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Arrays of sharp conical spike microstructures are created by repeatedly irradiating silicon surfaces with focused femtosecond laser pulses in SF6. The absorbance of light is increased to approximately 90% in a wavelength range from the near ultraviolet (0.25 m) to the near infrared (2.5 m) by the microstructured silicon surface. The microstructured surface presents pitch-black because of enhanced absorption with a broad wavelength range, which is called black silicon. The unique microstructure morphology of black silicon surface formed by femtosecond laser can also bring a lot of other surface functions, for example, self-cleaning and field emission. These functions make black silicon highly desirable in solar energy, detectors and other fields. Therefore, the forming mechanism and conditions of fabrication optimization for black silicon microstructure have always been the focus of research. In our work, the sample is moved by motor-controlled stage while the laser beam is fixed. In the case of laser beam scanning, arrays of sharp conical spikes on the silicon are manufactured in 70 kPa SF6. The aim of the experiment is to find how to optimize the distribution of the laser energy in a number of laser accumulation pulses (the combination of single pulse energy and pulse number) to control the surface morphology of the black silicon. Experimental results show that there appears a bottleneck effect of morphology size growth with the increase of laser irradiation (improving the single pulse energy or increasing pulse accumulation number). Excessive energy accumulation brings no extra effect on optimizing and controlling of microstructure morphology on the surface. Based on theoretical results obtained from a physical model we proposed, we find that the reason for this phenomenon is that the microstructure morphology induced by former sequence pulse modulates the laser energy absorption of current laser pulse, and changes the laser ablation efficiency of the current pulse. According to this physical mechanism, we propose a new way of optimizing surface morphology, with fixing the total laser irradiation energy. And the size and distribution of surface morphology can be achieved by optimizing the distribution of the laser energy in a number of laser accumulation pulses. This approach can not only improve the efficiency of silicon surface preparation of microstructures but also reduce the surface defects and damage. Furthermore, the proposed method can reduce the energy consumption in the process of femtosecond machining. It is of great significance for the engineering application of black silicon.

The radio-frequency (RF) emissions in a range from 30 MHz to 800 MHz from the plasma, which is produced by the nanosecond laser (532 nm, 8 ns) induced breakdown of atmospheric air, are presented. A spectrum analyzer which can scan over a spectral range of 9 kHz-26.5 GHz is used to record the RF-range radiation intensities of the emission from the plasma. RF electromagnetic radiations from the laser induced breakdown of atmospheric air are obtained for different input laser energies. A half-wave plate and a Glan prism are used to vary the input laser energy. Experimental results show that the intensities of RF radiation in a range of 30-200 MHz increase with the increase of laser energy, but the intensities of RF radiation in a 360-600 MHz frequency range decrease. To study the effect of input laser polarization on the RF radiation, we adopt the input lasers with vertical and horizontal polarization respectively. When the polarizations of the input laser and the antenna are the same, the RF radiation intensity is relatively high, and the frequency lines are relatively abundant. The changing relationship between the total power of RF radiation and the energy of the input laser is calculated and analyzed. It is observed that the total power of RF radiation first increases and then decreases with the increase of input laser energy. The influences of the plasma electron density on the plasma frequency and the plasma attenuation coefficient are investigated to explain the relationship between the total power of the RF radiation and the laser energy. The RF radiation is caused by the following processes. The generated electrons and ions are accelerated away from the core by their thermal pressures. This leads to charge separation and forming the electric dipole moments. These oscillating electric dipoles radiate electromagnetic waves in the RF range. Furthermore, the interactions of electrons with atomic and molecular clusters within the plasma play a major role in RF radiation, and the low frequency electromagnetic radiation takes place from the plasma that is far from fully ionized state. Further study of the characteristics of RF electromagnetic radiation is of great significance for understanding the physical mechanism of the interaction between laser and matter.

