Niek F. van Hulst

Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna

Directed Emission The atomlike features of quantum dots—the discrete energy levels and subsequent emission of light at discrete wavelengths—make them key building blocks in optoelectronic circuits and optical communication. However, the emitted light tends to be omnidirectional, which limits applications that require accurate transmission between sender and receiver. Curto et al. (p. 930; see the Perspective by Giessen and Lippitz) have designed an optical antenna, a shrunk-down version of the Yagi-Uda design used in microwave and radio communication, and show that coupling the quantum dot to the antenna provides control over the direction of the emitted light. An antenna designed for optical wavelengths is used to control the direction of the light emitted from a quantum dot. Nanoscale quantum emitters are key elements in quantum optics and sensing. However, efficient optical excitation and detection of such emitters involves large solid angles because their interaction with freely propagating light is omnidirectional. Here, we present unidirectional emission of a single emitter by coupling to a nanofabricated Yagi-Uda antenna. A quantum dot is placed in the near field of the antenna so that it drives the resonant feed element of the antenna. The resulting quantum-dot luminescence is strongly polarized and highly directed into a narrow forward angular cone. The directionality of the quantum dot can be controlled by tuning the antenna dimensions. Our results show the potential of optical antennas to communicate energy to, from, and between nano-emitters.

Enhanced directional excitation and emission of single emitters by a nano-optical Yagi-Uda antenna

We demonstrate by 3D numerical calculations that the interaction of a single quantum emitter with the electromagnetic field is both enhanced and directed by a nano-optical Yagi-Uda antenna. The single emitter is coupled in the near field to the resonant plasmon mode of the feed element, enhancing both excitation and emission rates. The angular emission of the coupled system is highly directed and determined by the antenna mode. Arbitrary control over the main direction of emission is obtained, regardless of the orientation of the emitter. The directivity is even more increased by the presence of a dielectric substrate, making such antennas a promising candidate for compact easy-to-address planar sensors.

Microdomains of the C-type lectin DC-SIGN are portals for virus entry into dendritic cells

The C-type lectin dendritic cell (DC)–specific intercellular adhesion molecule grabbing non-integrin (DC-SIGN; CD209) facilitates binding and internalization of several viruses, including HIV-1, on DCs, but the underlying mechanism for being such an efficient phagocytic pathogen-recognition receptor is poorly understood. By high resolution electron microscopy, we demonstrate a direct relation between DC-SIGN function as viral receptor and its microlocalization on the plasma membrane. During development of human monocyte-derived DCs, DC-SIGN becomes organized in well-defined microdomains, with an average diameter of 200 nm. Biochemical experiments and confocal microscopy indicate that DC-SIGN microdomains reside within lipid rafts. Finally, we show that the organization of DC-SIGN in microdomains on the plasma membrane is important for binding and internalization of virus particles, suggesting that these multimolecular assemblies of DC-SIGN act as a docking site for pathogens like HIV-1 to invade the host.

A detailed analysis of the optical beam deflection technique for use in atomic force microscopy

A Michelson interferometer and an optical beam deflection configuration (both shot noise and diffraction limited) are compared for application in an atomic force microscope. The comparison shows that the optical beam deflection method and the interferometer have essentially the same sensitivity. This remarkable result is explained by indicating the physical equivalence of both methods. Furthermore, various configurations using optical beam deflection are discussed. All the setups are capable of detecting the cantilever displacements with atomic resolution in a 10 kHz bandwidth.

Quantum Coherent Energy Transfer over Varying Pathways in Single Light-Harvesting Complexes

Coherence in Photosynthesis It is unclear how energy absorbed by pigments in antenna proteins is transferred to the central site of chemical catalysis during photosynthesis. Hildner et al. (p. 1448) observed coherence—prolonged persistence of a quantum mechanical phase relationship—at the single-molecule level in light-harvesting complexes from purple bacteria. The results bolster conclusions from past ensemble measurements that coherence plays a pivotal role in photosynthetic energy transfer. Hayes et al. (p. 1431, published online 18 April) examined a series of small molecules comprised of bridged chromophores that also manifest prolonged coherence. A phase relation observed in ensemble measurements of photosynthetic proteins is borne out at the single-molecule level. The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction center. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.

