Purcell enhancement can be realized using hyperbolic metamaterials (HMMs) composed of alternating metal/dielectric multilayers of subwavelength thickness. By adjusting the filling fraction of the metal layer, this structure possesses an effective hyperbolic dispersion and can access to epsilon-near-zero (ENZ) with one of the principal components of the permittivity tensor passes through zero. The unique property theoretically yields a large local density of state (LDOS) enabling to support a high Purcell factor and enhanced spontaneous emission rate of a quantum emitter in the vicinity. However, the property of the fabricated HMM deviates from the ideal characteristics estimated by effective medium theory (EMT) due to the finite thickness of the unit cell. Therefore, the actual LDOS and Purcell factor reduce significantly. Additionally, the outcoupling of the high-k waves from HMM remains challenging. It relies on small-area nanostructure due to the incapability of large-area nanofabrication. In this paper, we experimentally and theoretically study the effect of the unit cell thickness in Ag/ITO HMMs on the enhancement of QD emission. The study on 320 nm thick HMM formed by three different unit cell thicknesses ranging from 80 to 20 nm suggested that the Purcell factor increases as the unit cell thickness decreases. We also demonstrate a large-area outcoupling method using self-assembled nanoparticle monolayer to promote the detectable QD emission in the far field. A maximum enhancement factor of ~40 was observed by incorporating the nanoparticle monolayer. This enhancement technique and large-area outcoupling will find applications in display and biosensing.
A. Kannegulla, Y.-C. Wang, Y. Liu, B. Wu, and L.-J. Cheng, "Large-area outcoupling of quantum dot emission on multilayer hyperbolic metamaterials," Proc. SPIE 10719, Metamaterials, Metadevices, and Metasystems 2018, 107192K (Presented at SPIE Nanoscience + Engineering: August 23, 2018; Published: 19 September 2018); https://doi.org/10.1117/12.2322085.
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