Random optical media (ROM) are a novel class of photonic materials characterized by a disordered assembly of the elementary constituents (such as particles, wires and fibers), that determines unique scattering, absorption and emission properties. The propagation of light in ROM is affected by the size and optical properties (refractive index, absorption and emission wavelengths) of their components, as well as by the overall 3-dimensional architecture. So far, most of the investigated ROM have been realized using liquid dispersions or bulk samples embedding colloidal nanoparticles or porous systems. While nanowire-based ROM are poorly investigated, such materials can feature new optical effects related to the elongated shape of their building blocks and to their light-transport properties. Here we report on the fabrication and on the morphological and spectroscopic characterization of hybrid organic-inorganic nanowires, realized by doping polymers with dielectric nanoparticles. We investigate light diffusion and multi-scattering properties of 3- dimensional ROM formed by organic and hybrid nanowires, as well as field localization in 2-dimensional networks. The influence of nanowire geometry and composition on the scattering properties is also discussed.
Electrospinning technologies for the realization of active polymeric nanomaterials can be easily up-scaled, opening perspectives to industrial exploitation, and due to their versatility they can be employed to finely tailor the size, morphology and macroscopic assembly of fibers as well as their functional properties. Light-emitting or other active polymer nanofibers, made of conjugated polymers or of blends embedding chromophores or other functional dopants, are suitable for various applications in advanced photonics and sensing technologies. In particular, their almost onedimensional geometry and finely tunable composition make them interesting materials for developing novel lasing devices. However, electrospinning techniques rely on a large variety of parameters and possible experimental geometries, and they need to be carefully optimized in order to obtain suitable topographical and photonic properties in the resulting nanostructures. Targeted features include smooth and uniform fiber surface, dimensional control, as well as filament alignment, enhanced light emission, and stimulated emission. We here present various optimization strategies for electrospinning methods which have been implemented and developed by us for the realization of lasing architectures based on polymer nanofibers. The geometry of the resulting nanowires leads to peculiar light-scattering from spun filaments, and to controllable lasing characteristics.
We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.
We present a computational model for the simulation of optically interacting nano-structures immersed in a viscous fluid. In this scheme, nanostructures are represented by aggregates of small spheres. All optical and hydrodynamic interactions, including thermal fluctuations, are included. As an example, we consider optical binding of dielectric nanowires in counterpropagating plane waves. In particular, the formation of stable, ladder like structures, is demonstrated. In these arrangements, each nanowire lies parallel to the polarization direction of the beams, with their centres of mass colinear.
We consider the trapping of low refractive index objects, such as ultrasound contrast agent microbubbles, in a dual-beam fibre-optic trap. We confirm numerically that such a configuration results in stable trapping and we present the calculated trapping forces and spring constants. Furthermore we calculate the photonic stress profile over the surface of the trapped microbubble using both ray optics and Mie scattering approaches, and compare the results. We then find the optical stress-induced deformation of the microbubble for both the ray optics and Mie scattering stress profiles using linear elastic membrane theory. We suggest that this method could be a useful tool for quantifying the mechanical properties of the shell material of an ultrasound contrast agent microbubble.
We present a study of the manipulation of microparticles and the formation of optically bound structures of
particles in evanescent wave traps. Two trapping geometries are considered: the first is a surface trap where
the evanescent field above a glass prism is formed by the interference of a number of laser beams incident on
the prism-water interface; the second uses the evanescent field surrounding a biconical tapered optical fiber that
has been stretched to produce a waist of submicron diameter. In the surface trap we observe optical binding
of microparticles in to one-dimensional chain structures. In the tapered optical fiber trap we demonstrate
both particle transport for long distances along the fiber, and the formation of stable arrays of particles. In
both experiments we use video microscopy to track the particle locations and make quantitative measures of
the particle dynamics. The experimental studies of particle structures are complemented by light scattering
calculations based on Mie theory to infer how the geometries of the observed particle structures are controlled
by the underlying incident and scattered optical fields.
We present the result of an investigation into the optical trapping of micropaticles using laser beams with a spatially inhomogeneous polarization (cylindrical vector beams). We perform three-dimensional tracking of the Brownian fluctuations in position of a trapped particle and extract the trap spring constants. We characterize the trap geometry by the aspect ratio of spring constants in the directions transverse and parallel to the beam propagation direction and evaluate this figure of merit as a function of polarization angle. We show that the additional degree of freedom present in cylindrical vector beams (CVBs) allows us to control the optical trap strength and geometry by adjusting the polarization of the trapping beam only. Experimental results are compared with a theoretical model of optical trapping using CVBs derived from electromagnetic scattering theory in the T-matrix framework.
We investigate experimentally and theoretically plasmon-enhanced optical trapping of metal nanoparticles. We calculate
the optical forces on gold and silver nanospheres through a procedure based on the Maxwell stress tensor in the transition
T-matrix formalism. We compare our calculations with experimental results finding excellent agreement. We also
demonstrate how light-driven rotations can be generated and detected in non-symmetric nanorods aggregates. Analyzing
the motion correlations of the trapped nanostructures, we measure with high accuracy both the optical trapping
parameters, and the rotation frequency induced by the radiation pressure.