Optical Nanorod Antennas Modeled as Cavities for Dipolar Emitters: Evolution of Sub- and Super-Radiant Modes

Optical antennas link objects to light. Here we derive an analytical model for the interaction of dipolar transitions with radiation through nanorod antenna modes, by modeling nanorods as cavities. The model includes radiation damping, accurately describes the complete emission process, and is summarized in a phase-matching equation. We analytically discuss the quantitative evolution of antenna modes, in particular the gradual emergence of subradiant, super-radiant, and dark modes, as antennas become increasingly more bound, i.e., plasmonic. Our description is valid for the interaction of nanorods with light in general and is thus widely applicable.Optical antennas link objects to light. Here, we analyze metal nanorod antennas as cavities with variable reflection coefficients to derive the interaction of dipolar transitions with radiation through the antenna modes. The presented analytical model accurately describes the complete emission process, and is summarized in a phase-matching equation. We show how antenna modes evolve as they become increasingly more bound, i.e. plasmonic. The results illustrate why efficient antennas should not be too plasmonic, and how subradiant even modes can evolve into weakly-interacting dark modes. Our description is valid for the interaction of nanorods with light in general, and is thus widely applicable. Electronic address: Tim.Taminiau@icfo.es ICREA Institució Catalana de Recerca i Estudis Avançats, Spain.

Visualizing and controlling vibrational wave packets of single molecules

The active steering of the pathways taken by chemical reactions and the optimization of energy conversion processes provide striking examples of the coherent control of quantum interference through the use of shaped laser pulses. Experimentally, coherence is usually established by synchronizing a subset of molecules in an ensemble with ultra-short laser pulses. But in complex systems where even chemically identical molecules exist with different conformations and in diverse environments, the synchronized subset will have an intrinsic inhomogeneity that limits the degree of coherent control that can be achieved. A natural—and, indeed, the ultimate—solution to overcoming intrinsic inhomogeneities is the investigation of the behaviour of one molecule at a time. The single-molecule approach has provided useful insights into phenomena as diverse as biomolecular interactions, cellular processes and the dynamics of supercooled liquids and conjugated polymers. Coherent state preparation of single molecules has so far been restricted to cryogenic conditions, whereas at room temperature only incoherent vibrational relaxation pathways have been probed. Here we report the observation and manipulation of vibrational wave-packet interference in individual molecules at ambient conditions. We show that adapting the time and phase distribution of the optical excitation field to the dynamics of each molecule results in a high degree of control, and expect that the approach can be extended to achieve single-molecule coherent control in other complex inhomogeneous systems.

Quantifying the magnetic nature of light emission

Tremendous advances in the study of magnetic light-matter interactions have recently been achieved using man-made nanostructures that exhibit and exploit an optical magnetic response. However, naturally occurring emitters can also exhibit magnetic resonances in the form of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic nature of light emission using energy- and momentum-resolved spectroscopy, and leverage a pair of spectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuum fluctuations in the electric and magnetic fields at the nanometre scale. These results reveal a new tool for nano-optics: an atomic-size quantum emitter that interacts with the magnetic component of light.

Aluminum for nonlinear plasmonics: resonance-driven polarized luminescence of Al, Ag, and Au nanoantennas.

Resonant optical antennas are ideal for nanoscale nonlinear optical interactions due to their inherent strong local field enhancement. Indeed second- and third-order nonlinear response of gold nanoparticles has been reported. Here we compare the on- and off-resonance properties of aluminum, silver, and gold nanoantennas, by measuring two-photon photoluminescence (TPPL). Remarkably, aluminum shows 2 orders of magnitude higher luminescence efficiency than silver or gold. Moreover, in striking contrast to gold, the aluminum emission largely preserves the linear incident polarization. Finally, we show the systematic resonance control of two-photon excitation and luminescence polarization by tuning the antenna width and length independently. Furthermore, we analyze this tuning of the polarization with the rod dimensions by measuring the angular emission of TPPL via back focal plane imaging. Our findings point to aluminum as a promising metal for nonlinear plasmonics.

Multipolar Interference for Directed Light Emission

By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Here we exploit the interference among the electric dipole, quadrupole, and magnetic dipole moments of a split-ring resonator to experimentally realize a compact directional optical antenna. This single-element antenna design robustly directs emission even when covered with nanometric emitters at random positions, outperforming previously demonstrated nanoantennas with a bandwidth of 200 nm and a directivity of 10.1 dB from a subwavelength structure. The advantages of this approach bring directional optical antennas closer to practical applications.