We employ heterodyne interferometry to investigate the effect of a single organic molecule on the phase of a propagating laser beam. We report on the first phase-contrast images of individual molecules and demonstrate a single-molecule electro-optical phase switch by applying a voltage to the microelectrodes embedded in the sample. Our results may find applications in single-molecule holography, fast optical coherent signal processing, and single-emitter quantum operations.
1. a) The experimental setup. BS: beam splitter; BP: bandpass filter; LP: low-pass filter; S: sample; SIL: solid-immersion lens; AOM: acousto-optical modulator; PD: photodetector. The inset exemplifies raw data of the beating signal recorded in a start-stop configuration using the signals of PD1 and PD4, respectively. Green solid and dashed lines indicate the two detuned laser beams after the AOM. The red solid line signifies the fluorescence signal from the sample. b) Laser beam attenuation of 18% by a single molecule recorded on PD2 in reflection.
We have shown that a single organic molecule embedded in a solid matrix can shift the phase of a propagating light beam by several degrees. Improvements to the emitter-light coupling toward the ultimate limit of perfect reflection can make our scheme highly attractive for quantum gate operations. A particularly attractive on-chip solution in this regard is offered by the near-field coupling of molecules to light in metallic or dielectric nanoguides. This arrangement can be conveniently combined with integrated interferometers and provides the possibility to amplify the phase shift by coupling to several molecules placed in series within a few hundred nanometers. Such an approach is simpler, more compact and more robust than those based on microcavities because the system occupies a much smaller volume and does not rely on stringent resonant conditions of high-finesse cavities. Extension of the concepts demonstrated in this work to emitters with A or V energy level schemes would allow all-optical phase switching at the single-photon level. However, the strong optical nonlinearity of single molecules would also be helpful in the classical regime, where effects such as finite carrier lifetime and two-photon absorption under intense illumination currently limit the state-of-the-art phase switches