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Scientists take step towards practical metamaterial light emitters

Hyperbolic metamaterial structure can both enhance and capture quantum dot light emissions

A team from City College, City University of New York (CUNY), Purdue University and the University of Alberta has demonstrated how to both enhance light emission and capture light from metamaterials embedded with light emitting quantum dots. The breakthrough, they believe, could lead to a range of applications including ultrafast LEDs, nanoscale lasers and efficient single photon sources.

In the demonstration, the team, led by physicist Vinod Menon, used what are known as hyperbolic metamaterials to enhance the light emission properties of the quantum dots, which are used for local excitation of high-k modes. 

Metamaterials are artificial materials created with subwavelength repeating structures that can control and process electromagnetic radiation in unusual ways. Hyperbolic metamaterials are a type of metamaterial designed to display indefinite dispersion. Their properties include enhancement of spontaneous emission, as well as superlensing effects.

While enhancement in light emission from such systems has been shown by different groups, including Menon's, in the past, they were not useful in a practical way since light did not come out easily from them. The present work takes the first step towards developing practical light emitters based on metamaterials.

The active hyperbolic metamaterial structure consisted of seven alternating layers of Al2O3 at 20nm thickness, silver at 12nm, with 1.2nm germanium inbetween. A spin-coated active layer of quantum dots was embedded in the middle of the fifth Al2O3 layer. The team engineered an efficient light extraction scheme using a high-index-contrast bullseye grating, which was deposited on top.

"We've shown both an increase in light emission and were able to extract light." added Professor Menon, whose team included City College PhD students Tal Galfsky and H.N.S. Krishnamoothy

"˜Active hyperbolic metamaterials: enhanced spontaneous emission and light extraction' by Galfsky et al, appears in Optica, Vol. 2, Issue 1, pp. 62-65 (2015) http://dx.doi.org/10.1364/OPTICA.2.000062

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