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Effects of molecular asymmetry of optically active molecules on the polarization properties of multiply scattered light

I. Alex Vitkin, Richard D. Laszlo, and Claire L. Whyman

Open Access Open Access

Abstract

The use of polarized light for investigation of optically turbid systems has generated much recent interest since it has been shown that multiple scattering does not fully scramble the incident polarization states. It is possible under some conditions to measure polarization signals in diffusely scattered light, and use this information to characterize the structure or composition of the turbid medium. Furthermore, the idea of quantitative detection of optically active (chiral) molecules contained in such a system is attractive, particularly in clinical medicine where it may contribute to the development of a non-invasive method of glucose sensing in diabetic patients. This study uses polarization modulation and synchronous detection in the perpendicular and in the exact backscattering orientations to detect scattered light from liquid turbid samples containing varying amounts of L and D (left and right) isomeric forms of a chiral sugar. Polarization preservation increased with chiral concentrations in both orientations. In the perpendicular orientation, the optical rotation of the linearly polarized fraction also increased with the concentration of chiral solute, but in different directions for the two isomeric forms. There was no observed optical rotation in the exact backscattering geometry for either isomer. The presence of the chiral species is thus manifest in both detection directions, but the sense of the chiral asymmetry is not resolvable in retro-reflection. The experiments show that useful information may be extracted from turbid chiral samples using polarized light.

©2002 Optical Society of America

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Figures (4)
Figure 1:
Figure 1: Schematic of the polarization measurement apparatus, showing both perpendicular (90°) and exact backscattering (180°) detection geometries. P’s = linear polarizers; A’s = apertures; PEM = photoelastic modulator; PMT = photomultiplier tube; TIA = transimpedance amplifier; CD = chopper driver (135 Hz); PD = PEM driver (50 kHz).

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Figure 2:
Figure 2: (a) Typical data set and analytical fit from Equation (1) for a sample containing 0.38M L arabinose with 0.21% microspheres in the perpendicular orientation. Points represent scaled experimental data ratios while the smooth curve is the theoretical fit, described by α=0.53±0.18° and β=(33.3±0.2)%, with a goodness of fit R2=0.99941. (b) Typical data set and analytical fit from Equation (2) for a sample containing 0.52M L arabinose with 0.21% microspheres in the backscattering orientation. The fit is optimal for α=-0.12 ±0.60° and β=(15.9±0.4)%, with R2=0.9922.

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Figure 3:
Figure 3: (a) Induced optical rotation as a function of concentration of L and D forms of arabinose in solutions containing 0.21% microspheres, measured in the perpendicular direction. Error bars represent regression-fit uncertainties in derived parameters. Selected reproducibility results from repeated measurements are also displayed. (b) Degree of polarization as a function of concentration of L and D forms of arabinose, derived from the same experimental sets as the results of 3(a).

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Figure 4:
Figure 4: (a) Mean optical rotation as a function of concentration of arabinose in solutions containing 0.21% microspheres in the backscattering orientation. The points are the means of the four separately derived α‘s (two from L and two from D measurements), and the error bars represent the standard deviations of the means. (b) Corresponding mean backscattering results for the degree of polarization. Meaning of symbols and error bars is the same as in 4(a).

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Equations (5)

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S 2 f S dc ( ⊥ ) = 2 β J 2 ( δ o ) sin 2 ( α + θ ) 1 + β J o ( δ o ) sin ( α + θ )
S 2 f S dc ( ⇔ ) = 2 J 2 ( δ o ) A J o ( δ o ) A + B + C
A = 1.82 β [ 0.58 cos ( 2 α ) sin ( 2 ( θ + 90 ) ) − sin ( 2 α ) cos ( 2 ( θ + 90 ) ) − 0.82 sin ( 2 α ) ]
B = 1.82 [ 1 + 0.82 cos ( 2 ( θ + 90 ) ) ]
C = − 0.176 β [ { 0.815 + cos ( 2 α ) cos ( 2 ( θ + 90 ) ) } − 0.102 sin ( 2 α ) sin ( 2 ( θ + 90 ) ) ]
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