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.

The ability to detect light over a broad spectral range is central for practical optoelectronic applications, and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS 2 . However, polarization sensitivity within such a photodetector remains elusive. Here we demonstrate a linear-dichroic broadband photodetector with layered black phosphorus transistors, using the strong intrinsic linear dichroism arising from the in-plane optical anisotropy with respect to the atom-buckled direction, which is polarization sensitive over a broad bandwidth from about 400 nm to 3750 nm. Especially, a perpendicular built-in electric field induced by gating in the transistor geometry can spatially separate the photo-generated electrons and holes in the channel, effectively reducing their recombination rate, and thus enhancing the performance for linear dichroism photodetection. This provides practical functionality using anisotropic layered black phosphorus, thereby enabling novel optical and optoelectronic device applications. Corresponding author: hyhwang@stanford.edu, yicui@stanford.edu. 2Confined electronic systems in layered two-dimensional (2D) crystals are host to many emerging electronic, spintronic and photonic phenomena, 1, 2, 3 including quantum Hall and Dirac electrons in graphene 4, 5, 6 and topological surface states in topological insulators 7, 8 . Experimentally identifying new functionalities of two-dimensional materials is a challenging and rewarding frontier, enabled by recent advances in materials and device fabrication. One example is the valley polarization control using circularly polarized light in the non-centrosymmetric MoS 2 monolayer and resulting potential valleytronics applications. 9, 10,11 Other examples include recent demonstrations of novel electronic and optoelectronic applications of the well-known layered material black phosphorus (BP), such as high-mobility field effect transistors and linear-polarization dependent optical absorption. 12,13,14 Therefore, further discovering new properties and functionalities utilizing known layered materials is of practical importance and great current interest. 14,15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 As a potential functionality for layered 2D materials, linear dichroism (LD) is an electromagnetic spectroscopy probing different absorption of light polarized parallel or perpendicular to an orientation axis. It directly depends on the conformation and orientation of material/device structures, where they are either intrinsically oriented in an anisotropic crystal structure 27, 28 or extrinsically oriented in anisotropic device patterns 29, 30 . Compared to the hexagonal in-plane lattice in other 2D materials such as graphene and MoS 2 , which are insensitive to the linear polarization of incident light, the layered BP crystal with a rectangular in-plane lattice has a highly-anisotropic structure along the x and y directions (defined in Fig. 1a), where every two rows of P atoms alternatel...

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.

Multipolar transitions other than electric dipoles are generally too weak to be observed at optical frequencies in single quantum emitters. For example, fluorescent molecules and quantum dots have dimensions much smaller than the wavelength of light and therefore emit predominantly as electric dipoles. Here we demonstrate controlled emission of a quantum dot into multipolar radiation through selective coupling to a linear nanowire antenna. The antenna resonance tailors the interaction of the quantum dot with light, effectively creating a hybrid nanoscale source beyond the simple Hertz dipole. Our findings establish a basis for the controlled driving of fundamental modes in nanoantennas and metamaterials, for the understanding of the coupling of quantum emitters to nanophotonic devices such as waveguides and nanolasers, and for the development of innovative quantum nano-optics components with properties not found in nature.

Controlling light emission from quantum emitters has important applications ranging from solid-state lighting and displays to nanoscale single-photon sources. Optical antennas have emerged as promising tools to achieve such control right at the location of the emitter, without the need for bulky, external optics. Semiconductor nanoantennas are particularly practical for this purpose because simple geometries, such as wires and spheres, support multiple, degenerate optical resonances. Here, we start by modifying Mie scattering theory developed for plane wave illumination to describe scattering of dipole emission. We then use this theory and experiments to demonstrate several pathways to achieve control over the directionality, polarization state, and spectral emission that rely on a coherent coupling of an emitting dipole to optical resonances of a Si nanowire. A forward-to-backward ratio of 20 was demonstrated for the electric dipole emission at 680 nm from a monolayer MoS 2 by optically coupling it to a Si nanowire. Main text:Achieving control over the radiation properties of quantum emitters is key to improving efficiency and realizing new functionality in optoelectronic systems. Bulky optical components have been developed for many years and are extremely effective in controlling the angular, polarization, and spectral properties of light emission. Recent advances in the fields of metallic and dielectric optical metamaterials and nanoantennas have now also enabled effective integration of solid-state emitters and control elements into inexpensive platforms. 1-3 Such structures can manipulate light emission in the near-field of an emitter and thus hold a real promise to achieve even greater control over the emission process. For example, we will show how the undesired losses due to radiation of quantum emitters into a high-index substrate can be reduced by redirecting the emission upward with an antenna.Whereas structures based on noble metals are currently most advanced in manipulating light-matter interaction at the nanoscale, they typically are complex in shape, display undesired optical losses, and are not compatible with most semiconductor device processing technologies. High-index semiconductor antennas can circumvent these issues while providing complex electrical and optical functions. 2,4-14 Based on the mature fabrication infrastructure, silicon nanostructures appear particularly promising for optoelectronic applications. 4,[9][10][11][12][15][16][17] Semiconductor nanoparticles of simple geometric shapes have displayed directional scattering of plane waves when the renowned Kerker conditions are satisfied. 12,16,18 When these conditions are met, directionality is naturally achieved through the coherent excitation of electric and magnetic dipole resonances in the particle and tuning the interference of the associated scattered fields. 12,19,20 Thanks to their high refractive indices, semiconductor nanoparticles can satisfy the Kerker conditions in the visible spectral range. 16,18,21 Given the ever-in...