Coexistence of strong and weak coupling in ZnO nanowire cavities. T. Michalsky, H. Franke, R. Buschlinger, U. Peschel, M. Grundmann and R. Schmidt-Grund in Eur. Phys. J. Appl. Phys. 74:30502 (2016).  What the paper says!?

This is a report of polaritons in ZnO nanowires coated in a concentric DBR cavity.

In this work we demonstrate a nanowire based cavity which exhibits high Quality factors and a high spectral mode density, enabling the observation of strong coupling between several photonic modes and the excitonic system

of ZnO up to room temperature.

The work interests me primarily for their simultaneous observation of weak and strong coupling, but this is attributed to multimodes of their cavity resulting, not only in several polaritons (including middle-polariton branches) but also no-polariton when those modes do not penetrate the ZnO nanowire:

we simultaneously observe modes in the weak coupling regime due to a miss- ing mode overlap with the core excitons as we have shown by means of FDTD simulations recovering the spatial

mode distributions inside the cavity structure.

There is a brief overview of various techniques to realize nanowire cavities.

In literature dealing with wire-like cavities often two mode families are discussed. One type are the so-called

whispering gallery modes (WGMs) [...] The other type are Fabry-Pérot modes (FPM)

with interesting variations, such as:

the end facets of the wires act as intrinsic mirrors.

Here, «nanowire cavities are coated concentrically with DBRs». They surround a ZnO nanowire (a) with DBR (b) to confined excitons; cuts in (c):

The dispersion behavior shows immediately, that the mode family mentioned first is in the weak coupling regime (WCR) whereas the latter ones can be attributed to be in the strong coupling regime (SCR). The simultaneous appearance of modes in WCR and SCR results from the

two dimensional design of the cavity

Their model is a simple three-oscillator models, cf. Eq. (1).

By applying a coupled oscillator model for the modes which are supposed to be in the SCR, including the A and B excitonic ground state energies F XA and F XB as well as three bare cavity modes Ecav,i (i = 1, 2, 3),

we can reproduce the experimental results very well