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...
Metasurfaces offer the possibility to shape optical wavefronts with an ultracompact, planar form factor. However, most metasurfaces are static, and their optical functions are fixed after the fabrication process. Many modern optical systems require dynamic manipulation of light, and this is now driving the development of electrically reconfigurable metasurfaces. We can realize metasurfaces with fast (>105 hertz), electrically tunable pixels that offer complete (0- to 2π) phase control and large amplitude modulation of scattered waves through the microelectromechanical movement of silicon antenna arrays created in standard silicon-on-insulator technology. Our approach can be used to realize a platform technology that enables low-voltage operation of pixels for temporal color mixing and continuous, dynamic beam steering and light focusing.
SUMMARYEnergy and exergy analysis were carried out for a combined-cycle power plant by using the data taken from its units in operation to analyse a complex energy system more thoroughly and to identify the potential for improving efficiency of the system. In this context, energy and exergy fluxes at the inlet and the exit of the devices in one of the power plant main units as well as the energy and exergy losses were determined. The results show that combustion chambers, gas turbines and heat recovery steam generators (HRSG) are the main sources of irreversibilities representing more than 85% of the overall exergy losses. Some constructive and thermal suggestions for these devices have been made to improve the efficiency of the system.
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