Simulation on tip-assisted focusing of laser energy for sub-surface photon heating

The extensive confinement of electromagnetic field and the resulting localized heating effect induced by metallic nanotips are of great interest which promise novel applications including material surface processing and laser-assisted manufacturing. In this work, the sub-surface heating effect of silicon and monolayer graphene induced by gold nanotip-light interaction is studied through numerical simulations. The optimized optical enhancement around a free-standing gold tip is firstly studied with respect to the incident laser wavelength and incident angle. The maximum electric intensity enhancement is up to ∼66 times within sub-10 nm region around the tip apex which is obtained under a 532 nm p-polarized laser incidence. With the presence of silicon under the nanotip, the optical field in the gap is enhanced further, generating a heat source at the sub-surface (∼2 nm depth) of silicon. The heat source energy density is as high as 3 × 10¹⁵ W m⁻³ with a moderate incident intensity of 2 × 10⁷ W m⁻². As for gold nanotip-monolayer graphene configuration with a separation distance of 1 nm, the enhanced electric field can penetrate into monolayer carbon atoms and induce intensive heating by 55 times in the graphene and by 10⁴ times across the carbon layer surface, which could be used for localized modifications of graphene surface like surface cleaning and property tuning. Simulation results in this work provide a guide for the laser-assisted manufacturing/modification not only in the bulk materials but also in the 2D nanostructures.