Light assisted micro-manipulation techniques have provided a non-invasive technique to investigate microscopic world. Employing micro-optical elements for laser beam shaping can aggrandize the trapping capabilities of conventional optical tweezers system. In the paper we report the design parameters and fabrication techniques of diversified micro-optical elements. High quality Laguerre-Gaussian beams, Bessel beams, self-imaged bottle beams and fractional Bessel beams were achieved using such micro-optical elements. We further integrated these micro-elements in conventional optical tweezers systems for various trapping applications. Such micro-elements were high efficient (in terms of power conversion) which is an important criteria to be incorporated with the optical trapping systems. Due to its micro-size such micro-elements are potential candidates to be integrated with lab-on-a-chip techniques to realize next generation "miniaturized optical trap".
Optical trapping and manipulation are based on three types of the force and momentum, in the range of pN, to grab, lift and rotate microparticles due to the light intensity or phase distribution. In the first type, particles are trapped in the highest intensity region of the beam due to optical gradient force for high refractive index particles with respective to its surrounding medium. Secondly, orbital angular momentum (OAM) of a beam can be transferred to a particle due to phase singularity within a beam. Thirdly, spin angular momentum (SAM) can be transferred to the particles due to the circular polarization of the beam. In recent years, microfabricated optical elements have been used to modulate the amplitude and phase of the optical beam to create new generation of optical tweezers with additional manipulation dimensions. In this paper, a review of our recent discoveries of optical trapping and manipulation using micro-beam shaping and micro-optical elements will be presented.
A simple reflow technique and reactive-ion etching are employed to fabricate and integrate a refractive square-apertured arch Si microlens array (MLA) on the back of an IR focal plane array device (IRFPA), resulting in the formation of a monolithic MLA/IRFPA device. The fabricated on-chip Si MLA behaves as optical concentrators and is used to collect most of the incident light from each pixel area on a smaller photosensitive area of the IRFPA, causing the IR response characteristics of the monolithic device to be improved greatly when compared with an ordinary IRFPA device without the MLA. The advantages of employing the reflow technique and the reactive-ion etching lie not only in the excellent surface smoothness and dimensional uniformity of the fabricated MLA, but also as a cost-effective and mass production technology.
A hybrid encryption and decryption technique for optical information security is proposed. In this method, the iterative Fourier transform algorithm is employed to optimize the encrypted hologram and the decryption key as binary phase-only diffractive optical elements, which were fabricated by electron-beam lithography. In a simple optical setup, the optical decryption is implemented by superimposing the encrypted hologram and the decryption key. Numerical simulation and optical experiment confirm the proposed technique as a simple and easy implementation for optical decryption.
Recently, the optical trapping technique has been employed to construct three-dimensional microstructures. Such three-dimensional microstructures are created from the fact that optical beams such as, a single focused Gaussian beam, multiple beams from Laguerre-Gaussian (LG) interference patterns, and Bessel beams, are able to stack microparticles on top of the other. Once these microparticles are stacked, the optical forces from the optical beam were able to hold them in place.
In this paper, we demonstrate that by lowering the focused point of the LG beam below the cover slip (sample). The LG beam possesses the ability to stack multiple microparticles around its annular intensity rings and thus form a three-dimensional cubic structure. Hence we proposed a new technique of constructing microstructures, which is by creating optical fields with designed optical vortex shape. These microparticles will then be stacked according to the shape of the optical beam. This is an alternative method to obtain a desired three-dimensional crystalline structure, where shaping the optical vortices beam is used instead of using multiple beams.
This paper describes a cost-effective and high-volume soft-lithography method for building microlenses in hybrid sol-gel glass. The fabrication processes comprise the following three steps, namely fabricating microlens array in photoresist as a master lenses, molding replication of the lenses in poly-dimethylsiloxane (PDMS) as elastometric molds and embossing to press the PDMS replica onto the hybrid sol-gel glass. During the embossing process, while the PDMS mold is applied, the sol-gel sample was cured by UV exposure for densification. In this work, the master microlenses were patterned in photoresist using the reflow technique, where the authors took full advantage of the matured photosensitive material and fabrication technologies as the first and transitional step. This method enables us to fabricate thick micro-optical elements in sol-gel glass and it will be suitable for a range of applications in free-space and guided wave optics.