Spectroscopic study of emitter assemblies coupled to Plasmonic nano-cavities and metamaterials

Abstract

The plasma oscillations of noble metals as silver and gold are well understood from the free electron model of metals. In the past two decades, due to the availability of various physical and chemical fabrication methods for nano materials, the optical properties of nanostructures of noble metals gained interest. The plasma oscillations of the noble metal nanostructures have unique optical properties. The free electrons are confined by the geometry of nanostructure and give rise to spatially localized plasma oscillations. Various modes of the electrostatic multi-pole potential problem can be realized in such metal nanostructures. The localized dipolar plasma modes can be coupled to various light emitters, by placing the emitters, near these noble metal nanostructures. In this study, various aspects of interaction emitters coupled to silver nanostructures are explored.

The spontaneous emission properties of alloyed quantum dot films is discussed. The quantum dot monolayer samples are used for measuring angle resolved emission pattern, using a home-built angle resolved emission spectroscopy (ARES) system operating in both transmission and reflection modes. The ARES system is bench marked and photoluminescence (PL) emission anisotropy is quantified as anisotropy coefficient. The anisotropy coefficient indicates the orientation of emission transition dipole moment (TDM) relative to the substrate. The time resolved PL emission measurement is used to estimate the TDM magnitude.

The spontaneous emission properties of quantum dots coupled to silver nanoplatelets and silver nanowires are discussed. The nanowire and nanoplatelet cavity modes are in infrared region and quantum dot emission is in visible region of electromagnetic spectrum. Due to off-resonant weak coupling between quantum dots and silver nanowires/silver nanoplatelets lead to inhibition of the spontaneous emission rate. The spontaneous emission rate inhibition is measured in terms of Purcell factors less than unity. The emitter-cavity interaction is in weak coupling regime, as indicated by the Purcell inhibition of spontaneous emission.

The hyperbolic metamaterial (HMM) is introduced as an ordered hexagonal array of silver nanowires in an Aluminium oxide dielectric host. The HMM undergoes an optical topological transition and can support large number of cavity modes, which are the Bloch modes of the nanowire plasma resonances. It is shown that the Purcell enhancement of spontaneous emission on HMM is at least 4.6 fold. The vertically oriented nanowire array is optimally oriented for coupling the in-plane oriented excitons. Monolayer MoS2 is an ideal emitter to couple with HMM as its A and B excitons have unusually large TDMs. Rabi splitting is observed for B excitons, whose position is nearly resonant with the transition set-in point. The avoided crossing of strongly coupled Exciton-Polariton states is demonstrated. The A excitons do not show Rabi splitting. This selective strong coupling of B excitons is attributed to the inbuilt electrical field gradient of the HMM topological transition.