Experimental and Theoretical Analysis of the Nanoscale Crater Generation With a Near Field Scanning Optical Tip

Different nano-patterns have been generated with the same near field scanning optical microscope (NSOM) tips with multiple femtosecond laser pulses in different background gases. It is demonstrated that significant energy was transferred from the NSOM probe to a pure silicon surface for the generation of nano-protrusions and nano-craters, which shows the possibility of nano-fabrication with the present experimental configuration. In order to understand the heating effect of the target and the relationship between the generations of nano-craters, a corresponding theoretical analysis considering the wave format light propagation within a single tapering NSOM probe (first order approximation) and the subsequent light absorption in a target is established. This analysis show that electron temperature of around the nano-scale laser spot of target can be very high (>∼10,000 K) during the laser pulse. However, both the photoexcited electron number density and lattice temperature are much less the threshold for a thermal and non-thermal evaporation. Therefore, supplementary mechanisms in additional to pure pulsed light absorption are required for generation of nano-craters on a target if a single tapering angle NSOM probe is applied.