We demonstrate that efficient shape control may be achieved in the shell of colloidally grown semiconductor nanocrystals (independent of the core), allowing the combination of a 0-D spherical CdSe core with a 1-D rodlike CdS shell. Besides exhibiting linearly polarized emission with a room-temperature quantum efficiency above 70%, these mixed-dimensionality colloidal heterostructures display large, length-dependent Stokes shifts as well as giant extinction coefficients approaching 10 7 cm -1 M -1 .

We report the synthesis and characterization of highly luminescent colloidal nanocrystals consisting of CdSe cores protected with double inorganic shells (core−shell−shell nanocrystals). The outer ZnS shell provides efficient confinement of electron and hole wave functions inside the nanocrystal as well as high photochemical stability. Introducing the middle shell (CdS or ZnSe) sandwiched between CdSe core and ZnS outer shell allows considerable reducing strain inside nanocrystals because CdS and ZnSe have the lattice parameter intermediate to those of CdSe and ZnS. In contrast to CdSe/ZnS core−shells, in the core−shell−shell nanocrystals ZnS shell grows nearly defect free. Due to high quality of the ZnS shell, the core−shell−shell nanocrystals exhibit PL efficiency and photostability exceeding those of CdSe/ZnS nanocrystals. Preferential growth of the middle CdS shell in one crystallographic direction allows engineering the shape and luminescence polarization of the core−shell−shell nanocrystals.

Synthetic polymers are widely used materials, as attested by a production of more than 200 millions of tons per year, and are typically composed of linear repeat units. They may also be branched or irregularly crosslinked. Here, we introduce a two-dimensional polymer with internal periodicity composed of areal repeat units. This is an extension of Staudinger's polymerization concept (to form macromolecules by covalently linking repeat units together), but in two dimensions. A well-known example of such a two-dimensional polymer is graphene, but its thermolytic synthesis precludes molecular design on demand. Here, we have rationally synthesized an ordered, non-equilibrium two-dimensional polymer far beyond molecular dimensions. The procedure includes the crystallization of a specifically designed photoreactive monomer into a layered structure, a photo-polymerization step within the crystal and a solvent-induced delamination step that isolates individual two-dimensional polymers as free-standing, monolayered molecular sheets.

Fundamentally secure quantum cryptography has still not seen widespread application owing to the difficulty of generating single photons on demand. Semiconductor quantum-dot structures have recently shown great promise as practical single-photon sources, and devices with integrated optical cavities and electrical-carrier injection have already been demonstrated. However, a significant obstacle for the application of commonly used III-V quantum dots to quantum-information-processing schemes is the requirement of liquid-helium cryogenic temperatures. Epitaxially grown gallium nitride quantum dots embedded in aluminium nitride have the potential for operation at much higher temperatures. Here, we report triggered single-photon emission from gallium nitride quantum dots at temperatures up to 200 K, a temperature easily reachable with thermo-electric cooling. Gallium nitride quantum dots also open a new wavelength region in the blue and near-ultraviolet portions of the spectrum for single-photon sources.

We observe antibunching in the photons emitted from a strongly-coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single photon source operating in the strong coupling regime, with a Purcell factor of 61 ± 7 and quantum efficiency of 97%.PACS numbers: 78.67. Hc, 78.55.Cr, 78.90.+t Cavity quantum electrodynamics (CQED), addressing the interaction between a quantum emitter and a cavity, has been a central topic in atomic physics for decades [1,2,3,4] and has recently come to the forefront of semiconductor physics [5,6,7,8]. If the coupling between the single quantum emitter and cavity mode is strong compared to their decay rates, the emitter and cavity coherently exchange energy back and forth leading to Rabi oscillations. This strong coupling (SC) regime is of great interest for a variety of quantum information applications, especially with a solid-state implementation. A SC QD-microcavity system could lead to a nearly ideal single photon source (SPS) for quantum information processing, with extremely high efficiency and photon indistinguishability [9]. The same technology could be applied as an interface between a spin qubit and single photon qubit in a quantum network [10].SC between a single atom and a cavity was first achieved more than a decade ago [4]. An analogous system in the solid-state is the excitonic transition of a semiconductor quantum dot (QD) together with a semiconductor microcavity. Several groups have recently reported SC between a single (In,Ga)As QD and either micropillar [5], photonic crystal [6], or microdisk [7] resonators. SC can also occur between a single cavity mode and a collection of degenerate emitters, such as an ensemble of atoms or a quantum well [11]. However, in the latter case the behavior is classical: adding or removing one emitter or one photon from the system has little effect.In previous studies of QD-cavity SC [5,6,7] it was argued that the spectral density of QDs was sufficiently low that it is unlikely that several degenerate emitters contributed to the anticrossing. However, it was not verified that the system had one and only one emitter. There was a surprisingly large amount of emission from the cavity mode when the QD was far detuned. It was unclear whether this emission originated from the particular single QD or from many background emitters. An important step to establish SC in solid-state CQED is verification that the double-peaked spectrum originates from a single quantum emitter, not a collection of emitters, interacting with the cavity mode.In this Letter we present proof that the emission from a strongly-coupled QD-microcavity system is dominated by a single quantum emitter....

