Karen Volke-Sepúlveda

Orbital angular momentum of a high-order Bessel light beam

The orbital angular momentum density of Bessel beams is calculated explicitly within a rigorous vectorial treatment. This allows us to investigate some aspects that have not been analysed previously, such as the angular momentum content of azimuthally and radially polarized beams. Furthermore, we demonstrate experimentally the mechanical transfer of orbital angular momentum to trapped particles in optical tweezers using a high-order Bessel beam. We set transparent particles of known dimensions into rotation, where the sense of rotation can be reversed by changing the sign of the singularity. Quantitative results are obtained for rotation rates. This papers animations are available from the Multimedia Enhancements page.

Revolving interference patterns for the rotation of optically trapped particles

Abstract Optically trapped objects are rotated controllably in the interference pattern between a Laguerre–Gaussian (LG) beam and a Gaussian beam. In this work the interference pattern is analysed and its properties as it propagates are modelled, showing the important role played by the Guoy-phase of the two interfering beams. An analysis of producing controlled rotation of the interference pattern using a glass plate is presented demonstrating the ease with which the rotation can be controlled.

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.

Modulated optical sieve for sorting of polydisperse microparticles

We present an all-optical technique that permits sorting within a polydisperse sample of microparticles in the absence of any microfluidic flow. We can sort colloidal samples based on their size and their refractive index. We show experimental and theoretical data for this method. It is based on the specific response of different microparticles to an interference pattern of fringes vibrating with an asymmetric time modulation. The size selectivity arises from the spatial fringe periodicity whereas selection based on refractive index is controlled by the beam power.

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.

Moving interference patterns created using the angular Doppler-effect

We use the angular Doppler-effect to obtain stable frequency shifts from below one Hertz to hundreds of Hertz in the optical domain, constituting a control of 1 part in 1014. For the first time, we use these very small frequency shifts to create continuous motion in interference patterns including the scanning of linear fringe patterns and the rotation of the interference pattern formed from a Laguerre-Gaussian beam. This enables controlled lateral and rotational movement of trapped particles.

Experimental generation and analysis of first-order TE and TM Bessel modes in free space

We demonstrate the experimental generation at optical frequencies of the lowest- and first-order Bessel beams with TE and TM polarizations in free space by means of a Mach-Zehnder interferometer. The polarization and angular momentum properties of these waves are analyzed and discussed.

General construction and connections of vector propagation invariant optical fields: TE and TM modes and polarization states

Rigorous solutions of the Maxwell equations describing propagation invariant optical fields are presented in general; the elements for their specific applications to the Bessel, Mathieu and Weber families are also provided. Electric and magnetic transverse modes, and several polarization state solutions, are constructed; the connections between them are explicitly established. Their respective energy densities and Poynting vectors are also evaluated, in order to exhibit their propagation invariant nature. The experience with Bessel beams allows us to recognize that vector modes exhibit new and important features compared with the corresponding scalar fields; the results of this work constitute a first step towards the analysis of the dynamical properties of vector Mathieu and Weber beams.

A demonstration of rotating sound waves in free space and the transfer of their angular momentum to matter

We describe an apparatus for generating rotating sound waves in free space by superimposing two orthogonal standing modes with a quarter wave phase lag. The creation of a standing wave from the superposition of two counter-rotating waves is also possible with the same apparatus. The experiment permits direct measurement of both the amplitude and phase structure of the sound waves. A demonstration of angular momentum transfer from rotating acoustic waves to matter in free field is also described.

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.