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Evanescent-wave and ambient chiral sensing by signal-reversing cavity ringdown polarimetry


Detecting and quantifying chirality is important in fields ranging from analytical and biological chemistry to pharmacology1 and fundamental physics2: it can aid drug design and synthesis, contribute to protein structure determination, and help detect parity violation of the weak force. Recent developments employ microwaves3, femtosecond pulses4, superchiral light5 or photoionization6 to determine chirality, yet the most widely used methods remain the traditional methods of measuring circular dichroism and optical rotation. However, these signals are typically very weak against larger time-dependent backgrounds7. Cavity-enhanced optical methods can be used to amplify weak signals by passing them repeatedly through an optical cavity, and two-mirror cavities achieving up to 105 cavity passes have enabled absorption and birefringence measurements with record sensitivities8,9,10. But chiral signals cancel when passing back and forth through a cavity, while the ubiquitous spurious linear birefringence background is enhanced. Even when intracavity optics overcome these problems11,12,13,14,15, absolute chirality measurements remain difficult and sometimes impossible. Here we use a pulsed-laser bowtie cavity ringdown polarimeter with counter-propagating beams16,17 to enhance chiral signals by a factor equal to the number of cavity passes (typically >103); to suppress the effects of linear birefringence by means of a large induced intracavity Faraday rotation; and to effect rapid signal reversals by reversing the Faraday rotation and subtracting signals from the counter-propagating beams. These features allow absolute chiral signal measurements in environments where background subtraction is not feasible: we determine optical rotation from α-pinene vapour in open air, and from maltodextrin and fructose solutions in the evanescent wave produced by total internal reflection at a prism surface. The limits of the present polarimeter, when using a continuous-wave laser locked to a stable, high-finesse cavity, should match the sensitivity of linear birefringence measurements8 (3 × 10−13 radians), which is several orders of magnitude more sensitive than current chiral detection limits7,14,15 and is expected to transform chiral sensing in many fields.

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Figure 1: Cavity-enhanced polarimeter for chiral sensing.
Figure 2: Gas-cell optical rotation.
Figure 3: Open-air optical rotation.
Figure 4: Evanescent-wave optical rotation.


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This research was supported by the ERC grant TRICEPS (grant no. 207542), and the FP7 IAPP Programme SOFORT (PIAPGA-2009-251598). B.L. acknowledges the FP7 Infrastructure programme ESMI (CP&CSA-2010-262348) for partial support. We thank P. Tzallas for access to the Attosecond labs at IESL-FORTH.

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Authors and Affiliations



L.B. constructed the experiment, performed the gas-cell and open-air experiments, and analysed the data. D.S. performed the evanescent-wave experiments and analysed the data. G.E.K. and A.K.S. developed the data acquisition and analysis software, and assisted in the experiments. G.E.K. prepared the figures. B.L. derived the evanescent-wave optical rotation equations. T.P.R. had the idea for and directed the experiments, and wrote the manuscript. All authors provided important suggestions for the experiments, discussed the results and contributed to the manuscript.

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Correspondence to T. Peter Rakitzis.

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The authors declare no competing financial interests.

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Sofikitis, D., Bougas, L., Katsoprinakis, G. et al. Evanescent-wave and ambient chiral sensing by signal-reversing cavity ringdown polarimetry. Nature 514, 76–79 (2014).

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