Plasmonic nanostructures are known to influence the emission of near-by emitters. They can enhance the absorption and modify the external quantum efficiency of the coupled system. To evaluate the possibility of using plasmonics to enhance the light emission of a phosphor-converted LED device and create an efficient directional light source, regular arrays of aluminium nanoparticles covered with a red dye layer are investigated. In arrays of aluminum nanocylinders with a diameter of ca 140 nm combined with a thin (650 nm) layer of luminescent material, very narrow resonances have been observed, which lead to large enhancement factors of up to 70 and 20 for excitation with a directional blue laser source and a lambertian LED respectively, in a small spectral range for particular angles. The measured resonances agree very well with finite-difference time-domain numerical simulations. These changes in the angular emission profile of the red dye as well as the spectral shape of its emission can help to optimize the efficacy of phosphor-converted LED modules and increase the amount of useable light in a certain angular cone. Using Fourier microscopy, large modifications of the angular emission profile as well as spectral shaping are observed for these plasmonic LED devices if compared to reference samples without plasmonic nanostructures.
Photoluminescence from finite semiconductor nanowires is theoretically investigated. We show experimentally the directional emission of polarized light from single InP nanowires through Fourier microphotoluminescence, thus demonstrating semiconductor nanowires behave as efficient optical nanoantennas. Numerical calculations for finite nanowires confirm such enhanced and directional emission. We anticipate the relevance of these results for the development of nanowire photon sources with optimized efficiency and controlled emission.
We demonstrate experimentally that ensembles of conically shaped GaP nanorods form layers of graded refractive
index due to the increased filling fraction of GaP from the top to the bottom of the layer. Graded refractive
index layers reduce the reflection and increase the coupling of light into the substrate, leading to broadband
and omnidirectional antireflection surfaces. This reduced reflection is the result of matching the refractive index
at the interface between the substrate and air by the graded index layer. The layers can be modeled using a
transfer-matrix method for isotropic layered media. We show theoretically that the light coupling efficiency into
silicon can be higher than 95% over a broad wavelength range and for angles up to 60. by employing a layer with
a refractive index that increases parabolically. Broadband and omnidirectional antireflection layers are specially
interesting for enhancing harvesting of light in photovoltaics.
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