Huang Wen-hao

A two-photon femtosecond laser system for three-dimensional microfabrication and data storage

Utilizing the well-focused femtosecond laser with extreme high pulse intensity, we built a two-photon microfabrication and data storage system, which was introduced through several functional parts. Based on this homemade system, several three-dimensional microstructures were fabricated by two-photon polymerization, and three-dimensional data storage of six-layers was achieved by two-photon excitation with a photochromic material.

The possibility of trapping and manipulating a nanometer scale particle by the SNOM tip

Calculations show that the SNOM tip can be used to trap and move a nanometer scale particle. If the gradient force acting on the particle is positive, in the lateral direction, there exist a one-dimensional trap along the aperture edge and a two-dimensional trap along the z-axis. If the gradient force is negative, there is only a one-dimensional trap very close to the aperture. Generally, the force is large enough to push or pull the particle directly. On the other hand, we can generate a trap along the z-axis by adding an external field. The depth of the trap is large enough to suppress the thermal motion of the particle.

Some effects on SPM based surface measurement

The scanning probe microscope (SPM) has been used as a powerful tool for nanotechnology, especially in surface nanometrology. However, there are a lot of false images and modifications during the SPM measurement on the surfaces. This is because of the complex interaction between the SPM tip and the surface. The origin is not only due to the tip material or shape, but also to the structure of the sample. So people are paying much attention to draw true information from the SPM images. In this paper, we present some simulation methods and reconstruction examples for the microstructures and surface roughness based on SPM measurement. For example, in AFM measurement, we consider the effects of tip shape and dimension, also the surface topography distribution in both height and space. Some simulation results are compared with other measurement methods to verify the reliability.

Al superhydrophobic surfaces fabricated with femtosecond laser pulses

The Al surfaces are scanned with femtosecond laser pulses and the superhydrophobic surfaces are prepared. The topological and chemical changes on Al surfaces after femtosecond laser irradiation are studied. The wettability of Al surface reveals the competition between two different factors, which are hydrophilic trend due to the formation of aluminum oxides and enhanced hydrophobic trend due to increased roughness during laser pulse treatments.

Resonant Response of Rectangular AFM Cantilever in Liquid

Dynamic characteristics of atomic force microscopy (AFM) cantilevers can be influenced by their working media. We perform an experimental study on the resonant responses of rectangular AFM cantilevers with different sizes immersed in various viscous fluids. The measured resonance frequencies in liquids are used to validate several theoretical models. Comparison shows the analytical model proposed by Sader [J. Appl. Phys. 84 (1998) 64] can give the best agreement with the experimental results with the maximum relative error nearly 16% for all the cantilevers in different liquids. The ratio between the resonant frequencies in air and water is almost independent of the cantilever length, which is consistent with the theoretical analyses.

Femtosecond laser cleaning the surfaceof reflective mirror in telescope

Metallic mirror is the core component of the imaging system in the reflective astronomical telescope. However, because the telescopesare usually located in the wild, they are often contaminated by dust and other pollutants which will greatly reduce the imaging quality. Therefore, the surface of reflective mirror in telescope should be cleaned routinely in order to recover high performance of telescope. Currently, the artificial ice cleaning is the most common method employed in the cleaning of mirror surface, which is complicated and costly. Besides, the micron particles adhered on the surface of the mirror are especially difficult to be removed by dry ice flow when their diameters are less than 20 μ m because of the Van der Waals force between the particles and metal mirror. Moreover, an ordinary method with wipe and cleanser is also used in which the friction between wipe and mirror may damage the telescope. To overcome these shortcomings, this work proposed a laser cleaning strategy in which amplified femtosecond pulsed lasers was used to clean the reflecting mirror surface. As a new cleaning method, laser cleaning technology which is non-contact and frictionless has been applied successfully in the cleaning of industrial molds, buildings, precision machinery components. For the advantages of femtosecond laser, such as short pulse duration, small thermal effects, high pulse energy, it has been used in the cleaning of cultural relics and has good achievement in recent years. This article firstly studied the interaction between the femtosecond laser and the reflective aluminum mirror. The laser damage threshold of aluminum mirror was measured about 60 mJ/cm2, which would reduce to 57 mJ/cm2 for dirty mirror because of the microparticles adhered on the surface of reflector mirror. Then, by optimizing laser parameters including the scan rate, the scan pitch, and the laser energy, it could be found that when the laser energy density ranges from 30 to 55 mJ/cm2, micron dust particles with diameters less than 20 μ m on mirror surface could be cleaned with excellent clean effect and the reflectivity in the visible light region has been greatly improved. Finally, through the dust particles and aluminum mirror substrate spectrum analysis, we ruled out that the removed microparticles were thermal melted by laser energy. The results indicated that thermal expansion plays the key role in laser cleaning of metal mirror. This work shows that femtosecond laser cleaning has a good effect on the removing of micron particles. As a simple and easy way, femtosecond laser cleaning has a good prospect in cleaning of astronomical telescope.

The experiment and theoretical analysis of the completely inelastic collision of microspheres and optical microrotor

Optical tweezers have been successfully utilized in various scientific and engineering fields such as medicine, biology and MEMS. However, there is still a lack of a sophisticated model for optical tweezers on driving complex objects. In this paper a new electromagnetic model of optical tweezers based on moment method is presented. Optical force acting on a single microsphere and two microspheres is experimentally analyzed and the experimental results are compared with the simulated ones which are obtained by our model. A microrotor which can be used as a mixer in a lab-on-chip is also analyzed by the model. The results indicate that the algorithm presented here can be easily extended to multi complex structures and to torque computation algorithms, thus providing a highly flexible and universally useable computation engine.