Friedrich-Alexander-Universität Erlangen-Nürnberg

Doppler measurement of particle guided in hollow-core photonic crystal fibre

Doppler Measurement of Particle Guided in Hollow-core Photonic Crystal Fibre

In the 70’s Ashkin proposed to use radiation pressure to control micro-particles, opening the possibility to trap or accelerate particles [1]. In the early 90’s, Kawata and Sugiura used the evanescent field of a laser at a surface to accelerate polystyrene and glass spheres [2]. By contrast to this pioneering work, where the trapping field was highly asymmetric, hollow-core photonic crystal fibres (HC-PCF), where the light is highly confined in guided mode, offer a two-dimension trap where the radiation pressure can propel the particle along the third dimension. In many fields, especially cell biology, manipulation and measurement of microscopic objets can be extremely important.

In one experiment, we showed that radiation pressure used to propel a micrometer-sized particle in a liquid-filled HC-PCF (Fig.1) could be balanced by the fluidic counterflow of D2O. This allows a precise measurement of the microfluidic drag force on the microparticle [3,4].

Fig. 3: Loading, launching, and guidance of a particle of 6 µm diameter – fig. from ref. [3]

The speed of the guided particle moving inside the core of the HC-PCF can be measured by Doppler velocimetry [5]. In this scheme a small fraction of light is back-scattered by the moving particle at a Doppler-shiftedfrequency. Mixed with the unshifted light, reflected at the input of the fibre, this results in characteristic beating, which can be recorded by a photo-diode. The speed of the particle can be extracted from the Fourier transform of the recorded signal (Fig. 2). The observation of periodic variations in the particle velocity could be associated with the presence of two beating low-order guided modes. This scheme also allows analysing two particles simultaneously propagating inside the HC-PCF.

Fig. 4: (b) Schematic of the setup: BS, beam splitter, PD, photodiode. (c) Intensity beating detected by PD for a particle with R = 3.25µm, moving at Vp = 240 µm/s. One period corresponds to a displacement of 400nm. Fig. from ref. [4]

The trapping environment could be extended from liquid to gaseous media [6], which is the first step to particle guidance in evacuated fibres.


[1]     A. Ashkin, “Acceleration and trapping of particles by radiation pressure,” Phys. Rev. Lett. 24, 156–159 (1970)

[2]     Satoshi Kawata and Tadao Sugiura, "Movement of micrometer-sized particles in the evanescent field of a laser beam," Opt. Lett. 17, 772-774 (1992)

[3]     T. G. Euser, M. K. Garbos, J. S. Y. Chen, and P. St.J. Russell, "Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber," Opt. Lett. 34, 3674-3676 (2009)

[4]     M. K. Garbos, T. G. Euser, and P. St.J. Russell, "Optofluidic immobility of particles trapped in liquid-filled hollow-core photonic crystal fiber," Opt. Express19, 19643-19652 (2011)

[5]     M. K. Garbos, T. G. Euser, O. A. Schmidt, S. Unterkofler, and P. St.J. Russell, "Doppler velocimetry on microparticles trapped and propelled by laser light in liquid-filled photonic crystal fiber," Opt. Lett. 36, 2020-2022 (2011)

[6]     O. A. Schmidt, M. K. Garbos, T. G. Euser, and P. St.J. Russell, "Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber," Opt. Lett. (accepted, 2011)


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