Experimental Characterization of Laser Trepanned Microholes in Superalloy GH4220 with Water-Based Assistance.

An experiment using water-assisted millisecond laser trepanning on superalloy GH4220 was carried out, and the effects of pulse energy on the hole entrance morphology, diameter, roundness, cross-section morphology, taper angle, sidewall roughness, and recast layer in air and with water-based assistance were compared and analyzed. The results show that, compared with the air condition, the water-based assistance improved the material removal rate and hole quality, increased the diameter of the hole entrance and exit, increased the hole roundness, decreased the hole taper angle, decreased the hole sidewall roughness, and reduced the recast layer thickness. In addition, under the combined action of water and steam inside the hole, the sidewall surface morphology quality was improved. Compared with the air condition, the spatter around the hole entrance was reduced, but the oxidation phenomenon formed by the thermal effect surrounding the hole entrance with water-based assistance was more obvious. The research provided technical support for the industrial application of millisecond laser drilling.

Experimental investigation of femtosecond laser through-hole drilling of stainless steel with and without transverse magnetic assistance.

An experimental investigation of femtosecond laser through-hole drilling of stainless-steel 304 with and without transverse magnetic assistance was conducted. The characteristics of the through-hole geometry and sidewall as well as the chemical composition of the through-hole sidewall surface were analyzed. In addition, a theoretical analysis of magnetic-field-assisted femtosecond laser through-hole drilling is proposed. The results showed that transverse magnetic assistance could improve both the femtosecond laser through-hole drilling quality (through-hole geometry and sidewall characteristics) and efficiency. The primary reason is that transverse magnetic assistance changes the distribution of plasma and reduces the plasma density, which weakens the shielding effect of the plasma. However, compared with nanosecond laser drilling, the effect of the magnetic field on the femtosecond laser through-hole drilling was not obvious. A noticeable thermal effect appeared near the through-hole entrance at a pulse repetition rate of 500 kHz, and a heat affected zone and oxidation zone were produced, which is disadvantageous to laser drilling. This research has good prospects for industrial applications.

Theoretical study on femtosecond laser optical breakdown threshold in water mediated by aluminum nanoparticle coated with silica.

A strongly coupled finite element model of the optical breakdown during femtosecond laser pulse interaction, with different morphology of aluminum nanoparticles in water, was developed. This model provided new insight into the optical breakdown dependence on the nanoparticles' morphology and assembly. This model was used to theoretically investigate a 300 fs laser pulse interaction with uncoupled and plasmon coupled aluminum coated silica shell nanoparticles. This study revealed how the nanoparticles' one-dimensional assembly affected the optical breakdown threshold of its surrounding mediums. The optical breakdown threshold had much stronger dependence on the optical near-field enhancement than on the nanostructure's extinction cross-section. The maximum electric field that is outside of the aluminum nanoparticles, with 2 nm silica shell and 2 nm gap, was more than 4 times greater to the one inside of the aluminum nanoparticles. For dimer and trimer configuration, the calculated lattice cross-section temperatures at each breakdown threshold were below their melting point. It is suggested that water could be ionized by aluminum/silica (core/shell) nanostructure during femtosecond laser exposures without nanoparticles consumption. This model could increase understanding of the aluminum nanoparticle-mediated optical breakdown in water.

Broadband visible-light absorber via hybridization of propagating surface plasmon.

We demonstrate a broadband visible-light absorber based on excitation of multiple propagating surface plasmon (PSP) resonances. The simple structure is constructed of continuous gold/silica multi-layers covered by a one-dimensional gold grating. The broadening of bandwidth arises from the inter-layer hybridization and spectral superposition of PSPs, which is predicted with the analytical coupled oscillator model and validated using the RCWA simulation. The average absorption increases with the number of gold/silica pairs and exceeds 95% over the whole visible spectrum when only five pairs are included. Moreover, results show that the absorption can be further enhanced by grading the thickness of silica layers. The presented design might enable promising applications in the fields of photovoltaic cells and thermal emitters, owing to its advantages of wideband, near-unity absorption and simple fabrication simultaneously.

[Research on several times of abruptly changes in the ultrafast transient reflectivity in Co films].

In the present paper, cobalt films, chromium and cobalt double layer films, and silver and cobalt double layer films were deposited by magnetron sputtering technique. The transient reflectivity response in cobalt thin films and their double layer films was studied by using femtosecond laser pump-probe transient reflection experimental technology. The result shows that, under the condition of the cobalt films of the same thickness, when the pump power increases, the heating time of the electrons in cobalt thin films is independent of the pump power, and is 0. 134 4 ps. And for the cobalt films of different thickness, the electron thermalization time is directly related to the film thickness. In addition, when the laser pulse power is high enough, two or three transient reflectivity decline signals on glass substrate films are found which is different with previous researches, when the cobalt films were thin, there are three times of transient reflectivity abruptly decline signals, and when the cobalt films were thick, there are two times of transient reflectivity abruptly decline signals, so the thickness of the cobalt films determines the times of the changes of the ultrafast dynamics in cobalt films.

Dual-medium quantitative measurement simulation on cells.

For research on inhomogeneous cells, we present a simulation method called the dual-medium quantitative (DMQ) measurement simulation method, which is realized by combining phase-shifting digital holography with DMQ analysis. The reliability of this method is confirmed by comparing the simulated phase map with the experimental one by the Hilbert phase microscope [J. Phys. Chem. A 113, 13327 (2009)10.1021/jp904746r], and its ability for studying inhomogeneous cells is demonstrated with measurements of a simulated HeLa cell. The average deviation and the relative deviation of physical thickness and axially averaged refractive index are 0.0339 µm, 0.69% and 0.0013, 0.094%, respectively. This approach can provide good guidance for experimental research on inhomogeneous cells.

[The theory and the influential factors of laser induced breakdown spectroscopy].

The laser-induced breakdown spectroscopy (LIBS) technique, due to its lack of pretreatment of the material and the speed of analysis, has shown a great potential for a wide range of industrial applications. The current situation of application and study of LIBS are reviewed. Meanwhile, the laser used to explode the Cr and Co film had a pulse duration of 10 ns, the highest single pulse energy was 50 mJ at a repetition rate of 20 Hz, and a wavelength of 1064 nm. When the excitation energy was less than 10 mJ, there was little difference in spectral signal excited by different energy for Cr film. Otherwise, the signal showed obvious change when excitation energy was higher than 10 mJ. The principle, characters, history and recent trend of LIBS were introduced systematically. The factors influencing LIBS performances, including the characteristic of laser, delay time, experiment setting, the kind and the pressure of surrounding. Gas, single or dual pulse, and the analysis method were analyzed.

[Application of laser induced breakdown spectroscopy (LIBS) to microdetection of Au film machining].

Film micro-machining is a core in micro- and nano- technology, micro-electro-mechanical systems and photoelectron field. However, it is difficult to control the quality of machining in processing film. In the present paper the authors propose a method to solve this problem by using laser induced breakdown spectroscopy (LIBS). The authors investigated the relation between excitation condition and the quantity of plasma in the process of micro-machining by pulse laser. At low frequency (<20 Hz), when the excitation energy was confirmed, the quantity of plasma was steady and independent of the number of pulses. When the authors used single pulse to excite the film, the quantity of plasma increased with the energy ascension. Based on the quantity of plasma and the species of elements, the authors realized the real-time control of micro-machining become true.