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Mie Scattering Distinguishing OAM published in New J. Phys.

Mie scattering distinguishes the topological charge of an optical vortex: A homage to Gustav Mie

Mie scattering distinguishes the topological charge of an optical vortex: A homage to Gustav Mie
Valeria Garbin, Giovanni Volpe, Enrico Ferrari, Michel Versluis, Dan Cojoc & Dmitri Petrov
New Journal of Physics 11, 013046 (2009)
DOI: 10.1088/1367-2630/11/1/013046

One century after Mie’s original paper, Mie scattering is still a fertile field of scientific endeavor. We show that the Mie scattering distinguishes the topological charge of light beams with phase dislocations. We experimentally and numerically study the scattering of highly focused Laguerre–Gaussian beams by dielectric and metal spheres, and show that the scattered field is sensitive to the modulus and to the sign of the topological charge. The implications for position detection systems are also discussed.

Enhanced Detection Range for PFM published in Rev. Sci. Instrumen.

10-fold detection range increase in quadrant-photodiode position sensing for photonic force microscope

10-fold detection range increase in quadrant-photodiode position sensing for photonic force microscope
Sandro Perrone, Giovanni Volpe & Dmitri Petrov
Review of Scientific Instruments 79(10), 106101 (2008)
DOI: 10.1063/1.2993177

We propose a technique that permits one to increase by one order of magnitude the detectionrange of position sensing for the photonic force microscope with quadrant photodetectors(QPDs). This technique takes advantage of the unavoidable cross-talk between output signals of the QPD and does not assume that the output signals are linear in the probe displacement. We demonstrate the increase in the detection range from 150 to 1400 nm for a trapped polystyrene sphere with radius of 300 nm as probe.

Stochastic Resonant Damping published in Phys. Rev. E

Stochastic resonant damping in a noisy monostable system: Theory and experiment

Stochastic resonant damping in a noisy monostable system: Theory and experiment
Giovanni Volpe, Sandro Perrone, J. Miguel Rubi & Dmitri Petrov
Physical Review E 77(5), 051107 (2008)
DOI: 10.1103/PhysRevE.77.051107

Usually in the presence of a background noise an increased effort put in controlling a system stabilizes its behavior. Rarely it is thought that an increased control of the system can lead to a looser response and, therefore, to a poorer performance. Strikingly there are many systems that show this weird behavior; examples can be drawn form physical, biological, and social systems. Until now no simple and general mechanism underlying such behaviors has been identified. Here we show that such a mechanism, named stochastic resonant damping, can be provided by the interplay between the background noise and the control exerted on the system. We experimentally verify our prediction on a physical model system based on a colloidal particle held in an oscillating optical potential. Our result adds a tool for the study of intrinsically noisy phenomena, joining the many constructive facets of noise identified in the past decades—for example, stochastic resonance, noise-induced activation, and Brownian ratchets.

Surface Plasmon Optical Tweezers published in Phys. Rev. Lett.

Surface plasmon optical tweezers: Tunable optical manipulation in the femtonewton range

Surface plasmon optical tweezers: Tunable optical manipulation in the femtonewton range
Maurizio Righini, Giovanni Volpe, Christian Girard, Dmitri Petrov & Romain Quidant
Physical Review Letters 100(18), 186804 (2008)
DOI: 10.1103/PhysRevLett.100.186804

We present a quantitative analysis of 2D surface plasmon based optical tweezers able to trap microcolloids at a patterned metal surface under low laser intensity. Photonic force microscopy is used to assess the properties of surface plasmon traps, such as confinement and stiffness, revealing stable trapping with forces in the range of a few tens of femtonewtons. We also investigate the specificities of surface plasmon tweezers with respect to conventional 3D tweezers responsible for their selectivity to the trapped specimen’s size. The accurate engineering of the trapping properties through the adjustment of the illumination parameters opens new perspectives in the realization of future optically driven on-a-chip devices.

Singular Point Characterization in Microscopic Flows published in Phys. Rev. E

Singular point characterization in microscopic flows

Singular point characterization in microscopic flows
Giorgio Volpe, Giovanni Volpe & Dmitri Petrov
Physical Review E 77(3), 037301 (2008)
DOI: 10.1103/PhysRevE.77.037301
arXiv: 0711.0923

We suggest an approach to microrheology based on optical traps capable of measuring fluid fluxes around singular points of fluid flows. We experimentally demonstrate this technique, applying it to the characterization of controlled flows produced by a set of birefringent spheres spinning due to the transfer of light angular momentum. Unlike the previous techniques, this method is able to distinguish between a singular point in a complex flow and the absence of flow at all; furthermore it permits us to characterize the stability of the singular point.

