Xianhua Wang

Maskless fabrication of concave microlens arrays on silica glasses by a femtosecond-laser-enhanced local wet etching method

A simple and efficient technique for large-area manufacturing of concave microlens arrays (MLAs) on silica glasses with femtosecond (fs)-laser-enhanced chemical wet etching is demonstrated. By means of fs laser in situ irradiations followed by the hydrofluoric acid etching process, large area close-packed rectangular and hexagonal concave MLAs with diameters less than a hundred of micrometers are fabricated within a few hours. The fabricated MLAs exhibit excellent surface quality and uniformity. In contrast to the classic thermal reflow process, the presented technique is a maskless process and allows the flexible control of the size, shape and the packing pattern of the MLAs by adjusting the parameters such as the pulse energy, the number of shots and etching time.

Anisotropic wetting on microstrips surface fabricated by femtosecond laser.

In this paper, we present a new method to realize anisotropy by restricting a droplet on an unstructured Si hydrophobic domain between two superhydrophobic strips fabricated by femtosecond laser. The water contact angles and corresponding water baseline length were investigated. The results showed that anisotropy would vary with the volume-induced pinning-depinning-repinning behavior of the droplet. Furthermore, through the observation of water response on small Si domain, the adhesive force of the structure is proven to be the key factor giving rise to the anisotropy wetting. This phenomenon could potentially be used as a model for fundamental research, and such structures could be utilized to control large volume in microfluidic devices, lab-on-chip system, microreactors, and self-cleaning surfaces.

Versatile route to gapless microlens arrays using laser-tunable wet-etched curved surfaces

This work reveals a cost-efficient and flexible approach to various microlens arrays on polymers, which is essential to micro-optics elements. An 800-nm femtosecond laser is employed to control the hydrofluoric (HF) acid etching process on silica glasses, and concave microstructures with smooth curved surfaces are produced by this method. Then, the micro-structured glass templates can serve as molds for replicating microlenses on polymers. In this paper, a high-ordered microlens array with over 16,000 hexagonal-shaped lenses is fabricated on poly (dimethyl siloxane) [PDMS], and its perfect light-gathering ability and imaging performance are demonstrated. The flexibility of this method is demonstrated by successful preparation of several concave molds with different patterns which are difficult to be obtained by other methods. This technique provides a new route to small-scaled, smooth and curved surfaces which is widely used in micro-optics, biochemical analysis and superhydrophobic interface.

Fabrication of bioinspired omnidirectional and gapless microlens array for wide field-of-view detections

Microlens arrays on curvilinear surfaces are highly desirable for wide field-of-view imaging and sensing systems. However, it is technically challenging to fabricate these structures. This letter reports a simple method to machine close-packed microlenses on curvilinear surfaces as inspired by the insect eyes, which involves a femtosecond-laser-based microfabrication and a thermomechanical bending process. Over 7600 hexagonal-shaped microlenses with a diameter of 50 μm were fabricated on a hemispherical poly (methyl methacrylate) shell, which is similar to the compound eyes of insects. The optical performances of the microlens array were demonstrated by the abilities of high-resolution imaging and large view-angle focusing.

Fabrication qualities of micro-gratings encoding dependence on laser parameters by two-beam femtosecond lasers interference

Femtosecond laser interference is a promising tool for micro-fabrication and micromachining of periodical structures on the surface of samples or inside transparent materials, but femtosecond laser pulses are very hard to interfere due to their spectrum widths may reach to several tens of nanometers, and their spectrum widths will be stretched by shorting the duration according to the Fourier transform. We realized two 25 fs pulses interference and encoded micro-gratings on Au-Cr thin films using this interference pattern. The interference patterns of two laser pulses with different pulse durations in sub-hundred femtosecond time domain were calculated to explore the influence of pulse durations on processing qualities of encoded micro-gratings. The results show that, the shorter pulses are preferable to fabricate micro-gratings with fine resolution on intractable materials, and longer pulses are helpful to improve encoding efficiency and contrast ratio of bright & dark interfered fringes. The differences between encoded micro-gratings on Au-Cr thin film using these interference patterns validated our analysis, which are hardly observed when pulse duration is longer than 100 fs mainly because the size of interfered area is larger than the focal spots. Moreover, the distance between two focal spots also has been chosen to identify our calculations, and the experimental results are agreement with the calculations.