László Oroszi

Protein-based integrated optical switching and modulation

The static and dynamic response of optical waveguides coated with a thin protein film of bacteriorhodopsin was investigated. The size and kinetics of the light-induced refractive index changes of the adlayer were determined under different conditions of illumination. The results demonstrate the applicability of this protein as an active, programmable nonlinear optical material in all-optical integrated circuits.

Light sailboats: Laser driven autonomous microrobots

We introduce a system of light driven microscopic autonomous moving particles that move on a flat surface. The design is simple, yet effective: Micrometer sized objects with wedge shape are produced by photopolymerization, and they are covered with a reflective surface. When the area of motion is illuminated perpendicularly from above, the light is deflected to the side by the wedge shaped objects, in the direction determined by the position and orientation of the particles. The momentum change during reflection provides the driving force for an effectively autonomous motion. The system is an efficient tool to study self propelled microscopic robots.

Fast integrated optical switching by the protein bacteriorhodopsin

State-of-the-art photonic integration technology is ready to provide the passive elements of optical integrated circuits, based either on silicon, glass or plastic materials. The bottleneck is to find the proper nonlinear optical (NLO) materials in waveguide-based integrated optical circuits for light-controlled active functions. Recently, we proposed an approach where the active role is performed by the chromoprotein bacteriorhodopsin as an NLO material, that can be combined with appropriate integrated optical devices. Here we present data supporting the possibility of switching based on a fast photoreaction of bacteriorhodopsin. The results are expected to have important implications for photonic switching technology.

Optical tweezers with tips grown at the end of fibers by photopolymerization.

We present a method to build an optical tip at the end of a single-mode optical fiber. The tip is grown by a self-writing process: photopolymerization by the light coming from the optical fiber. We developed a technique to produce a flat end surface on the tip. The good optical quality of the tip and the output laser beam was demonstrated by the fact that a counterpropagating optical trap could be constructed by using the tips with parameters comparable to regular fiber traps. Because of the small size of the tips, the tweezers require a much smaller space than regular fiber traps.

Control of electro-osmostic flow by light

The author introduce a technique for the optical control of electro-osmosis. One wall of the microfluidic channel is formed by a photoconductive material. Illumination of the photoconductor results in a local drop of the electric field in the microchannel, reducing the electro-osmotic driving force. Fluid flow can be effectively modulated by light utilizing this effect. They verified the phenomenon in linear channels and further developed the method by building a Y junction, an optically controlled microfluidic switch. This unit is the basis of more complex flow patterns: it demonstrates the feasibility of dynamic optical control of microfluidic devices.

Modeling of ionic relaxation around a biomembrane disk

A simplified Brownian dynamics model and the corresponding software implementation have been developed for the simulation of electrolyte dynamics on the mesoscopic scale. In addition to direct control simulations, the model system has been verified by a quantitative comparison with the Debye-Hückel theory. As a first application, the model was used to simulate ionic relaxation processes following abrupt intramembrane charge rearrangements in the case of a disk shaped membrane. In addition to its general implications, the obtained properties of the relaxation kinetics confirm the assumptions of the theory of the so-called suspension method, a technique capable of tracing molecular charge motions of membrane proteins in three dimensions.

Theory of electric signals of membrane proteins in three dimensions

Abstract. Transmembrane ion pumps are often investigated experimentally by photoelectric measurements in model systems. In addition to the most widely used systems based on model membranes, a fundamentally different class is represented by the so-called suspension methods. In this technique the electric signal is measured on a bulk suspension of oriented ion pumps in the form of a displacement current. On this system, electric and spectroscopic experiments can be performed simultaneously. Using the information from both types of measurements, and utilizing the three-dimensional nature of the system, it is possible to follow the intramolecular charge motions in all three spatial directions. The derivable dipole moment changes associated with conformational transitions allow the verification of molecular dynamic models. In this work a theory is presented to describe the suspension method; samples with different symmetry properties and the possibilities of photoselection to obtain the desired three-dimensional information are analyzed.

An all optical microfluidic sorter.

We present a microfluidic cell sorter that is able to count, characterize and sort micrometer sized particles and cells. In addition to optical counting and characterization, also sorting is performed by optical forces. The device is optimized for simplicity. The microfluidic channels and optical waveguides that carry the illuminating, detecting and sorting light form a single integrated structure, all built from the same material in a single photopolymerization step.

Dimensionality constraints of light-induced rotation

We have studied the conditions of rotation induced by collimated light carrying no angular momentum. Objects of different shapes and optical properties were examined in the nontrivial case where the rotation axis is perpendicular to the direction of light propagation. This geometry offers important advantages for application as it fundamentally broadens the possible practical arrangements to be realised. We found that collimated light cannot drive permanent rotation of 2D or prism-like 3D objects (i.e. fixed cross-sectional profile along the rotation axis) in the case of fully reflective or fully transparent materials. Based on both geometrical optics simulations and theoretical analysis, we derived a general condition for rotation induced by collimated light carrying no angular momentum valid for any arrangement: Permanent rotation is not possible if the scattering interaction is two-dimensional and lossless. In contrast, light induced rotation can be sustained if partial absorption is present or the object has specific true 3D geometry. We designed, simulated, fabricated, and experimentally tested a microscopic rotor capable of rotation around an axis perpendicular to the illuminating light.

Optical Waveguide Lightmode Spectroscopy and Biocomputing

A concept of a new application based on the outstanding nonlinear optical properties of the chromoprotein bacteriorhodopsin was investigated. Using the Optical Waveguide Lightmode Spectroscopy technique on dried bacteriorhodopsin films gave us the possibility to exploit the large refractive index changes corresponding to the absorption changes during the photocycle. The results demonstrate the applicability of this protein as an active nonlinear optical material in all-optical integrated circuits.