(Invited) Enhancement of Light Emission in Luminescent Structures within Nano-Arrays

Abstract

Nanostructures can dramatically alter the light emission in luminescent structures by strongly altering the photonic densities of states at the location of the luminescent emitter. We describe and develop two complementary platforms that tailor luminescent emission through i) quantum dots in nanocup arrays and ii) corrugated nanoarrays in organic light-emitting structures.We fabricated nanocup arrays with pitch ~750 nm by replica molding in polymer molds. A thin non-conformal gold film was sputter-deposited on the polymer film with angle-directed deposition. The gold film was continuous with a variation of spatial thickness, with the film being optically thin at the bottom of the nanocup [1]. Optical measurements revealed an extraordinary optical transmission (EOT) peak at a wavelength slightly smaller than the pitch. Although no holes were etched in the film the optically thin metal at the bottom allowed resonant light transmission. This is a simple pathway to fabricate large area (>cm2) nano-arrays with EOT without expensive lithography which limits such nano-arrays to very small areas. Quantum dots were embedded in these nanoarrays and revealed a marked reduction of photo-luminescence lifetime by ~4 compared to the control samples, when the emitting wavelength was tuned to the optical resonance of the nanoarray. Scattering matrix and FDTD simulations both show an enhancement of the field by a factor >100. The optically enhanced field in nanocups enhanced the spontaneous emission of embedded quantum dots. The simulated Purcell factor is enhanced by >80 at the bottom of the nanocup, and by a factor of 3-15 averaged over the nanocup height, in agreement with measured photoluminescence lifetimes that decreased by a factor of ~4 for a quantum dot ensemble.We also study and simulate light emission from organic light emitting diodes (OLEDs) -where only 20% of the light is emitted to air and remainder (~80%) is trapped as internally waveguided modes and surface plasmon modes at the cathode-organic interface. We develop rigorous scattering matrix simulations [2] to demonstrate the emission of light can be increased by enhancement factors >3, when the OLEDs are integrated with periodic nanoarrays. The emissive region conformally follows the periodicity of the nano-array. The periodic nanostructures diffract trapped waveguided modes and plasmonic modes to the air cone and are a pathway to achieving light outcoupling efficiencies exceeding 60%. The predicted dependence of the outcoupled light emission on the pitch and height of the nano-arrays will be presented. Acknowledgements: Partially supported by NSF grants CMMI-1265844 and CMMI-170648.