Expansion of arbitrary electromagnetic fields in terms of vector spherical wave functions.

Since 1908, when Mie reported analytical expressions for the fields scattered by a spherical particle upon incidence of plane-waves, generalizing his analysis for the case of an arbitrary incident wave has been an open question because of the cancellation of the prefactor radial spherical Bessel function. This cancellation was obtained before by our own group for a highly focused beam centered in the objective. In this work, however, we show for the first time how these terms can be canceled out for any arbitrary incident field that satisfies Maxwells equations, and obtain analytical expressions for the beam shape coefficients. We show several examples on how to use our method to obtain analytical beam shape coefficients for: Bessel beams, general hollow waveguide modes and specific geometries such as cylindrical and rectangular. Our method uses the vector potential, which shows the interesting characteristic of being gauge invariant. These results are highly relevant for speeding up numerical calculation of light scattering applications such as the radiation forces acting on spherical particles placed in an arbitrary electromagnetic field, as in an optical tweezers system.

Long-distance laser propulsion and deformation- monitoring of cells in optofluidic photonic crystal fiber.

We introduce a unique method for laser-propelling individual cells over distances of 10s of cm through stationary liquid in a microfluidic channel. This is achieved by using liquid-filled hollow-core photonic crystal fiber (HC-PCF). HC-PCF provides low-loss light guidance in a well-defined single mode, resulting in highly uniform optical trapping and propulsive forces in the core which at the same time acts as a microfluidic channel. Cells are trapped laterally at the center of the core, typically several microns away from the glass interface, which eliminates adherence effects and external perturbations. During propagation, the velocity of the cells is conveniently monitored using a non-imaging Doppler velocimetry technique. Dynamic changes in velocity at constant optical powers up to 350 mW indicate stress-induced changes in the shape of the cells, which is confirmed by bright-field microscopy. Our results suggest that HC-PCF will be useful as a new tool for the study of single-cell biomechanics.