Investigations of local currents in a semiconductor by single-molecule spectroscopy

Single organic molecules spread on a thin semiconductor film probe local electrical currents. For low DC voltages, we measure strong quadratic line shifts with thresholds at higher voltages. Under AC voltages, the shifts of most molecules display resonances between 10kHz and 1 MHz, with frequencies and shapes differing from molecule to molecule within the laser spot. For high enough DC voltages, auto-oscillations of the optical frequency can appear. These surprising effects do not arise from a standard Stark effect, nor from lattice heating, and might be related to current instabilities or slow recharging waves.