We present experiments where a single subwavelength scatterer is used to examine and control the back-scattering induced coupling between counterpropagating high-Q modes of a microsphere resonator. Our measurements reveal the standing wave character of the resulting symmetric and antisymmetric eigenmodes, their unbalanced intensity distributions, and the coherent nature of their coupling. We discuss our findings and the underlying classical physics in the framework common to quantum optics and provide a particularly intuitive explanation of the central processes.The radiative properties of atoms can be strongly modified by coupling them to resonators [1]. A historical corner stone of this field of research, known as Cavity Quantum Electrodynamics (CQED), was set in 1946 by E. M. Purcell who proposed that the radiation rate of an oscillating dipole at wavelength λ can be enhanced by a factor F = 3Qλ 3 /4π 2 V m in a resonant cavity of quality factor Q and mode volume V m [1]. This socalled Purcell effect holds in the dissipative weak coupling regime where the cavity finesse is small so that the atomic radiation remains dominated by its coupling to the bath of the electromagnetic modes. In the strong coupling regime, coherent exchange of energy between the atom and the resonator causes the atomic resonance to lose its identity and to become replaced by a doublet. These phenomena have been studied for more than three decades [2,3,4,5] although the in-situ manipulation of a single emitter in a single mode of a high-Q microresonator remains a challenge [4,5]. In this Letter, we consider the controlled coupling of a classical nano-object to a high-finesse whispering-gallery mode (WGM) microresonator. We discuss both theoretically and experimentally the resulting coherent coupling between two degenerate counterpropagating WGMs and the modification of the Rayleigh scattering rate. Our findings show that the concepts of the strong and weak coupling play a central role even in this fully classical system. * Electronic address: oliver.benson@physik.hu-berlin. The resonators in our work consist of microspheres melted at the end of silica fibers [6]. Such spheres support very high-Q WGMs and have been studied by several groups [7,8,9]. About ten years ago, it was discovered that the high-Q resonances of these cavities are often composed of doublets [10]. Such a mode splitting has been since discussed in conjunction with various WGM resonators [8,9,11,12,13]. It turns out that mode splitting has been observed in other ring resonators and has been explained as the result of the coupling between the electric fields E c and E cc of the degenerate clockwise (c) and counter clockwise (cc) modes via back scattering. The new superpositions states (+) and (−) are described byHere a and b are complex coefficients. In the simplest case, the coupling between E c and E cc can be caused by a reflector [14,15]. In the case of WGM resonators, however, it has been suggested that backscattering from a distribution of residual subwavelength in...

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities or waveguides. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second 'gating' beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Single emitters have been considered as sources of single photons in various contexts such as cryptography, quantum computation, spectroscopy, and metrology 1,2,3 . The success of these applications will crucially rely on the efficient directional emission of photons into well-defined modes. To accomplish a high efficiency, researchers have investigated microcavities at cryogenic temperatures 4 , photonic nanowires 5, and near-field coupling to metallic nano-antennas 6 . However, despite an impressive progress, the existing realizations substantially fall short of unity collection efficiency. Here we report on a theoretical and experimental study of a dielectric planar antenna, which uses a layered structure for tailoring the angular emission of a single oriented molecule. We demonstrate a collection efficiency of 96% using a microscope objective at room temperature and obtain record detection rates of about 50 MHz. Our scheme is wavelength-insensitive and can be readily extended to other solid-state emitters such as color centers 7 and semiconductor quantum dots 8 .One of the most powerful and versatile approaches to the generation of single photons exploits the property that a single quantum mechanical two-level system cannot emit two photons simultaneously since each excitation and emission cycle requires a finite time. Unfortunately, such single-photon sources (SPS) are intrinsically inefficient because their radiation spreads over a 4π solid angle and cannot be fully captured by conventional optics. Several years ago, a simple avenue for efficient photon collection was proposed by Koyama et al. in the context of fluorescence microscopy 9 , where emitters were placed at the interface between two media with large refractive index contrast 9,10 . Such a structure can be viewed as a dielectric antenna 11 in which the dipolar radiation of the emitter is funneled into the high-index substrate. The black trace in Fig. 1a shows the angular emission of a dipole sitting close to an interface and oriented perpendicular to it. Despite the strongly modified radiation pattern, one finds that 14% of the light is still lost to the upper half-space, and more importantly, a considerable amount of light is directed to very large angles in the lower substrate, which are not accessible by the collection optics. In this Letter, we remedy these issues by embedding the emitter in a dielectric layer that we engineer on top of the highindex substrate and obtain unprecedented photon collection efficiencies, directionality, and count rates.To provide an intuitive explanation of our antenna design, let us decompose the radiation of a dipolar emitter into plane waves and consider the propagation of each component 12. This