We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. As well as corroborating existing measurements of spin momentum, our results reveal its dynamic effect. We show how the under-damped motion of probe particles in structured light fields can expose the nature and morphology of optical momentum flows, and provide a testbed for elementary non-equilibrium statistical mechanics.
Antibiotics cure infections by influencing bacterial growth or viability. Antibiotics can be divided to two groups on the basis of their effect on microbial cells through two main mechanisms, which are either bactericidal or bacteriostatic. Bactericidal antibiotics kill the bacteria and bacteriostatic antibiotics suppress the growth of bacteria (keep them in the stationary phase of growth). One of many factors to predict a favorable clinical outcome of the potential action of antimicrobial chemicals may be provided using in vitro bactericidal/bacteriostatic data (e.g., minimum inhibitory concentrations-MICs). Consequently, MICs are used in clinical situations mainly to confirm resistance, and to determine the in vitro activities of new antimicrobials. We report on the combination of data obtained from MICs with information on microorganisms' "fingerprint" (e.g., DNA/RNA, and proteins) provided by Raman spectroscopy. Thus, we could follow mechanisms of the bacteriostatic versus bactericidal action simply by detecting the Raman bands corresponding to DNA. The Raman spectra of Staphylococcus epidermidis treated with clindamycin (a bacteriostatic agent) indeed show little effect on DNA which is in contrast OPEN ACESSMolecules 2013, 18 13189 with the action of ciprofloxacin (a bactericidal agent), where the Raman spectra show a decrease in strength of the signal assigned to DNA, suggesting DNA fragmentation.
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...
We examine the rotational dynamics of spheroidal particles in an optical trap comprising counter-propagating Gaussian beams of opposing helicity. Isolated spheroids undergo continuous rotation with frequencies determined by their size and aspect ratio, whilst pairs of spheroids display phase locking behaviour. The introduction of additional particles leads to yet more complex behaviour. Experimental results are supported by numerical calculations.
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