Near-Field Optical Spectroscopy of Individual Molecules in Solids

Near-field single-molecule spectroscopy was performed on a crystal of pentacene-doped $p$-terphenyl using an Al-coated optical fiber tip with 60 nm diam aperture as a near-field light source. Individual molecules located ≅400 nm from the tip had linewidths of 10-20 MHz at 1.8 K. The background fluorescence provided a useful distance-sensing signal, as it increased exponentially during approach with a characteristic length of ≅100 nm. Single molecules closest to the tip were identified by Stark shifts.