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Excitable particles in an optical torque wrench


The optical torque wrench is a laser trapping technique capable of applying and directly measuring torque on microscopic birefringent particles using spin momentum transfer, and has found application in the measurement of static torsional properties of biological molecules such as single DNAs. Motivated by the potential of the optical torque wrench to access the fast rotational dynamics of biological systems, a result of its all-optical manipulation and detection, we focus on the angular dynamics of the trapped birefringent particle, demonstrating its excitability in the vicinity of a critical point. This links the optical torque wrench to nonlinear dynamical systems such as neuronal and cardiovascular tissues, nonlinear optics and chemical reactions, all of which display an excitable binary (‘all-or-none’) response to input perturbations. On the basis of this dynamical feature, we devise and implement a conceptually new sensing technique capable of detecting single perturbation events with high signal-to-noise ratio and continuously adjustable sensitivity.

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Figure 1: Measurements of torque on a birefringent particle.
Figure 2: The potential experienced by the birefringent particle, and corresponding phase plots.
Figure 3: Demonstration of the excitability of a rotating birefringent cylinder in an optical torque wrench.
Figure 4: Probability distribution for the interspike time of events triggered by thermal noise.
Figure 5: Sensing through excitability.

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We thank A. La Porta for graciously sharing key aspects of the construction of OTW, M. Wang for discussion of polarization, E. van der Drift, M. Zuiddam, A. van Run and M. van der Krogt of the Delft Nanofacility and J. Andreasson of Stanford University for advice on the microfabrication of quartz cylinders, M. Wiggin for help in fabricating them, J. van der Does, D. de Roos and J. Beekman for help with instrumentation and infrastructure, J. Kerssemakers, I. de Vlaminck, M. van Loenhout, S. Klijnhout and J. Lipfert for stimulating discussions, and the TU Delft, FOM (Dutch Foundation for Research on Matter), NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) and the European Science Foundation for financial support.

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F.P. and Z.H. contributed equally to this work. F.P., S.B. and N.H.D. conceived and designed the experiments. F.P., M.v.O. carried out the experiments. Z.H. developed the nanofabrication protocols. F.P., Z.H., S.B. and M.v.O. analysed the data. Z.H. and S.B. contributed materials and analysis tools. F.P., Z.H., S.B. and N.H.D. wrote the paper.

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Correspondence to Nynke H. Dekker.

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The authors declare no competing financial interests.

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Pedaci, F., Huang, Z., van Oene, M. et al. Excitable particles in an optical torque wrench. Nature Phys 7, 259–264 (2011).

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