Anna Bezryadina

Dipole solitons in optically induced two-dimensional photonic lattices

Dipole solitons in a two-dimensional optically induced photonic lattice are theoretically predicted and experimentally demonstrated for the first time to our knowledge. It is shown that such dipole solitons are stable and robust under appropriate conditions. Our experimental results are in good agreement with theoretical predictions.

Observation of two-dimensional lattice vector solitons

We demonstrate the formation of fundamental and dipolelike vector solitons in an optically induced two-dimensional photonic lattice. Such vector solitons are realized by mutual trapping of two beams in the lattice. Our theoretical results are in good agreement with experimental observations.

Self-trapping and flipping of double-charged vortices in optically induced photonic lattices.

We report what is believed to be the first observation of self-trapping and charge-flipping of double-charged optical vortices in two-dimensional photonic lattices. Both on- and off-site excitations lead to the formation of rotating quasi-vortex solitons, reversing the topological charges and the direction of rotation through a quadrupole-like transition state. Experimental results are corroborated with numerical simulations.

Air stability of TiO2/PbS colloidal nanoparticle solar cells and its impact on power efficiency

The short-term (less than 1 hour) exposure of TiO2/PbS quantum dot photovoltaics to air increases the open circuit voltage (Voc) and fill factor (FF) while slightly decreasing the short circuit current density (Jsc), leading to a power conversion efficiency above 4% and a peak external quantum efficiency over 80% for 1.1 eV PbS. The resulting Jsc, Voc, and FF under 100 mW/cm2 AM1.5 are 18.6 mA/cm2, 0.517 V, and 42% for 1.1 eV PbS and 8.03 mA/cm2, 0.655 V, and 35% for 1.7 eV PbS, respectively. Long-term air exposures result in much lower conductivities. Furthermore, short-term air exposure effects are fully reversible upon removal from air, and longer-term effects are mostly reversible through soaking in 1,2-ethanedithiol.

OBSERVATION OF ONE- AND TWO-DIMENSIONAL DISCRETE SURFACE SPATIAL SOLITONS

The recent theoretical predictions and experimental observations of discrete surface solitons propagating along the interface between a one- or two-dimensional continuous medium and a one- or two-dimensional waveguide array are reviewed. These discrete solitons were found in second order (periodically poled lithium niobate) and third order nonlinear media, including AlGaAs, photorefractive media and glass, respectively.

Experiments on Gaussian beams and vortices in optically induced photonic lattices

We investigate experimentally the propagation of fundamental Gaussian beams and vortices in a two-dimensional photonic lattice optically induced with partially coherent light. We focus on soliton-lattice interactions and vortex-lattice interactions when the lattice is operated in a nonlinear regime. In this case a host of novel phenomena is demonstrated, including soliton-induced lattice dislocation-deformation, soliton hopping and slow-down, and creation of structures akin to optical polarons. In addition, we observe that the nonlinear interaction between a vortex beam and a solitonic lattice leads to lattice twisting due to a transfer of the angular momentum carried by the vortex beam to the lattice. Results demonstrating a clear transition from discrete diffraction to the formation of two-dimensional, discrete fundamental and vortex solitons in a linear lattice are also included.

Mid-gap trap states in CdTe nanoparticle solar cells

Thin film solar cells comprised of quantum-confined CdTe nanoparticles are shown to have a low intrinsic density of mid-gap trap states relative to their equivalent bulk film, indicating that the ligands are effective at electrically passivating surface states. Sintering the nanoparticles into a poly-crystalline thin film increases device performance but also increases the density of mid-gap trap states due to doping from the CdCl treatment and the formation of long range disorder such as grain boundaries and dislocations. Long term aging under illumination increases the density of mid-gap traps in the unsintered films due to degradation of the ligands.

Guiding and nonlinear coupling of light in plasmonic nanosuspensions

We demonstrate two different types of coupled beam propagation dynamics in colloidal gold nanosuspensions. In the first case, an infrared (IR) probe beam (1064 nm) is guided by a low-power visible beam (532 nm) in a gold nanosphere or in nanorod suspensions due to the formation of a plasmonic resonant soliton. Although the IR beam does not experience nonlinear self-action effects, even at high power levels, needle-like deep penetration of both beams through otherwise highly dissipative suspensions is realized. In the second case, a master/slave-type nonlinear coupling is observed in gold nanoshell suspensions, in which the nanoparticles have opposite polarizabilities at the visible and IR wavelengths. In this latter regime, both beams experience a self-focusing nonlinearity that can be fine-tuned.

Optical disassembly of cellular clusters by tunable ‘tug-of-war’ tweezers

Bacterial biofilms underlie many persistent infections, posing major hurdles in antibiotic treatment. Here we design and demonstrate ‘tug-of-war’ optical tweezers that can facilitate the assessment of cell–cell adhesion—a key contributing factor to biofilm development, thanks to the combined actions of optical scattering and gradient forces. With a customized optical landscape distinct from that of conventional tweezers, not only can such ‘tug-of-war’ tweezers stably trap and stretch a rod-shaped bacterium in the observing plane, but, more importantly, they can also impose a tunable lateral force that pulls apart cellular clusters without any tethering or mechanical movement. As a proof of principle, we examined a Sinorhizobium meliloti strain that forms robust biofilms and found that the strength of intercellular adhesion depends on the growth medium. This technique may herald new photonic tools for optical manipulation and biofilm study, as well as other biological applications.

Nonlinear self-action of light through biological suspensions

It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.