Nonlinear Plasmonics: Four-photon Near-field Photolithography using Optical Antennas

Recent experiments on four-photon nonlinear exposure of photoresist near nanoplasmonic structures raise a question: What nonlinear processes are responsible for the observed profile? Here, we study the nonlinear exposure of poly(methyl methacrylate) (PMMA) in the near-field of gold nanoantennas. We consider six possible nonlinear processes and study them in terms of the developed volumes and the exposure profiles in photoresist. We find that the direct fourth harmonic generation (4HG) in gold is the dominant nonlinear process. The developed volume from 4HG and the exposure profiles both match closely with the experiments. The next strongest process is direct four-photon absorption (4PA) in PMMA. The strength of 4PA process is about one order of magnitude weaker than 4HG. The developed volume and exposure profiles predicted from 4PA process clearly deviate from experiments.