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Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials



Plasmonic nanostructures have been extensively used to manipulate the spontaneous light emission rate of molecules and their radiative efficiency1,2,3,4,5,6. Because molecules near a metallic surface experience a different environment than in free space, their spontaneous radiative emission rate is generally enhanced7,8,9. Such enhancement, measured by means of the Purcell factor, arises as a consequence of the overlap between the surface plasmon mode frequency and the emission spectrum of the molecule. However, such overlap is available only for a few narrow bands of frequency due to the limited plasmonic materials existing in nature10,11. Although this limitation can be overcome by using hyperbolic metamaterials (HMMs)—a type of nanoscale artificial material with hyperbolic dispersion relations—the Purcell factor and the radiative power have remained relatively low12,13,14,15,16. Here, we show that by nanopatterning a hyperbolic metamaterial made of Ag and Si multilayers, the spontaneous emission rate of rhodamine dye molecules is enhanced 76-fold at tunable frequencies and the emission intensity of the dye increases by 80-fold compared with the same hyperbolic metamaterial without nanostructuring. We explain these results using a dynamic Lorentzian model in the time domain.

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Figure 1: Nanopatterned multilayer HMMs.
Figure 2: Comparison of Purcell factors for Ag–Si multilayer HMMs and a pure Ag single layer.
Figure 3: Comparison of Purcell enhancement for uniform and nanopatterned multilayer HMMs.
Figure 5: Experimental demonstration of the geometrical dependence of Purcell and fluorescence intensity enhancement on nanopatterned multilayer HMMs.
Figure 4: Experimental measurements and theoretical fit of time-resolved fluorescence for dye molecules on different samples.

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The authors acknowledge financial support from an Office of Naval Research (ONR) Young Investigator Award (no. N00014-13-1-0535), the National Science Foundation–Electrical, Communications and Cyber Systems (NSF-ECCS) (grant no. 0969405) and the National Science Foundation–Civil, Mechanical and Manufacturing Innovation (NSF-CMMI) (grant no. 1120795). The authors thank P. Zhang for help with STEM characterization and M. Digman, E. Gratton, L. Feng, H. Chen and Y. Liu for assistance during various phases of simulations and experiments. The time-resolved photoluminescence measurements were carried out at the Laboratory for Fluorescence Dynamics (LFD) at the University of California, Irvine (UCI). The LFD is supported jointly by the National Center for Research Resources (5P41RR003155-27), the National Institute of General Medical Sciences of the National Institutes of Health (8P41GM103540) and UCI.

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Z.L. conceived the concept. D.L. performed numerical modelling and simulations, in detail. J.J.K. and D.L. fabricated the samples. D.L. performed the optical experiments and prepared all the figures. D.L., Z.L. and J.J.K. wrote the manuscript. All authors discussed the results and commented on the manuscript. Z.L. supervised the project.

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Correspondence to Zhaowei Liu.

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Lu, D., Kan, J., Fullerton, E. et al. Enhancing spontaneous emission rates of molecules using nanopatterned multilayer hyperbolic metamaterials. Nature Nanotech 9, 48–53 (2014).

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