The interesting propagation properties of periodical gratings have been widely studied in the last decades. A special characteristic involving periodical structures is the self-imaging, in free propagation. Commonly the Talbot effect only consider the scalar nature of the fields, however the vectorial nature of the fields also plays an important role in free propagation. In this work, we study theoretically and experimentally the free propagation at fractional Talbot distances of inhomogeneous polarization periodical gratings, which are characterized by orthogonal polarization states. We found that the polarization states change in a peculiar way at different planes. We also generate some particular polarization gratings by means of a spatial light modulator and demonstrate that the theoretical and experimental results are in good agreement.
In this work, we discuss the generation of spatially inhomogeneous polarization beams (SIPBs), by using two liquid crystal polarization holograms (PHs). The first PH generates two circularly opposed polarized scalar beams, which are collinearly recombined by the second PH. Owing to the tunability of the liquid crystal birefringence, we demonstrate experimentally the generation of SIPBs with efficiency near to 100%. By taking advantage of the diffraction properties, the high efficiency, and the intrinsic achromaticity of the polarization holograms, the method aims to overcome the limitations related to stability and efficiency, making it attractive for applications.
Here we report the creation and manipulation of colloidal crystals by inducing temperature gradients in a colloidal suspension of silica microparticles. A colloidal crystal is an ordered array of colloid particles analogous to their atomic or molecular counterparts with proper scaling considerations. The generation and properties of colloidal crystals have been of great interest for diverse science applications such as photonic crystals, chemical sensors among others. We report a technique that utilizes particles of silica of different diameters to form colloidal crystals by temperature gradients produced by light absorption at a metallic thin film deposited on one of the substrates. Moreover, we study the behavior of the particles by having different number of hot zones.
In this work, we show the conversion of a Gaussian beam into an annular vortex beam (AVB) by means of an optical vortex element (OVE). This is a simple phase plate which generates the AVB at a determined distance without the use of external optical elements such as lenses and axicons. We discuss the interesting features and the advantages of the OVE respect to other methods to generate AVB such as the conventional vortex (CV) and the helical axicon (HA). The OVE presents the highest intensity peak respect to both the CV and the HA. Another important feature is that the OVE and the HA maintain a fixed annular radius; in contrast the CV changes the annular radius, while the topological charge is modified. The OVE is displayed on a spatial light modulator (SLM) in order to generate experimentally the AVBs. We demonstrate the features of the AVB generated and measure the high angular velocities achieved due to the angular momentum transfer to 3 μm particles.
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.
In recent years, particle transport at microscopic level has become an important research topic which has led to the
understanding of directed particle transport subjected to thermal fluctuations. Brownian motors (also called ratchet
mechanism) are one of the most interesting phenomena of work generation in nonequilibrium systems under
random external forces. In this work, we report Brownian movement rectification of 0.5 μm diameter latex
particles using pulsating ratchets. In order to implement the ratchets, an asymmetric 2D potential saw tooth phase
pattern is displayed on a spatial phase modulator and then transformed into an intensity pattern by using the phase
contrast method. This pattern is focused down with a 100x microscope objective obtaining a pattern of ~40x40
μm2 at the focal plane. The patterns parameters can be dynamically controlled: periodicity, asymmetry, and
on/off rate, which allows optimization of directed transport. We found that there is an optimum value for the
on/off rate and particle diameter/spatial period obtaining an average speed of 0.6 μm/s. The 2D pattern allow us to
manipulate a large number of particles, in contrast to previous studies were only one particle has been studied,
opening the opportunity to massive sorting of particles.
A numerical and experimental comparison between different synthetic holographic codes is presented. Its performance
is evaluated considering the generation of Bessel and Laguerre-Gaussian beams, as examples. Some
reviews of computer generated holograms (CGHs) have been published in the literature but none of them have
included a detailed comparison of their performance in the encoding of structured optical fields. The numerical
evaluation includes an analysis of the theoretical features of each hologram and a calculation of Signal-to-Noise
Ratio of the reconstructed field while the experimental evaluation assume the implementation of the holograms
using a pixelated phase modulator.
The use of spatial light modulators to generate arbitrary optical field distributions has been extensively used to trap and manipulate dynamically a large number of particles. Here we show that by using phase computer generated holograms displayed on a spatial light modulator (SLM) sorting of microparticles can be achieved at relatively low power. The algorithm used for the generation of the PCGH is based on iterative Fourier transform algorithm which generate a spots array in the Fourier plane, then controlling some parameters as: the spot separation, the direction and velocity of the pattern displacement, optical sorting of micron-sized particles can be achieved.
A spatial light modulator (SLM) is a very useful optical tool due its versatility to be manipulated dynamically. We
propose a characterization method for reflective SLMs in quasi-normal configuration. This device can work as either
amplitude-mostly or phase-mostly modulator. To achieve the modulation, the SLM can be accompanied by two
polarizes, or additionally using a quarter-wave plate retarder. By simulating the behaviour of the optical setup for this
device; we establish an experimental Jones matrix that represent the modulator. In this work we present a more realistic
modulator characterization, only assuming that the modulator Jones matrix should be unitary (the modulator do not
absorb light). We report this model for characterization of the Holoeye LCR2500 SLM.
We experimentally investigated the transmission variation of a nonlinear optical loop mirror (NOLM). We analyzed the transmission evolution of the NOLM based on the input intensity. The NOLM is formed by a symmetrical coupler, 500 m of highly twisted low-birefringence fiber and a quarter-wave retarder plate in the loop. If we rotate the quarter-wave retarder plate is possible to change the transmission behavior. Using circular polarization of the input beam is possible to get a high contrast between the maximum and minimum of the transmission. With this characteristic, we have the possibility to reduce the amplitude fluctuations and the pedestal in a train of optical pulses.