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Resonant optical tweezers in photonic crystal cavities

Posted March 21, 2013


19.03.2013 – EPFL scientists have achieved optical trapping in a photonic crystal cavity embedded in a microfluidics environment.

These new type of optical tweezers were used to trap sub-micrometre particle sized with unprecedented low power. The trapping phenomenon is also accompanied with cavity back-action effects, which result from the mutual interaction between the position of the trapped particle and the cavity field.

Optical tweezers were invented in the 1980s and since then they have become a wide-spread technique to manipulate particles ranging from a few hundred nanometres to a few micrometers. The technique makes use of the mechanical forces generated by light on a small object and usually a tightly focused laser beam is used. Near the focus, small dielectric objects experience strong mechanical forces deriving from the large local field gradients. Nevertheless, optical tweezers have proven to be of limited use for the optical trapping of dielectric particles smaller than 100 – 200 nm in diameter because as the particle size diminishes, the requested power to generate a strong enough optical gradient force dramatically increases. On a practical aspect, optical manipulation often requires bulky and complex optical systems and there is only a weak, if any, selectivity of the trapped particle with respect of its, shape, size or refractive index.

A large variety of schemes have been proposed in order to overcome these limitations. Most of them make use of the large field gradients existing in the vicinity of light guiding structure like integrated waveguides, optical cavities, dielectric ridge waveguides or plasmonic structures. These approaches are limited by the large amount of optical power required and allow particle confinement only in one or two directions.

Read more at: EPFL News

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