terms of the different control parameters, such as the size of the particles, the period and asymmetry
of the fringes, the amplitude and frequency of the rocking mechanism, and the power level in the sample.
We propose a technique for the characterization of a 1D-periodic optical potential by studying the dynamics of
non-brownian microscopic particles immerse in water (negligible thermal noise). It has been demonstrated that
in the Mie regime, a periodic light pattern applied to a particle acts as an effective potential that depends on
the size of the particle respect to the period of the optical landscape [I. Ricardez-Vargas, et.al. Appl. Phys.
Lett. 88, 121116 (2006)]. We verify this fact by studying the dynamics of a particle moving within the pattern
due to the effect of a known constant external force. The periodic light pattern is generated with interference
techniques whereas the external force is applied by means of a controlled inclination of the sample cell. We fit
the experimental results for the ensemble average of particle position against time with a theoretical model of
the physical situation. In this way we obtain a curve for the optical force as a function of particle's position for
Optical vortices became a hot topic since almost two decades ago, when it was recognized that Laguerre-Gaussian laser
modes carry orbital angular momentum [Allen et al. Phys Rev A 45, 8185 (1992)] related with a screw phase dislocation,
and different from the spin angular momentum associated to circular polarization. In 1995, this dynamical quantity was
transferred to matter in an optical micromanipulation system for the first time [H. He, et al., Phys. Rev. Lett. 75, 826
(1995)], and since then, a number of studies on angular momentum of light have unveiled different interesting aspects on
the subject. However, there are still open questions, which have arisen together with the generation of novel light beams,
such as vector vortices, for instance. In contrast with scalar vortices, with usual polarization states (linear, circular,
elliptical), the orientation and magnitude of the electric field of vector vortices (solutions of the vector wave equation) is
a function of space and time. In this work, we present an experimental study of the local angular momentum density of a
Bessel vector vortex of first order by means of an optical trap. For this purpose, we used different probe particles in order
to sense the local contribution to the optical angular momentum in each region of the beam. But optical fields are not the
only wave fields that may exhibit phase dislocations or singularities. There are close analogies between light and sound
fields that can be exploited in order to get a better understanding of common phenomena and study new aspects in both
branches of physics. Here we also present the first experimental demonstration and theoretical analysis of acoustical
vortices in free field, with similar properties to those of the optical vortices, including the angular momentum that can be
transferred to matter. The corresponding analogies and differences with the optical case turn out to be very enlightening
for the understanding of the phenomenon of angular momentum in wave fields.
We demonstrate the use of supercontinuum radiation to provide enhanced guiding distances of microscopic particles
compared to the standard continuous wave or femtosecond lasers. Our technique relies on the chromatic aberration of the
lens used to form an elongated focal region within which guiding takes place. The resulting beam profile has been
modelled and shows that for a Gaussian input beam, the intensity profile after the lens can be considered as a sum of
Gaussians, one for each wavelength but with varying focal position due to dispersion. Our experimental investigations
compare radiation from continuous wave (bandwidth <1nm) and femtosecond pulsed (bandwidth > 100nm) lasers as
well as supercontinuum radiation (bandwidth > 450nm) and show good agreement with theory.
We present a detailed theoretical discussion and experimental analysis of an interferometric optical trapping device that allows efficient sorting of particles, including biological samples, either by size or refractive index. This technique involves no microfluidic flow, but it is based on the specific response of different microparticles to an interference pattern of fringes vibrating with a periodic but non-symmetric time modulation function. The performance of the system is analyzed in terms of the different control parameters, such as the period of the fringes, the vibration amplitude and frequency, and the power level in the sample. We discuss the possibility of using this system to characterize unknown samples.
The experimental generation of vector Bessel beams of different orders with TE and TM polarizations is demonstrated in
the optical frequencies domain and in free space by means of a Mach-Zehnder interferometer. For the zero order beams,
these modes correspond to the well known azimuthal and radial polarization states, whereas higher order versions
become vector vortices, thus carrying orbital angular momentum. The case of the lowest order TE and TM vector
vortices, corresponding to a topological charge of l=1, is particularly interesting due to their off-axis location and the
non-null intensity at the beam center, unlike the case of topological charges l>1. The polarization and angular momentum
properties of these waves are thoroughly analyzed and discussed for different cases.
The use of axicons or conical lenses has been implemented and proposed in different optical traps, either to create propagation invariant optical fields like Bessel beams or to generate other kinds of traps like hollow beams. In this work we show that the optical field emerging from a system formed by the combination of two axicons can have very different configurations and propagation properties depending upon a few relevant parameters of the system, such as the internal angles of the axicons, the distance between them and their diameters. In particular, by varying only the distance between the axicons, the emerging field may change from a single Bessel beam to a collimated hollow beam or to an array of optical bottle beams, which corresponds to the interference of two Bessel beams with different parameters. Based on these results, we propose a new dynamically reconfigurable optical trap.
We report on the observation of a normal streak effect on hollow micron sized spheres when illuminated by a focused Gaussian beam in a conventional optical tweezers setup. The hollow microspheres suspended in water can be optically trapped at the center of the illuminating beam. When the microsphere is illuminated off center, an emerging beam approximately perpendicular respect to the incoming beam is generated. This effect due to total internal reflections has been observed in microspheres with different external diameters, ranging from 5-20 microns. The generated normal beam is used to either pull or push other particles or objects around the microsphere or to remove particles stuck to the sphere due to radiation pressure.
We demonstrate the use of the angular Doppler effect to obtain continuous motion of interference patterns. A small frequency shift between two beams can create such a moving pattern. By rotating a half wave plate in one arm of an interferometer, frequency shifts in the optical domain from less than 1 Hertz to kHz are achieved. We apply moving interference patterns in an optical tweezers set-up to enable controlled and continuous motion of optically trapped particles and structures.
Optical guiding of micron-sized particles is shown using both Gaussian and zeroth-order Bessel light beams. Axial and transverse forces for guiding in both beams are calculated. Experiments show that the Bessel beam allows for extended guiding distances compared to a Gaussian beam, at the expense of guiding velocity.