Impulsively-Driven Motion of 100 nm Particles in Hollow Core Photonic Crystal Fibre at Atmospheric Pressure

Hollow-core photonic crystal fibre (HC-PCF) provides a convenient means of optically trapping and transporting dielectric particles [1] and even cold atoms [2] over long distances. The particles are trapped by gradient forces in the transverse plane, and in the axial direction strong gradient traps can be formed, for sub <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-\lambda/2$</tex> particles, at field maxima in the interference pattern between counter-propagating beams. Introducing a frequency difference causes the fringes to move, creating a particle conveyor belt at low air pressure [3], [4]. Here we study this phenomenon at atmospheric pressure, when viscous drag is strong. Light from a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda=1064$</tex> nm CW Nd:YAG laser is launched into both ends of a single-ring HC-PCF (core diameter 44 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{m}$</tex>), resulting in a fringe spacing of 532 nm, which is large enough to create axial traps for the 100 nm silica particles used in the experiment. One of the trapping beams is phase-modulated (using a Pockels cell) with a sawtooth waveform of amplitude <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2\pi$</tex> and frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\mathrm{S}}$</tex>, resulting in an effective fringe velocity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\nu_{f}=f_{S}\lambda/2$</tex>. The particle velocity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$v_{p} &lt; v_{f}$</tex> oscillates as it is momentarily captured and accelerated by each passing fringe, followed by release and deceleration by viscous forces (Fig. 1(a)).