Splitting light in a chiral medium

A “light-beam” will split into two at the boundary of a liquid if the liquid contains chiral molecules. This effect was predicted by Fresnel in 1822. In 2006 we succeeded in imaging this small effect (see Figure).

Our work has appeared in a number of news reports.

The significance of this optical effect is that it permits detection of chiral molecules in nanoliters, rather than the large sample-volumes that are currently needed in laboratory polarimeters. For the last 200 years, chiral molecules have been detected in polarimeters by observing the rotation of a plane polarized light beam that traverses an “optically active” liquid. This requires long path-lengths and large sample-volumes, whereas “light-splitting” occurs within a micron at an interface and can be observed in minute volumes [1].
The effect may not only be observed in refraction, as predicted by Fresnel, but also in reflection [1] and in diffraction [2].

Figure: Intensity plots of a laser with Gaussian beam profile. The images correspond to a passage through 8, 12, 16, and 20 chiral interfaces and show the gradual chirality-induced splitting into two circularly polarized beams. The last image shows the light split into its two cp components.

The angle between the refracted/reflected beams is at best ~microradians for neat liquids. Typically, one needs to detect small concentrations of chiral molecules, and so detection of deflections down to a few nanoradians is needed. Using position sensitive detectors (PSD) and polarization modulators we can routinely measure angular deflections of around 50 nanoradians. To increase the sensitivity down to a few nanoradians, we are  borrowing ideas from quantum measurements. Using so called “weak measurements” it is possible to detect Angstrom light-deflections.

[1] A. Ghosh and P. Fischer, “Chiral Molecules Split Light: Reflection and Refraction in a Chiral Liquid”, Phys. Rev. Lett., 97, (2006), 173002. [pdf]

[2] A. Ghosh, F.M. Fazal, and P. Fischer, “Circular differential double diffraction in chiral media”, Opt. Lett., 32, (2007), 1836. [pdf]

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