Giorgio Volpe – Page 3 – Soft Matter Lab

Active Brownian Motion Tunable by Light published in J. Phys. Condens. Matter

Active Brownian motion tunable by light
Ivo Buttinoni, Giovanni Volpe, Felix Kümmel, Giorgio Volpe & Clemens Bechinger
Journal of Physics: Condensed Matter 24(28), 284129 (2012)
DOI: 10.1088/0953-8984/24/28/284129
arXiv: 1110.2202

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle’s self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

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.

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.

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

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
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.