Abstract: Radiation pressure is used to push gold nanorods on multilayer graphene and create hybrid active surfaces for surface-enhanced Raman spectroscopy (SERS) in liquid. As a proof of concept, ultrasensitive detection of bovine serum albumin is shown and the aggregation kinetics is studied as a function of the irradiation time. We compare the results on graphene with experiments on glass and gold surfaces. Optical aggregation on graphene occurs on time scales of 20 min, ca. 3.5 times slower than on glass. No stable aggregation is obtained on gold. We attribute the differences to the destabilization effect of the standing wave produced on the metallic substrates, due to their higher reflectivity, and to the reduced thermophoretic effects, related to the higher heat dissipation. Despite the slowdown of the aggregation kinetics, the usage of graphene as substrate offers manifold benefits: an almost negligible fluorescence background when using near-infrared light (785 nm), the absence of thermal absorption as well as the possibility to easily functionalize the surface to enhance the affinity with the analytes. Our results enlarge the spectrum of materials that can be used for optical aggregation and SERS detection of biomolecules, highlighting the importance of controlling the physical properties of the surfaces. Graphic abstract: [Figure not available: see fulltext.]

Optical tweezers, tools based on strongly focused light, enable optical trapping, manipulation, and characterisation of a wide range of microscopic and nanoscopic materials. In the limiting cases of spherical particles either much smaller or much larger than the trapping wavelength, the force in optical tweezers separates into a conservative gradient force, which is proportional to the light intensity gradient and responsible for trapping, and a non-conservative scattering force, which is proportional to the light intensity and is generally detrimental for trapping, but fundamental for optical manipulation and laser cooling. For non-spherical particles or at intermediate (meso)scales, the situation is more complex and this traditional identification of gradient and scattering force is more elusive. Moreover, shape and composition can have dramatic consequences for optically trapped particle dynamics. Here, after an introduction to the theory and practice of optical forces with a focus on the role of shape and composition, we give an overview of some recent applications to biology, nanotechnology, spectroscopy, stochastic thermodynamics, critical Casimir forces, and active matter.

Optical trapping of hybrid core-shell gold-polymer particles is studied. Optical forces are measured for different gold core size and polymer shell thickness, revealing how a polymer shell increases the trapping efficiency with respect to the bare gold nanoparticles. Data are in agreement with calculations of optical trapping based on electromagnetic scattering theory in the T-matrix approach. The scaling behaviour of optical forces with respect to the ratio between polymer layer thickness and the whole particle radius is found and discussed.

In this work, we present a comparative theoretical study about the optical absorption coefficient calculated in ordered nanopillar and nanohole photonic crystal silicon structures for solar energy applications. In particular, we investigate the ultimate efficiency at normal incidence condition of such structures for several fill factors and lattice constants. We find that, except for small ranges of frequency where an inversion of tendency is observed, the total absorption coefficient in nanopillar arrays is greater than the one calculated in nanohole arrays. Moreover, optimized silicon nanopillar arrays show percentage improvements of the ultimate efficiency up to 138% with respect to the case of a silicon thin film of equivalent thickness. Finally, we report preliminary experimental results about the realization of a silicon photonic crystal with a nanopillar array structure to be exploited as an optical trapping film in solar cells. © 2013 SPIE.

We use laser beams with radial and azimuthal polarization to optically trap carbon nanotubes. We measure force constants and trap parameters as a function of power showing improved axial trapping efficiency with respect to linearly polarized beams. The analysis of the thermal fluctuations highlights a significant change in the optical trapping potential when using cylindrical vector beams. This enables the use of polarization states to shape optical traps according to the particle geometry, as well as paving the way to nanoprobe-based photonic force microscopy with increased performance compared to a standard linearly polarized configuration. (C) 2012 Optical Society of America

A digital holographic characterization of Bessel beams produced by polymeric microaxicons is reported. Both intensity and phase of the beam can be numerically reconstructed in whichever point starting from a single acquired hologram. Optical parameters such as the full width at half maximum, the focal length and the depth of focus of the axicon lens are experimentally measured. The Bessel beam exiting from the axicon, with a very large depth of focus with respect to that of a Gaussian beam, is successfully exploited for optical trapping of micrometric objects.

Silicon nanoparticles obtained by ball-milling of a 50% porosity silicon layer have been optically trapped when dispersed in a water-surfactant environment. We measured the optical force constants using linearly and radially polarized trapping beams finding a reshaping of the optical potential and an enhanced axial spring constant for the latter. These measurements open perspectives for the control and handling of silicon nanoparticles as labeling agents in biological analysis and fluorescence imaging techniques.

The authors present the design and optimization of an optofluidic monolithic chip, able to provide optical trapping and controlled stretching of single cells. The chip is fabricated in a fused silica glass substrate by femtosecond laser micromachining which can produce both optical waveguides and microfluidic channels with great accuracy. A new fabrication procedure adopted in this work allows the demonstration of microchannels with a square cross-section, thus guaranteeing an improved quality of the trapped cell images. Femtosecond laser micromachining emerges as a promising technique for the development of multifunctional integrated biophotonic devices that can be easily coupled to a microscope platform, thus enabling a complete characterization of the cells under test.

We present a novel all-fiber optical tweezer (OT) for biological applications. The tweezer is based on a new approach relying on total internal reflection in an annular core fiber or into a fiber bundle. The proposed device, whose trapping efficacy has been recently demonstrated experimentally, is extremely promising, also because optical manipulation and analysis functions can be easily added to the tweezer basic structure, leading to the realization of a powerful biotool. In this paper, a detailed numerical analysis of the structure properties and of its efficiency is carried out in the Mie regime. Moreover, by defining a new parameter to evaluate the trapping efficiency, we perform a comparison between the proposed tweezer structure and a standard OT based on a strongly focused Gaussian beam.

We have developed and characterized a new method to produce a continuous beam of cold atoms from a standard vapour cell magneto-optical trap (MOT). The experimental apparatus is very simple. Using a laser beam it is possible to hollow out in the source MOT a direction of unbalanced radiation pressure along which cold atoms can be accelerated out of the trap. The transverse cooling process that takes place during the extraction reduces the beam divergence. The atomic beam is used to load a magneto-optical trap operation in an ultra high vacuum environment. At avapour pressure of of 10^-8 mbar in the loading cell. we have produced a continuous flux of 7 x 10^7 atoms/s at the recapture cell with a mean velocity of 14 m/s. A comparison of this method with a pulsed transfer scheme is presented.