Estimation of statistical properties of rough surface profiles from the Hurst exponent of speckle patterns.

We applied the Hurst exponent technique to an experimental study of rough metallic surface profiles and the speckle patterns generated by them. Characterization of important statistical properties of the surface profile and speckle patterns were performed. We observed a clear correlation between the Hurst exponent of a surface profile and the one calculated from the associated speckle patterns. Therefore, in principle, information of the Hurst exponent of the profile can be obtained from the Hurst exponent of speckle patterns. Range and sampling analyses were performed in the Hurst exponent calculations showing the robustness of the method. As an additional application, we performed a basic simulation to show that the Hurst exponent is sensitive to surface waviness.

Non-Markovianity through quantum coherence in an all-optical setup.

We propose an all-optical experiment to quantify non-Markovianity in an open quantum system through quantum coherence of a single quantum bit. We use an amplitude damping channel implemented by an optical setup with an intense laser beam simulating a single-photon polarization. The optimization over initial states required to quantify non-Markovianity is analytically evaluated. The experimental results are in very good agreement with the theoretical predictions.

Orbital angular momentum symmetry in a driven optical parametric oscillator.

We investigate the dynamics of a driven optical parametric oscillator under the injection of orbital angular momentum. The injected mode is adiabatically driven through arbitrary transformations on the Poincaré sphere of first-order paraxial beams. As a result, the down-converted beam conjugated to the seed is shown to follow a path imposed by a nontrivial symmetry on the Poincaré sphere. This symmetry allows controllable distinguishability between the spatial modes of the down-converted beams. In this Letter, we provide convincing experimental evidence of this effect.

Speckle patterns produced by an optical vortex and its application to surface roughness measurements.

In this work, we report on the analysis of speckle patterns produced by illuminating different rough surfaces with an optical vortex, a first-order (l=1) Laguerre-Gaussian beam. The generated speckle patterns were observed in the normal direction exploring four different planes: the diffraction plane, image plane, focal plane, and exact Fourier transform plane. The digital speckle patterns were analyzed using the Hurst exponent of digital images, an interesting tool used to study surface roughness. We show a proof of principle that the Hurst exponent of a digital speckle pattern is more sensitive with respect to the surface roughness when the speckle pattern is produced by an optical vortex and observed at a focal plane. We also show that Hurst exponents are not so sensitive with respect to the topological charge l. These results open news possibilities of investigation into speckle metrology once we have several techniques that use speckle patterns for different applications.

Tripartite nonseparability in classical optics.

It is possible to prepare classical optical beams which cannot be characterized by a tensor product of vectors describing each of their degrees of freedom. Here we report the experimental creation of such a nonseparable, tripartite GHZ-like state of path, polarization, and transverse modes of a classical laser beam. We use a Mach-Zehnder interferometer with an additional mirror and other optical elements to perform measurements that violate Mermin's inequality. This demonstration of a classical optical analogue of tripartite entanglement paves the path to novel optical applications inspired by multipartite quantum information protocols.

Topological phase structure of vector vortex beams.

The topological phase acquired by vector vortex optical beams is investigated. Under local unitary operations on their polarization and transverse degrees of freedom, the vector vortices can only acquire discrete geometric phase values, 0 or π, associated with closed paths belonging to different homotopy classes on the SO(3) manifold. These discrete values are demonstrated through interferometric measurements, and the spin-orbit mode separability is associated to the visibility of the interference patterns. The local unitary operations performed on the vector vortices involved both polarization and transverse mode transformations with birefringent wave plates and astigmatic mode converters. The experimental results agree with our theoretical simulations and generalize our previous results obtained with polarization transformations only.

Transfer of orbital angular momentum in a multimode parametric oscillator.

We investigate the transfer of orbital angular momentum (OAM) in a type-II optical parametric oscillator (OPO). We produce different kinds of transverse modes on the downconverted beams by pumping the OPO with a first-order Laguerre-Gaussian beam well above threshold. Different operation regimes are attained showing a rich variety of transverse modes bearing OAM.

Topological phase for spin-orbit transformations on a laser beam.

We investigate the topological phase associated with the double connectedness of the SO(3) representation in terms of maximally entangled states. An experimental demonstration is provided in the context of polarization and spatial mode transformations of a laser beam carrying orbital angular momentum. The topological phase is evidenced through interferometric measurements, and a quantitative relationship between the concurrence and the fringes visibility is derived. Both the quantum and the classical regimes were investigated.

Topological defects in moiré fringes with spiral zone plates.

We present a study of spatial structures created by superposition of spiral zone plates used for generating optical beams with phase singularities. Moiré fringes are observed that show topological defects similar to those appearing in interference patterns of optical vortices. A brief theoretical discussion is included that supports the similarities between the two phenomena. Our results may lead to interesting applications to digital information processing by optical means.