Biomedical subjects

A Dogariu

Publications and source records attributed to A Dogariu.

14 recordsLinked to original sources

PubMed2006

All-ultraviolet time-resolved coherent anti-Stokes Raman scattering.

We report all-UV coherent anti-Stokes Raman scattering (CARS) in calcite with 250-280 nm pump, Stokes, probe, and anti-Stokes light. UV CARS efficiency is approximately 7x higher than for comparable scattering in the visible, 480-540 nm. Time-resolved UV CARS reveals lengthening of the dephasing time of 1086 cm(-1) CO3(2-) internal vibrations from 4 to 7 ps with increasing vibrational excitation, consistent with a phonon depletion model.

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PubMed2005

Optical polarimetry of random fields.

The state of polarization of an optical field provides detailed information concerning both the radiation emission processes and the intricate interaction between light and matter. We report here a novel approach for characterizing the polarization properties of electromagnetic fields for which the electric field vector at a point may fluctuate in three dimensions. Using probes which couple all three components of the field, we were able to extract the polarized and unpolarized components of such fields. Our results constitute the proof of concept for what could be called three-dimensional optical polarimetry.

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PubMed2005

Non-Gaussian statistics of optical near-fields.

Optical fields in the proximity of random media can be modeled as a superposition of waves with random phases. We demonstrate that, depending on the experimental geometry, both Gaussian and non-Gaussian regimes can be established for the statistics of scattered intensity. In a reflection-emission configuration, the first-order statistics of the scattered light is non-Gaussian and it can be used to retrieve information about the physical interface.

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PubMed2005

Generation of correlated photon pairs in a microstructure fiber.

We propose and experimentally demonstrate a new method of generating correlated photons in a microstructure fiber by means of a reversed degenerate four-wave-mixing process. Here one photon is annihilated from each of the bichromatic pump pulses to generate a pair of photons at the mean frequency. For a microstructure fiber as short as 1.5 m the measured coincidence counting rate is approximately eight times that of the accidental coincidences with a peak pump power of 0.25 W.

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PubMed2005

Generation of complete coherence in Young's interference experiment with random mutually uncorrelated electromagnetic beams.

The recently developed theory that unifies the treatments of polarization and coherence of random electromagnetic beams is applied to study field correlations in Young's interference experiment. It is found that at certain pairs of points the transmitted field is spatially fully coherent, irrespective of the state of coherence and polarization of the field that is incident on the two pinholes.

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PubMed2004

Light spectra in the near field of random media.

The spectrum of light in close proximity to a highly scattering, randomly inhomogeneous medium is investigated. We find differences between the far- and near-field spectra that are due to evanescent wave contributions. We show that these spectral modifications depend on the coherence properties of an equivalent planar source, which, in turn, relates to the statistical characteristics of the interface.

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PubMed2003

Spatial correlations in the near field of random media.

The conventional coherence theory suggests that the fields radiated by statistically homogeneous sources correlate over spatial regions of the order of the wavelength irrespective of the distance from the surface of the source. Contrary to these predictions, we show that the spatial correlations of optical fields in close proximity of highly scattering, randomly inhomogeneous media depend on this distance and, moreover, their extent can be significantly smaller than the wavelength. The contribution of evanescent fields is experimentally demonstrated and the coherence length in the near field is shown to relate to the coherence properties at the surface which are, in turn, determined by the structural characteristics of the random media.

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PubMed2003

Coherence properties near interfaces of random media.

The spatial coherence theory of optical fields radiated by statistically homogeneous, infinite, planar sources does not account for detailed surface characteristics. However, coherence properties change with the angular distribution of the intensity radiated from such a source, which, in turn, depends on the physical properties of the sample. We show that second-order correlations of the optical field measured at several wavelengths away from the surface of a highly inhomogeneous medium relate to the statistical characteristics of the interface.

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PubMed2002

Spectral anomalies at wave-front dislocations.

In a recent Letter, Gbur, Visser, and Wolf [Phys. Rev. Lett. 88, 013901 (2002)] predict that remarkable spectral changes can take place in the neighborhood of phase singularities of a diffracted focused wave. We report here the experimental observation of this anomalous spectral behavior and show that this is a general characteristic of optical vortices. Using a high-resolution interferometric technique, we are able to measure directly both the spectral intensity distribution and the spectral phase at the singular points of optical waves.

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PubMed2002

Spatially resolved microrheology using localized coherence volumes.

We propose an optical technique, based on dynamic light scattering, for investigating the local rheological response of a complex fluid over a frequency range larger than that provided by standard mechanical instrumentation. The low-coherence radiation used in a fiber optics configuration allows the measurements to be confined to a small volume around a tenth of a picoliter. The ability of the method to accurately measure both loss and storage moduli has been tested using both simple Newtonian liquids and viscoelastic, complex fluids. The possibility to monitor liquid-gel transitions in polymer solutions has also been demonstrated. The unique capability of the technique to localize the measurement volume suggests that this novel approach can be used for three-dimensional mapping of rheological properties in heterogeneous systems.

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PubMed2001

Signal velocity, causality, and quantum noise in superluminal light pulse propagation.

We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.

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PubMed2000

Gain-assisted superluminal light propagation

Einstein's theory of special relativity and the principle of causality imply that the speed of any moving object cannot exceed that of light in a vacuum (c). Nevertheless, there exist various proposals for observing faster-than-c propagation of light pulses, using anomalous dispersion near an absorption line, nonlinear and linear gain lines, or tunnelling barriers. However, in all previous experimental demonstrations, the light pulses experienced either very large absorption or severe reshaping, resulting in controversies over the interpretation. Here we use gain-assisted linear anomalous dispersion to demonstrate superluminal light propagation in atomic caesium gas. The group velocity of a laser pulse in this region exceeds c and can even become negative, while the shape of the pulse is preserved. We measure a group-velocity index of n(g) = -310(+/- 5); in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. The observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components in an anomalous dispersion region.

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PubMed2000

Evidence of scattering anisotropy effects on boundary conditions of the diffusion equation

We present experimental evidence that the diffusion process close to the boundary is fundamentally different from the bulk diffusion in the sense that it is affected by the scattering properties of the individual centers. Systematic measurements show a nontrivial behavior of the extrapolation length ratio on the scattering anisotropy of individual particles. The results are compared with other works and are supported by an independent Monte Carlo simulation. A simple analytical model that emphasizes the influence of the last scattering event successfully describes the results.

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PubMedDate not supplied

The influence of drying on quantitative laser fluorescence and optical pathlengths in incipient natural caries lesions.

Drying effects in 14 natural lesions were studied with quantitative light-induced fluorescence and optical pathlength spectroscopy. Results were compared with clinical judgments of the lesion surface and microradiographical characterizations of the lesions. Relative fluorescence and average pathlength decreased as a function of drying time with a decay time ranging from 35.5 to <1 min. Depth and mineral loss correlated with average pathlength total changes (r = -0.79/-0.60, respectively) and poorly with total fluorescence changes (r approximately = 0.3). The decay time of the drying process for the relative fluorescence correlated well with a theoretical model based on water diffusion in lesion and surface layer, but only for large decay times. Clinical judgments could not be related to the surface layer properties or the changes in the average pathlength, but were weakly related to the changes in the relative fluorescence. We conclude that (i) fluorescence effects are mostly due to the screening by the lesion of the fluorescence from the dentin and enamel-dentin junction; (ii) water evaporation in lesions conforms to the diffusion laws only in large lesions with low surface layer penetrability; (iii) the evaporation process is controlled by the surface layer only for small surface penetrabilities (approximately 0.1 vol% microm(-1)).

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