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Single Molecules as Optical Probes for Structure and Dynamics

Michel Orrit

Molecular Nano-Optics and Spins, Leiden Institute of Physics, Leiden University, The Netherlands

3:30 pm Thursday, 2 July 2009, EN101 (Ground Floor, EN Building), Hawthorn.

As compared to electron microscopy and to scanning probe microscopies, the optical selection of individual molecules or nanoparticles in a far-field microscope has several advantages. Laser excitation often is non-invasive, can reach much beyond surface layers, and commands a wide range of time-resolved and frequency-resolved spectroscopic techniques. Optical signals provide unique insights into the dynamics of nano-objects. I shall illustrate the applications of single-molecule optics to dynamics with recent topics from our group.
iWe studied the time-resolved absorption of single gold nanoparticles by interferometry, and detected acoustic oscillations launched by an intense pump pulse. This opens the field of nano-acoustics, which combines picosecond temporal resolution with nanometer spatial resolution.

i We probed the approach of the glass transition in a supercooled molecular liquid by following the rotational diffusion of single fluorescent molecules . We found large differences in local viscosity, with exceedingly long memory times, days or longer. We associate this heterogeneity of the supercooled liquid to a solid-like structure, present already 30 K above the glass transition temperature. Macroscopic rheology experiments confirm that the solid-like structure confers many of the well-known attributes of soft glassy rheology (yield-stress, shear-thinning, aging,...) to supercooled glycerol. The shear modulus of the material changes by more than two orders of magnitude upon two-week aging at 205 K.

Experiments on single nano-objects demonstrate the power of local observations at nanometer scales. They reveal previously unobserved and unsuspected properties, throwing new light even onto such long-standing questions as the glass transition. Beside molecular cell biology, single molecules can be very useful in soft matter, chemical physics, and material science.

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