When a ray of light is refracted by a particle, part of its momentum is transferred to the particle, resulting in a force [Ashkin (1986)]. For spherical dielectric particles near the focus of a laser beam, the resulting gradient force pushes the particle towards the high intensity central part of the beam. This gradient force is exploited in optical tweezers, which has become a widely used tool in biophysics. We are carrying out detailed studies of optical trapping and propulsion using the well-defined optical guided mode in a hollow-core photonic crystal fibres. In this case the lateral trapping and propulsive forces are constant along the entire length of the fibre, provided the optical losses are sufficiently small. Our goal is study opto-mechanical interactions between guided optical modes and trapped particles, and to explore novel applications.
A fundamental limitation of conventional optical tweezers is that a strong trapping force is only present near the focus of a laser beam. This can be overcome by making use of diffractionless single-mode propagation in the hollow core of a photonic crystal fibre. Particles can remain trapped near the centre of the PCF core by the gradient force, while undergoing continuous acceleration in the axial direction due to the scattering force (see figure). The particles reach a terminal velocity that is limited by the drag force. In evacuated fibres, however, the practical speed limit will be determined by the lateral trapping potential, which will need to be strong enough to counter-balance the centripetal acceleration due to unavoidable slight bends along the fibre length. Otherwise the particles will crash into the side-walls of the core [Benabid (2002)]. At extremely high velocities, even in a fully evacuated fibre van-der-Waals forces between the particle and the core walls will provide an additional drag force.
Interesting applications arise when we combine single mode guidance in hollow core PCF with microfluidic flow. Figure 3(a) shows a photonic crystal fibre that was designed, following known scaling laws [Birks (2004)], for single-mode guidance at a wavelength of 1064 nm when filled with deuterium oxide. Light propagates in a single optical mode and particles can thus be optically trapped and guided through the fibre core. Using the setup in the figure, micron-sized glass particles are first tweezered and then launched into the core. Once inside the fibre, a particle can be moved to and fro by adjusting the laser power. The low optical attenuation, in combination with the absence of diffraction, allows particles to be moved at will over extended distances (tens of cm). They can also be held stationary against a liquid counter-flow by carefully balancing the drag and the optical forces [Euser (2009)]. The technique offers a unique way of studying drag forces induced by a liquid flow around single particles in a microfluidic channel. The counter-flowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles or cells.
There are many potential applications for this new system. Very small flows (tens of pL/sec) can be counter-balanced at moderate optical powers (tens of mW), allowing for example cells or vesicles to be held motionless while drugs or chemicals flow past in highly controlled quantities [Unterkofler (2013)]. Side-illumination through the transparent cladding would allow photo-activation of, e.g., novel anti-cancer compounds in the liquid, and fluorescence could be monitored either through the cladding or along the guiding core.
We have reported that "flying particles" optically trapped in the core of a HC-PCF can be used as remote multi-sensors [Bykov (2015)]. The figure shows the set-up for an electric field sensor in which a charged particle is optically propelled along the HC-PCF. Any transverse electric field in the vicinity of the fibre will cause the particle to be displaced from core centre, which will changes the light scattered by the particle and thus the transmitted optical signal. With proper calibration, the change in the transmission can be precisely correlated to the amplitude of the electric field, with an experimentally demonstrated spatial resolution of better than 50 µm. The system is of particular interest for use in highly radioactive environments where conventional solid-glass cores rapidly darken due to radiation damage. We are currently working on implementing this technology at the Cooler Synchrotron (COSY) at the Institute of Nuclear Physics in Jülich. The possibility of using particles with different physical properties allows measurement of many different quantities and offers unique flexibility compared to conventional fibre sensors. For example radioluminescent particles could be used to probe irradiation levels inside hostile environments. When the guided particle passes a radioactive region, part of the emitted luminescence will be captured by the fibre and transmitted back to the fibre input for detection.