Shima Fardad

Optical nonlinearities and enhanced light transmission in soft-matter systems with tunable polarizabilities.

We demonstrate a new class of synthetic colloidal suspensions capable of exhibiting negative polarizabilities, and observe for the first time robust propagation and enhanced transmission of self-trapped light over long distances that would have been otherwise impossible in conventional suspensions with positive polarizabilities. Such light penetration through the strong scattering environment is attributed to the interplay between optical forces and self-activated transparency effects while no thermal effect is involved. By judiciously mixing colloidal particles of both negative and positive polarizabilities, we show that the resulting nonlinear response of these systems can be fine-tuned. Our experimental observations are in agreement with theoretical analysis based on a thermodynamic model that takes into account particle-particle interactions. These results may open up new opportunities in developing soft-matter systems with engineered optical nonlinearities.

Plasmonic resonant solitons in metallic nanosuspensions.

Robust propagation of self-trapped light over distances exceeding 25 diffraction lengths has been demonstrated for the first time in plasmonic nanosuspensions. This phenomenon results from the interplay between optical forces and enhanced polarizability that would have been otherwise impossible in conventional dielectric dispersions. Plasmonic nanostructures such as core-shell particles, nanorods, and spheres are shown to display tunable polarizabilities depending on their size, shape, and composition, as well as the wavelength of illumination. Here we discuss nonlinear light-matter dynamics arising from an effective positive Kerr effect, which in turn allows for deep penetration of long needles of light through dissipative colloidal media. Our findings may open up new possibilities toward synthesizing soft-matter systems with customized optical nonlinearities.

Trapping and rotating microparticles and bacteria with moiré-based optical propelling beams

We propose and demonstrate trapping and rotation of microparticles and biological samples with a moiré-based rotating optical tweezers. We show that polystyrene beads, as well as Escherichia coli cells, can be rotated with ease, while the speed and direction of rotation are fully controllable by a computer, obviating mechanical movement or phase-sensitive interference. Furthermore, we demonstrate experimentally the generation of white-light propelling beams and arrays, and discuss the possibility of optical tweezing and particle micro-manipulation based on incoherent white-light rotating patterns.

Interactions between self-channeled optical beams in soft-matter systems with artificial nonlinearities

We demonstrate optical interactions between stable self-trapped optical beams in soft-matter systems with pre-engineered saturable self-focusing optical nonlinearities. Our experiments, carried out in dilute suspensions of particles with negative polarizabilities, show that optical beam interactions can vary from attractive to repulsive, or can display an energy exchange depending on the initial relative phases. The corresponding observations are in good agreement with theoretical predictions.

Generalized Mie theory of optical forces

The theory of optical forces on spherical scatterers is here generalized to arbitrary incident fields. The interaction between spherical harmonics of different order, and the degree and azimuthal parity, is studied in detail. The resulting force from all the contributing components is presented in analytical form. A further generalization of this formulation to nonspherical scatterers is also discussed.

High efficiency cholesteric liquid crystal lasers with an external stable resonator

An amplified cholesteric liquid crystal (CLC) laser performance is demonstrated by utilizing a binary-dye mixture (with 62 wt% DCM and 38 wt% PM597) as the active medium and an external stable resonator. The measured results show that the laser efficiency is enhanced as compared to the highest efficiency of each individual dye. Furthermore, using such an active CLC in an external stable resonator leads to a approximately 92X improved efficiency over the single CLC laser. In this instance, the binary-dye doped CLC simultaneously functions as laser oscillator and amplifier.

Tunable polarizability and self-trapping of light in colloidal suspensions of gold nanoparticles

Aqueous suspensions containing pure gold nanoparticles and silica-gold core-shells are shown to exhibit different polarizibilities, thus allowing self-trapping of long needles of light. The different nonlinear mechanisms behind these processes are investigated.

Dielectric and metallic nanosuspensions with tunable optical nonlinearities

We provide a brief report on our recent work on dielectric and metallic colloidal nanosuspensions with negative polarizability where we observed robust propagation of self-trapped light over a long distance. Our results open up new opportunities in developing soft-matter systems with tunable optical nonlinearities.

Formation and interaction of self-guided optical beams in a pre-engineered soft-matter system

We demonstrate stable beam self-trapping in soft-matter systems with artificial saturable self-focusing nonlinearities. Our experiments reveal optical beam interactions that can vary from attractive to repulsive as well as an energy exchange.

Anomalous optical forces on a Mie-particle in a transverse Poynting vector flow

We show how a superposition of non-interfering plane waves can generate a transverse Poynting vector, which in turn leads to transverse optical forces on a fully isotropic dielectric scatterer, in contrast with conventional radiation-pressure forces.