Appl Phys B (2012) 107:257–262 DOI 10.1007/s00340-012-5009-6 RAPID COMMUNICATION Direct observation of optical excitation transfer based on resonant optical near-field interaction W. Nomura · T. Yatsui · T. Kawazoe · M. Naruse · E. Runge · C. Lienau · M. Ohtsu Received: 21 February 2012 / Published online: 21 April 2012 © Springer-Verlag 2012 Abstract This article reports the direct observation of long- distance optical excitation transfer based on resonant optical near-field interactions in randomly distributed quantum dots (QDs). We fabricated optical excitation transfer paths based on randomly distributed QDs by using CdSe/ZnS core–shell QDs and succeeded for the first time in obtaining output sig- nals resulting from a unidirectional optical excitation trans- fer length of 2.4 μm. Furthermore, we demonstrate that the optical excitation transfer occurs via the resonant excited levels of the QDs with a comparative experiment using non- resonant QDs. This excitation-transfer mechanism allows for intersecting, non-interacting nano-optical wires. W. Nomura ( ) · T. Yatsui · T. Kawazoe · M. Ohtsu School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan e-mail: nomura@nanophotonics.t.u-tokyo.ac.jp W. Nomura · T. Yatsui · T. Kawazoe · M. Ohtsu The Nanophotonics Research Center, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan M. Naruse National Institute of Information and Communications Technology, 4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan E. Runge Institut für Physik, Technische Universität Ilmenau, 98693 Ilmenau, Germany C. Lienau Institut für Physik, Carl von Ossietzky Universität, 26111 Oldenburg, Germany 1 Introduction Novel optical devices, nanofabrication technologies, and systems utilizing energy transfer between nanomaterials by means of optical near field have been developed during re- cent years [1]. Several theoretical studies performed in par- allel with the advancements in near-field technologies de- scribe the optical near field as a field in which a material excitation is coupled with photons and which may be ap- propriately viewed as photons carrying a material excitation (dressed photons) [2]. According to the dressed photon the- ory, optical near-field interactions, i.e. the transfer of elec- tromagnetic energy through near-field interactions between nanomaterials, is interpreted as the exchange of dressed pho- tons. Since dressed photons that occur in nanomaterials are localized in nanometric dimensions [3], the long-wavelength approximation that has been used for conventional light– electron interactions does not hold. In particular, it is possi- ble to excite electrons and excitons to energy levels that are dipole-forbidden [4]. This has brought about qualitative in- novations in optical devices and their applications. One ex- ample of a recently developed practical nanophotonic device that operates based on this property is a nanodimensional logic gate array that operates at room temperature [5]. In order to utilize optical near-field excitation transfer based on the exchange of dressed photons for a device or a system, it is necessary to pay sufficient care to the trans- fer of optical signals between functional elements. In order to ensure stability of a nanophotonic device, which operates based on very weak signals, wiring (we use the term to in- clude optical signal connections) that does not involve signal reflections from the subsequent stage is required. In general, it is difficult to eliminate such signal reflections in optical waveguides, such as those using surface plasmon polaritons or photonic crystals [6, 7]. A substantial challenge consists