Browsing by Author "Decca, R. S."

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    The effects of spherical aberration on multiphoton fluorescence excitation microscopy
    (Wiley Blackwell (Blackwell Publishing), 2011-05) Young, P. A.; Clendenon, S. G.; Byars, J. M.; Decca, R. S.; Dunn, K. W.; Department of Medicine, IU School of Medicine
    Multiphoton fluorescence excitation microscopy is almost invariably conducted with samples whose refractive index differ from that of the objective immersion medium, conditions that cause spherical aberration. Due to the quadratic nature of multiphoton fluorescence excitation, spherical aberration is expected to profoundly affect the depth dependence of fluorescence excitation. In order to determine the effect of refractive index mismatch in multiphoton fluorescence excitation microscopy, we measured signal attenuation, photobleaching rates and resolution degradation with depth in homogeneous samples with minimal light scattering and absorption over a range of refractive indices. These studies demonstrate that signal levels and resolution both rapidly decline with depth into refractive index mismatched samples. Analyses of photobleaching rates indicate that the preponderance of signal attenuation with depth results from decreased rates of fluorescence excitation, even in a system with a descanned emission collection pathway. Similar results were obtained in analyses of fluorescence microspheres embedded in rat kidney tissue, demonstrating that spherical aberration is an important limiting factor in multiphoton fluorescence excitation microscopy of biological samples.
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    Probing the screening of the Casimir interaction with optical tweezers
    (American Physical Society, 2021) Pires, L. B.; Ether, D. S.; Spreng, B.; Araújo, G. R. S.; Decca, R. S.; Dutra, R. S.; Borges, M.; Rosa, F. S. S.; Ingold, G.-L.; Moura, M. J. B.; Frases, S.; Pontes, B.; Nussenzveig, H. M.; Reynaud, S.; Viana, N. B.; Maia Neto, P. A.; Physics, School of Science
    We measure the colloidal interaction between two silica microspheres in an aqueous solution in the distance range from 0.2 to 0.5 μm with the help of optical tweezers. When employing a sample with a low salt concentration, the resulting interaction is dominated by the repulsive double-layer interaction which is fully characterized. The double-layer interaction is suppressed when adding 0.22 M of salt to our sample, thus leading to a purely attractive Casimir signal. When analyzing the experimental data for the potential energy and force, we find good agreement with theoretical results based on the scattering approach. At the distance range probed experimentally, the interaction arises mainly from the unscreened transverse magnetic contribution in the zero-frequency limit, with nonzero Matsubara frequencies providing a negligible contribution. In contrast, such unscreened contribution is not included by the standard theoretical model of the Casimir interaction in electrolyte solutions, in which the zero-frequency term is treated separately as an electrostatic fluctuational effect. As a consequence, the resulting attraction is too weak in this standard model, by approximately one order of magnitude, to explain the experimental data. Overall, our experimental results shed light on the nature of the thermal zero-frequency contribution and indicate that the Casimir attraction across polar liquids has a longer range than previously predicted.