Antonio A. R. Neves

Laser emission from electrospun polymer nanofibers

althoughelectrospinning (ES), based on the stretching of a polymersolution under electrostatic forces, represents a practicallyunique technology that combines low cost and high through-put. Moreover, the addition of active components (i.e.,nanoparticles or molecular species) to the ES polymersolution allows one to obtain composite nanofibers withspecific functionalities.

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. We compare experimental rotation frequencies to calculations based on T-Matrix formalism, which accurately reproduces measured data, thus providing a comprehensive description of trapping and rotation dynamics of the linear nanostructures.

Two-photon patterning of a polymer containing Y-shaped azochromophores

We report on the patterning of the free surface of azo-based polymer films by means of mass migration driven by one- or two-photon absorption. A symmetric donor-acceptor-donor structured Y-shaped azochromophore is specifically synthesized to enhance two-photon absorption in the polymer. The exposure of the polymer film to a focused laser beam results in light-driven mass migration for both one- and two-photon absorptions. Features with subdiffraction resolution (250 nm) are realized and the patterning dynamics is investigated as a function of the light dose. Furthermore, functional photonic structures, such as diffraction gratings with periods ranging between 0.5 and 2.0 μm, have been realized.

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.

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. This is accomplished using extended precision computation for a wide range of commonly used size parameters and indices of refraction. The revised formula for the finite number of terms can be a promising and valuable approach for efficient modeling light scattering phenomena.

Photonic Nanojets in Optical Tweezers

Abstract Photonic nanojets have been brought into attention ten years ago for potential application in ultramicroscopy, because of its sub-wavelength resolution that can enhance detection and interaction with matter. For these novel applications under development, the optical trapping of a sphere acts as an ideal framework to employ photonic nanojets. In the present study, we generated nanojets by using a highly focused incident beam, in contrast to traditional plane waves. The method inherits the advantage of optical trapping, especially for intracellular applications, with the microsphere in equilibrium on the beam propagation axis and positioned arbitrarily in space. Moreover, owing to optical scattering forces, when the sphere is in equilibrium, its center shifts with respect to the focal point of the incident beam. However, when the system is in stable equilibrium with a configuration involving optical tweezers, photonic nanojets cannot be formed. To overcome this issue, we employed double optical tweezers in an unorthodox configuration involving two collinear and co-propagating beams, the precise positioning of which would turn on/off the photonic nanojets, thereby improving the applicability of photonic nanojets.

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. We show several examples on how to use our method to obtain analytical beam shape coefficients for: Bessel beams, general hollow waveguide modes and specific geometries such as cylindrical and rectangular. Our method uses the vector potential, which shows the interesting characteristic of being gauge invariant. These results are highly relevant for speeding up numerical calculation of light scattering applications such as the radiation forces acting on spherical particles placed in an arbitrary electromagnetic field, as in an optical tweezers system.

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.

Optical cooling and trapping: introduction

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 33 papers, including both experimental and theoretical works, which span this wide range of activities.

Two-photons micro-structuring of a polymer containing Y-shape azo-chromophores

We report about the possibility to drive mass migration into an azo-based polymer by means of two-photon absorption. A symmetric donor-acceptor-donor structured Y-shape azo-chromophore is specifically synthesized to enhance two-photon absorption in the polymer. Features resulting from two-photon absorption in the near-infrared show sub-diffraction limit linear dimensions (250nm).