Transverse spin forces and non-equilibrium particle dynamics in a circularly polarized vacuum optical trap.

We provide a vivid demonstration of the mechanical effect of transverse spin momentum in an optical beam in free space. This component of the Poynting momentum was previously thought to be virtual, and unmeasurable. Here, its effect is revealed in the inertial motion of a probe particle in a circularly polarized Gaussian trap, in vacuum. Transverse spin forces combine with thermal fluctuations to induce a striking range of non-equilibrium phenomena. With increasing beam power we observe (i) growing departures from energy equipartition, (ii) the formation of coherent, thermally excited orbits and, ultimately, (iii) the ejection of the particle from the trap. As well as corroborating existing measurements of spin momentum, our results reveal its dynamic effect. We show how the under-damped motion of probe particles in structured light fields can expose the nature and morphology of optical momentum flows, and provide a testbed for elementary non-equilibrium statistical mechanics.

Optical trapping of microalgae at 735-1064 nm: photodamage assessment.

Living microalgal cells differ from other cells that are used as objects for optical micromanipulation, in that they have strong light absorption in the visible range, and by the fact that their reaction centers are susceptible to photodamage. We trapped cells of the microalga Trachydiscus minutus using optical tweezers with laser wavelengths in the range from 735 nm to 1064 nm. The exposure to high photon flux density caused photodamage that was strongly wavelength dependent. The photochemical activity before and after exposure was assessed using a pulse amplitude modulation (PAM) technique. The photochemical activity was significantly and irreversibly suppressed by a 30s exposure to incident radiation at 735, 785, and 835 nm at a power of 25 mW. Irradiance at 885, 935 and 1064 nm had negligible effect at the same power. At a wavelength 1064 nm, a trapping power up to 218 mW caused no observable photodamage.

Static and dynamic behavior of two optically bound microparticles in a standing wave.

It is generally accepted that the interaction between particles mediated by the scattered light (called optical binding) is very weak. Therefore, the optical binding is usually neglected in a multi-particle trapping in distinct optical traps. Here we show that even the presence of only two dielectric particles confined in the standing wave leads to their significantly different behavior comparing to the case of a single trapped particle. We obtained persuading coincidence between our experimental records and the results of the deterministic and stochastic theoretical simulations based on the coupled dipole method.

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.

[Experience in data management of the clinical retrospective project in Czech and Slovak oncology centres (IKARUS Project)]. / Zkusenosti s managementem dat retrospektivního klinického projektu v podmínkách ceské a slovenské onkologie (projekt IKARUS).

BACKGROUND: The retrospective part of the IKARUS Project (Incidence of Skeletal Related Events in Breast Cancer) was focused on monitoring the incidence of skeletal related events in patients with metastatic breast cancer treated in the Czech and Slovak Republics.The aim was to describe the experience with data collection management in the conditions of the Czech and Slovak Republics. SUBJECTS AND METHODS: Retrospective collection of data in multi-centre, non-interventional, epidemiological and explorative studies. Female patients diagnosed since 2000 were involved in the project in order to respect the five-year period of monitoring and to describe the treatment of the period. RESULTS: During the initiation phase of the retrospective study each of the 18 Complex Cancer Centres in the Czech Republic (see www.linkos.cz) and 18 chosen oncology centres in the Slovak Republic were addressed. In the end, data were collected from 13 oncology centres in the Czech Republic and 12 oncology centres in the Slovak Republic. The initial plan to enrol 650 patients was not completed; data on 254 patients from the Czech Republic and 125 patients from the Slovak Republic were finally analysed.The effectiveness of retrospective data collection in the conditions of Czech and Slovak oncology corresponded with the possibilities of access to data of formerly diagnosed and treated patients. In searching for retrospective data the present hospital information systems could not be used in most oncology centres.Therefore, the cost of retrospective data collection was estimated and was shown to be relatively high. CONCLUSION: The binding methodical conclusion is that unless a systemic change is made in the functionality of hospital information systems and standardised electronic documentation is introduced, the retrospective collection of clinical data in our conditions will be associated with high costs and a relatively low recovery factor.

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.

Long-range one-dimensional longitudinal optical binding.

We create extended longitudinally optically bound chains of microparticles with the use of counterpropagating "nondiffracting" light fields, the so-called Bessel beams. The beam homogeneity and extended propagation range allow the creation of 200 microm long chains of organized microparticles. We observe short-range multistability within a single chain and long-range multistability between several distinct chains. Our observations are supported by theoretical results of the coupled dipole method.

Single-beam trapping in front of reflective surfaces.

We show that the optical trapping of dielectric particles by a single focused beam in front of a weakly reflective surface is considerably affected by interference of the incident and reflected beams, which creates a standing-wave component of the total field. We use the two-photon-excited fluorescence from a trapped dyed probe to detect changes in the distance between the trapped beam focus as the focus approaches the reflective surface. This procedure enables us to determine the relative strengths of the single-beam and the standing-wave trapping forces. We demonstrate that, even for reflection from a glass-water interface, standing-wave trapping dominates, as far as 5 mum from the surface.

Optical trapping of nanoparticles and microparticles by a Gaussian standing wave.

The optical trapping of nanoparticles and microparticles by a Gaussian standing wave is experimentally demonstrated for the first time to the authors' knowledge. The standing wave is obtained under a microscope objective as a result of the interference of an incoming laser beam and a beam reflected on a microscope slide that has been coated with a system of reflective dielectric layers. Experimental results show that three-dimensional trapping of nanoparticles (100-nm polystyrene spheres) and one or more vertically aligned micro-objects (5-mum polystyrene spheres, yeast cells) can easily be achieved by use of even highly aberrated beams or objectives with low numerical apertures.

Use of the beat frequency between two modes for frequency stabilization of internal-mirror lasers.

We present a simple method of frequency stabilization of a laser with two orthogonal modes that uses the effects of anomalous dispersion at the active line. The result of this effect is that the frequency difference between oscillation modes varies during the frequency tuning along the Doppler line of the transition and has its minimum or maximum when the modes are symmetrically placed around the central part of the gain curve. We applied the semiclassical approach to describe the laser, and we established the rules for laser parameters to obtain the most convenient conditions for stabilization. We stabilized the thermally compensated experimental laser tube and reached frequency stabilities of 3 × 10(-9) and 3 × 10(-10) for integrating times of 1 and 134 s, respectively.