We present here a detailed study of the behaviour of a three dimensional Brownian motor based on cold atoms in a double optical lattice [P. Sjölund et al., Phys. Rev. Lett. 96, 190602 (2006)]. This includes both experiments and numerical simulations of a Brownian particle. The potentials used are spatially and temporally symmetric, but combined spatiotemporal symmetry is broken by phase shifts and asymmetric transfer rates between potentials. The diffusion of atoms in the optical lattices is rectified and controlled both in direction and speed along three dimensions. We explore a large range of experimental parameters, where irradiances and detunings of the optical lattice lights are varied within the dissipative regime. Induced drift velocities in the order of one atomic recoil velocity have been achieved.PACS. 32.80.Lg Mechanical effects of light on atoms, molecules, and ions -05.40.Jc Brownian motion -32.80.Pj Optical cooling of atoms; trapping arXiv:0704.3955v1 [physics.atom-ph]
We study the influence of the lattice topography and the coupling between motion in different directions, for a three-dimensional Brownian motor based on cold atoms in a double optical lattice. Due to controllable relative spatial phases between the lattices, our Brownian motor can induce drifts in arbitrary directions. Since the lattices couple the different directions, the relation between the phase shifts and the directionality of the induced drift is non trivial. Here is therefore this relation investigated experimentally by systematically varying the relative spatial phase in two dimensions, while monitoring the vertically induced drift and the temperature. A relative spatial phase range of 2π × 2π is covered. We show that a drift, controllable both in speed and direction, can be achieved, by varying the phase both parallel and perpendicular to the direction of the measured induced drift. The experimental results are qualitatively reproduced by numerical simulations of a simplified, classical model of the system.
Dietary overload of toxic, free metabolic intermediates leads to disrupted insulin signalling and fatty liver disease. However, it was recently reported that this pathway might not be universal: depletion of histone deacetylase (HDAC) enhances insulin sensitivity alongside hepatic lipid accumulation in mice, but the mechanistic role of microscopic lipid structure in this effect remains unclear. Here we study the effect of Entinostat, a synthetic HDAC inhibitor undergoing clinical trials, on hepatic lipid metabolism in the paradigmatic HepaRG liver cell line. Specifically, we statistically quantify lipid droplet morphology at single cell level utilizing label-free microscopy, coherent anti-Stokes Raman scattering, supported by gene expression. We observe Entinostat efficiently rerouting carbohydrates and free-fatty acids into lipid droplets, upregulating lipid coat protein gene Plin4, and relocating droplets nearer to the nucleus. Our results demonstrate the power of Entinostat to promote lipid synthesis and storage, allowing reduced systemic sugar levels and sequestration of toxic metabolites within protected protein-coated droplets, suggesting a potential therapeutic strategy for diseases such as diabetes and metabolic syndrome.
Nonlinear plasmonics opens up for wavelength conversion, reduced interaction/emission volumes, and nonlinear enhancement effects at the nanoscale with many compelling nanophotonic applications foreseen. We investigate nonlinear plasmonic responses of nanoholes in thin gold films by exciting the holes individually with tightly focused laser beams, employing a degenerated pump/probe and Stokes excitation scheme. Excitation of the holes results in efficient generation of both narrowband four-wave mixing (FWM) and broadband multiphoton excited luminescence, blueshifted relative to the excitation beams. Clear enhancements were observed when matching the pump/probe wavelength with the hole plasmon resonance. These observations show that the FWM generation is locally excited by nanoholes and has a resonant behavior primarily governed by the dimensions of the individual holes. Nanostructures provide a unique and effective means to concentrate and manipulate light at the nanoscale through the excitation of collective electron oscillations known as surface plasmons. While so-called surface plasmon polaritons (SPPs) propagate along metal surfaces, metal nanostructures can also sustain spatially confined localized surface plasmons (LSPs), the resonances (LSPRs) of which depend on the size, shape and, higher-order arrangements of the nanostructures. While the linear regime of plasmonics has been extensively investigated over the past decade, the nonlinear, multiphoton excited correspondent remains relatively unexplored. The idea of downscaling nonlinear interaction phenomena by inducing nonlinear plasmonic effects in nanosized objects is highly attractive, as they could in contrast to the linear correspondents, generate coherent fields, offer frequency conversion and nonlinear enhancement effects, presenting new dimensions in the development of laser-like nanoemitters, nanosized devices for long-range information transfer/optical storage, and nanosensors [1-6]. The experimentally most straightforward schemes of nonlinear plasmon excitation, second-and higher-harmonic generation as well as multiphoton excited luminescence (MPEL), have been studied on different nanostructures [7][8][9][10][11][12][13]. More recently, a series of reports on the more complex but interesting four-wave mixing (FWM) process has appeared [14][15][16][17][18][19]. In one of the most common FWM excitation schemes, two of the three initial fields are degenerate, requiring only two excitation beams: the pump/probe and Stokes at wavelengths λ 1 and λ 2 , respectively (where λ 1 < λ 2 ). This results in a blueshifted FWM signal emitted at 1∕λ FWM 2∕λ 1 − 1∕λ 2 [ Fig. 1(a)]. Furthermore, the FWM intensity, I FWM , depends quadratically on the intensity of the pump/probe beam, I 1 , and linearly on the intensity of the Stokes beam, I 2 ∶I FWM ∝ I 2 1 × I 2 , resulting in strong signal generation though spatially localized to high excitation intensity regions only. In smooth gold films excitation of SPPs at FWM frequency have been shown for free-space excita...
We have realized real-time steering of the directed transport in a Brownian motor based on cold atoms in optical lattices, and demonstrate drifts along pre-designed paths. The transport is induced by spatiotemporal asymmetries in the system, where we can control the spatial part, and we show that the response to changes in asymmetry is very fast. In addition to the directional steering, a real-time control of the magnitude of the average drift velocity and an on/off switching of the motor are also demonstrated. We use a non-invasive real-time detection of the transport, enabling a feedback control of the system.
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