Guoliang Deng

Effective random laser action in Rhodamine 6G solution with Al nanoparticles

We have studied the random laser action in Rhodamine 6G (Rh6G) ethylene glycol solution with Al nanoparticles. The experiment results are obtained by pumping with a nanosecond (7 ns) laser pulse, which demonstrated the existence of effective random laser emission. It is found that the threshold of the random laser depends on the concentration of the Rh6G and the concentration of Al nanoparticles. The concentration and diameter of Al nanoparticles have effects on the optical path; a higher concentration or a larger diameter results in a shorter optical path length. Also multimode survival and mode competition have been observed at a relatively high concentration (0.08 M) of Rh6G, where the concentration of Al nanoparticles is 0.0015 M.

Temperature dependence of laser-induced micro/nanostructures for femtosecond laser irradiation of silicon

The temperature dependence (from 25°C to 350°C) of laser-induced micro/nanostructures for multiple linearly polarized femtosecond laser pulse (pulse duration τ=35  fs, wavelength λ=800  nm) irradiation of silicon in air is studied experimentally. Distinct micro/nanostructures are fabricated at elevated temperature. Low spatial frequency, laser-induced periodic ripple structures (LSFL), which are perpendicular to the polarization of the laser beam, are formed at all temperatures. Micrometer-size grooves, which are oriented perpendicular to the LSFL ripples, have been observed in the central part of the irradiated area above 150°C. The threshold to fabricate the LSFL ripples goes from 1.65 to 2  kJ/m2 while the temperature of the substrate increases from 25°C to 350°C. The possible mechanism of the temperature dependence of the micro/nanostructure generation is also discussed. These results demonstrate that temperature is an important parameter to be tuned to tailor the micro/nanostructure fabrication.

Crystalline micro/nanostructures fabrication on silicon using femtosecond laser

The laser induced micro/ nano-meter size surface structures are fabricated by multi linear polarized femtosecond laser pulses (pulse duration τ=35 fs, wavelength λ=800 nm) irradiation at room temperature(25 ℃ ) and 400 ℃. The structures fabricated at these two temperatures show distinct temperature dependence. The grooves, which are parallel to the polarization of the laser light, can be clearly observed at almost all the structured area formed at 400 ℃ while laser induced period structures(ripples) are the most pronounced surface structure in the crater formed at room temperature. The crystallinity of these surface structures are investigated by using Raman spectroscopy. The Raman spectrum shows that all the structured area formed at 400 ℃ is crystalline(or poly-crystalline) while amorphous silicon can be observed within the structures formed at room temperature (25 ℃). These results indicate that temperature is an important parameter to be tuned to tailor the micro/nano-structure fabrication.

Fiber Optical Tweezers for the Operation of Particles

We take example by the fabrication of nano-probe chemical etching process to make optical tapered fiber, and we use the semiconductor laser as the light source to design a single-fiber optical tweezers system, thus achieve the capture of yeast cells and the thrust of hollow glass beads. At the same time, we use FDTD method to simulate the force analysis of the yeast and hollow glass beads in Gaussian light field. The optical tweezers system has concise structure; the preparation of optical tweezers is simple and flexible. Its easy to achieve non-contact operation of biological cells, extended to the micro-operation of other particles and of some significance in materials and biological research field.

Experimental and FDTD study of silicon surface morphology induced by femtosecond laser irradiation at a high substrate temperature

Substrate temperature is an important parameter for controlling the properties of femtosecond laser induced surface structures besides traditional ways. The morphology on silicon surface at different temperatures are studied experimentally. Compared to those formed at 300 K, smoother ripples, micro-grooves and nano/micro-holes are formed at 700 K. A two temperature model and FDTD method are used to discuss the temperature dependence of surface structures. The results show that the increased light absorption at elevated temperature leads to the reduction of surface roughness. The type-g feature in the FDTD-η map at 700 K, which corresponds to the energy deposition modulation parallel to the laser polarization with a periodicity bigger than the wavelength, is the origin of the formation of grooves. This work can benefit both surface structures based applications and the study of femtosecond laser-matter interactions.

Laser effects based optimal laser parameter identifications for paint removal from metal substrate at 1064 nm: a multi-pulse model

Abstract Paint removal by laser ablation is favoured among cleaning techniques due to its high efficiency. How to predict the optimal laser parameters without producing damage to substrate still remains challenging for accurate paint stripping. On the basis of ablation morphologies and combining experiments with numerical modelling, the underlying mechanisms and the optimal conditions for paint removal by laser ablation are thoroughly investigated. Our studies suggest that laser paint removal is dominated by the laser vaporization effect, thermal stress effect and laser plasma effect, in which thermal stress effect is the most favoured while laser plasma effect should be avoided during removal operations. Based on the thermodynamic equations, we numerically evaluated the spatial distribution of the temperature as well as thermal stress in the paint and substrate under the irradiation of laser pulse at 1064 nm. The obtained curves of the paint thickness vs. threshold fluences can provide the reference standard of laser parameter selection in view of the paint layer with different thickness. A multi-pulse model is proposed and validated under a constant laser fluence to perfectly remove a thicker paint layer. The investigations and the methods proposed here might give hints to the efficient operations on the paint removal and lowering the risk of substrate damages.

Femtosecond Laser-Induced Nanometer-Scale Rods on Silicon Surface

We report that silicon surface develops nanometer-scale rods when irradiated with two steps of six femtosecond laser pulses aggregately of 800 nm, 35fs duration in air. The evolution of the rods also has been studied. The result indicates that the rods disappear and only ripples with direction being perpendicular to the polarization of the laser in the second step remain, when the number of the laser pulses increase from three to five. The formation of the nano-scale rods involes several processes which include interference of refracted light and scattered light, surface capillary exceted, re-solidation of silicon surface, roughness-enhenced optical absorption.

Application of Wavelet Digital Filter in the Radial Shearing Interferometry

For the problem of spectrum aliasing of radial shearing interference fringes, a new technique for removing the background components and high frequency components of interference fringes based on discrete wavelet transform (DWT) is established. This image pre-filtering stage not only enables the FFT method to demodulate interference fringes with larger bandwidths but also reduces speckle noise significantly. When the wavefront is complex, the reconstruction RMS error is better than /25 by FFT method, however, by using the new method suggested here it is better than /40. Comparison analysis showed that the new method has strong capabilities to anti-noise and anti-aliasing of spectrum, reduces the frequency domain filter design requirements and improves the accuracy and stability of the wavefront detection.