Laser Doppler velocimetry is used to measure very accurately the velocity and position of a microparticle propelled and guided by laser light in liquid-filled photonic crystal fiber. Periodic variations in particle velocity are observed that correlate closely with modal beating between the two lowest order guided fiber modes.

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells.

Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm·s(-1) W(-1) (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles.

Since 1908, when Mie reported analytical expressions for the fields scattered by a spherical particle upon incidence of an electromagnetic plane-wave, generalizing his analysis to the case of an arbitrary incident wave has proved elusive. This is due to the presence of certain radially-dependent terms in the equation for the beam-shape coefficients of the expansion of the electromagnetic fields in terms of vector spherical wave functions. Here we show for the first time how these terms can be canceled out, allowing analytical expressions for the beam shape coefficients to be found for a completely arbitrary incident field. We give several examples of how this new method, which is well suited to numerical calculation, can be used. Analytical expressions are found for Bessel beams and the modes of rectangular and cylindrical metallic waveguides. The results are highly relevant for speeding up calculation of the radiation forces acting on spherical particles placed in an arbitrary electromagnetic field, such as in optical tweezers.PACS numbers: 03.50. De,42.25.Bs Gustav Mie, in his celebrated 1908 paper [1], used the vector spherical wave function (VSWF), or partial wave expansion (PWE), of a linear polarized plane-wave to generalize scattering theories to spherical particles of any size, from geometrical optics to the Rayleigh regime, and thus was able to clarify many phenomena, for example in atmospheric physics. He obtained analytical expressions for the expansion coefficients based on special mathematical identities related to a plane-wave. This beam expansion was necessary for applying boundary conditions at a spherical interface. Since then, with the arrival of lasers and optical waveguides, the diversity and complexity of possible incident fields has become enormous so that the restriction to an incident plane-wave has become unrealistic.Different experiments, ranging from particle levitation and trapping [2,3], to the ultrahigh-Q microcavities used in cavity QED experiments [4,5], use different beams. For example, very high numerical aperture beams are used in optical tweezers and confocal microscopy [6-8], evanescent fields in near-field microscopy [9,10], and the waveguide modes of a fiber taper are employed to couple light to the whispering gallery modes of spherical microcavities [11]. Optical forces, absorption, Raman scattering and fluorescence can be greatly enhanced inside spherical microcavities at Mie resonances [12][13][14][15]. Laguerre-Gaussian, Hermite-Gaussian and Bessel beams [16,17], and the internal electromagnetic field of hollow core photonic crystal fibers [18,19], are used to trap and transport particles. The understanding of all these phenomena requires a precise knowledge of the VSWF coefficients of the incident beams. A generalized Lorenz-Mie theory was developed to handle the many variants of beams beyond classical planewaves, and the expansion coefficients in these cases are known as beam shape coefficients (BSC) [20,21]. Moreover, because the VSWFs form an orthogonal complete ...

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. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)