Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Site-Selective, Two-Photon Plasmonic Nanofocusing on a Single Quantum Dot for Near-Room-Temperature Operation Su-Hyun Gong, Sejeong Kim, Je-Hyung Kim,† Jong-Hoi Cho, and Yong-Hoon Cho* Department of Physics and KI for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: Although the study of single quantum dot (QD) properties without the background noise and dephasing processes caused by surrounding carriers is a crucial issue, the spatial-selective excitation of a single QD is still challenging, due to the diffraction nature of light. Here, we demonstrate a deep subwavelength excitation of a single QD using twophoton plasmonic nanofocusing. Self-aligned plasmonic nanofocusing on a single QD was achieved using metal coated nanopyramid structures. The highly enhanced local electric field generated by the plasmonic nanofocusing gives rise to a large increase in the optical nonlinear effect (i.e., two-photon excitation). As a result of the enhanced field enhancement on the metal-pyramid hybrid structure, the two-photon luminescence intensity was enhanced by a factor of 5000, and the selective excitation of a single QD enabled us to observe InGaN QD emission at near room temperature, due to the large suppression of the background emission. Our approach opens promising perspectives for quantum optics experiments with highly reduced background emissions. KEYWORDS: self-aligned plasmonic nanofocusing, single quantum dot spectroscopy, nonlinear, two-photon excitation, site-controlled quantum dot
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included to separate the emissions of the nanoemitters from the excitation laser. Instead of frequency-selective excitations, it is also possible to use spatial-selective excitations. A near-field scanning optical microscope (NSOM) system has been widely used to overcome diffraction limited focusing or imaging.8−10 The spatial resolution of excitation using a NSOM is directly limited by the aperture size of the probe, which is typically on the order of 50−100 nm. However, it is difficult to achieve a highcollection efficiency of photons from the aperture, since fewer photons are collected as the aperture size is reduced for high resolution. Therefore, subwavelength focusing beyond the diffraction limit, using far-field microscopy, becomes important. Twophoton spectroscopy provides a high degree of spatial selectivity in three dimensions. Two-photon spectroscopy generally requires a high excitation power, since two-photon absorption relies on nonlinear effects, and this produces other problems, such as Joule heating.11,12 Another interesting way to obtain a spatial selective excitation is using metal nanostructures that form localized electric fields in subwavelength scale, and enhanced nonlinear effects. Although the plasmon-induced enhancement of fluorescence under two-photon excitation have been demon-
olid-state nanoemitters, such as quantum dots and diamond color centers, have wide potential value for quantum optics and quantum information science.1−3 Microscope-based spectroscopy is widely used to examine the optical properties of single nanoemitters. However, the large scale mismatch between a single nanoemitter (
