Xun Gao

The role of spatial confinement on nanosecond YAG laser-induced Cu plasma

The role of spatial confinement in nanosecond laser-induced Cu plasma is investigated by optical emission spectroscopy, fast imaging and optical shadow imaging. Significant spectral enhancements of the atomic and ionic emissions are observed when two Al-plates are used to confine the plasma. The experimental results show that parameters such as the plasma temperature, electron density, and spectral signal-to-background ratio all increase in the presence of spatial confinement, and the distance of the two walls are critical for the spectral enhancement, the enhancement time duration, and the delay time at which the maximum spectral enhancement is observed. These effects occur later and become weaker with increasing wall distance. The fast imaging and shadowgraph results show that the shape of the plasma plume becomes narrower and longer with spatial confinement, and is also dependent on the wall distance. The spectral enhancement is attributed to the reflection of the shock wave by the wall plates, which compresses the plasma to a small local region and induces further collision excitation.

Effects of ambient conditions on femtosecond laser-induced breakdown spectroscopy of Al

Aluminum alloy was analysed by using femtosecond laser-induced breakdown spectroscopy under argon, air and helium environments at pressures ranging from 1 to 80?kPa. The results reveal that both spectra intensity and lines detection are significantly influenced by the ambient conditions. In all ambient gases, as the pressure increases the emitted light initially increases, attains its maximum intensity and then decreases with further increase in pressure. It is also observed that some lines are well detected at low pressures in argon while they are absent at the same pressures in helium. In addition, plasma parameters such as electron densities and electron temperature have been investigated at different pressures in the three gases. Hotter and denser plasma has been observed in argon than that in air and helium. Furthermore, it is noted that plasma parameters at relative low pressures of argon (1?kPa) are similar to those obtained at relative high pressures of helium (80?kPa). The optimum conditions for the use of argon and helium as ambient gases have been determined. In fact, argon provides the best environment of femtosecond laser-induced breakdown spectroscopy only at relative low pressures while helium constitutes a good environment only at relative high pressures.

Triggering and guiding high-voltage discharge in air by single and multiple femtosecond filaments.

The abilities to trigger and guide high-voltage discharge by using single and <em>multiple filaments</em> (MFs) are experimentally studied. It is shown that the discharge voltage threshold can be reduced significantly in both regimes of single and MF; however, the MF does not gain a larger reduction than a single filament. This behavior of the MF is attributed to the single discharge path rather than simultaneous multiple ones as one might expect during the discharge process.

Formation of strong light-trapping nano- and microscale structures on a spherical metal surface by femtosecond laser filament

Nano- and microscale structures on a material surface formed by femtosecond laser processing have greatly changed optical characteristics, wettability, as well as other properties of the material. In this work, we report the formation of nano- and microscale structures on a spherical Al surface with femtosecond laser filament, and find that the filament-processed surface has a strong light-trapping ability from UV to IR (0.2–2.5 μm). Our result demonstrates that this method can be used to process a spherical surface without the complexity of a 4-axis sample control, and in principle, it is applicable to any non-planar sample.

Characteristics of plasma plume expansion from Al target induced by oblique incidence of 1064 and 355?nm nanosecond Nd?:?YAG laser

Evolution of a plasma plume from an Al target ablated with a nanosecond 1064 and 355?nm laser respectively under oblique incidence in air is studied using the time-resolved shadowgraph imaging technique. The characteristics of plasma plume expansion with different focusing conditions (focal point on, ahead of and after the target surface) are experimentally investigated. Experimental results show that the evolution of the plasma plume is strongly influenced by air breakdown which occurs prior to the laser beam reaching the target. Without the occurrence of air breakdown, the temporal evolution of the Al plasma plume with both UV and IR ablation laser wavelengths shows the plume expansion with an ellipsoid-shaped plume front travelling mainly against the incoming laser beam due to the formation of a laser-supported detonation wave at the initial stage of laser ablation, and then the shape of the plume front turns into a sphere. Experimental results also show that a higher portion of the laser pulse energy reaches the target surface at UV laser wavelength than that of an IR laser due to the higher penetrating ability of the UV laser wavelength to the plasma.

Imaging Ultrafast Plasmon Dynamics within a Complex Dolmen Nanostructure Using Photoemission Electron Microscopy

We report direct nanoscale imaging of ultrafast plasmon in a gold dolmen nanostructure excited with the 7 fs laser pulses by combining the interferometric time-resolved technology with the three-photon photoemission electron microscopy (PEEM). The interferometric time-resolved traces show that the plasmon mode beating pattern appears at the ends of the dimer slabs in the dolmen nanostructure as a result of coherent superposition of multiple localized surface plasmon modes induced by broad bandwidth of the ultrafast laser pulses. The PEEM measurement further discloses that in-phase of the oscillation field of two neighbor defects are surprisingly observed, which is attributed to the plasmon coupling between them. Furthermore, the control of the temporal delay between the pump and probe laser pluses could be utilized for manipulation of the near-field distribution. These findings deepen our understanding of ultrafast plasmon dynamics in a complex nanosystem.

Interference-induced filament array in fused silica

One- and two-dimensional filament arrays are obtained in fused silica by using two and three interfered femtosecond laser beams, respectively. By modulating the number, cross angle, and azimuth of the beams, the dimension, period, orientation, and geometry of the filament-array can be controlled. The multiple beams interference method provides a convenient and effective method to generate and control the filament array in optical media with multiple degrees of freedom but without any external pulse modulation or focal element.

Characteristics of shock wave from nanosecond laser-produced aluminum plasma in air

Characteristics of shock wave as well as its evolution of aluminum plasma produced by nanosecond YAG laser is investigated by time-resolved optical shadowgraph images. Experimental results show that shock wave is strongly influenced by the laser parameters and target arrangement. Shock waves from aluminum plasma and air plasma are observed simultaneously by shadowgraphs when the distance from lens to target surface (DLTS) is longer than the lens focal length, and a narrow bright “line” is observed in the region where shock waves from Al plasma and air plasma meet. The longitudinal expansion velocity of shock wave from Al plasma is largely influenced by DLTS and laser intensity as well, and it increases with laser intensity at the early stage of plasma expansion and reach to a maximum of 8.1×104 m/s.

Combined effects of ambient gas pressures and magnetic field on laser plasma expansion dynamics

In this work, we investigated the influence of air gas pressures on the expansion features of nanosecond laser ablated aluminum plasma in the absence and presence of a nonuniform magnetic field using fast photography. A particular emphasis was given to the plume dynamics (shape, size) with the combined effects of ambient gas pressures and an external magnetic field. Free expansion, sharpening effect, and hemi-spherical structures of the aluminum plasma were observed without a magnetic field under different gas pressures. Analysis of the resulting plume images with the combined effects of air gas pressures and a magnetic field show significant changes, such as plume splitting, elliptical geometry changes, radial expansion, and plume confinement. Furthermore, the total size of the plasma plume with a magnetic field was measured to be smaller than the plasma plume without a magnetic field at several background pressures.