T. Čižmár

High quality quasi-Bessel beam generated by round-tip axicon

We study theoretically and experimentally the spatial intensity distribution of the zero-order Bessel beam formed by the axicon which possess a rounded tip. Such a tip generates a refracted beam that interferes with the quasi-Bessel beam created behind the axicon. In turn an undesired intensity modulation occurs that significantly disturbs the unique properties of the quasi-Bessel beam--namely the constant shape of the lateral intensity distribution and the slow variation of the on-axis beam intensity along the beam propagation. We show how the spatial filtration of the beam in the Fourier plane improves this spatial beam distribution and removes the undesired modulation. We use an efficient numerical method based on Hankel transformations to simulate the propagation of the beam behind the axicon and filter. We experimentally measure the intensity distribution of the beam in many lateral planes and subsequently reconstruct the spatial intensity distribution of the beam. Computed and measured beam distributions are compared and the obtained agreement is very good.

Optical sorting and detection of submicrometer objects in a motional standing wave

An extended interference pattern close to the surface may result in either a transmissive or an evanescent surface field for large-area manipulation of trapped particles. The affinity of differing particle sizes to a moving standing-wave light pattern allows us to hold and deliver them in a bidirectional manner and demonstrate experimentally particle sorting in the submicrometer region. This is performed without the need of fluid flow (static sorting). Theoretical predictions support the experimental observations that certain sizes of colloidal particles thermally hop more easily between neighboring traps. A generic method is also presented for particle position detection in an extended periodic light pattern and applied to characterization of optical traps and particle behavior.

Sub-micron particle organization by self-imaging of non-diffracting beams

We present the first theoretical and experimental study of dielectric sub-micron particle behaviour in an optical field generated by interference of co-propagating non-diffracting beams of different propagation constants. In such a field, there are periodic oscillations of the on-axial intensity maxima (self- imaging) that are frequently mentioned as useful for optical trapping. We show thatinthreedimensionsthisistrueonlyforverysmallparticlesandtheincreasing number of interfering beams does not enable confinement of substantially bigger particles under the studied conditions. Experimentally, we succeeded in optical confinement of beads radii from 100nm up to 300nm but only with the help of fluid flow against the beams propagation. We observed self-organization of the particles into the periodic 1D array with the interparticle distance equal to 7.68 µm. We observed how a bead jump from one trap to the neighbouring- occupied trap caused a domino effect propagating with constant velocity over the subsequent occupied traps. Phase shift in one beam induced controlled bi-directional shift of the whole structure over a maximal distance of 250 µm in two co-propagating Bessel beams.

Bidirectional Optical Sorting of Gold Nanoparticles

We present a generic technique allowing size-based all-optical sorting of gold nanoparticles. Optical forces acting on metallic nanoparticles are substantially enhanced when they are illuminated at a wavelength near the plasmon resonance, as determined by the particles geometry. Exploiting these resonances, we realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.

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.

Formation of long and thin polymer fiber using nondiffracting beam

We present a unique method that utilizes high intensity core of the zero-order nondiffracting beam (NDB) to fabricate a homogeneous polymer fiber as narrow as 2 mum and as long as centimeters. The constant diameter of the fiber along all its length is done by the propagation invariant properties of the NDB. The length of the fiber is determined by the maximum propagation distance of the NDB which is much longer than the classical Gaussian beam of comparable width. Moreover, we also proved that the self-writing waveguide mechanism prolongs the length of the developed fibers. Circular movement of the NDB creates hollow fiber, several co-axial, or overlapping fibers.

Optical tracking of spherical micro-objects in spatially periodic interference fields

We present a new method that provides precise detection of micro-object position with respect to a spatially periodic illumination field. Altering the mutual position of the object and the illumination field causes that a pattern of scattered light detected perpendicularly by a CCD camera changes. We present a procedure how to employ this pattern changes to track micrometer-size object in the standing wave and how to apply this method to optical tracking of Brownian particle even in periodic illumination field in motion.

Orbital angular momentum of mixed vortex beams

The orbital angular momentum (OAM) of the single vortex beam depends on its power and wavefront helicity. In the paper, this relation is generalized for mixed vortex beams composed of several coaxial vortices with different topological charges. The presented interference law indicates interference effects of the OAM resulting in local spatial gradients of the OAM density. Description of the OAM of mixed vortex beams is used for demonstration of a possibility to tune the OAM density of a composite vortex field without changing topological charges or intensity distribution. Experimental realization of the OAM tuning is discussed for interference of two focused vortex beams generated by means of a spiral phase mask.

A biophotonics platform based on optical trapping of photonic membranes

We present a biophotonics platform based on the optical manipulation of photonic membranes via holographical tweezers. We review the fabrication and manipulation protocol which grants full six-degrees-of-freedom control over these membranes. This is despite the membranes having extreme aspect ratios, being 90 nm in thickness and 15 - 20 μm in side length. The photonic properties of the trapped membranes can be tailored to very specific applications, by structuring their topology carefully. Our method merges the flexibility of photonic design of optical meta-surfaces with the advanced manipulation capability offered by holographic optical tweezers. Here we demonstrate the validity of our approach, discussing the peculiar mechanical properties of trapped photonic membranes. Specifically, we focus on imaging and surface-enhanced Raman spectroscopy applications.