Influence of noise on force measurements
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 104(17), 170602 (2010)
We demonstrate how the ineluctable presence of thermal noise alters the measurement of forces acting on microscopic and nanoscopic objects. We quantify this effect exemplarily for a Brownian particle near a wall subjected to gravitational and electrostatic forces. Our results demonstrate that the force-measurement process is prone to artifacts if the noise is not correctly taken into account.
Novel perspectives for the application of total internal reflection microscopy
Giovanni Volpe, Thomas Brettschneider, Laurent Helden & Clemens Bechinger
Optics Express 17(26), 23975—23985 (2009)
Total Internal Reflection Microscopy (TIRM) is a sensitive non-invasive technique to measure the interaction potentials between a colloidal particle and a wall with femtonewton resolution. The equilibrium distribution of the particle-wall separation distance z is sampled monitoring the intensity I scattered by the Brownian particle under evanescent illumination. Central to the data analysis is the knowledge of the relation between I and the corresponding z, which typically must be known a priori. This poses considerable constraints to the experimental conditions where TIRM can be applied (short penetration depth of the evanescent wave, transparent surfaces). Here, we introduce a method to experimentally determine I(z) by relying only on the distance-dependent particle-wall hydrodynamic interactions. We demonstrate that this method largely extends the range of conditions accessible with TIRM, and even allows measurements on highly reflecting gold surfaces where multiple reflections lead to a complex I(z).
Quantitative assessment of non-conservative radiation forces in an optical trap
Giuseppe Pesce, Giorgio Volpe, Anna Chiara De Luca, Giulia Rusciano & Giovanni Volpe
EPL (Europhysics Letters) 86(3), 38002 (2009)
The forces acting on an optically trapped particle are usually assumed to be conservative. However, the presence of a non-conservative component has recently been demonstrated. Here, we propose a technique that permits one to quantify the contribution of such a non-conservative component. This is an extension of a standard calibration technique for optical tweezers and, therefore, can easily become a standard test to verify the conservative optical force assumption. Using this technique, we have analyzed optically trapped particles of different size under different trapping conditions. We conclude that the non-conservative effects are effectively negligible and do not affect the standard calibration procedure, unless for extremely low-power trapping, far away from the trapping regimes usually used in experiments.
Thermal noise suppression: How much does it cost?
Giovanni Volpe, Jan Wehr, Dmitri Petrov & J. Miguel Rubi
Journal of Physics A: Mathematical and Theoretical 42(9), 095005 (2009)
In order to stabilize the behavior of noisy systems, confining it around a desirable state, an effort is required to suppress the intrinsic noise. This noise suppression task entails a cost. For the important case of thermal noise in an overdamped system, we show that the minimum cost is achieved when the system control parameters are held constant: any additional deterministic or random modulation produces an increase of the cost. We discuss the implications of this phenomenon for those overdamped systems whose control parameters are intrinsically noisy, presenting a case study based on the example of a Brownian particle optically trapped in an oscillating potential.
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)
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.
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)
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 in a noisy monostable system: Theory and experiment
Giovanni Volpe, Sandro Perrone, J. Miguel Rubi & Dmitri Petrov
Physical Review E 77(5), 051107 (2008)
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: Tunable optical manipulation in the femtonewton range
Maurizio Righini, Giovanni Volpe, Christian Girard, Dmitri Petrov & Romain Quidant
Physical Review Letters 100(18), 186804 (2008)
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
Giorgio Volpe, Giovanni Volpe & Dmitri Petrov
Physical Review E 77(3), 037301 (2008)
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.
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)
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.