The lack of symmetry between electric and magnetic charges, a fundamental consequence of the small value of the fine-structure constant, is directly related to the weakness of magnetic effects in optical materials. Properly tailored plasmonic nanoclusters have been proposed recently to induce artificial optical magnetism based on the principle that magnetic effects are indistinguishable from specific forms of spatial dispersion of permittivity at optical frequencies. In a different context, plasmonic Fano resonances have generated a great deal of interest, particularly for use in sensing applications that benefit from sharp spectral features and extreme field localization. In the absence of natural magnetism, optical Fano resonances have so far been based on purely electric effects. In this Letter, we demonstrate that a subwavelength plasmonic metamolecule consisting of four closely spaced gold nanoparticles supports a strong magnetic response coupled to a broad electric resonance. Small structural asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Our findings are supported by excellent agreement with simulations and analytical calculations, and represent an important step towards the quest for artificial magnetism and negative refractive index metamaterials at optical frequencies.

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Plasmonic cavities represent a promising platform for controlling light-matter interaction due to their exceptionally small mode volume and high density of photonic states. Using plasmonic cavities for enhancing light's coupling to individual two-level systems, such as single semiconductor quantum dots (QD), is particularly desirable for exploring cavity quantum electrodynamic (QED) effects and using them in quantum information applications. The lack of experimental progress in this area is in part due to the difficulty of precisely placing a QD within nanometers of the plasmonic cavity. Here, we study the simplest plasmonic cavity in the form of a spherical metallic nanoparticle (MNP). By controllably positioning a semiconductor QD in the close proximity of the MNP cavity via atomic force microscope (AFM) manipulation, the scattering spectrum of the MNP is dramatically modified due to Fano interference between the classical plasmonic resonance of the MNP and the quantized exciton resonance in the QD. Moreover, our experiment demonstrates that a single two-level system can render a spherical MNP strongly anisotropic. These findings represent an important step toward realizing quantum plasmonic devices.optical spectroscopy | hybrid nanostructures | quantum systems | plasmonic cavities | Fano resonance M any quantum network and quantum information processing schemes build upon the enhanced light-matter interaction between a single quantum emitter and a cavity, enabling the effective conversion between photonic and matterbased quantum states (1-4). For example, if the absorption of a photon by a single atom placed inside a cavity can render it transparent to a second photon, then a variety of promising quantum information processing devices can be envisioned including quantum phase gates and repeaters (5). Such QED effects require a high atomic cooperativity c ¼ g 2 γk , where the coupling strength g 2 ∝ 1=V is inversely proportional to the volume of the cavity mode V (6). γ and k are the linewidth of the atomic transition and the cavity mode, respectively. Typically, a high cavity quality factor Q (or low k) of conventional photonic cavities is required to compensate for relatively large (diffraction-limited) mode volumes and comes at a cost: The narrow linewidth of cavity modes places stringent requirements on their spectral alignment with the frequencies of quantum transitions. Plasmonic cavities, on the other hand, achieve high values of C while maintaining moderate Q values because of their ultrasmall modal volume (7-10). The relaxed spectral alignment requirements facilitate the experimental realization of various quantum phenomena, such as collective photon emission from a small ensemble of emitters (11) and single photon sources with tunable statistical properties (12).Prior experiments exploring cavity QED effects associated with single emitters coupled to plasmonic cavities or waveguides focused almost exclusively on the observations of reducing the emitter's lifetime due to the enhanced radiative (propor...

A comprehensive study of the photoluminescence dynamics in newly developed CdSe/ZnS quantum dots with alloyed core/shell interfaces is presented. Time-correlated single photon counting is used to measure the decay of exciton luminescence from both the ensemble and individual quantum dots. For decreasing emission wavelength (i.e., for smaller dots), the ensemble data reveal increasing total decay rates with greater variation. This systematic change is expected for emitters with stronger quantum confinement and more influenced by the surface/interface trap states. In experiments performed on single quantum dots, the photoluminescence trajectories exhibit two-state blinking behavior. The distributions of the "off"-state probability density are described by an average power-law exponent of 1.5 6 0.2, while the average decay rate of emission from the threshold-discriminated "on"-states is estimated to be 0.035 6 0.004 ns À1 . We suggest that in core/shell quantum dots with a large bandgap offset, the compositionally graded energy profile at the interface may not be smooth enough to suppress nonradiative Auger recombination and prevent blinking.

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We investigate photoluminescence from individual “giant” CdSe/CdS core/thick-shell quantum dots (gQDs) placed near an epitaxial Ag film with an atomically smooth surface. The key observation is that the lifetimes of the gQDs are drastically reduced and exhibit a remarkably narrow distribution compared to the gQDs deposited on a thermally deposited Ag film. The larger variations in gQDs’ lifetimes on the thermally deposited Ag film arise from excitonic coupling to localized surface plasmons associated with nanoscale surface corrugations of different heights. A calculation is performed based on a simple model system of a QD coupled to a metallic nanosphere. The calculation shows that the QD lifetime initially shortens and reaches a saturated value with increasing radius of the metal nanoparticle (MNP). Because the epitaxial film can be treated as a sphere with an infinitely large radius, the calculation confirms and explains the different QD dynamics near the two types of Ag films as observed experimentally. Our studies demonstrate that epitaxial Ag films serve as an ideal material platform for reliable control over the QD lifetime and may lead to improved photodetectors and light emitting devices requiring fast response or modulation.

Articles you may be interested inDetermination of the inner diameter of a double-walled carbon nanotube from its Raman spectra J. Appl. Phys. 113, 064304 (2013); 10.1063/1.4790162Micro-Raman scattering of selenium-filled double-walled carbon nanotubes: Temperature study Four types of filled double-walled carbon nanotubes ͑DWNTs͒ ͑Se@DWNT; Te@DWNT; HgTe@DWNT; and PbI 2 @ DWNT͒ have been studied by high-resolution transmission electron microscopy and micro-Raman spectroscopy in the temperature interval from 80 to 700 K employing 785 nm excitation wavelength. The temperature dependence of the dominant bands ͑D-band, G-band, and the ͑2D͒-band͒ are analyzed in terms of the model developed by Klemens, Hart, Agraval, Lax, and Cowley and extended by Balkanski for anharmonic decay of optical phonons. The quasiharmonic frequencies and the anharmonicity constants were obtained from the temperature dependences of the analyzed Raman bands. The findings were compared to analogous study for empty DWNTs. The strength of the van der Waals interaction between the guest material and the carbon nanotube ͑CNT͒ estimated through the quasiharmonic frequencies was found to decrease in the following order: Se@DWNT; Te@DWNT; PbI 2 @ DWNT, and HgTe@DWNT. In agreement with this, the anharmonicity due to the phonon-phonon interactions was found to decrease in the same order.

Time-resolved photon counting technique was employed to study dynamics of photoluminescence from the ensemble and single CdSe/ZnS quantum dots with the alloyed core/shell interfaces. The ensemble data revealed enhanced effect of disorder-induced trap states for increasing emission energy, as implied from the changes in the distribution of total decay rates. The emission trajectories collected for single quantum dots showed familiar, two-state blinking pattern. It suggests that in a large-band-offset CdSe/ZnS system, the introduced alloying of the core-to-shell region cannot smooth enough the confinement potential in order to suppress nonradiative Auger recombination and blinking.