## Light induced collective dynamics and long-range interactions between nanoparticles

Juan_José Sáenz
juanjo.saenz@dipc.org
Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018-Donostia-San Sebastián, Spain

We review a number of intriguing predictions regarding the dynamics of plasmonic nanoparticles under crossed laser fields [1]. As a recent example, we will discuss the self-organized collective behavior of gold nanoparticles moving in aqueous solution under a non-conservative optical vortex lattice. As we will see, above a critical field intensity and concentration, the interplay between optical forces, thermal fluctuations and hydrodynamic pairing leads to a spontaneous transition towards synchronized motion [2].

Light induced forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. It has been recently shown that non-equilibrium, quasi-monochromatic, random fluctuating light fields can be used to induce and control isotropic, translational invariant, dispersion forces between small colloidal particles [3]. Interestingly, when the light frequency of a quasi-monochromatic isotropic random field is tuned to an absorption line (at the so-called Fröhlich resonance) we will see that the attractive force between two identical molecules or resonant nanoparticles follows a gravity-like inverse square distance law [4]. Our results generalize Lorentz’s [5] (and Spitzer-Gamow’s “Mock Gravity” [6]) electromagnetic version of the remarkable Fatio-LeSage’s corpuscular theory of gravity introduced as early as in 1690.

[1] Albaladejo, S., et al., Nano letters, 9, 3527 (2009); Zapata I, et al., Phys. Rev. E 93, 062130 (2016); Luis-Hita J. et al., ACS Photonics, 3, pp.1286 (2016); Meléndez. M. et al., Phys. Rev. E 99, 022603 (2019).

[2] Delgado-Buscalioni, R. et al., Phys. Rev. E 00, 002600 (2018)

[3] Brügger, G. et al., Nat. Commun., 6, 7460 (2015)

[4] Luis-Hita, J. et al., arXiv:1802.05648 (2018)

[5] Lorentz, H.A., Lectures on Theoretical Physics. (1927)

[6] Gamow, G., Rev. Mod. Phys., 21, 367 (1949)