Wavelength dependent atomic force microscope tip-enhanced laser ablation

The role of laser wavelength in atomic force microscopy (AFM) tip-enhanced laser ablation was studied using an apertureless tip and a nanosecond pulsed laser. An optical parametric oscillator (OPO) laser wavelength tunable from 410 to 2400 nm was used to irradiate a gold-coated silicon AFM probe held 15 nm above the surface of an anthracene film. The absorption of laser energy by the tip at 532 nm is sufficient to melt the gold coating and increase the diameter of the tip from about 100 nm to approximately 1 µm. The ablation crater volume was measured and found to have a maximum at 500 nm and an approximately linear drop to 800 nm. Craters could not be produced between 800 and 1200 nm and the crater was slightly smaller at 450 nm compared to 500 nm. A crater rim was observed with a volume comparable to that of the crater but lower density. The mechanism of ablation is postulated to be the result of energy absorption by the tip through plasmon resonance of the gold coating followed by melting of the anthracene by ballistic, contact, or radiative heating of the anthracene film.