Faster and More Accurate Geometrical-Optics Optical Force Calculation Using Neural Networks.

Optical forces are often calculated by discretizing the trapping light beam into a set of rays and using geometrical optics to compute the exchange of momentum. However, the number of rays sets a trade-off between calculation speed and accuracy. Here, we show that using neural networks permits overcoming this limitation, obtaining not only faster but also more accurate simulations.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers.

Optical forces on microspheres inside hollow core photonic crystal fibers (HC-PCFs) are often predicted using a ray optics model, which constrains its validity based on wavelength and microsphere sizes. Here, we introduce a rigorous treatment of the electromagnetic forces based on the Lorenz-Mie theory, which involves analytical determination of beam shape coefficients for the optical modes of a HC-PCF. The method is more practicable than numerical approaches and, in contrast with ray optics models, it is not limited by system size parameters.

Changes in fertilization medium viscosity using hyaluronic acid impact bull sperm motility and acrosome status.

The female reproductive tract, in particular the composition of the uterine and oviduct fluids, is responsible, at least in part, for triggering sperm cell modifications, essential for the acquisition of fertilization ability. Hyaluronic acid (HA) is a glycosaminoglycan present in these fluids, and its role in the fertilization process and sperm functionality is still barely understood. This work was designed to (a) determine the rheological characteristics of the fertilization medium by the addition of HA and (b) determine the HA influence on sperm motility and functional status.

Experimental optical trapping with frozen waves.

We report, to the best of our knowledge, the first optical trapping experimental demonstration of microparticles with frozen waves. Frozen waves are an efficient method to model longitudinally the intensity of nondiffracting beams obtained by superposing copropagating Bessel beams with the same frequency and order. Based on this, we investigate the optical force distribution acting on microparticles of two types of frozen waves.

Expansion of arbitrary electromagnetic fields in terms of vector spherical wave functions.

Since 1908, when Mie reported analytical expressions for the fields scattered by a spherical particle upon incidence of plane-waves, generalizing his analysis for the case of an arbitrary incident wave has been an open question because of the cancellation of the prefactor radial spherical Bessel function. This cancellation was obtained before by our own group for a highly focused beam centered in the objective. In this work, however, we show for the first time how these terms can be canceled out for any arbitrary incident field that satisfies Maxwells equations, and obtain analytical expressions for the beam shape coefficients.

Focus issue introduction: optical cooling and trapping.

The year 2015 is an auspicious year for optical science, as it is being celebrated as the International Year of Light and Light-Based Technologies. This Focus Issue of the journals Optics Express and Journal of the Optical Society of America B has been organized by the OSA Technical Group on Optical Cooling and Trapping to mark this occasion, and to highlight the most recent and exciting developments in the topics covered by the group. Together this joint Focus Issue features 32 papers, including both experimental and theoretical works, which span this wide range of activities.

Effect of finite terms on the truncation error of Mie series.

The finite sum of the squares of the Mie coefficients is very useful for addressing problems of classical light scattering. An approximate formula available in the literature, and still in use today, has been developed to determine a priori the number of the most significant terms needed to evaluate the scattering cross section. Here, we obtain an improved formula, which includes the number of terms needed for determining the scattering cross section within a prescribed relative error.

Two-photon continuous flow lithography.

A new approach for microfluidics-based production of polymeric particles, namely two-photon continuous flow lithography, is reported. This technique takes advantage of two-photon lithography to create objects with sub-micrometer and 3D features, and overcomes the traditional process limitations of two-photon lithography by using multiple beam production under continuous flow. Polymeric fibers, helical and bow-tie particles with sub-diffraction resolution and surface roughness as low as 10 nm are demonstrated.

Rotational dynamics of optically trapped nanofibers.

We report on the experimental evidence of tilted polymer nanofiber rotation, using a highly focused linear polarized Gaussian beam. Torque is controlled by varying trapping power or fiber tilt angle. This suggests an alternative strategy to previously reported approaches for the rotation of nano-objects, to test fundamental theoretical aspects.

Axial optical trapping efficiency through a dielectric interface.

Axial trapping through a dielectric interface is investigated in the framework of the angular spectrum representation and of the generalized Lorenz-Mie theory. We determine the optical force for an arbitrarily polarized non-paraxial, strongly aberrated, axially symmetric focusing beam and apply this description to the case of an arbitrarily positioned dielectric microsphere, commonly employed in optical tweezers, not taking into account the contribution of evanescent waves at the interface. We derive the analytical expression of the force profile, finding that the incident polarization does not affect the axial optical force.

Electromagnetic forces for an arbitrary optical trapping of a spherical dielectric.

A double tweezers setup was employed to perform ultra sensitive force measurements and to obtain the full optical force curve as a function of radial position and wavelength. The light polarization was used to select either the transverse electric (TE), or transverse magnetic (TM), or both, modes excitation. Analytical solution for optical trapping force on a spherical dielectric particle for an arbitrary positioned focused beam is presented in a generalized Lorenz-Mie diffraction theory.

Exact partial wave expansion of optical beams with respect to an arbitrary origin.

Using an analytical expression for an integral involving Bessel and Legendre functions, we succeed in obtaining the partial wave decomposition of a general optical beam at an arbitrary location relative to the origin. We also showed that solid angle integration will eliminate the radial dependence of the expansion coefficients. The beam shape coefficients obtained are given by an exact expression in terms of single or double integrals.