Soft Matter Lab

Comparison Between Force Measurement Methods published in Phys. Rev. E

Force measurement in the presence of Brownian noise: Equilibrium distribution method vs. drift method
Thomas Brettschneider, Giovanni Volpe, Laurent Helden, Jan Wehr & Clemens Bechinger
Physical Review E 83(4), 041113 (2011)
DOI: 10.1103/PhysRevE.83.041113
arXiv: 1009.2386

The study of microsystems and the development of nanotechnologies require alternative techniques to measure piconewton and femtonewton forces at microscopic and nanoscopic scales. Among the challenges is the need to deal with the ineluctable thermal noise, which, in the typical experimental situation of a spatial diffusion gradient, causes a spurious drift. This leads to a correction term when forces are estimated from drift measurements [G. Volpe, L. Helden, T. Brettschneider, J. Wehr, and C. Bechinger, Phys. Rev. Lett. 104, 170602 (2010)]. Here we provide a systematic study of such an effect by comparing the forces acting on various Brownian particles derived from equilibrium-distribution and drift measurements. We discuss the physical origin of the correction term, its dependence on wall distance and particle radius, and its relation to the convention used to solve the respective stochastic integrals. Such a correction term becomes more significant for smaller particles and is predicted to be on the order of several piconewtons for particles the size of a biomolecule.

Work Done by Rotational Force Fields published in J. Opt.

Influence of rotational force fields on the determination of the work done on a driven Brownian particle
Giuseppe Pesce, Giovanni Volpe, Alberto Imparato, Giulia Rusciano & Antonio Sasso
Journal of Optics 13(4), 044006 (2011)
DOI: 10.1088/2040-8978/13/4/044006
arXiv: 1006.4534

For a Brownian system the evolution of thermodynamic quantities is a stochastic process, in particular the work performed on a driven colloidal particle held in an optical trap, changes for each realization of the experimental manipulation, even though the manipulation protocol remains unchanged. Nevertheless, the work distribution is governed by established laws. Here, we show how the measurement of the work distribution is influenced by the presence of rotational, i.e. nonconservative, radiation forces. Experiments on particles of different materials show that the rotational radiation forces, and therefore their effect on the work distributions, increase with the particle’s refractive index.

Fractal Plasmonics published in Opt. Express

Fractal plasmonics: Subdiffraction focusing and broadband spectral response by a Sierpisky nanocarpet
Giorgio Volpe, Giovanni Volpe & Romain Quidant
Optics Express 19(4), 3612—3618 (2011)
DOI: 10.1364/OE.19.003612

Plasmonic nanostructures offer a great potential to enhance light-matter interaction at the nanometer scale. The response upon illumination at a given wavelength and polarization is governed by the characteristic lengths associated to the shape and size of the nanostructure. Here, we propose the use of engineered fractal plasmonic structures to extend the degrees of freedom and the parameters available for their design. In particular, we focus on a paradigmatic fractal geometry, namely the Sierpinski carpet. We explore the possibility of using it to achieve a controlled broadband spectral response by controlling the degree of its fractal complexity. Furthermore, we investigate some other arising properties, such as subdiffraction limited focusing and its potential use for optical trapping of nano-objects. An attractive advantage of the focusing over more standard geometries, such as gap antennas, is that it occurs away from the metal surface (≈ 80nm) at the center of the nanostructure, leaving an open space accessible to objects for enhanced light-matter interaction.

Influence of Noise on Force Measurements published in Phys. Rev. Lett.

Influence of noise on force measurements
Giovanni Volpe, Laurent Helden, Thomas Brettschneider, Jan Wehr & Clemens Bechinger
Physical Review Letters 104(17), 170602 (2010)
DOI: 10.1103/PhysRevLett.104.170602
arXiv: 1004.0874

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.

Enhanced TIRM published in Opt. Express

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)
DOI: 10.1364/OE.17.023975
arXiv: 0909.5131

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).

Non-conservative Forces in Optical Traps published in EPL

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)
DOI: 10.1209/0295-5075/86/38002
arXiv: 0902.4178

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.

Cost of Thermal Noise Suppression published in J. Phys. A.

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)
DOI: 10.1088/1751-8113/42/9/095005
arXiv: 0711.0923

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 Distinguishing OAM published in New J. Phys.

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
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
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.