A. Lencina

Symmetric Airy beams

In this Letter a new class of light beam arisen from the symmetrization of the spectral cubic phase of an Airy beam is presented. The symmetric Airy beam exhibits peculiar features. It propagates at initial stages with a single central lobe that autofocuses and then collapses immediately behind the autofocus. Then, the beam splits into two specular off-axis parabolic lobes like those corresponding to two Airy beams accelerating in opposite directions. Its features are analyzed and compared to other kinds of autofocusing beams; the superposition of two conventional Airy beams having opposite accelerations (in rectangular coordinates) and also to the recently demonstrated circular Airy beam (in cylindrical coordinates). The generation of a symmetric Airy beam is experimentally demonstrated as well. Besides, based on its main features, some possible applications are also discussed.

Analysis and applications of the speckle patterns registered in a photorefractive BTO crystal

The features of the volume speckle pattern recording in a sillenite BTO crystal are investigated. The modulation of the speckle grains appears when the image of a random diffuser is formed onto the crystal by an optical system whose pupil consists of two identical apertures. In fact, the volume nature of the grains generates speckle overlapping regions where the modulation takes place. The 3-D modulated speckle pattern stored in the crystal leads to a space-charge field by drift of photocarriers, resulting in a refractive index modulation. By observing the diffracted read-out light is established that the index modulation in the speckle overlapping regions lead to a remarkable angular selectivity. The statistical nature behavior of the speckle distributions is considered in the theoretical analysis. A comparison between the theoretical predictions and the experimental results is done. The analysis leads to the adequate selection of the write-in parameters as it is illustrated in the optical processing applications presented.

Dynamic behaviour of speckle cluster formation

In this work, we analyse the speckle cluster structure generated when a coherently illuminated diffuser is imaged by introducing a multiple aperture pupil mask in front of the lens plane. We demonstrate that the speckle cluster originates from the complex speckle modulation generated by multiple interferences among the wavefront passing through each aperture. The auto-correlation function of the intensity distribution when using a multiple aperture pupil arrangement is calculated. Besides, we demonstrate that the autocorrelation function and the intensity corresponding to a single scattering element of the input are coincident. This result allows interpretation of the dynamics behaviour of the speckle cluster formation by considering the result obtained by a single scattering element. Then, we determine the pupil mask geometrical parameters that control the cluster behaviour and therefore the condition for obtaining a highly repetitive cluster structure that we define as a ‘regular cluster’. The theoretical simulations based on the random walk model are in agreement with the experimental results supporting the validity of our approach.

Intensity-symmetric Airy beams

Theoretical, numerical, and experimental research on a novel family of Airy beams in rectangular coordinates having a symmetric transverse pattern of light intensity is presented. The intensity-symmetric Airy beams include both the symmetric Airy beam whose field amplitude is an even function of the transverse coordinates and the antisymmetric Airy beam whose field amplitude is an odd function of such coordinates. The theoretical foundations are based on the relationship of the symmetries of the spectral phase with the cosine and sine Fourier transforms. These beams are analyzed in a propagation range also including the region preceding the Fourier plane. These beams exhibit autofocusing, collapse, self-bending, and reversal propagation. Moreover, the intensity distribution is strongly asymmetric with respect to the Fourier plane. All these peculiar features were not reported for other classes of paraxial beams in a rectangular frame. The experimental generation of intensity-symmetric Airy beams is demonstrated supporting the theoretical predictions. Possible applications in planar waveguide writing and optical trapping are also discussed.

High-quality optical vortex-beam generation by using a multilevel vortex-producing lens

In the present work, we propose a method to generate high-quality optical vortices with a reduced number of phase levels by using multilevel vortex-producing lenses (VPLs). The VPL is implemented in a liquid-crystal spatial light modulator with limited capacity to project phase levels. The proposed method significantly improves the quality of the optical vortex obtained by employing spiral phase plates with the same number of phase levels. Simulations and experimental results confirming the effectiveness of the method are presented.

Generation of optical vortices by using binary vortex producing lenses

Experimental high-quality optical vortices of different topological charges are generated by using a vortex producing lens with two phase levels. In our setup, the lens is displayed on a liquid-crystal spatial light modulator that only attains phase modulation of around 1.2π. This achievement opens the real possibility of creating high-quality optical vortices with devices of very low phase modulation capacity. The experimental setup is fully described, and the considerations to set the optimal parameters to obtain high-quality optical vortices are discussed and experimentally established. The phase and intensity of the optical vortices are recovered. The phase is obtained through a phase-shifting method that is directly programmed onto the modulator avoiding any class of mechanical displacement.

Three-dimensional clustered speckle fields: theory, simulations and experimental verification

Clustered speckle patterns are a particular type of speckles that appear when a coherently illuminated diffuser is imaged through a multiple aperture pupil mask attached to a lens. The cluster formation is the result of the complex speckle modulations of the multiple interferences produced by the apertures. In this paper, a three-dimensional analytical approach to simulate cluster speckles everywhere after the lens is presented. This approach has the possibility of including multiple aperture masks at the lens and at the diffuser, in contrast to previous works which were also limited to the description of the patterns only at the image plane. This model contributes to the development of tailor made speckle patterns that can be used in diverse optical applications, including those lying in the focus region. The approach is validated under different conditions by comparing experimental results with simulations on a statistical basis. Some aspects of possible uses of these clusters are briefly revised, such as optical trapping, manipulation and metrology.

Caustic beams from unusual powers of the spectral phase

Caustic optical beams arising from a spectral phase whose power lies in an unusual range of values less than two are presented. Unlike what happens for conventional phase powers greater than two, it is feasible to generate caustic structures having properties that do not follow the established sorting. For instance, an asymptotic cusp caustic beam having a cusp point at infinity is demonstrated. For the sake of completeness, the caustic beam properties are analyzed within the whole real range of the phase power. Accurate behavior rules between the symmetries of the beam spectral phase and its intensity distribution are found. These findings strengthen the fundamentals and engineering on caustic beams in diverse optical and physical branches.

Optical transmittance of plane-parallel-faces saturable nonlinear media

The transmittance of saturable nonlinear media under plane-wave excitation is analyzed for a Fabry-Perot geometry. The medium response is studied in terms of both a squared-field amplitude modulus and a time-averaged Poynting vector modulus. It was observed that both cases provide similar transmittance for the parameter range considered. For the nonlinearity proportional to the time-averaged Poynting vector modulus, a simple analytical expression for transmittance is derived as an Airy-type pattern. This allows us to perform a global analysis on the optical properties in terms of the nonlinear parameters and medium thickness. Several types of behavior were found at different parameter regions. Bistability happens only for excitation intensities around the saturation value. Finally, some possible implications of our results for bistable solitons are proposed.