Martin Šiler

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.

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.

Following the mechanisms of bacteriostatic versus bactericidal action using Raman spectroscopy.

Antibiotics cure infections by influencing bacterial growth or viability. Antibiotics can be divided to two groups on the basis of their effect on microbial cells through two main mechanisms, which are either bactericidal or bacteriostatic. Bactericidal antibiotics kill the bacteria and bacteriostatic antibiotics suppress the growth of bacteria (keep them in the stationary phase of growth). One of many factors to predict a favorable clinical outcome of the potential action of antimicrobial chemicals may be provided using in vitro bactericidal/bacteriostatic data (e.g., minimum inhibitory concentrations—MICs). Consequently, MICs are used in clinical situations mainly to confirm resistance, and to determine the in vitro activities of new antimicrobials. We report on the combination of data obtained from MICs with information on microorganisms’ “fingerprint” (e.g., DNA/RNA, and proteins) provided by Raman spectroscopy. Thus, we could follow mechanisms of the bacteriostatic versus bactericidal action simply by detecting the Raman bands corresponding to DNA. The Raman spectra of Staphylococcus epidermidis treated with clindamycin (a bacteriostatic agent) indeed show little effect on DNA which is in contrast with the action of ciprofloxacin (a bactericidal agent), where the Raman spectra show a decrease in strength of the signal assigned to DNA, suggesting DNA fragmentation.

Optical forces induced behavior of a particle in a non-diffracting vortex beam.

An interaction between a light field with complex field spatial distribution and a micro-particle leads to forces that drag the particle in space and may confine it in a stable position or a trajectory. The particle behavior is determined by its size with respect to the characteristic length of the spatially periodic or symmetric light field distribution. We study theoretically and experimentally the behavior of a microparticle near the center of an optical vortex beam in a plane perpendicular to the beam propagation. We show that such particle may be stably trapped either in a dark spot on the vortex beam axis, or in one of two points placed off the optical axis. It may also circulate along a trajectory having its radius smaller or equal to the radius of the first bright vortex ring.

Surface delivery of a single nanoparticle under moving evanescent standing-wave illumination

We study the delivery of a submicrometre-sized spherical dielectric particle suspended in water and confined in an evanescent field in the proximity of a glass–water interface. When illuminated by a single evanescent wave, the particle is propelled along the glass surface by the radiation pressure. Illumination by two counter-propagating and coherent evanescent waves leads to the formation of a surface-bound evanescent standing wave serving as a one-dimensional array of optical traps for the stable confinement of the particle. These traps can be translated simultaneously along the surface by shifting the phase of one of the two interfering evanescent waves, carrying the confined particle along in an optical conveyor belt (OCB). However, due to the thermal activation, the particle jumps between neighboring optical traps, and its delivery conditions in the OCB are thus more complex than in the case of the single evanescent wave propulsion. We analyze the delivery speed of a single particle confined in the OCB moving with different speeds and formed by optical traps of different depths. We present a theoretical description of the particle delivery speed in the OCB and compare it with the delivery speed in the single evanescent wave. We support our theoretical conclusions by experimental observations and demonstrate that especially particles having diameters smaller than ~220 nm are delivered faster in the OCB using the same total optical power.

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.

Complex rotational dynamics of multiple spheroidal particles in a circularly polarized, dual beam trap

We examine the rotational dynamics of spheroidal particles in an optical trap comprising counter-propagating Gaussian beams of opposing helicity. Isolated spheroids undergo continuous rotation with frequencies determined by their size and aspect ratio, whilst pairs of spheroids display phase locking behaviour. The introduction of additional particles leads to yet more complex behaviour. Experimental results are supported by numerical calculations.

Optical sorting of nonspherical and living microobjects in moving interference structures

Contactless, sterile and nondestructive separation of microobjects or living cells is demanded in many areas of biology and analytical chemistry, as well as in physics or engineering. Here we demonstrate advanced sorting methods based on the optical forces exerted by travelling interference fringes with tunable periodicity controlled by a spatial light modulator. Besides the sorting of spherical particles we also demonstrate separation of algal cells of different sizes and particles of different shapes. The three presented methods offer simultaneous sorting of more objects in static suspension placed in a Petri dish or on a microscope slide.

Candida parapsilosis Biofilm Identification by Raman Spectroscopy

Colonies of Candida parapsilosis on culture plates were probed directly in situ using Raman spectroscopy for rapid identification of specific strains separated by a given time intervals (up to months apart). To classify the Raman spectra, data analysis was performed using the approach of principal component analysis (PCA). The analysis of the data sets generated during the scans of individual colonies reveals that despite the inhomogeneity of the biological samples unambiguous associations to individual strains (two biofilm-positive and two biofilm-negative) could be made.