Optical near field enhancement on substrate can be achieved by localizing femtosecond laser energy with nanoparticles. The enhanced field is located in the region between nanoparticles and the substrate. The localized femtosecond optical field is of great significance for fabricating the micro/nano structure with characteristic size beyond the diffraction limit. Up to now, femtosecond processing nanohole assisted by particle array is only possible for metal particle (Au) and low-refractive-index dielectric polystyrene particle. However, previous research results show that it cannot be realized for metal particle arrays (Au) to form periodic nanohole arrays, and it is limited for polystyrene particle to choose the corresponding substrate. In this paper, a novel method is proposed, in which high refractive index TiO2 arrayed particles are placed on the substrate to achieve laser induced near field enhancement. This makes feasible the nanoscale processing beyond the diffraction limit. In this paper, near field distributions of TiO2 particle array on Si, Pt and SiO2 substrates are simulated by the finite-difference time-domain (FDTD) method. The results show that TiO2 particles concentrate the laser energy to a region with a diameter of 100 nm around the particle and the near field enhancement is 140 times higher than the incident laser intensity, which is beneficial to fabricating the nanostructure of super diffraction limit, such as sub-hundred nanometer nanohole ablation by femtosecond laser. For Si substrate, the near field enhancement is only about 30% lower for TiO2 particle array than that for single TiO2 particle. In order to explore the influence mechanism of the substrate material parameters on the near field enhancement of TiO2 nanoparticle array, we further simulate the enhancement factor for the substrates of different refractive indices. It is found that the near field is enhanced with the increase of substrate refractive index, and this is attributed to an increased interaction of the particle with the near field of substrate and the scattering effect in which the TiO2 particle supports forward near field intensity pattern. Moreover, the image charge model is introduced to analyze the effect of substrate optical parameters on local field enhancement. Results in this paper can be applied to most metals as well as dielectric substrate surfaces, and they open a new way for femtosecond laser near field nano-processing with characteristic size beyond the diffraction limit.

The paper proposes two kinds of filter structures based on LTCC technology, which includes single layer stepped-impedance microstrip line structure and multilayer structure. According to the design of requirements, two kinds of filter with different structure are designed finally. And the difference between these two kinds of structures is proposed from physical dimensions and performance respectively. Filter in this paper is needed that the cut-off frequency is 1.7 GHz, the attenuation is larger than 30dB at 3GHz and less than 2dB in the pass band. The simulation results show that the two kinds of structures of LTCC filter can meet the desired demand. The single layer stepped-impedance microstrip line LPF has no advantage on physical size and the attenuation rate is lower than the multilayer LPF after 1.7 GHz. So the multilayer structure is dominant in the design of LPF.

The heavy metal elements contained in soil samples which are collected from Changchun train station, CUST campus, South Lake and Jingyue Lake park are separately analyzed by using the orthogonal dual laser pulses induced breakdown spectroscopy (DP-LIBS). The elements Mn, Cr, Cu and Pb are qualitatively analyzed according to the LIBS spectral intensity. It is shown that the intensity of the spectrum is greatly enhanced by using the DP-LIBS and the enhancement is closely related to the delay time between the double pulses. Enhancement variation with delay time presents double-peak changing when 532 nm and 1064 nm laser pulse energies are 70 mJ and 100 mJ respectively. The enhancement of Mn I 406.4 nm is 2.75 at a delay time of 20 s and it is 2.4 at a delay time of 30 s when the second peak appears. The enhancement mechanism of orthogonal pre-ablation DP-LIBS is further discussed.

Thermal characteristics of tightly-contacted copper-gold double-layer thin film target under ablation of femtosecond laser pulses are investigated by using a two-temperature theoretical model. Numerical simulation shows that electron heat flux varies significantly on the boundary of copper-gold film with different maximal electron temperature of 1.15 ×103 K at 5 ps after ablating laser pulse in gold and copper films, which can reach a balance around 12.6 ps and 8.2 ps for a single and double pulse ablation, respectively, and in the meantime, the lattice temperature difference crossing the gold-copper interface is only about 0.04 × 103 K at the same time scale. It is also found that electron-lattice heat relaxation time increases linearly with laser fluence in both single and double pulse ablation, and a sudden change of the relaxation time appears after the laser energy density exceeds the ablation threshold.