Resonant optical tweezers in photonic crystal cavities

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.

Recently Prof. R. Houdré's group presented the first experimental demonstration of an integrated hollow optical cavity, which selectively traps sub-wavelength particles with unprecedented low optical power of the order of 100 μW. The optical nanocavities are designed within so-called photonic crystal structures, which are a recently invented nanostructured photonic material. The optical traps are then implemented in an optofluidic chip. They have also shown that the dynamic perturbation of the cavity mode by the trapped particle generates a rapidly varying resonance frequency shift associated with the constrained Brownian motion of the particle within the resonant trap. One of the major consequences is the appearance of back-action effects due to the mutual interaction between the resonantly exited optical field and the particle movement. In particular, they have demonstrated the existence of two distinct trapping regimes. While one of these regimes is comparable in nature to a classical optical tweezers, the second is very unique as the optical field generating the restoring force is only present when the particle is moving toward the outside of the trap and vanishes when the particle is well located within the centre of the trap.

Future prospects include the use of microfluidic integrated hollow photonic crystal cavities for single particle sorting and trapping in a well controlled orientation, analysis, advanced spectroscopy, the investigation and manipulation of biological microorganisms such as small bacteria, cell organelles and viruses. These results constitute a first step towards the manipulation of single particles of the size of a virus on a CMOS-compatible chip using ultralow optical powers. This could potentially lead to new generations of portable Lab-on-a-Chip devices for Point-of-Care diagnosis.