Non-conservative instabilities in optical vacuum traps

0532398 - ÚPT 2021 RIV US eng C - Conference Paper (international conference)
Svak, Vojtěch - Arita, Y. - Simpson, Stephen Hugh - Brzobohatý, Oto - Šiler, Martin - Jákl, Petr - Kaňka, Jan - Zemánek, Pavel - Dholakia, K.
Non-conservative instabilities in optical vacuum traps.
Proceedings of SPIE. In: Complex Light and Optical Forces XIV. Proceedings of SPIE. Vol. 11297. Bellingham: SPIE, 2020, č. článku 112970F. ISBN 978-1-5106-3358-2. ISSN 0277-786X. E-ISSN 1996-756X.
[Conference on Complex Light and Optical Forces /14./. San Francisco (US), 04.02.2020-05.02.2020]
R&D Projects: GA ČR(CZ) GA19-17765S
Institutional support: RVO:68081731
Keywords : optical force * spin * birefringence * instability
OECD category: Optics (including laser optics and quantum optics)

Particles held in optical tweezers are commonly thought to be at thermodynamic equilibrium with their environment. Under this assumption the elastic energy of the trap is equal to the thermal energy. As a result the variance of the particle position is completely independent of viscosity and inversely proportional to the optical power in the trap. Here we show that these conditions only hold for very high symmetry cases e.g. perfectly spherical particles in unaberrated, linearly polarized Gaussian traps. Here we show that any reduction in symmetry leads to asymmetrically coupled degrees of freedom. The associated force field is linearly non-conservative and the tweezer is no longer at equilibrium. In overdamped systems the effect is a underlying systematic bias to the Brownian motion. In underdamped systems, this systematic component can accumulate momentum, eventually destabilizing the trap. We illustrate this latter effect with reference to two systems, (i) an isotropic sphere in a circularly polarized trap, and (ii) a birefringent sphere in a linearly polarized trap. In both cases the instability can be approached either by decreasing air pressure or by increasing optical power. Close to instability, the trapped particle executes increasingly coherent motion that is highly sensitive to external perturbations. Potential applications to weak force sensing are discussed.
Permanent Link: http://hdl.handle.net/11104/0310902