Chemical patterning of ultrathin polymer films by direct-write multiphoton lithography.

We applied 2-photon laser ablation to write subdiffraction nanoscale chemical patterns into ultrathin polymer films under ambient conditions. Poly(ethylene glycol) methacrylate brush layers were prepared on quartz substrates via surface-initiated atom-transfer radical polymerization and ablated to expose the underlying substrate using the nonlinear 2-photon absorbance of a frequency-doubled Ti:sapphire femtosecond laser. Single-shot ablation thresholds of polymer films were ~1.5 times smaller than that of a quartz substrate, which allowed patterning of nanoscale features without damage to the underlying substrate. At a 1/e(2) laser spot diameter of 0.86 μm, the features of exposed substrate approached ~80 nm, well below the diffraction limit for 400 nm light. Ablated features were chemically distinct and amenable to chemical modification.