Fine structure of coupled optical modes in photonic molecules

We report on the coherent coupling of whispering gallery modes (WGM) in a photonic molecule formed from two melamine-formaldehyde spherical microcavities with a thin shell of CdTe nanocrystals. Utilizing a microporous polymer structure to orient the photonic molecule, we have excited the photonic molecule both on and off axis. This controllable geometry has allowed the observation of an off-axis fine structure that consists of very sharp peaks resulting from the removal of the WGM degeneracy with respect to the azimuthal quantum number $m$. The mode splittings are in very good agreement with theory.

Figure 1

(a) The PL spectrum of a PM accommodated in a microwell. Arrows indicate the coupled modes. (b) The PL spectra of noninteracting microspheres. Inset: microscope image of the PM in the microwell. The dark cross indicates the excitation position.

Figure 2

(a) The PL spectra of the PM formed by two almost identical microspheres with off-axis excitation and detection. The vertical lines indicate the spectral position of the WGM detected in spectra of the individual spheres. (b) The spacing between neighboring peaks. The dashed lines are a guide to the eye. Inset: microscope image of the PM. The dark cross indicates the excitation position.

Figure 3

Spacing between adjacent peaks estimated for different values of azimuthal quantum number $\left(m\right)$ from calculation of the splitting between bonding and antibonding modes. The dashed line is a guide to the eye.

Figure 4

Quality factor calculated for $m$ resonances forming bonding (squares) and antibonding (triangles) branches of PM. Quality factor estimated for one of individual spheres forming the PM (circles) is given for comparison. Inset: The deconvolution of the line shape of $m$ resonances belonging the ${\mathit{TE}}_{21}^1$ antibonding branch of a photonic molecule.