Gallium nanoparticles have attracted widespread interest of scientists in recent years because gallium is not only a metal with surface plasmon resonance in the ultraviolet region, but also a phase change material with a low melting point (29.7 ° C). Potential applications of ultraviolet plasmon resonance for gallium nanoparticles include light storage, environmental restoration, fluorescence, and surface enhanced Raman spectroscopy. Compared to other metals that exhibit plasmon resonance in the ultraviolet region, gallium nanoparticles are stable in the atmosphere due to their thin natural oxide shell. In addition, another interesting feature of gallium nanoparticles is the so-called undercooling / overheating phenomenon, and the phase transition temperature largely depends on its size. Therefore, this unique property of gallium nanoparticles allows us to systematically study the linear and nonlinear optical properties of liquid gallium nanoparticles, the manipulation of directional radiation, and the effect of phase changes on their optical properties at room temperature.
In recent years, high-efficiency broadband nano-scale light emitters operating at optical frequencies have attracted great interest, such as ultra-compact optical chips, bio-imaging, nano-spectroscopy, and active photonic devices. The realization of effective nanoscale white light sources remains the main fundamental challenge. The surface plasmon resonance of large metal nanoparticles usually looks very wide and the electric field enhancement is relatively weak. So using femtosecond laser pulses to excite metal nanoparticles will only produce very weak photoluminescence. However, this can be significantly changed by bringing them to the surface of a thin metal film. It is known that mirror dipoles of metal nanoparticles can be induced through a thin metal film, and the interaction between a horizontal electric dipole and its mirror dipole can form a magnetic dipole with a narrow line width. If femtosecond laser pulses are used to excite the metal nanoparticles at the mirror magnetic dipole resonance, the photoluminescence can be effectively enhanced. In addition, the coherent interaction of the wide mode (bright mode) and the narrow mode (dark mode) can form so-called Fano resonances, resulting in a significantly enhanced electric field, which is very useful for enhancing optical nonlinearity.
The research team of Professor Lan Sheng from the School of Information Optoelectronics Technology of South China Normal University reported the first method of preparing spherical gallium nanoparticles with a diameter ranging from 50 to 300 nm by using femtosecond laser ablation. They found that the prepared gallium nanospheres consisted of a liquid gallium core and a solid gallium dioxide shell, and exhibited surface plasmon resonance across the ultraviolet to near-infrared spectral region. Under the excitation of femtosecond laser, only the second harmonic was observed on the gallium nanospheres placed on the glass. In sharp contrast, when these gallium nanospheres are placed on a thin silver film, they can produce significant white light radiation when excited by a femtosecond laser. Through numerical simulation, it was found that a Fano resonance derived from the interference between the mirror-induced magnetic dipole resonance and the gap plasma mode was formed in the backscattering spectrum of the gallium nanosphere. By resonantly excited magnetic dipole resonance or Fano resonance, efficient white light emission can be achieved. By analyzing the dependence of the luminous intensity and excitation intensity of the gallium nanospheres, the white light emitted by the gallium nanospheres belongs to the thermionic band fluorescence. The experimental observations are in good agreement with the numerical simulation results based on the complex refractive index of liquid gallium.