S. H. Tao

Fractional optical vortex beam induced rotation of particles

We experimentally demonstrate optical rotation and manipulation of microscopic particles by use of optical vortex beams with fractional topological charges, namely fractional optical vortex beams, which are coupled in an optical tweezers system. Like the vortex beams with integer topological charges, the fractional optical vortex beams are also capable of rotating particles induced by the transfer of orbital angular momentum. However, the unique radial opening (low-intensity gap) in the intensity ring encompassing the dark core, due to the fractional nature of the beam, hinders the rotation significantly. The fractional vortex beams orbital angular momentum and radial opening are exploited to guide and transport microscopic particles.

Sequence of focused optical vortices generated by a spiral fractal zone plate

We propose a method for producing a sequence of focused optical vortices along the propagation direction by using a spiral fractal zone plate. The generated beam possesses the optical vortices embedded at subsidiary foci as well as the major ones of the fractal zone plate. The experimental results are obtained in good agreement with the simulations.

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.

Variable-radius focused optical vortex with suppressed sidelobes

We propose a design for a phase mask for generating an optical vortex with suppressed sidelobes in the focal plane where the radius of the intensity ring is variable. A radial modulation added to conventional phase mask exp(iltheta) projects the light diffracted from different annular zones into a single intensity ring in the focal plane.

Collinear superposition of multiple helical beams generated by a single azimuthally modulated phase-only element.

We propose a highly efficient approach to generating multihelix beams that contain more than one helical mode, and the power distribution over helical modes is adjustable. A multihelix beam embedded with three collinear helical modes is demonstrated by use of a spatial light modulator.

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.

The generation of an array of nondiffracting beams by a single composite computer generated hologram

Nondiffracting beams have generated great interest recently owing to their potential applications and unique properties. In this paper, we propose and validate a simple and effective method to generate arrays of various nondiffracting modes using a composite computer-generated hologram, which comprises N × N holograms each generating an individual nondiffracting beam. Employing the composite hologram approach, we can modulate individual nondiffracting beam in an array by varying the location, dimension, and phase information coded on each hologram. We experimentally generated regular arrays of Bessel beams, customized-shaped arrays of Bessel beams, and arrays of self-imaged optical bottle beams. The interference among the beams in the arrays was found to be weak within the nondiffracting distance.

Residue orbital angular momentum in interferenced double vortex beams with unequal topological charges

When two vortex beams with unequal topological charges superpose coherently, orbital angular momentum (OAM) in the two beams would not be cancelled out completely in the interference. The residual OAMs contained by the superposed beam are located at different concentric rings and may have opposite orientations owing to the difference of the charges. The residual OAM can be confirmed by the rotation of microparticles when difference between the charges of two interfering beams is large.

Deterministic approach to the generation of modified helical beams for optical manipulation

We present a deterministic method to generate modified helical beams which create optical vortices with desired dark core intensity patterns in the far-field. The experiments are implemented and verified by a spatial light modulator (SLM), which imprints a phase function onto the incident wavefront of a TEM00 laser mode to transform the incident beam into a modified helical beam. The phase function can be calculated once a specific dark core shape of an optical vortex is required. The modified helical beam is exploited in optical manipulation with verification of its orbital angular momentum experimentally.