Joint Colloquium Distinguished Lecture Series

Light in Scattering Media: From Finding Cancer to Building a Better Lens

Wednesday, November 1st 306 Soda Hall 4:00 - 5:00 pm Kevin Webb Purdue University

Abstract:

Seeing through random scattering media ultimately involves some form of characterization of the influence of the scatter. This characterization can form the basis of images that have importance in soft tissue imaging, among other areas. One measure is the ensemble averaged temporal impulse response of the medium. I show how this temporal response can be obtained from third order intensity correlations in frequency of speckle patterns, under the assumption of circular Gaussian fields. With significant scatter in a random medium, light becomes completely depolarized, and a diffusion model can be applied. However, with lesser scatter, information is retained in the polarization, which leads to the merits of polarization-dependent impulse responses. Assuming a diffusion equation model, I describe an optimization approach for obtaining images of absorption and scatter from modulated light measurements. I also present an expansion of this representation to two wavelengths, which allows fluorescence imaging, thereby enhancing the contrast and also providing for fluorescent labeling with the help of cancer targeting agents.

While all naturally occurring materials have a positive refractive index, by synthesizing a suitable metamaterial and operating it beyond the electric and magnetic dipole resonance, it's possible to achieve a negative refractive index, with concomitant negative refraction. Perhaps the most interesting consequence of a negative index is the possibility of evanescent field growth and circumventing the wavelength limit for image resolution. The potential technological impact is as diverse as memory density, lithography, and microscopy. A passive causal resonant system, as dictated by the Kramers-Kronig relations, should have some degree of loss, and I present the consequence of perturbational loss on the possible resolution. Interestingly, with such imperfect lenses, we find vortices, which themselves may be useful. A relatively simple way to achieve subwavelength resolution, using the appropriate polarization, is with a multilayer metal-insulator system, and I present some preliminary results. While these metamaterials are periodic, they don't need to be, and I generalize with the concept of irregular field transformation structures, which offer some remarkable features for optical signal processing, and for sources and detectors.

Finally, I show how fields between metal surfaces can be resonantly enhanced, thereby providing a new mechanism for surface enhanced Raman scattering and also for nanoparticle waveguides. These structures should be important in spectroscopy and integrated optics.

Biography:

Kevin Webb is a Professor in the ECE Department of Purdue University. Hi research focuses on nanophotonics, electromagnetics, optical imaging in scattering media and semiconductor devices.

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