Control over the interaction between single photons and individual optical emitters is an outstanding problem in quantum science and engineering. It is of interest for ultimate control over light quanta, as well as for potential applications such as efficient photon collection, single-photon switching and transistors, and long-range optical coupling of quantum bits. Recently, substantial advances have been made towards these goals, based on modifying photon fields around an emitter using high-finesse optical cavities. Here we demonstrate a cavity-free, broadband approach for engineering photon-emitter interactions via subwavelength confinement of optical fields near metallic nanostructures. When a single CdSe quantum dot is optically excited in close proximity to a silver nanowire, emission from the quantum dot couples directly to guided surface plasmons in the nanowire, causing the wire's ends to light up. Non-classical photon correlations between the emission from the quantum dot and the ends of the nanowire demonstrate that the latter stems from the generation of single, quantized plasmons. Results from a large number of devices show that efficient coupling is accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission, in good agreement with theoretical predictions.

* These authors contribute equally to this work.Photonic circuits can be much faster than their electronic counterparts, but they are difficult to miniaturize below the optical wavelength scale. Nanoscale photonic circuits based on surface plasmon polaritons (SPs) are a promising solution to this problem because they can localize light below the diffraction limit 1-8 . However, there is a general tradeoff between the localization of an SP and the efficiency with which it can be detected with conventional far-field optics. Here we describe a new all-electrical SP detection technique based on the near-field coupling between guided plasmons and a nanowire field-effect transistor. We use our detectors to electrically detect the plasmon emission from an individual colloidal quantum dot coupled to an SP waveguide. The detectors are both nanoscale and highly efficient (0.1 electrons/plasmon), and a plasmonic gating effect can be used to amplify the signal even higher (up to 50 electrons/plasmon). These results enable new on-chip optical sensing applications and fulfill a key requirement for "dark" optoplasmonic nanocircuits in which SPs can be generated, manipulated, and detected without involving far-field radiation.2 SPs are charge-density waves that propagate along metal-dielectric interfaces. They can be concentrated and guided by current carrying wires, suggesting an integrated approach to optical and electrical signal processing. Our near-field plasmon detection scheme consists of an Ag nanowire (NW) crossing a Ge NW field-effect transistor (Fig. 1, Methods). The Ag NW guides 10 SPs to the Ag/Ge junction, where they are converted to electron-hole (e-h) pairs 11-13 and detected as current through the Ge NW. The Ag NWs are highly crystalline and defect-free 8,14,15 , allowing SPs to propagate over distances of several microns without scattering into free-space photons. The Ge NWs are lightly p-doped, covered with a thin native oxide layer, and sensitive to visible light 16 .Electrical plasmon detection is demonstrated by scanning a focused laser beam across an Ag/Ge crossbar device and recording the current (I) through the Ge NW as a function of the diffraction-limited laser spot position. These data, recorded at V b = V gate = 0, show that current flows through the Ge NW only when the laser beam is focused on four distinct spots on the device (Fig. 1b). First, current is detected when the laser is focused near the Ag/Ge junction, due to the direct photoresponse of the Ge NW 16 . The photocurrent induced on the left (I left ) and right (I right ) sides of the junction have opposite signs (discussed below). Moreover, current through the Ge NW (I plas ) is recorded when the laser is focused at either end of the Ag NW.This I plas signal is the key signature for electrical SP detection. Propagating SPs can be launched in the Ag NW only when the excitation laser is incident on the Ag NW ends 15 . Away from the ends, free space photon-to-SP conversion is strongly suppressed by the wave vector mismatch between the two ...

We propose and demonstrate a new approach for achieving enhanced light-matter interactions with quantum emitters. Our approach makes use of a plasmon resonator composed of defect-free, highly crystalline silver nanowires surrounded by patterned dielectric distributed Bragg reflectors. These resonators have an effective mode volume (V(eff)) 2 orders of magnitude below the diffraction limit and a quality factor (Q) approaching 100, enabling enhancement of spontaneous emission rates by a factor exceeding 75 at the cavity resonance. We also show that these resonators can be used to convert a broadband quantum emitter to a narrow-band single-photon source with color-selective emission enhancement.

We report the synthesis and characterization of axial nanorod heterostructures composed of cadmium selenide (CdSe) and cadmium sulfide (CdS). The synthesis employs a solution-liquid-solid (SLS) mechanism with the assistance of bismuth nanocrystals adhered to a substrate (silicon or a III-V semiconductor). Transmission electron microscopy (TEM) and diffraction studies show that CdSe and CdS segments exhibit the wurtzite (hexagonal) crystal structure with <5% stacking faults. Both of these segments grow along the [002] direction with an epitaxial interface between them. Energy-dispersive X-ray (EDX) spectrometry using a high-resolution TEM operating in scanning mode confirms the alloy-free composition modulation in the nanorod heterostructures, showing that Se and S are localized in the CdSe and CdS portions of the nanorod heterostructures, respectively. This study demonstrates that SLS synthesis provides an alternate route to prepare axial nanorod heterostructures that have been difficult to generate using either vapor-liquid-solid growth or catalyst-free solution-phase synthesis.

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