Large emission enhancement and emergence of strong coupling with plasmons in nanoassemblies: Role of quantum interactions and finite emitter size

The Purcell effect has been the basis for several decades in understanding enhancement of photonic efficiency and decay rates of emitters through their coupling to cavity modes and metal nanostructures. However, it is not clear whether this regime of radiative enhancements can be extended to ultrasmall nanoparticle sizes or interparticle distances. Here we report large radiative enhancements of quantum dot assemblies with extremely small metal nanoparticles and emitter-particle separations $R$ of a few nanometers, where Purcell effect would lead to either no enhancements or quenching. We invoke a new regime of radiative enhancements to explain the experimental data and also correctly predict the emergence of strong coupling below certain $R$, as observed in experiments. In addition, we show that the widely used point emitter approximations diverge from actual observations in the case of finite size emitters at such small separations.

Figure 1

(a) TEM images of QD-AuNP monolayer film. Dark features represent AuNPs. (b) AFM scanning image represents the packed arrangement of AuNP and QDs. (c) Interactions and components of decay in the model; ${\mathrm{\Gamma }}_0^r$ and $\mathrm{\Gamma }$ represent the decay of independent QE and AuNP, and ${\mathrm{\Gamma }}_{\text{leak}}$ is due to strong coupling with the AuNP, respectively. The golden sphere represents the metal nanoparticle (AuNP) which was modeled as a multipole sphere. The blue sphere represents a finite size emitter constituted of three spatially separated but coupled dipoles, to simulate a multipole quantum emitter (QE) with three possible modes. Three little violet balls with green arrows represent these three dipoles with random polarizations.

Figure 2

Steady-state and time-resolved PL data for various AuNP-doped QD films. (a) The individual PL emission spectra for different $R$. Green curve shows the behavior of the system in presence of larger nanoparticle. (b) The enhancement factor $\left({\text{EF}}_{\text{PL}}\right)$ is quantified from the PL measurement as a function of $R$. (c) Individual decay spectra for different films. (d) Enhancement factor in decay rate $\left({\text{EF}}_{\text{exp}}\right)$ is calculated as the ratio of decay rates of AuNP doped QD film to the reference QD film.

Figure 3

Results from calculations of QD-AuNP interactions. (a) Predictions of efficiency using different theoretical models for $Q_o$ = 0.5. It represents enhancements in emission for a single excitation. (b) Enhancement factors $\left({\text{EF}}_{\text{theory}}\right)$ predicted for repeated excitations.

Figure 4

Strong coupling between QDs and AuNPs. (a) Individual spectra corresponding to reference QD film $\left(Q_{4}C\right)$ and AuNP-doped QD film $\left(A_{5}C\right)$. (b) Energy splitting in PL emission varying with the separation $R$ due to strong coupling among QDs and AuNPs. (c) Theoretically predicted energy shifts for emitting charge distributions of finite sizes; note log scale and actual variations between model predictions at larger separations $R$ are extremely small compared to variations at smaller separations on the left of the figure.