H. B. Niu

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.

Fabrication of efficient microaxicon by direct electron-beam lithography for long nondiffracting distance of Bessel beams for optical manipulation

We demonstrate a fabrication technique in the realization of microaxicon by single-step processing via electron-beam lithography. Microaxicon is used for the generation of propagation-invariant Bessel beams which find tremendous applications in optical trapping. The proposed technique is a simple, reliable, and reproducible method for the production of high-quality Bessel beams with long propagation-invariant distances, in our configuration, in excess of 20cm. Such Bessel beams with long nondiffracting distances are essential for optical tweezers systems in many cases.

Generation of femtosecond optical vortices using a single refractive optical element

It is shown experimentally for small spectral bandwidth that spiral phase plates (SPPs) can generate optical vortices in femtosecond beams without the need for additional dispersion compensation elements. An autocorrelation measurement performed on the obtained optical vortex (OV) beam showed that the SPP did not significantly change the pulse duration. Hence the single element SPP, already an established beam shaping technique in continuous wave and high power regimes, is presented as an efficient and practical means to obtain OVs that are free from spatial chirp in femtosecond beams.

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.

Wavelength-multiplexing phase-sensitive surface plasmon imaging sensor

A wavelength-multiplexing phase-sensitive surface plasmon resonance (SPR) imaging sensor offering wide dynamic detection range and microarray capability is reported. Phase detection is accomplished by performing self-interference between the s- and p- polarizations within the signal beam. A liquid crystal tunable filter is used to sequentially select the SPR excitation wavelength from a white light source. This wavelength-multiplexing approach enables fast detection of the sensors SPR phase response over a wide range of wavelengths, thereby covering literally any regions of interest within the SPR dip and thus maintaining the highest sensitivity point at all times. The phase-sensitive approach is particularly important for imaging SPR sensing applications because of its less stringent requirements for intensity signal-to-noise ratio, which also means the possibility of using uncooled modest resolution analog-to-digital conversion imaging devices. Experimental results demonstrate a resolution of 2.7×10(-7) RIU with a dynamic range of 0.0138 RIU.

Microfabricated-composite-hologram-enabled multiple channel longitudinal optical guiding of microparticles in nondiffracting core of a Bessel beam array

In this letter, we report multiple-channel longitudinal optical guiding of microparticles using an array of Bessel beams generated from a composite hologram fabricated by ultraviolet lithography. The optical guiding efficiency of each Bessel beam in the optical array is investigated experimentally. The rod-like core of each Bessel beam, with its nondiffracting and self-reconstruction property, has been shown to offer strong capability for optical guiding along the propagation axis possibly even in multiple-microfluidic channels massively.

Generation of high-quality optical vortex beams in free-space propagation by microfabricated wedge with spatial filtering technique

The authors propose a low-pass 2f spatial filter for improving optical vortex beams in terms of circular geometry generated by a microfabricated wedge in free space. Compared with a conventional wedge, they are able to realize high-quality vortex beams with Q value close to 1 in free-space propagation. Physical mechanism behind the technique is found that it removes the diffraction artifacts as well as higher spatial frequency components of the quasivortex beams based on a wedge and hence the filtered beam is reconstructed with minimized distortions.

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.

Dynamic optical manipulation using intensity patterns directly projected by a reflective spatial light modulator

We propose a simple method to manipulate microparticles dynamically with intensity-modulated patterns projected by a spatial light modulator (SLM), on which the patterns are controlled by a computer directly. The patterns are intensity–intensity modulated by the SLM without involving any computation or algorithm. With the dynamic patterns we can manipulate particles interactively and visibly by drawing or mouse-dragging pictures or even playing a video file on the computer screen. Experimental observations verified the feasibility of the proposed technique as a simple and direct solution for interactive optical manipulation.

Self-processing solgel material for one-step fabrication of micrometer-period sinusoidal phase gratings using the Lloyd's mirror scheme

A novel self-processing silica-zirconia hybrid solgel material has been developed and employed in one-step fabrication of micrometer-period sinusoidal phase gratings. In the process, the gratings with a sinusoidal profile were corrugated on the surface of the solgel film by UV exposure using the Lloyds mirror setup. No further development or etching step is needed to reveal the sinusoidal profile because the corrugation is formed due to the self-processing property of the solgel material, which is robust enough to be used as an end product. The period, amplitude, diffraction efficiency of the +/-1 order, and surface roughness of one of the fabricated gratings are 0.99 microm, 330 nm, 30.56%, and 1.27 nm, respectively. The new self-processing material is practical and promising for fabrication of micro-optical elements.