Detection of biological particles by the use of circular dichroism measurements improved by scattering theory

Abstract

Light scattered from optically active spheres was theoretically analyzed for biodetection. The circularly polarized signal of near-forward scattering from circularly dichroic spheres was calculated. Both remote and point biodetection were considered. The analysis included the effect of a circular aperture and beam block at the detector. If the incident light is linearly polarized, a false signal would limit the sensitivity of the biodetector. If the incident light is randomly polarized, shot noise would limit the sensitivity. Suggested improvements to current techniques include a beam block, precise angular measurements, randomly polarized light, index-matching fluid, and larger apertures for large particles.

Light Scattering from an Optically Active Sphere into a Circular Sensor Aperture

“…In earlier work [11], two of us (Rosen and Pendleton) applied Bohren's scattering theory in the first simulation of scattering from optically active spheres into sensor apertures. The experimental configurations considered in that work [11] are characterized by the presence of a sensor with a circular aperture.…”

“…The experimental configurations considered in that work [11] are characterized by the presence of a sensor with a circular aperture. The assumption is that all light scattered into the solid angle intercepted by the circular aperture is focused onto the sensor.…”

“…This means that we obtain realistic simulations of the measured signals not by computing the Stokes parameters for a single direction but by obtaining the average of each Stokes parameter over the aperture solid angle. Our earlier paper [11] very briefly outlined the theory but emphasized a discussion of computational results, omitting any discussion of the algorithm used for the computations. To provide a better basis for future computations of this type, we discuss the theory of two relevant algorithms here.…”

“…The earlier computations [11] were performed with a numerical integration over the aperture solid angle. We developed this method by adapting a twodimensional Legendre-Gaussian numerical integration algorithm originally designed by Pierce [12] for integration over a circle.…”

“…A succinct description of the method is given in section 3. A computer program based on this method was developed and run on a personal computer, giving the computational results reported earlier [11]. This numerical integration method is computationally simple, but the results must be checked for convergence; i.e., the computation must be repeated with an increasing number of points in the computational grid until convergence is obtained.…”