Polarimetric detection of cached objects and chiral solutes by light scattering in turbid media

Abstract

Photoelastic modulation (PEM) and synchronous detection of laser light scattered from an optically dense turbid medium can reveal the presence and topographical features of embedded targets invisible to naked-eye observation under ambient illumination, as well as provide a quantitative measure of the optical rotation, and therefore the concentration, of chiral constituents dissolved in a turbid fluid. A 544 nm helium-neon probe beam phase-modulated at f = 50 kHz was scanned across the front surface of a scattering cell containing an optically dense suspension of micron-sized polystyrene microspheres and different types of embedded targets. Backscattered light was analyzed for signals at the modulation frequency 1(f) and first harmonic I(2f), which gave nearly instantaneous measures (i.e. approximately over a modulation period T = 1 7 is) of the difference in intensities of orthogonal states of circular and linear polarizations, respectively. Examination of different targets showed sensitivity of polarimetnc imaging to edges, surface texture, and absorption. In another set of experiments the optical rotation and degree ofpolarization ofphase-modulated light was observed by forward, lateral, and back scattering from solutions of the enantiomer D-glucose containing a suspension of polystyrene microspheres. Optical rotations increased linearly with glucose concentration at a rate dependent on the microsphere concentration, and were large even at optical thicknesses sufficiently great to extinguish transmission of the incident beam. Applications of the techniques to remote viewing and biochemical analyses can be envisioned..