J. Bu

Generating radial or azimuthal polarization by axial sampling of circularly polarized vortex beams

A laser beam with circular polarization can be converted into either radial or azimuthal polarization by a microfabricated spiral phase plate and a radial (or azimuthal)-type linear analyzer. The resulting polarization is axially symmetric and is able to produce tightly focused light fields beyond the diffraction limit. We describe in detail the theory behind the technique and the experimental verification of the polarization both in the far field and at the focus of a high numerical aperture lens. Vector properties of the beam under strong focusing conditions were observed by comparing the fluorescence images corresponding to the focal intensity distribution for both radial and azimuthal polarizations. The technique discussed here may easily be implemented to a wide range of optical instruments and devices that require the use of tightly focused light beams.

Simple reflow technique for fabrication of a microlens array in solgel glass.

A simple reflow method for fabrication of refractive microlens arrays in inorganic-organic SiO2-ZrO2 solgel glass is presented. To our knowledge, this is the first report that presents a simple reflow technique for transforming a negatively induced hybrid solgel material into desirable spherical microlenses. It is shown that the microlenses have excellent smooth surfaces and uniform dimensions. The reflow technique is considerably cheaper than use of a high-energy beam-sensitive gray-scale mask and is suitable for mass production.

Direct noninterference cylindrical vector beam generation applied in the femtosecond regime

This letter describes the generation of femtosecond cylindrical vector beams by passing the optical pulse through a microfabricated spiral phase plate and an azimuth-type polarization analyzer. The resulting beam resembles a doughnut and has a polarization that is axially symmetric. Portions of the wave front were sampled via the GRENOUILLE frequency resolved optical grating device; it was found that the original pulse of 200fs was stretched slightly by ∼13fs. Compared to an interferometric beam combination technique, this experiment geometry is simple, minimizes material dispersion, and has negligible spatial chirp.

Single-step fabrication of a microlens array in sol-gel material by direct laser writing and its application in optical coupling

This paper presents a kind of inorganic–organic hybrid SiO2/ZrO2 sol–gel material successfully developed for use in a single-step fabrication of a refractive spherical microlens array (MLA). The single-step fabrication method employs direct laser writing technique to generate a designed spherical MLA on a layer of the sol–gel film. The method offers unique advantages over the conventional photoresist-based fabrication technology by eliminating the need for using a complex ion beam etching process and costly high-energy beam-sensitive grey scale mask. The measured results show that the microlenses have uniform dimensions, smooth surfaces, and strong focusing function. Furthermore, the fabricated MLA, as a coupler between a laser diode and a single mode fibre, has greatly improved the coupling efficiency between them.

Design and fabrication of a double-axicon for generation of tailorable self-imaged three-dimensional intensity voids

We propose a new design for fabrication of a highly power-efficient double axicon to generate self-imaged three-dimensional intensity voids along the propagation of a beam. The conventional conical structure of an axicon is modified and shaped like an axiconlike structure with a double-gradient surface profile. The gradient conical surfaces generate Bessel beams with varying radial wave vectors that are superimposed and interfere to generate a sequence of three-dimensional intensity voids. The proposed element was fabricated using electron-beam lithography, and experimental verification of the design is reported.

Large-scale optical traps on a chip for optical sorting

The authors present a power-efficient large-scale lensless optical traps on a chip (OTOCs) as an optofluidic element for optical sorting of microparticles. Based on the well-known Talbot self-imaging effect in the Fresnel region, the OTOC makes use of a two-dimensional microfabricated chessboardlike structure to create an optical lattice near its emergent plane. Simultaneous trapping of hundreds of microparticles in a regular array (>200×200μm2) is proved experimentally without adopting an external optical projection lens configuration. Furthermore, the authors demonstrate experimental results for large-scale sorting of microparticles by sizes using the OTOC.

Soft-lithography-enabled fabrication of large numerical aperture refractive microlens array in hybrid SiO2–TiO2 sol-gel glass

This paper describes a low-cost soft-lithography method for building large-numerical-aperture microlenses in hybrid sol-gel glass. The fabrication processes comprise three steps, namely fabrication of large numerical aperture microlens array in photoresist as a master, replication of the master in poly-dimethylsiloxane (PDMS) as elastomeric molds and reproduction of the PDMS replica onto the hybrid sol-gel glass as an end structure. Comparing with the direct UV fabrication in the photosensitive sol-gel glass, the proposed method provides a unique advantage in terms of fabrication freedom in shape and thickness of micro-optical elements in the sol-gel glass. This eliminates the difficulties in direct photolithography in sol-gel glass for arbitrary shapes and great thickness due to ever increasing demands.

Low-cost and efficient coupling technique using reflowed sol-gel microlens.

In this paper, we present a novel reflow technique for the fabrication of an elliptical microlens array in sol-gel glass. The fabricated refractive microlens array has a very smooth surface, and structural and dimensional conformity with the designed parameters. It is shown that the microlens array can provide high coupling efficiency of a laser diode to a single mode fiber, and relax the lateral and axial misalignment tolerance. The microlens coupling technique has the advantages of low coupling loss, large misalignment tolerances, and small package volume.

Simultaneous optical trapping of microparticles in multiple planes by a modified self-imaging effect on a chip

The authors propose a three-dimensional (3D) optical trapping of microparticles in multiple planes simultaneously based on a modified self-imaging effect. Similar to the Talbot self-imaging effect, the modified self-imaging effect is induced by a layer of trapped particles and it is subsequently used as a periodic grating structure to generate its own self-imaging pattern in 3D. Based on this secondary layer-by-layer self-imaging effect, optical trapping of silica and polystyrene colloidal particles at different planes in a microchamber are demonstrated experimentally.

Two-layered metallic film induced surface plasmons for enhanced optical propulsion of microparticles

We demonstrate the use of surface plasmons (SPs) excited on a two-layered metallic film for enhanced particle propulsion. The extended SP field is predominantly attributed to propel particles with increased speed along metal-deposited regions. Various silver-gold combinations were used to quantify for the SP field enhancement. The underlying silver yields better SP evanescent field enhancement, while the overlying gold ensures that the stability of the sensing surface is not compromised. In comparison to the conventional single gold film SP resonance configuration for particle manipulation, the two-layered metallic structure enhances the propulsive force, as verified experimentally by ∼2.0 times increase in particle velocities.