In the last decade, several efforts have been spent in the study of near-field coupled systems, in order to induce
hybridization of plasmonic modes. Within this context, particular attention has been recently paid on the possibility to
couple conventional bright and dark modes. As a result of such phenomenon, a Fano resonance appears as a
characteristic sharp dip in the scattering spectra. Here we show how, gradually coupling a single rod-like nanostructure
to an aligned nanoantenna dimer, it is possible to induce the near-field activation of an anti-bonding dark mode. The high
polarization sensitivity presented by the far-field response of T-shape trimer, combined with the sharp Fano resonance
sustained by this plasmonic device, opens interesting perspectives towards a new era of photonic devices.
The ability to confine light in small volumes, associated to low background signals, is an important technological
achievement for a number of disciplines such as biology or electronics. In fact, decoupling the source position from the
sample area allows an unprecedented sensitivity which can be exploited in different systems. The most direct
implications are however related to either Surface Enhanced Raman Scattering (SERS) or Tip Enhanced Raman
Scattering (TERS). Furthermore, while the combination with super-hydrophobic patterns can overcome the typical
diffusion limit of sensors, focused surface plasmons decaying into hot electrons can be exploited to study the electronic
properties of the sample by means of a Schottky junction. Within this paper these techniques will be briefly described
and the key role played by both surface and localized plasmons will be highlighted.
Trapping and manipulation of microscopic objects using fiber optical traps is gaining considerable interest, as these objects can be manipulated inside complex environments, thus removing the limitation of short working distance of the conventional optical tweezers. We show that an axicon like structure built on the tip of a single mode optical fiber produces a focused beam shape with a central hole, implying a very small fraction of the power traveling with rays nearly parallels to the optical axis. Interesting transportation behavior of polystyrene particles using the scattering forces from such an axicon tip fiber was observed. As the distance of the particle from the fiber tip increased, since almost no rays interact with the particle, the scattering forces decreased substantially. Therefore, velocity of the particle at different distances was found to depend much more critically on the particle size in contrast to the beam generated by the bare fiber. While the speed of transport could be increased linearly by increasing the laser power in both axicon tipped fiber and bare fiber, increased speed was observed for particles of larger sizes for both the fiber types. However, the fractional increase in speed for increased size of particles was found to be quite large for axicon tipped fiber as compared to the bare fiber. Use of the observed differences in speed of transportation of microscopic objects could be used to sort them based upon their size.
Since the low index particles are repelled away from the highest intensity point, trapping them optically requires either a rotating Gaussian beam or optical vortex beams focused by a high numerical microscope objective. However, the short working distance of these microscope objectives puts a limit on the depth at which these particles can be manipulated. Here, we show that axicon like structure built on tip of a single mode optical fiber produces a focused beam that is able to trap low index particles. In fact, in addition to transverse trapping inside the dark conical region surrounded by high intensity ring, axial trapping is possible by the balance of scattering force against the buoyancy of the particles. The low-index particle system consisted of an emulsion of water droplets in acetophenone. When the fiber was kept horizontal, the low index spheres moved away along the beam and thus could be transported
by influence of the scattering force. However in the vertical position (or at an angle) of the fiber, the particles could be trapped stably both in transverse and axial directions. Chain of such particles could also be trapped and transported together by translation of the fiber. Using escape force technique, transverse trapping force and thus efficiency for particle in Mie regime was measured. Details of these measurements and theory showed that trapping of Raleigh particle is possible with such axicon-tip fibers. This ability to manipulate low-index spheres inside complex condensed environments using such traps will throw new insights in the understanding of bubble-bubble and bubble-wall interactions, thus probing the physics behind sonoluminescence and exploring new applications in biology and medicine.
The present research work is devoted to the realization of an efficient fiber-waveguide optical coupling between single-mode fiber and rectangular waveguide. The outcomimg laser beam exiting from the fiber has a gaussian transversal field distribution. On the contrary, the single-mode waveguide has an asymmetric transversal field distribution in X and Y-axis. To transform the outcoming circular laser beam onto a rectangular, size adjusted, spot we have used a multilevel diffractive phase element fabricated directly on the top of the fiber by means of nanolitography. The diffractive phase element is calculated to focus and reshape the gaussian symmetric beam exiting a single-mode fiber into a desired asymmetric intensity distribution at the waveguide input plane. Phase modulation is obtained by multilevel profiling a polymeric material coated on the top of the fiber by means of a specific fabrications process including e-beam lithography and chemical etching. Experimental results obtained for fiber-waveguide coupling with a 20 microns diameter diffractive element are also presented.