Unidirectional signal transfer in quantum-dot systems via optical near-field interactions

To decrease the sizes of photonic devices beyond the diffraction limit of light, we propose nanophotonic devices based on optical near-field interactions between semiconductor quantum dots (QDs). To drive such devices, an optical signal guide whose width is less than several tens of nanometers is required. Furthermore, unidirectional signal transfer is essential to prevent nanophotonic devices operating incorrectly due to signals reflected from the destination. For unidirectional signal transfer at the nanometer scale, we propose a nanophotonic signal transmitter based on optical nearfield interactions between small QDs of the same size and energy dissipation in larger QDs that have a resonant exciton energy level with the small QDs. To confirm such unidirectional energy transfer, we used time-resolved photoluminescence spectroscopy to observe exciton energy transfer between the small QDs via the optical near-field, and subsequent energy dissipation in the larger QDs. We estimated that the energy transfer time between resonant CdSe/ZnS QDs was 135 ps, which is shorter than the exciton lifetime of 2.10 ns. Furthermore, we confirmed that exciton energy did not transfer between nonresonant QD pairs. These results indicated that the proposed nanophotonic signal transmitters based on optical near-field interactions and energy dissipation could be used to make multiple transmitters and selfdirectional interconnections.