Controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force of femtosecond laser pulses

Abstract

Nanoparticles made of different materials usually support optical resonances in the visible to near infrared spectral range, such as the localized surface plasmons observed in metallic nanoparticles and the Mie resonances observed in dielectric ones. Such optical resonances, which are important for practical applications, depend strongly on the morphologies of nanoparticles. Laser irradiation is a simple but effective way to modify such optical resonances through the change in the morphology of a nanoparticle. Although laser-induced shaping of metallic nanoparticles has been successfully demonstrated, it remains a big challenge for dielectric nanoparticles due to their larger Young’s modulus and smaller thermal conductivities. Here, we proposed and demonstrated a strategy for realizing controllable shaping of high-index dielectric nanoparticles by exploiting the giant optical force induced by femtosecond laser pulses. It was found that both Si and Ge nanoparticles can be lit up by resonantly exciting the optical resonances with femtosecond laser pulses, leading to the luminescence burst when the laser power exceeds a threshold. In addition, the morphologies of Si and Ge nanoparticles can be modified by utilizing the giant absorption force exerted on them and the reduced Young’s modulus at high temperatures. The shape transformation from sphere to ellipsoid can be realized by laser irradiation, leading to the blueshifts of the optical resonances. It was found that Si and Ge nanoparticles were generally elongated along the direction parallel to the polarization of the laser light. Controllable shaping of Si and Ge can be achieved by deliberately adjusting the excitation wavelength and the laser power. Our findings are helpful for understanding the giant absorption force of femtosecond laser light and are useful for designing nanoscale photonic devices based on shaped high-index nanoparticles.