Introduction

Light is massless, yet it carries momentum. Therefore, when light is deflected by a particle, there is an equal-and-opposite change in momentum of the particle.

First proposed in 1619 by Johannes Kepler to explain why the tails of comets always point away from the sun, this /radiation pressure/ is very weak, and affects smaller particles disproportionately strongly. As well as reflecting or scattering, a transparent object can /refract/ the light, changing its direction, and this too leads to a force on the particle, known as the optical gradient force. With tightly focused laser light, small transparent objects, including biological cells, can be manipulated in a technique known as /laser tweezers/. Such tweezing is typically in fluid, which can dissipate heat, and in a planar geometry, with the particle moved only transverse to the direction of the light. Particles can now be trapped in /vacuum/, where there is no viscous fluid to disturb the motion, and with very tight focusing, such that the /gradient force/ along the direction of the light can overcome the /scattering force/, which would otherwise push the particle away from the light source. This field has become known as /levitated optomechanics/.

Levitated Optomechanics uses tightly focused laser fields in high vacuum to control the motion of nanoparticles. The control and precision of this technique allows us to cool the particle towards the ground-state of harmonic motion, study non-equilibrium thermodynamics, explore matter-wave interferometry, and perhaps even test new physics, ranging from extensions to quantum mechanics designed to explain the so-called classical-to-quantum transition, to modifications to the r-squared law of gravity on very short length-scales.

Experimental work at Swansea uses fibre optics which provides an extremely stable and versatile platform, allows access to the mature technology of optical telecommunications, and permits geometries that in free-space would be extremely challenging. A large fraction of light scattered by the nanoparticle is collected, which allows one to approach the Standard Quantum Limit; complex interferometric optical paths can be constructed robustly; and kilometre-scale delay lines are feasible. Weak signals can be amplified /optically/, either with standard off-the-shelf Erbium Doped Fibre Amplifiers, with custom-built devices, or using emerging phase-sensitive amplification techniques.

Present research includes exploration of different methods of feedback control. The natural way to control a stochastic degree of freedom, in this case the nanoparticle position, is to measure, process, and adjust some control parameter; this is the basis for feedback control and is used extensively in classical systems.

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