Optical trapping and control of a dielectric nanowire by a nanoaperture
Optical trapping and control of a dielectric nanowire by a nanoaperture
Mehdi Shafiei Aporvari, Fardin Kheirandish & Giovanni Volpe
Optics Letters 40(20), 4807—4810 (2015)
DOI: 10.1364/OL.40.004807
arXiv: 1507.04044
We demonstrate that a single sub-wavelength nanoaperture in a metallic thin film can be used to achieve dynamic optical trapping and control of a single dielectric nanowire. A nanoaperture can trap a nanowire, control its orientation when illuminated by a linearly polarized incident field, and rotate the nanowire when illuminated by a circularly polarized incident field. Compared to other designs, this approach has the advantage of a low-power driving field entailing low heating and photodamage.
Aberrant cerebral network topology and mild cognitive impairment in early Parkinson’s disease
Aberrant cerebral network topology and mild cognitive impairment in early Parkinson’s disease
Joana B. Pereira, Dag Aarsland, Cedric E. Ginestet, Alexander V. Lebedev, Lars-Olof Wahlund, Andrew Simmons, Giovanni Volpe & Eric Westman
Human Brain Mapping 36(8), 2980—2995 (2015)
DOI: 10.1002/hbm.22822
The aim of this study was to assess whether mild cognitive impairment (MCI) is associated with disruption in large-scale structural networks in newly diagnosed, drug-na€ıve patients with Parkin- son’s disease (PD). Graph theoretical analyses were applied to 3T MRI data from 123 PD patients and 56 controls from the Parkinson’s progression markers initiative (PPMI). Thirty-three patients were classified as having Parkinson’s disease with mild cognitive impairment (PD-MCI) using the Movement Disorders Society Task Force criteria, while the remaining 90 PD patients were classified as cognitively normal (PD- CN). Global measures (clustering coefficient, characteristic path length, global efficiency, small-world- ness) and regional measures (regional clustering coefficient, regional efficiency, hubs) were assessed in the structural networks that were constructed based on cortical thickness and subcortical volume data. PD-MCI patients showed a marked reduction in the average correlation strength between cortical and subcortical regions compared with controls. These patients had a larger characteristic path length and reduced global efficiency in addition to a lower regional efficiency in frontal and parietal regions com- pared with PD-CN patients and controls. A reorganization of the highly connected regions in the network was observed in both groups of patients. This study shows that the earliest stages of cognitive decline in PD are associated with a disruption in the large-scale coordination of the brain network and with a decrease of the efficiency of parallel information processing. These changes are likely to signal further cognitive decline and provide support to the role of aberrant network topology in cognitive impairment in patients with early PD.
Formation, compression and surface melting of colloidal clusters by active particles
Formation, compression and surface melting of colloidal clusters by active particles
Felix Kümmel, Parmida Shabestari, Celia Lozano, Giovanni Volpe & Clemens Bechinger
Soft Matter 11(31), 6187—6191 (2015)
DOI: 10.1039/C5SM00827A
We demonstrate with experiments and numerical simulations that the structure and dynamics of a suspension of passive particles is strongly altered by adding a very small (o1%) number of active particles. With increasing passive particle density, we observe first the formation of dynamic clusters comprised of passive particles being surrounded by active particles, then the merging and compression of these clusters, and eventually the local melting of crystalline regions by enclosed active particles.
The Smoluchowski-Kramers limit of stochastic differential equations with arbitrary state-dependent friction
The Smoluchowski-Kramers limit of stochastic differential equations with arbitrary state-dependent friction
Scott Hottovy, Austin McDaniel, Giovanni Volpe & Jan Wehr
Communications in Mathematical Physics 336(3), 1259—1283 (2015)
DOI: 10.1007/s00220-014-2233-4
arXiv: 1404.2330
We study a class of systems of stochastic differential equations describing diffusive phenomena. The Smoluchowski-Kramers approximation is used to describe their dynamics in the small mass limit. Our systems have arbitrary state-dependent friction and noise coefficients. We identify the limiting equation and, in particular, the additional drift term that appears in the limit is expressed in terms of the solution to a Lyapunov matrix equation. The proof uses a theory of convergence of stochastic integrals developed by Kurtz and Protter. The result is sufficiently general to include systems driven by both white and Ornstein–Uhlenbeck colored noises. We discuss applications of the main theorem to several physical phenomena, including the experimental study of Brownian motion in a diffusion gradient.
