An important source of innovation in nanophotonics is the idea to scale down known radio wave technologies to the optical regime. One thoroughly investigated example of this approach are metallic nanoantennas which employ plasmonic resonances to couple localized emitters to selected far-field modes. While metals can be treated as perfect conductors in the microwave regime, their response becomes Drude-like at optical frequencies. Thus, plasmonic nanoantennas are inherently lossy. Moreover, their resonant nature requires precise control of the antenna geometry. A promising way to circumvent these problems is the use of broadband nanoantennas made from low-loss dielectric materials. Here, we report on highly directional emission from hybrid dielectric leaky-wave nanoantennas made of Hafnium dioxide nanostructures deposited on a glass substrate. Colloidal semiconductor quantum dots deposited in the nanoantenna feed gap serve as a local light source. The emission patterns of hybrid nanoantennas with different sizes are measured by Fourier imaging. We find for all antenna sizes a highly directional emission, underlining the broadband operation of our design.Nanoantennas have become valuable elements of the photonics toolbox to control and manipulate light on the nanoscale [1-3]. They allow for an efficient interconversion of localized excitations and propagating electromagnetic waves [4,5]. In receiving mode, nanoantennas can locally increase the light intensity by several orders of magnitude [6][7][8]. This property can be used for the efficient excitation of quantum emitters [9,10] and to boost nonlinear effects [11][12][13][14][15]. In transmitting mode, coupling of quantum emitters to nanoantennas allows for the control of the emission properties [16][17][18][19][20]. For instance, Curto et al reported on a highly directional plasmonic Yagi-Uda antenna [17] and Lee et al. demonstrated a planar dielectric antenna with near unity collection efficiency [20].Like their microwave counterparts, nanoantennas can be categorized based on their functional principle into two large groups: (i) resonant antennas and (ii) nonresonant traveling wave antennas. So far, most research has focused on resonant nanoantennas based either on plasmonic resonances in metals [18,[21][22][23] or on Mie resonances in highrefractive index dielectrics [24][25][26]. The latter offer the prospect of reducing dissipative losses while still providing large resonant enhancements of the electromagnetic near field. A recent review on optically resonant dielectric nanoantennas can be found in reference [27]. Moreover, dielectric antennas have been used in dielectric gradient metasurfaces as scattering elements [28]. In contrast to this, traveling wave antennas operating at optical frequencies have been studied considerably less. However, there is a growing interest in transferring the traveling wave concept to higher operating frequencies in order to achieve non-resonant broadband operation [29][30][31].Leaky-wave antennas are a subset of traveling wa...

Sheets of slab waveguides with sharp corners are investigated. By means of rigorous numerical experiments, we look at oblique incidence of semi-guided plane waves. Radiation losses vanish beyond a certain critical angle of incidence. One can thus realize lossless propagation through 90-degree corner configurations, where the remaining guided waves are still subject to pronounced reflection and polarization conversion. A system of two corners can be viewed as a structure akin to a Fabry-Perot-interferometer. By adjusting the distance between the two partial reflectors, here the 90-degree corners, one identifies step-like configurations that transmit the semi-guided plane waves without radiation losses, and virtually without reflections. Simulations of semi-guided beams with in-plane wide Gaussian profiles show that the effect survives in a true 3-D framework.

We investigate the phase-dependent excitation of localized surface plasmon polaritons in coupled nanorods by using nonlinear spectroscopy. Our design of a coupled three-nanorod structure allows independent excitation with cross-polarized light. Here, we show that the excitation of a particular plasmon mode can be coherently controlled by changing the relative phase of two orthogonally polarized light fields. Furthermore, we observe a phase relation for the excitation that is dominantly caused by damping effects.

We show how to optically connect guiding layers at different elevations in a 3-D integrated photonic circuit. Transfer of optical power carried by planar, semi-guided waves is possible without reflections or radiation losses, and over large vertical distances. This functionality is realized through simple step-like folds of high-contrast dielectric slab waveguides, in combination with oblique wave incidence, and fulfilling a resonance condition. Radiation losses vanish, and polarization conversion is suppressed for TE wave incidence beyond certain critical angles. This can be understood by fundamental arguments resting on a version of Snell's law. The two 90° corners of a step act as identical partial reflectors in a Fabry-Perot-like resonator setup. By selecting the step height, i.e., the distance between the reflectors, one realizes resonant states with full transmission. Rigorous quasi-analytical simulations for typical silicon/silica parameters demonstrate the functioning. Combinations of several step junctions can lead to other types of optical on-chip connects, e.g., U-turn- or bridge-like configurations.

This paper presents a numerical investigation of the effect of different back sweep angles and exducer widths on the steady-state impeller outlet flow pattern of a centrifugal compressor with a vaneless diffuser. The investigations have been performed with commercial computational fluid dynamics (CFD) and in-house programmed one-dimensional (1D) codes. CFD calculations aim to investigate how flow pattern from the impeller is quantitatively influenced by compressor geometry parameters; thereby, the location of wake and its magnitude (flow angle and relative velocity magnitude) are analyzed. Results show that the increased back sweep impeller provides a more uniform flow pattern in terms of velocity and flow deviation angle distribution, and offers better potential for the diffusion process inside a vaneless (or vaned) diffuser. Secondary flux fraction and flow deviation angle from CFD simulation are implemented into the 1D two-zone program to improve 1D prediction results.