Lingfei Ji

Enhancement of the dielectric permittivity of Ta2O5 ceramics by CO2 laser irradiation

The tantalum-pentoxide ceramics with significantly enhanced dielectric permittivities around 60–70 (up to 71.62) at 1MHz and 20°C were prepared by CO2 laser irradiation. The dielectric loss and the temperature coefficient of dielectric permittivity could be 0.0128 and 721ppm∕°C, respectively. Dielectric properties were investigated over the temperature range from −60 to 100°C at 1MHz. The frequency dependence of the dielectric properties was also investigated between 1kHz and 1MHz at 20°C. The stabilization of high-temperature phase at RT and crystallographic orientation in the ceramics by laser irradiation can explain the strong enhancement of the dielectric permittivity.

Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays

Here we report strong enhancement in ultraviolet-photoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 μm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency.

Investigations of dielectric enhancement in (Ta2O5)1-x(TiO2)x ceramics prepared by laser-sintering technique

The maximum dielectric permittivity of Ti-doped Ta2O5 ceramics may reach 450 by a laser-sintering technique. The aim of this study is to investigate the mechanisms of the dielectric enhancement based on the unique structural and morphological properties of the laser-sintered ceramics. The reason for the dielectric enhancement is due to the crystal structure distortion in the high-temperature phase, the oriented grain growth taking place in a direction deviating from [001] in the laser-sintered ceramics. The concurrent nature of quenching effects, a sharp temperature gradient and mass transfer in liquid phase originated from laser high energy irradiation with strict directivity leads to the structural and morphological properties.

Blue photoluminescence enhancement in laser-irradiated 6H-SiC at room temperature

Blue photoluminescence (PL) of 6H-SiC irradiated by an ultraviolet laser can be observed at room temperature in dark condition. PL spectra with Gaussian fitting curve of the irradiated SiC show that blue luminescence band (∼440 nm) is more pronounced than other bands. The blue PL enhancement is the combined result of the improved shallow N-donor energy level and the unique surface state with Si nanocrystals and graphene/Si composite due to the effect of photon energy input by the short-wavelength laser irradiation. The study can provide a promising route towards the preparation of well-controlled blue photoluminescence material for light-emitting devices.

Improvement of the electrical and ferromagnetic properties in La0.67Ca0.33MnO3 thin film irradiated by CO2 laser

The authors report significant improvement in the electrical and magnetic properties of La0.67Ca0.33MnO3 thin film irradiated by CO2 laser in air for several dozens of seconds. It is shown that after CO2 laser irradiation the crystallinity of the film is strongly enhanced, and the surface structure is remarkably modified and displays a plainlike morphology with a lower surface roughness. The authors find that CO2 laser irradiation results in a sharp decrease in the resistivity and a notable increase in the temperature coefficient of resistance, the insulator-metal transition temperature, and ferromagnetic transition temperature. The experimental results suggest that CO2 laser irradiation is a fast and effective method to optimize colossal magnetoresistance materials for technological applications.

Structural correlations of the enhancement of dielectric permittivity in the (Ta2O5)0.92(TiO2)0.08 system

A Ti-ion ordering which correlates to the significant enhancement of the dielectric constant was determined in the (Ta2O5)0.92(TiO2)0.08 system by high resolution electron microscopy and the corresponding simulations. The alien Ti ions were determined to selectively sit on the octahedral locations in the Ta2O5 base structure in an average of 9-time period along the ⟨110⟩ direction. The authors suggest this Ti-ion ordering induces the directional enhancement of the dielectric permittivity of the (Ta2O5)0.92(TiO2)0.08 system. The atomic mechanism of this modulated structure contributing to the enhancement of the dielectric constant was qualitatively interpreted using a one-dimensional chain model. The structural correlations of the dielectric enhancement revealed in this study suggest a way to search or design high-k materials.

Laser crack-free cutting technique for thick and dense ceramics

A new laser crack-free cutting technique for dense ceramics with thickness of ≥ 10 mm based on beam continuous piercing full through the workpieces is presented. The techology can be concluded as a feasible tool for cutting of thick and dense ceramics in complex path, even for 3D cutting.

Porous silicon with double band photoluminescence fabricated by chemical-assisted picosecond laser irradiation

The fabrication of porous silicon with double band photoluminescence (PL) by chemical-assisted 1064 nm picosecond (ps) laser irradiation of polysilicon is presented. The hybrid method includes controlling of the laser scanning interval, number of scans to form dense micropores, and subsequently, short-time acid corrosion to form fine nanostructure. Along with the stable red PL visible to the naked eye, a new violet PL band at 432 nm existed in the fabricated samples. The double band photoluminescence was mainly attributed to the prepared micro/nano hierarchical structure. Moreover, dangling bond defects formed by high-energy photon impinging acted as the catalyzer in the subsequent acid corrosion resulted in good passivation of the nanostructure with the formation of Si-H and Si-O bond, which also contributed to the stable photoluminescence. The maximum surface porosity of the samples was concluded to be 90.48% which also bring good hydrophobicity for the prepared samples.

Ti:sapphire femtosecond laser interaction with human dental dentine

Abstract This paper reports an investigation of the characteristics of human dentine ablated by a femtosecond laser of 800 nm wavelength for potential pain free dental restoration applications. The work has shown that precise femtosecond laser ablation with clear boundaries and without cracks and debris can be obtained. The transmittance of the dentine tissue was ∼2·7%, and the absorption coefficient was ∼3·19 cm−1 at 800 nm. The laser processed dentine morphology was investigated over a wide range of laser fluences and pulse numbers. The fluence threshold of dentine was determined as 0·45 J cm−2 according to the experimental dependence of the ablated depth versus laser fluence. The thermal energy conduction effects of dentine tubules were analysed. The SEM–EDX and Raman mapping analysis of phosphate indicate that the chemical components of the dentine remained intact before and after ablation.

Parallel-axis positioning device for laser processing: Design and applications

A parallel-axis positioning device with “twice positioning” method is presented to exectly locate the laser spot at a special position on workpiece for laser macro-processing. Position accuracy of the laser spot on workpiece reaches 0.01mm.