A step-by-step guide to the realisation of advanced optical tweezers
A step-by-step guide to the realisation of advanced optical tweezers
Giuseppe Pesce, Giorgio Volpe, Onofrio M. Maragò, Philip H. Jones, Sylvain Gigan, Antonio Sasso & Giovanni Volpe Journal of the Optical Society of America B 32(5), B84—B98 (2015)
DOI: 10.1364/JOSAB.32.000B84
arXiv: 1501.07894
Since the pioneering work of Arthur Ashkin, optical tweezers (OT) have become an indispensable tool for contactless manipulation of micro- and nanoparticles. Nowadays OT are employed in a myriad of applications demonstrating their importance. While the basic principle of OT is the use of a strongly focused laser beam to trap and manipulate particles, more complex experimental setups are required to perform novel and challenging experiments. With this article, we provide a detailed step-by-step guide for the construction of advanced optical manipulation systems. First, we explain how to build a single-beam OT on a homemade micro- scope and how to calibrate it. Improving on this design, we realize a holographic OT, which can manipulate independently multiple particles and generate more sophisticated wavefronts such as Laguerre–Gaussian beams. Finally, we explain how to implement a speckle OT, which permits one to employ random speckle light fields for deterministic optical manipulation.
Computational toolbox for optical tweezers in geometrical optics
Computational toolbox for optical tweezers in geometrical optics
Agnese Callegari, Mite Mijalkov, Burak Gököz & Giovanni Volpe
Journal of the Optical Society of America B 32(5), B11—B19 (2015)
DOI: 10.1364/JOSAB.32.000B11
arXiv: 1402.5439
Optical tweezers have found widespread application in many fields, from physics to biology. Here, we explain in detail how optical forces and torques can be described within the geometrical optics approximation, and we show that this approximation provides reliable results in agreement with experiments for particles whose characteristic dimensions are larger than the wavelength of the trapping light. Furthermore, we provide an object-oriented software package implemented in MATLAB for the calculation of optical forces and torques in the geometrical optics regime: Optical Tweezers in Geometrical Optics (OTGO). We provide all source codes for OTGO as well as documentation and code examples—e.g., standard optical tweezers, optical tweezers with elon- gated particles, the windmill effect, and Kramers transitions between two optical traps—necessary to enable users to effectively employ it in their research.
Longterm influence of fluid inertia on the diffusion of a Brownian particle
Longterm influence of fluid inertia on the diffusion of a Brownian particle
Giuseppe Pesce, Giorgio Volpe, Giovanni Volpe & Antonio Sasso
Physical Review E 90(4), 042309 (2014)
DOI: 10.1103/PhysRevE.90.042309
arXiv: 1402.6913
We experimentally measure the effects of fluid inertia on the diffusion of a Brownian particle at very long time scales. In previous experiments, the use of standard optical tweezers introduced a cutoff in the free diffusion of the particle, which limited the measurement of these effects to times comparable with the relaxation time of the fluid inertia, i.e., a few milliseconds. Here, by using blinking optical tweezers, we detect these inertial effects on time scales several orders longer up to a few seconds. The measured mean square displacement of a freely diffusing Brownian particle in a liquid shows a deviation from the Einstein-Smoluchowsky theory that diverges with time. These results are consistent with a generalized theory that takes into account not only the particle inertia but also the inertia of the surrounding fluid.
Speckle optical tweezers: Micromanipulation with random light fields
Speckle optical tweezers: Micromanipulation with random light fields
Giorgio Volpe, Lisa Kurz, Agnese Callegari, Giovanni Volpe & Sylvain Gigan
Optics Express 22(15), 18159—18167 (2014)
DOI: 10.1364/OE.22.018159
arXiv: 1403.0364
Current optical manipulation techniques rely on carefully engineered setups and samples. Although similar conditions are routinely met in research laboratories, it is still a challenge to manipulate microparticles when the environment is not well controlled and known a priori, since optical imperfections and scattering limit the applicability of this technique to real-life situations, such as in biomedical or microfluidic applications. Nonetheless, scattering of coherent light by disordered structures gives rise to speckles, random diffraction patterns with well- defined statistical properties. Here, we experimentally demonstrate how speckle fields can become a versatile tool to efficiently perform fundamental optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these “speckle optical tweezers” will greatly broaden the perspectives of optical manipulation for real-life applications.
Reply to comment on “Circular motion of asymmetric self-propelling particles”
Reply to comment on “Circular motion of asymmetric self-propelling particles”
Felix Kümmel, Borge ten Hagen, Raphael Wittkowski, Daisuke Takagi, Ivo Buttinoni, Ralf Eichhorn, Giovanni Volpe, Hartmut Löwen & Clemens Bechinger
Physical Review Letters 113(2), 029802 (2014)
DOI: 10.1103/PhysRevLett.113.029802
arXiv: 1407.4016
Simulation of the active Brownian motion of a microswimmer
Simulation of the active Brownian motion of a microswimmer
Giorgio Volpe, Sylvain Gigan & Giovanni Volpe
American Journal of Physics 82(7), 659—664 (2014)
DOI: 10.1119/1.4870398
Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations both in homogenous and complex environments. In particular, we show how active Brownian particles do not follow the Maxwell-Boltzmann distribution—a clear signature of their out-of-equilibrium nature—and how, unlike passive Brownian particles, microswimmers can be funneled, trapped, and sorted.
