Omnidirectional Transport in Fully Reconfigurable Two Dimensional Optical Ratchets

Abstract

A fully reconfigurable two-dimensional (2D) rocking ratchet system created with holographic optical micromanipulation is presented. We can generate optical potentials with the geometry of any Bravais lattice in 2D and introduce a spatial asymmetry with arbitrary orientation. Nontrivial directed transport of Brownian particles along different directions is demonstrated numerically and experimentally, including on-axis, perpendicular and oblique with respect to an unbiased ac driving. The most important aspect to define the current direction is shown to be the asymmetry and not the driving orientation, and yet we show a system in which the asymmetry orientation of each potential well does not coincide with the transport direction, suggesting an additional symmetry breaking as a result of a coupling with the lattice configuration. Our experimental device, due to its versatility, opens up a new range of possibilities in the study of nonequilibrium dynamics at the microscopic level.PACS numbers: 87.80. Cc, 05.60.Cd, 82.70.Dd Initially motivated by the understanding of biological engines and the design of artificial nanodevices, the emergence of directed transport in the presence of unbiased external forces due to a spatiotemporal symmetry breaking has become a major research topic in different scientific areas [1]. This intriguing phenomenon, known as ratchet effect, lies in the heart of nonequilibrium thermodynamics at microscopic scale. This model can explain the functioning of a number of systems in nature, such as molecular motors [2][3][4] or protein translocation processes [5]. Additionally, schemes based on this mechanism have been implemented to sort biomolecules [6] and inorganic microparticles [7], to rectify the motion of cold atoms in optical lattices [8,9] and vortices in superconductors [10], among others. The rich dynamics arising in ratchets becomes evident from the diverse phenomena that can be observed even in the simplest cases of onedimensional (1D) systems, such as bidirectional transport depending on size, chaotic behavior and current reversals [11][12][13][14][15]. This is due to the delicate interplay among a whole set of parameters, encompassing the structure of a spatial potential, the modulation of an external driving, the strength of thermal noise and the properties of the particles. Studies on the influence of these aspects have paved the way to broaden our understanding of transport processes at the micro and nanoscale, but this area is far from complete.Naturally, a degree of complexity and versatility is added in two-dimensional (2D) systems, which become very important in the context of electronic transport in 2D crystals like graphene [16] and semiconductor artificial nanomaterials [17], for example. Among the studies of 2D ratchets, an explored path has been the use of symmetric spatial potentials either with a temporally asymmetric drive [18][19][20] or with an induced symmetry breaking due to a synchronization and phase coupling of two ac signals: a flashing potential and a sym...

Brownian Asymmetric Simple Exclusion Process

“…Current experimental micro-manipulation techniques allow precise engineering and fine tuning of relevant aspects of the model: the external tilted periodic potential and the confinement. The precise control of the potential may be achieved with high precision using holographic optical tweezers as it was done in a related experimental work [20]. The confinement can be realized within microfluidic chips.…”

Multiple Current Reversals Using Superimposed Driven Lattices

“…Our scheme of multiple current reversal can be experimentally realized using cold atoms or colloids with optical lattices [14,32,38] and lattices designed using holographic trapping techniques [37,[54][55][56][57]. The background lattice can be formed by 2D optical lattices where the periodic potential is generated by counterpropagating laser beams of perpendicular polarization.…”

“…Due to novel experimental progress in atom trapping techniques, directed transport of atomic ensembles has been realized in ac-driven optical lattices [29,30] both in the ultracold quantum regime [31] and at micro kelvin temperatures where a classical dynamics approach successfully describes the experiments [14,32]. Apart from the vast majority of ratchet based setups in one spatial dimension (1D) [7,15,16,33,34], recent experiments have significantly progressed the realization of highly controllable two dimensional (2D) setups using ac-driven optical lattices [14,29,35,36] and holographic optical tweezers [37]. Due to such widespread applications of directed particle transport, the different mechanisms to control the transport have been a topic of ongoing research.…”

Microparticle transport networks with holographic optical tweezers and cavitation bubbles

“…Regular Brownian microbeads can also be driven out of equilibrium to induce directed motion when combined with potentials and external forces, making Brownian ratchets [15][16][17]. The main limitation of these ratchet systems is that many parameters have to be carefully tuned to enable directed transport: the potentials have to be periodic (usually spatially asymmetric) and external forces are required in most implementations.…”