Photonics Torque Microscopy published in Phys. Rev. E

Brownian motion in a non-homogeneous force field and photonic force microscope

Brownian motion in a non-homogeneous force field and photonic force microscope
Giorgio Volpe, Giovanni Volpe & Dmitri Petrov
Physical Review E 76(6), 061118 (2007)
DOI: 10.1103/PhysRevE.76.061118
arXiv: 0711.0923

The photonic force microscope (PFM) is an opto-mechanical technique that uses an optically trapped probe to measure forces in the range of pico to femto Newton. For a correct use of the PFM, the force field has to be homogeneous on the scale of the Brownian motion of the trapped probe. This condition implicates that the force field must be conservative, excluding the possibility of a rotational component. However, there are cases where these assumptions are not fulfilled. Here, we show how to expand the PFM technique in order to deal with these cases. We introduce the theory of this enhanced PFM and we propose a concrete analysis workflow to reconstruct the force field from the experimental time series of the probe position. Furthermore, we experimentally verify some particularly important cases, namely, the case of a conservative and of a rotational force field.

Back-scattering Position Detection for PFM published in J. Appl. Phys.

Back-scattering position detection for photonic force microscopy

Back-scattering position detection for photonic force microscopy
Giovanni Volpe, Gregory Kozyreff & Dmitri Petrov
Journal of Applied Physics 102(8), 084701 (2007)
DOI: 10.1063/1.2799047

An optically trapped particle is an extremely sensitive probe for the measurement of pico- and femto-Newton forces between the particle and its environment in microscopic systems (photonic force microscopy). A typical setup comprises an optical trap, which holds the probe, and a position sensing system, which uses the scattering of a beam illuminating the probe. Usually the position is accurately determined by measuring the deflection of the forward-scattered light transmitted through the probe. However, geometrical constraints may prevent access to this side of the trap, forcing one to make use of the backscattered light instead. A theory is presented together with numerical results that describes the use of the backscattered light for position detection. With a Mie–Debye approach, we compute the total (incident plus scattered) field and follow its evolution as it is collected by the condenser lenses and projected onto the position detectors and the responses of position sensitive detectors and quadrant photodetectors to the displacement of the probe in the optical trap, both in forward and backward configurations. We find out that in the case of backward detection, for both types of detectors the displacement sensitivity can change sign as a function of the probe size and is null for some critical sizes. In addition, we study the influence of the numerical aperture of the detection system, polarization, and the cross talk between position measurements in orthogonal directions. We finally discuss how these features should be taken into account in experimental designs.

Actin-cytoskeleton Depolymerisation Detection in a Single Cell published in Opt. Express

Real-time actin-cytoskeleton depolymerization detection in a single cell using optical tweezers

Real-time actin-cytoskeleton depolymerization detection in a single cell using optical tweezers
Anna Chiara de Luca, Giovanni Volpe, Anna Morales Drets, Maria Isabel Geli, Giuseppe Pesce, Giulia Rusciano, Antonio Sasso & Dmitri Petrov
Optics Express 15(13), 7922—7932 (2007)
DOI: 10.1364/OE.15.007922

The cytoskeleton provides the backbone structure for the cellular organization, determining, in particular, the cellular mechanical properties. These are important factors in many biological processes, as, for instance, the metastatic process of malignant cells. In this paper, we demonstrate the possibility of monitoring the cytoskeleton structural transformations in optically trapped yeast cells (Saccharomyces cerevisiae) by tracking the forward scattered light via a quadrant photodiode. We distinguished normal cells from cells treated with latrunculin A, a drug which is known to induce the actin-cytoskeleton depolymerization. Since the proposed technique relies only on the inherent properties of the optical trap, without requiring external markers or biochemical sensitive spectroscopic techniques, it can be readily combined with existing optical tweezers setups.

Torque Detection using Brownian Fluctuations published in Phys. Rev. Lett.

Torque detection using Brownian fluctuations

Torque detection using Brownian fluctuations
Giovanni Volpe & Dmitri Petrov
Physical Review Letters 97(21), 210603 (2006)
DOI: 10.1103/PhysRevLett.97.210603

We report the statistical analysis of the movement of a submicron particle confined in a harmonic potential in the presence of a torque. The absolute value of the torque can be found from the auto- and cross-correlation functions of the particle’s coordinates. We experimentally prove this analysis by detecting the torque produced onto an optically trapped particle by an optical beam with orbital angular momentum.

Also featured in “Focus: Hidden Twists and Turns”, Physics 18, 17 (December 1, 2006)

Surface Plasmon Radiation Forces published in Phys. Rev. Lett.

Surface plasmon radiation forces

Surface plasmon radiation forces
Giovanni Volpe, Romain Quidant, Gonçal Badenes & Dmitri Petrov
Physical Review Letters 96(23), 238101 (2006)
DOI: 10.1103/PhysRevLett.96.238101

We report the first experimental observation of momentum transfer from a surface plasmon to a single dielectric sphere. Using a photonic force microscope, we measure the plasmon radiation forces on different polystyrene beads as a function of their distance from the metal surface. We show that the force magnitude at resonance is strongly enhanced compared to a nonresonant illumination. Measurements performed as a function of the probe particle size indicate that optical manipulation by plasmon fields has a strong potential for optical sorting.