Mechanism for femtosecond laser-induced periodic subwavelength structures on solid surface: surface two-plasmon resonance

TL;DR

This paper proposes that surface two-plasmon resonance (STPR) is the key mechanism behind the formation of subwavelength ripple structures on metals irradiated by femtosecond lasers, supported by experiments and simulations.

Contribution

It introduces the STPR model as a self-consistent explanation for subwavelength ripple formation, including periods shorter than half the laser wavelength.

Findings

STPR causes self-formation of subwavelength ripples.

Experimental carving confirms independent wave patterns of STPR.

Time-resolved spectroscopy and simulations verify ablation dynamics and electron temperature.

Abstract

We present that surface two-plasmon resonance (STPR) in electron plasma sheet produced by femtosecond laser irradiating metal surface is the self-formation mechanism of periodic subwavelength ripple structures. Peaks of overdense electrons formed by resonant two-plasmon wave pull bound ions out of the metal surface and thus the wave pattern of STPR is "carved" on the surface by Coulomb ablation (removal) resulting from the strong electrostatic field induced by charge separation. To confirm the STPR model, we have performed analogical carving experiments by two laser beams with perpendicular polarizations. The results explicitly show that two wave patterns of STPR are independently carved on the exposure area of target surface. The time-scale of ablation dynamics and the electron temperature in ultrafast interaction are also verified by time-resolved spectroscopy experiment and numerical simulation, respectively. The present model can self-consistently explain the formation of subwavelength ripple structures even with spatial periods shorter than half of the laser wavelength, shedding light on the understanding of ultrafast laser-solid interaction.