Nonadiabatic optical near-field etching is a novel way of polishing surfaces of optical elements without any mechanical contact processes. It selectively induces photochemical reactions on the surface in regions where optical near fields are excited, namely, in the vicinity of portions having fine-scale rough structures, which leads to reduced surface roughness. In this paper, we analyze the surface roughness of optical elements planarized by nonadiabatic optical near-field etching based on power spectrum-based methods. Our analysis clearly reveals the effects of the near-field etching which are otherwise concealed when using conventional roughness measures such as the roughness average (Ra). We also investigate the effect of planarization via numerical simulations using surface profiles obtained experimentally before and after the near-field etching.
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