![]() ![]() ![]() In a 1D particle array, such as the one studied here, binding is possible because the optical fields propagate bidirectionally along the array. In these experiments, there is typically very-little multiple scattering between particles-a necessary prerequisite for optical binding 19, 20, 21, 22, which can only occur if the scattered field from one particle strongly interacts with the other particles in the array, and vice-versa. Multiple trapping sites have previously been created using interference 17 and holographic tweezers 18, allowing the formation of a lattice of trapped microparticles. ![]() Optical binding between arrays of trapped particles adds an additional dimension, and will result in rich dynamics, enabling access to collective coupling between high-Q mechanical oscillators and, potentially, simultaneous cooling of the mechanical motion of multiple particles. Recent advances include the use of feedback or cavity cooling of the centre-of-mass temperature towards the mechanical ground state 6, 7, 8, 9, 10, 11, permitting experimental tests of fundamental physics 12, 13, 14, and the use of individual levitated particles as point sensors 15, 16. This leads to very-high mechanical Q-factors and permits particle rotational speeds in the MHz range 4, 5. An optically tweezered particle, especially at low gas pressure, is isolated from the external environment, resulting in very low mechanical damping. In recent years, the emerging field of “levitated optomechanics” has attracted increasing interest. Since Ashkin’s first report of the acceleration and trapping of microparticles by optical forces 1, the use of optical tweezers has developed into a standard technique for biological manipulation and pico-Newton force sensing 2, 3, to mention just two examples from a wide range of applications. The HC-PCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a well-isolated and protected environment. The measured inter-particle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process. When evacuated to a gas pressure of 6 mbar, the collective mechanical modes of the bound-particle array are able to be observed. The levitated bound-particle array can be translated to-and-fro over centimetre distances along the fibre. Three polystyrene particles with a diameter of 1 µm are stably bound together with an inter-particle distance of ~40 μm, or 50 times longer than the wavelength of the trapping laser. Here we report the observation of long-range optical binding of multiple levitated microparticles, mediated by intermodal scattering and interference inside the evacuated core of a hollow-core photonic crystal fibre (HC-PCF). Optically levitated micro- and nanoparticles offer an ideal playground for investigating photon–phonon interactions over macroscopic distances. ![]()
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