Laser-induced nanoparticle ordering

Nanoparticles were produced on the surface of silicon upon pulsed-laser irradiation in the presence of an inert gas atmosphere at fluences close to the melting threshold. It was observed that nanoparticle formation required redeposition of ablated material. Redeposition took place in the form of a thin film intermixed with extremely small nanoparticles possibly formed in the gas phase. Through the use of nonpolarized laser light, it was shown that nanoparticles, fairly uniform in size, became grouped into curvilinear strings distributed with a short-range ordering. Microstructuring of part of the surface prior to the laser treatment had the remarkable effect of producing nanoparticles lying along straight and fairly long (approximately I mm) lines, whose spacing equaled the laser wavelength for normal beam incidence. In this work, it is shown that the use of polarized light eliminated the need of an aiding agent: nanoparticle alignment ensued under similar laser treatment conditions. The phenomenon of nanoparticle alignment bears a striking similarity with the phenomenon of laser-induced periodic surface structures (LIPSS), obeying the same dependence of line spacing upon light wavelength and beam angle of incidence as the grating spacing in LIPSS. The new results strongly support the proposition that the two phenomena, LIPSS and laser-induced nanoparticle alignment, evolve as a result of the same light interference mechanism.