W. C. Cheong

Optical vortex beam shaping by use of highly efficient irregular spiral phase plates for optical micromanipulation

Optical dark traps such as Laguerre-Gaussian beams, modulated optical vortices, and high-order Bessel beams have been used in the micromanipulation of microparticles. Such optical traps are highly versatile, as they are able to trap both high- and low-index microparticles as well as to set them into rotation by use of the orbital angular momentum of light. Holography has been widely used to modulate the shape of an optical vortex for new optical traps. We show that, by designing the shape of a spiral phase plate and using electron-beam lithography for fabrication, one can modulate the amplitude and the phase of an optical vortex with respect to the specific shape of the spiral phase plate as required. Furthermore, to the best of our knowledge this is the first report of transferring orbital angular momentum from a spiral phase plate to an absorptive microparticle in an experiment. Hence, with this technique, optical dark traps can easily be designed and fabricated.

Direct electron-beam writing of continuous spiral phase plates in negative resist with high power efficiency for optical manipulation

Laser beams propagating in Laguerre-Gaussian (LG) modes are of considerable interest due to their widespread applications in the areas of optical manipulation of microparticles, quantum entanglement of photons, nonlinear optics, optical vortex interactions, and atomic studies. However, the proliferation of LG beams has been hampered due to the absence of reliable and reproducible fabrication technologies in producing the required optical elements for their generation. In this letter, we describe a simple, reliable, and reproducible fabrication technique for a micron-sized spiral phase plate with high power efficiency (80%–90%) and good beam uniformity. This facilitates the widespread use of LG beams in various applications: as an example the fabricated elements can easily and readily be incorporated into an existing optical trapping system with minimum modification.

Fabrication of concave refractive microlens arrays in solgel glass by a simple proximity-effect-assisted reflow technique

We report a simple method for fabricating a concave refractive microlens array (MLA) in solgel glass by using a proximity-effect-assisted reflow technique. The solgel concave refractive MLA that we fabricated had excellent surface smoothness; good dimensional conformity, with an 8.23% nonuniformity of the microlens elements; and structural perfection, with a biggest deviation of 1% from a perfect concave spherical crown. The relative error between the measured and the designed values of the concave MLAs focal length was only 1.83%. Compared with the conventional fabrication techniques for concave MLAs, the proposed method has significant advantages including simplicity, low cost, good element conformity, and smooth device surface.

Optical steering of high and low index microparticles by manipulating an off-axis optical vortex

We demonstrate the use of a rotating off-axis optical phase singularity, generated through an intentional misalignment of a high optical efficiency spiral phase plate (SPP), to optically steer both high and low index microparticles trapped within the optical beam in a controlled manner. This intentional misalignment of the SPP creates an asymmetrical intensity beam pattern due to the optical vortex being displaced from the centre of the beam propagation axis. By using this optical beam pattern, we propose that a cell can be trapped by an optical beam of matching beam diameter while its internal structure can be manipulated by the off-axis optical vortex.

High-power efficient multiple optical vortices in a single beam generated by a kinoform-type spiral phase plate

We propose using a solitary kinoform-type spiral phase plate structure to generate an array of vortices located in a single beam. Kinoform-type spiral surfaces allow each wavelength component of the phase modulation value to be wrapped back to its 2 pi equivalent for optical vortices of high charge. This allows the surface-relief profiles of high-charge vortices to be microfabricated with the same physical height as spiral phase plates of unity-charged optical vortices. The m-charged optical vortex obtained interacts with the inherent coherent background, which changes the propagation dynamics of the optical vortex and splits the initial m charge into /m/ unity-charged optical vortices within the same beam. Compared to a hologram, a multistart spiral phase plate is more efficient in the use of available spatial frequencies and beam energy and also is computationally less demanding. Furthermore, using microfabrication techniques will allow for greater achievable tolerances in terms of smaller feature sizes.

Direct electron beam writing of kinoform micro-axicon for generation of propagation-invariant beams with long non-diffracting distance

To enhance the non-diffracting distance of propagation-invariant beams, fabrication of an axicon with large aperture diameter and height is essential. It is noted, however, that fabrication of continuous surface profile micro-optical elements with a large physical height (thick resist) is prone to various fabrication constraints and errors. We employ the kinoform technique to alleviate such problems in the fabrication of axicons. In this paper kinoform micro-axicons and kinoform micro-double-axicons are designed and fabricated employing electron beam lithography techniques to generate a Bessel beam and a self-imaged bottle beam with long non-diffracting distance. Furthermore, a detailed study of power conversion efficiency of kinoform structures is discussed.

Hybrid sample-inverted reflow and soft-lithography technique for fabrication of conicoid microlens arrays.

We report a cost-effective fabrication method, with a combination of the sample-inverted reflow technique and the soft-lithography replication method, to fabricate conicoid refractive microlens arrays (MLAs), including hyperboloid, paraboloid, and ellipsoid MLAs in inorganic-organic hybrid SiO2-ZrO2 solgel material. The fabrication procedures involve two basic steps. First, a master of the conicoid MLA was made in photoresist by the sample-inverted reflow technique. Second, we built a negative mold of the master by casting polydimethylsiloxane (PDMS) onto a silicone elastomer against the master, and then the profile was imprinted onto the solgel glass. As a result, the fabricated solgel MLAs have been obtained with excellent smooth profiles, having negligible discrepancies from the profiles of ideal conicoid MLAs.

Reflowed solgel spherical microlens for high-efficiency optical coupling between a laser diode and a single-mode fiber.

To improve the coupling efficiency between a laser diode and a single-mode fiber, we propose a two-microlens coupling scheme that uses two solgel spherical microlenses for high coupling efficiency. The conventional reflow technique was employed and extended to the inorganic-organic hybrid SiO2/ZrO2 solgel material to form the microlenses. Preliminary results show that the coupling efficiency was increased to--1.28 dB (74.5%) by the proposed scheme, compared with a coupling efficiency of--10.13 dB (9.7%) by the butt-joint method. The proposed fabrication technique demonstrates that use of a reflowed solgel spherical microlens is a cost-effective mass-production approach to application of micro-optical elements in optical communication.

Micro-optical elements for optical manipulation

Using micro-optical elements can be an effective, low-cost way to shape laser micro-beams, which are needed for optical trapping and manipulation. This article covers various designs of micro-optical elements that are used to generate the necessary optical vortices within a beam.

Hybrid encryption and decryption technique using microfabricated diffractive optical elements

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.