Vítězslav Karásek

Experimental and theoretical determination of optical binding forces

We present an experimental and theoretical study of long distance optical binding effects acting upon micro-particles placed in a standing wave optical field. In particular we present for the first time quantitatively the binding forces between individual particles for varying inter-particle separations, polarizations and incident angles of the binding beam. Our quantitative experimental data and numerical simulations show that these effects are essentially enhanced due to the presence of a reflective surface in a sample chamber. They also reveal conditions to form stable optically bound clusters of two and three particles in this geometry. We also show that the inter-particle separation in the formed clusters can be controlled by altering the angle of the beam incident upon the sample plane. This demonstrates new perspectives for the generation and control of optically bound soft matter and may be useful to understand various inter-particle effects in the presence of reflective surfaces.

Three-Dimensional Optical Trapping of a Plasmonic Nanoparticle using Low Numerical Aperture Optical Tweezers

It was previously believed that larger metal nanoparticles behave as tiny mirrors that are pushed by the light beam radiative force along the direction of beam propagation, without a chance to be confined. However, several groups have recently reported successful optical trapping of gold and silver particles as large as 250 nm. We offer a possible explanation based on the fact that metal nanoparticles naturally occur in various non-spherical shapes and their optical properties differ significantly due to changes in localized plasmon excitation. We demonstrate experimentally and support theoretically three-dimensional confinement of large gold nanoparticles in an optical trap based on very low numerical aperture optics. We showed theoretically that the unique properties of gold nanoprisms allow an increase of trapping force by an order of magnitude at certain aspect ratios. These results pave the way to spatial manipulation of plasmonic nanoparticles using an optical fibre, with interesting applications in biology and medicine.

Non-spherical gold nanoparticles trapped in optical tweezers: shape matters

We present the results of a theoretical analysis focused on three-dimensional optical trapping of non-spherical gold nanoparticles using a tightly focused laser beam (i.e. optical tweezers). We investigate how the wavelength of the trapping beam enhances trapping stiffness and determines the stable orientation of nonspherical nanoparticles in the optical trap which reveals the optimal trapping wavelength. We consider nanoparticles with diameters being between 20 nm and 254 nm illuminated by a highly focused laser beam at wavelength 1064 nm and compare our results based on the coupled-dipole method with published theoretical and experimental data. We demonstrate that by considering the non-spherical morphology of the nanoparticle we can explain the experimentally observed three-dimensional trapping of plasmonic nanoparticles with size higher than 170 nm. These results will contribute to a better understanding of the trapping and alignment of real metal nanoparticles in optical tweezers and their applications as optically controllable nanosources of heat or probes of weak forces and torques.

Dynamic size tuning of multidimensional optically bound matter

We generate and dynamically control one-, two- and three-dimensional optically bound structures of soft matter in the geometry of counter-propagating incoherent laser beams. We report results for the Bessel, Gaussian, and Laguerre-Gaussian laser modes and particularly focus on the influence of the lateral dimensions of the beam profile on the resulting self-arranged optically bound structures. Employing the transfer of the orbital angular momentum of light in the Laguerre-Gaussian beams, we show that optically bound structures can conserve their spatial arrangements even while orbiting along the beam circumference.

Extreme axial optical force in a standing wave achieved by optimized object shape

Standing wave optical trapping offers many useful advantages in comparison to single beam trapping, especially for submicrometer size particles. It provides axial force stronger by several orders of magnitude, much higher axial trap stiffness, and spatial confinement of particles with higher refractive index. Mainly spherical particles are nowadays considered theoretically and trapped experimentally. In this paper we consider prolate objects of cylindrical symmetry with radius periodically modulated along the axial direction and we present a theoretical study of optimized objects shapes resulting in up to tenfold enhancement of the axial optical force in comparison with the original unmodulated object shape. We obtain analytical formulas for the axial optical force acting on low refractive index objects where the light scattering by the object is negligible. Numerical results based on the coupled dipole method are presented for objects with higher refractive indices and they support the previous simplified analytical conclusions.

Optical binding in non-diffracting beams

Optical binding forces are present always when two and more colloidal particles are confined by external optical forces in a limited volume. They are caused by rescattering of the original incident field and they manifest themselves by a specific stationary displacements between confined micro-objects. Under certain circumstances the objects can self-organize into spatial arrangements creating so called optically bound matter. Detailed understanding of these forces might enable one to control this behavior. In this paper we present a theoretical study of the object self-organization in the field of two counter-propagating spatially incoherent non-diffracting beams. The field is uniform along the axis of propagation and so the studied phenomenon does not depend on the axial position of micro-objects. Therefore in this system the binding forces can be simply separated from the external forces of trapping field. We also compare this situation with setup of two counter-propagating Gaussian beams which are routinely used for studies of optical binding effects.

Optical binding of unlike particles

The self-arrangement of several micron-sized particles (due to their mutual scattering of light fields) is called optical binding. The scattered light is induced by incident laser fields which may be also used for partial localization (similarly to optical tweezers) of the particles in the water solution. We have previously experimentally studied the optical binding in the configurations employing two counter-propagation Bessel beams and these results were sufficiently supported by our numerical simulations (Coupled dipole method { CDM). Here we present numerical studies of configurations where some symmetries of the problem are broken. We study how the difference of sizes of two spherical particles influences their spatial stable separation. These results may serve for detection of particles of different composition, size or shape.

Behavior of microparticles in laser interference field

We describe a general way how to calculate optical forces and torque acting on colloids placed into laser interference field. In this paper we focus on a configuration with three interfering beams laying in one plane and we present a comprehensive analysis of trap properties and particle behaviour. We found that this arrangement can be used for sorting of particles according to their size or refractive index.

Optical trapping of non-spherical plasmonic nanoparticles

Laser manipulation with plasmonic nano-particles is a rapidly growing field with various practical applications stretching beyond physics towards biology and chemistry. For example gold nano-particles can be employed as local heat source, probes for surface enhanced Raman spectroscopy with a sensitivity going down to a single molecule or contact-less probe in scanning near-field optical microscope. A single tightly focused laser beam optical tweezers was also employed to three-dimensional trapping of gold and silver nano-particles with diameters between 20 to 250 nm. However, theoretical models assuming the spherical shape of a nano-particle predict spatial confinement only for particles with diameter lower than 100 nm. Our results indicate this discrepancy is caused by ignoring particles shape which is very important for understanding of light-matter interaction.

Dynamics of an optically bound structure made of particles of unequal sizes

This theoretical study based on the coupled dipoles model focuses on the dynamics of two optically bound dielectric spheres of unequal sizes confined in counter-propagating incoherent Bessel beams. We analyzed the relative motion of the particles with respect to each other and defined conditions where they form a stable optically bound structure (OBS). We also investigated the motion of the center of mass of the OBS and found that its direction depends on the particle separation in the structure. Besides the optical interaction between objects, we also considered a hydrodynamic coupling in order to obtain more precise results for moving an OBS.