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Department of Condensed Matter Theory

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Instituto de Ciencia de Materiales de Madrid
Consejo Superior de Investigaciones Cientificas, CSIC
Universidad Autonoma de Madrid, UAM
Campus de Cantoblanco
Madrid, 28049, Spain.


 

RESEARCH INTERESTS

 

NEAR FIELD OPTICS AND PHOTONIC FORCE MICROSCOPY

The demonstration of levitation and trapping of small particles by light was established since more than twenty years ago. This has lead to recent studies of particle manipulation by the action of light. The resultant product has been the optical tweezer (OT) for handling microparticles and other biological microstructures suspended in solution, and the photonic force microscope (PFM) used as a transducer of the interaction potentials in colloids, or the binding forces in macromolecule arrays.

Our work consists of putting forward models of force action between particles, surfaces and light beams on illumination of these systems. In this way, we predict the magnitudes of forces and potentials through the action of illuminating beams. This makes it possible a monitoring of the signal provided by nano and microparticles in the OT and the PFM. The objective is twofold: on the one hand nanoparticles can be used as nanodetectors in the PFM, yielding better signals resembling surface topography than the near field optical microscope. On the other hand, the control of electromagnetic potential wells, created by light illumination, leads to a device of particle ensembles on substrates through light action.

Of particular relevance is the presence of morphological resonances in dense dielectric particles and plasmon polariton resonances in metallic microparticles and nanoparticles. The presence of particle eigenmodes excitation leads to dramatic enhancements of the interaction potentials, and thus of the binding forces and imaging signals.

LIGHT PROPAGATION IN DISORDERED MEDIA

The phenomena of light localization in random media and photonic bandgaps in ordered structures have given rise to a series of interesting effects such as enhanced backscattering, long range correlations, new speckle statistics distributions, and photonic crystals. The influence of inhomogeneities and defects in the transmission properties of waveguides affects the wavefront and mode distribution, and thus has consequences for the transmission of information content of its degrees of freedom. This is characterized by the speckle pattern. Processing of these optical signals requires a detailed knowledge of the evolution of the speckle statistics on propagation. There already exist several analogies with electron transmission in nanowires. In this work, we analyze the transmittivity and reflectivity of surface disordered waveguides, the evolution of the speckle distribution, both in near field and far field (mode distribution), as the signal advances in the waveguide through the ballistic, diffusion and localization regimes.

DIFFUSE PHOTON DENSITY WAVES WITH SPECIAL EMPHASIS IN MEDICAL IMAGING

The study of light transport through strongly scattering media has recently received increased attention because of its applications to medical diagnosis. N particular, much research is motivated by the ability of optical radiation to diagnose tumors, brain oxigenation or blood clog formation. In most practical situations, the diffusion equation is suffciently accurate to describe visible or near-infrared radiation light transport within turbid media such as human tissues. A detailed analysis of these processes, both in the presence of interfaces between diffusive media, and at the separation of these from air or other non scattering regions, has been carried out. The resolution content and functional information of these waves has been analysed.

At present, we have put forward a direct scattering model that predicts the diffuse wavefront, in presence of resine and intralipid interfaces, of breasts with strange objects inside. The Green function or point source response of such a system has been successfully obtained. This allows the data inversion and the reconstruction of optical parameters of the hidden objects such as the scattering and absorption coefficients. A clinical application of this model, which is in progress, requires a subsequent development of a tomographic procedure in cylindrical geometries , a way to introduce specific contrast agents to enhance the signal scattered from the objects, and detailed clinical tests on the relationship between optical parameters and functional characteristics of tissue.

 

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Manuel Nieto Vesperinas
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