Junfeng Shao

Mesoporous Transition Metal Oxides for Supercapacitors

Recently, transition metal oxides, such as ruthenium oxide (RuO2), manganese dioxide (MnO2), nickel oxides (NiO) and cobalt oxide (Co3O4), have been widely investigated as electrode materials for pseudo-capacitors. In particular, these metal oxides with mesoporous structures have become very hot nanomaterials in the field of supercapacitors owing to their large specific surface areas and suitable pore size distributions. The high specific capacities of these mesoporous metal oxides are resulted from the effective contacts between electrode materials and electrolytes as well as fast transportation of ions and electrons in the bulk of electrode and at the interface of electrode and electrolyte. During the past decade, many achievements on mesoporous transition metal oxides have been made. In this mini-review, we select several typical nanomaterials, such as RuO2, MnO2, NiO, Co3O4 and nickel cobaltite (NiCo2O4), and briefly summarize the recent research progress of these mesoporous transition metal oxides-based electrodes in the field of supercapacitors.

Optimally enhanced optical emission in laser-induced air plasma by femtosecond double-pulse

In laser-induced breakdown spectroscopy, a femtosecond double-pulse laser was used to induce air plasma. The plasma spectroscopy was observed to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to femtosecond single-pulse laser. In particular, the optical emission intensity can be optimized by adjusting the delay time of femtosecond double-pulse. An appropriate pulse-to-pulse delay was selected, that was typically about 50 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy, plasma channel, and so on.

Comparison of plasma temperature and electron density on nanosecond laser ablation of Cu and nano-Cu

Laser-induced breakdown spectroscopy is performed through the collection of spectra by spectral detection equipment at different delay times and distances from targets composed of Cu and nano-Cu, which are ablated using a Nd:YAG laser (532 nm, 10 ns, 10 Hz) in our experiments. The measured wavelength range is from 475 nm to 525 nm. Using the local thermodynamic equilibrium model, we analyze the characteristics of the plasma temperature and the electron number density for different distances between the target surface and the lens. The results show that when compared with the nano-Cu plasma case, the temperature of the Cu plasma is higher, while its electron number density is lower.

Carbon-based optical limiting materials

In this mini-review, special attention has been paid to carbon-based optical limiting materials. After a brief introduction to optical limiting mechanisms of carbon-based optical materials and their characterization technique, this mini-review presents the recent progress of carbon-based optical limiting materials including carbon black suspensions (CBS), carbon nanotubes (CNTs), fullerenes, graphene and detonation nanodiamond. Finally, perspectives on carbon-based optical limiting are given.

Optimization of distances between the target surface and focal point on spatially confined laser-induced breakdown spectroscopy with a cylindrical cavity

The spatial confinement effect in laser-induced plasma with different distances between the target surface and focal point is investigated by optical emission spectroscopy. A Nd:YAG laser is used to produce plasma from a silicon sample in air atmosphere. When the appropriate distance is selected, the duration of spectral emission enhancement is much longer, and the enhancement effect is much stronger. The phenomenon is attributed to the aspect ratio of the lateral to axial direction of the plasma plume. The plasma plume of a large aspect ratio will interact with the reflected shockwave in a long range of delay time, leading to high particle density. This provided a better understanding about the effect of the distance between the target surface and focal point, leading to better conditions for spatially confined laser-induced breakdown spectroscopy.

Linear equations method for modal decomposition using intensity information

The linear equations method is proposed to calculate the complete modal content of the partially coherent laser beam using only the intensity information. This method could give not only the incoherent expansion coefficients of the modal decomposition but also the cross-correlation expansion coefficients using the intensity profiles in several planes of finite distance along the propagation direction. A simulation is also presented to verify the validity of this theory. In our algorithm, the minimum and maximum mode orders should be known a priori, so we provide an estimation method for the two parameters.

Effect of cylindrical cavity height on laser-induced breakdown spectroscopy with spatial confinement

In this paper, we present a study on the spatial confinement effect of laser-induced plasma with a cylindrical cavity in laser-induced breakdown spectroscopy (LIBS). The emission intensity with the spatial confinement is dependent on the height of the confinement cavity. It is found that, by selecting the appropriate height of cylindrical cavity, the signal enhancement can be significantly increased. At the cylindrical cavity (diameter=2 mm) with a height of 6 mm, the enhancement ratio has the maximum value (approximately 8.3), and the value of the relative standard deviation (RSD) (7.6%) is at a minimum, the repeatability of LIBS signal is best. The results indicate that the height of confinement cavity is very important for LIBS technique to reduce the limit of detection and improve the precision.

Ultrafast thermionic emission from metal irradiated using a femtosecond laser and an electric field in combination

Ultrafast thermionic emission from gold film irradiated with a femtosecond laser pulse in the presence of an additional electric field is analyzed using a two-temperature equation combined with a modified Richardson equation. The calculated results show that the duration of the emission is below 1 ps. Supplying an additional electric field is found to change the emission from the metal surface. Given the same laser fluence, this additional field reduces the work function of the metal, and thus improves the efficiency of thermionic emission. These results help to understand the mechanism and suggest ways to improve emissions in the context of ultrafast thermalized electron systems.

Study of jamming of the frequency modulation infrared seekers

The threat of the IR guidance missile is a direct consequence of extensive proliferation of the airborne IR countermeasure. The aim of a countermeasure system is to inject false information into a sensor system to create confusion. Many optical seekers have a single detector that is used to sense the position of its victim in its field of view. A seeker has a spinning reticle in the focal plane of the optical system that collects energy from the thermal scene and focuses it on to the detector. In this paper, the principle of the conical-scan FM reticle is analyzed. Then the effect that different amplitude or frequency modulated mid-infrared laser pulse acts on the reticle system is simulated. When the ratio of jamming energy to target radiation (repression) gradually increases, the azimuth error and the misalignment angle error become larger. The results show that simply increasing the intensity of the jamming light achieves little, but it increases the received signal strength of the FM reticle system ,so that the target will be more easily exposed. A slow variation of amplitude will warp the azimuth information received by the seeker, but the target can’t be completely out of the missile tracking. If the repression and the jamming frequency change at the same time, the jamming effects can be more obvious. When the jamming signal’s angular frequency is twice as large as the carrier frequency of the reticle system, the seeker will can’t receive an accurate signal and the jamming can be achieved. The jamming mechanism of the conical-scan FM IR seeker is described and it is helpful to the airborne IR countermeasure system.

Emission enhancement of laser-induced breakdown spectroscopy by increasing sample temperature combined with spatial confinement

To enhance spectral intensity in laser-induced breakdown spectroscopy, increasing the sample temperature and spatial confinement were used simultaneously to improve optical emission of plasmas induced from silicon target using a Nd:YAG laser pulse in air. The sample was uniformly heated to temperatures ranging from a low temperature (25 °C) to a high temperature (250 °C) with laser energy of 60 mJ, and a cylindrical cavity with a diameter of 6 mm and a depth of 6 mm was used to confine the plasma. The results illustrated that as the sample temperature increased, the spectral intensity of plasma without spatial confinement appeared to saturate at a sample temperature of 150 °C. When the cylindrical cavity was present, the spectral intensity increased monotonically along with the sample temperature. The enhancement effect of combining spatial confinement and increasing the sample temperature was stronger than that of spatial confinement or increasing the sample temperature alone. The experimental results indicated that the optical emission intensity can be further improved by combining the two enhancement effects.