Photonic molecules doped with semiconductor nanocrystals

We report on coherent cavity field coupling in linear chains and arrays of exactly size-matched spherical microcavities doped with $\mathrm{CdSe}$ quantum dots. The spatial distribution and the dominant polarization type of both the weakly and strongly coupled cavity resonances are studied spectrally and spatially resolved in various coupled resonator geometries. Both experiment and theory show strong photon mode coupling with pronounced mode splitting as well as weak coupling with no significant loss in $Q$-factor depending on the emitter position and orientation.

Figure 1

Emission spectra taken in $\mathrm{TE}\text{−}$sensitive detection (see text) at characteristic bisphere points. Upper panel: redshifted, bonding resonances measured at the intersection point (uppermost curve) and weaker, antibonding modes taken at the edge position; middle panel: spectra of the spectrally unshifted bisphere modes taken at detection points apart from the bisphere axis; lower panel: Mie-scattering spectrum for a single polystyrene sphere.

Figure 2

(Color) Polarization-sensitive intensity maps of a bisphere photonic molecule for a weakly coupled ${\mathrm{TE}}_{19}$-type resonance (b) and (c) and a strongly coupled bonding ${\mathrm{TE}}_{18}$-type resonance (d) (note the stronger intensity in the bisphere center compared to that at the poles indicating the bonding type of the mode). The orientation of the polarizer is given by arrows. In (a) the spectrally integrated, unpolarized emission is shown. For the spectral positions see Fig. 1.

Figure 3

(Color) Calculated emission spectrum and electric field intensity distributions ${\mid E\mid }^2$ excited by a dipole oscillating in $x$ direction and (a) located in the lower sphere $1.2\mu \mathrm{m}$ from the center at the equator and (b) at the pole on the bisphere axis (see scheme in the inset). The upper panel shows the spectra, the lower panel the ${\mid E\mid }^2$-distribution in xy plane in a logarithmic color-coded scale of $40\mathrm{dB}$. Spectrum (b) has an offset of $+1.5$ for clarity.

Figure 4

(Color) Polarization-sensitive intensity maps of a trisphere linear chain configuration. The intensity distribution of a ${\mathrm{TE}}_{18}$-type mode reveals similar characteristics compared to the bisphere system: vanishing intensity of the weakly coupled modes along the trisphere axis (b), complementary emission pattern of the strongly coupled bonding mode (c). The used polarization for the mode mapping is indicated by arrows.

Figure 5

(Color) Intensity characteristics of bonding resonances of triple (left) and quadruple (right) sphere configurations. In the first row the spectrally integrated intensity distributions are plotted, the other images show the intensity distribution for different polarizations of the mode mapping as indicated by arrows.

Figure 6

Emission spectra taken at symmetry-relevant detection points for a trisphere system (see inset). The used polarization for detecting the spectra is indicated by arrows; lower panel: Mie scattering spectrum for a corresponding single polystyrene sphere.