3D Speckle
When a laser beam strikes an optically rough surface such as frosted glass, the wavefront of the beam is modulated stochastically and its propagation in space becomes irregular. The intensity distribution observed at a certain distance from the glass has a deeply mottled appearance, in which light (bright) and dark (dark) spots, known as speckles, alternate in a highly disordered manner. By studying the statistical properties of speckles in three dimensions (3D), it is possible to characterize the roughness of the surface and define the spatial coherence volume of the speckle field.

Ghost Imaging
The Ghost Imaging technique allows the image of an object to be created without the need for the radiation transmitted by the object to be collected by a two-dimensional sensor, hence the term “ghost.” Using two twin speckle fields generated using a laser, a rotating frosted glass, and a beam splitter, it is possible to reconstruct the ghost image of the object by correlating the 2D intensity distribution of the first speckle field (called the reference, which does not interact with the object and is measured with a pixelated sensor) with the 1D signal obtained by collecting all the power transmitted by the object illuminated with the second speckle field (called the object), which is twin to the first.
Spatial coherence
The spatial coherence properties of radiation determine its ability to produce interference fringes when different portions of its wavefront are superimposed. This property is fundamental to all optical interferometry techniques. In the laboratory, laser radiation and frosted glass will be used to simulate pseudo-thermal radiation with different spatial coherence characteristics, and it will be shown how it is possible to measure the size of the coherence area using a Young interferometer.
Speckle velocimetry
Under appropriate conditions, the speckles produced by a system of moving particles move in unison with the particles. Consequently, by measuring the cross-correlation of two speckle fields acquired at a given time interval, it is possible to trace the 2D mapping of the particle velocity field. This technique will be applied in the laboratory to measure the (laminar) velocity profile of a fluid inside a duct and to characterize the sedimentation velocity of colloidal particles.
Brownian motion
A colloidal particle dispersed in a fluid is subject to thermal agitation known as Brownian motion. The mean square displacement of the particle depends on its translational diffusion coefficient, which, in turn, through the Stokes-Einstein relation, depends on the radius of the particle. Using a simple optical microscope interfaced with a high-speed camera, it is possible to visualize and track the motion of the particle, measure its mean square displacement, and consequently determine its radius.
Extinction at multiple wavelengths.
The spectral extinction technique allows samples with dimensions comparable to the wavelength of visible radiation to be studied. During the laboratory session, a miniaturized fiber optic spectrophotometer will be used for the granulometric analysis of particles of known diameter and for the study of colloidal aggregation processes.