When an optical lens is illuminated by a plane wave, the generated focal spot is given by the Abbe diffraction limit. However, arbitrary small spots, surrounded by additional lobes, can be obtained by illuminating the lens with a suitable light pattern. This is a manifestation of super-oscillation (SO), since the far field intensity pattern is band-limited by the ratio of the lens numerical aperture and the wavelength, but nevertheless the light beam at the focal plane can oscillate locally at much higher frequency. Here, we investigate a systematic method to structure the small lobes of SO function, by using Gaussian, Hermite-Gaussian, Laguerre-Gaussian and Airy functions. After experimentally realizing the subwavelength focusing of these structured super-oscillating optical beams we showed their capabilities to achieve high localization of nano-meter sized particles and observed unprecedented localization accuracy and trapping stiffness, significantly exceeding those provided by standard diffraction limited beams. Further, we envisage that the method of structuring super-oscillating functions shown here can be used in other fields, e.g. STED microscopy, nonlinear frequency conversion, lithography, plasmonics as well as in the time domain for structuring light pulses for supertransmission and for time-dependent focusing
We propose and demonstrate experimentally an optical analogue of the famous Archimedes' screw where airborne particles are conveyed down or upstream the photons momentum ow through the rotation of a helical optical beam. We also report on the action of such a rotating screw on low-absorbing particles in a solution.
We demonstrate experimentally, an optically controlled dynamic photonic bandgap material, and to characterize its
properties. The PhC device consists of colloidal particles assembled, using Holographic Optical Tweezers, into periodic
structures located between prepositioned optical fibers. The positions of the optical traps can be modified in-situ in a
well-controlled manner. The spectral transmission properties of the devices are experimentally to obtain the passbands
and bandgaps. The desired structure and photonic band diagram of the photonic materials are calculated and optimized
numerically using and plane wave simulations. Good agreement is found between the calculated band structure and the
measured transmission spectrum for the different photonic crystal arrangements.
We review our technique for tomographic phase microscopy with optical tweezers . This tomographic phase microscopy approach enables full 3-D refractive-index reconstruction. Tomographic phase microscopy measures quantitatively the 3- D distribution of refractive-index in biological cells. We integrated our external interferometric module with holographic optical tweezers for obtaining quantitative phase maps of biological samples from a wide range of angles. The close-tocommon- path, off-axis interferometric system enables a full-rotation tomographic acquisition of a single cell using holographic optical tweezers for trapping and manipulating with a desired array of traps, while acquiring phase information of a single cell from all different angles and maintaining the native surrounding medium. We experimentally demonstrated two reconstruction algorithms: the filtered back-projection method and the Fourier diffraction method for 3-D refractive index imaging of yeast cells.
Optical traps use forces exerted by specially structured beams of light to localize microscopic objects in three
dimensions. In the case of single-beam optical traps, such as optical tweezers, trapping is due entirely to gradients
in the light's intensity. Gradients in the light field's phase also control optical forces, however, and their quite
general influence on trapped particles' dynamics has only recently been explored in detail. We demonstrate
both theoretically and experimentally how phase gradients give rise to forces in optical traps and explore the
sometimes surprising influence of phase-gradient forces on trapped objects' motions.
Holographic optical trapping uses forces exerted by computer-generated holograms to organize microscopic materials
into three-dimensional structures. Achieving and verifying accurate three-dimensional placement requires
methods for assessing the accuracy of the projected traps' geometry, as well as methods for measuring trapped
objects' three-dimensional positions. Volumetric imaging of the projected trapping pattern solves one problem.
Holographic video microscopy addresses the other. The combination is exceptionally effective for organizing,
inspecting and analyzing soft-matter systems.
Colloidal particles driven by an optical vortex constitute a model driven-dissipative system in which both macroscopic
and microscopic aspects of transport can be studied. We find that the single-particle diffusion in an optical
vortex can be either normal super-diffusive or sub-diffusive depending on the number of particles in the vortex
and on the timescale over which the diffusion is measured. For a three particle system we find that the particles
dynamics can be either steady-state periodic or with weakly chaotic characteristics depending on the relative
efect of modulations in the intensity along the vortex and hydrodynamic interactions between the spheres. We
introduce the use of the N-fold bond orientational order parameter to characterize particle circulating in a ring
by one macroscopic quantity. for a three particle system we show that for short time scales the single-particle
super-diffusion corresponds to a super-diffusive motion of the order parameter. At longer time scales we find that
the order parameter asymptotes to the expected normal di.usion behavior for the steady state system, while
fractional dynamics develop in the weakly chaotic system. Moreover, we confirm a prediction that related the
power laws governing the fractional dynamics with those governing the weakly chaotic behavior.