Active Tuning of Hybridized Modes in a Heterogeneous Photonic Molecule

From fundamental discovery to practical application, advances in the optical and quantum sciences rely upon precise control of light-matter interactions. Systems of coupled optical cavities are ubiquitous in these efforts, yet the design and active modification of the hybridized mode properties remains challenging. Here, we demonstrate the design, fabrication, and analysis of a tunable heterogeneous photonic molecule consisting of a ring resonator strongly coupled to a nanobeam photonic crystal cavity. Leveraging the disparity in mode volume between these two strongly coupled cavities, we combine theory and experiment to establish the ability to actively tune the mode volume of the resulting supermodes over a full order of magnitude. As the mode volume determines the strength of light-matter interactions, this work illustrates the potential for strongly coupled cavities with dissimilar mode volumes in applications requiring designer photonic properties and tunable light-matter coupling, such as photonics-based quantum simulation.

Figure 1

(a) A SEM image of the SU-8 cladded coupled ring-resonator–nanobeam device with a 500-nm gap between the ring and the nanobeam at the point of closest separation. Scale bar: $5\mu \mathrm{m}$. (b) The $y$ component of the electric field profiles for the nanobeam-cavity mode (bottom) and the ring-resonator mode (top) studied. The system is modeled as a coupled oscillator, parametrized by an effective coupling strength $\sqrt{{\mathcal{G}}_{12}{\mathcal{G}}_{21}}$ and effective frequencies ${\mathrm{\Omega }}_i$ distinct from the bare resonant frequencies ${\omega }_i$. (c) Transmission spectra collected for four equally spaced temperatures (gray circles) with simultaneous least-squares fits to the model overlaid (red lines).

Figure 2

(a) Anticrossing resulting from strong coupling between the ring-resonator and nanobeam-cavity modes. Experimental data are shown as circles, while colored solid lines display the resulting least-squares fit to Eq. (6). The gray lines overlie the theoretical values of ${\omega }_{\pm }$, extrapolated via parameter values obtained from the fits. (b) The evolution of the supermode resonant frequencies as a function of the detuning. The black points correspond to experimentally measured peak transmission energies, while the error bars indicate the uncertainty in the peak energy due to the finite density of the transmission energies measured. The solid curves display the theoretical supermode energies computed from Eq. (4), parametrized through simultaneous fits to transmission measurements.

Figure 3

(a) The field profile for the upper- (top) and lower- (bottom) cavity polaritons at various temperatures. Both supermodes are dominated by the nanobeam field at all observed temperatures due to the weighting of ${\mathbf{f}}_1(\mathbf{x})$ and ${\mathbf{f}}_2(\mathbf{x})$ in Eq. (7). (b) The hybridized mode volumes $V_{+}$ (blue curve) and $V_{-}$ (red curve) of the upper- and lower-cavity polaritons. The gray region indicates the range of experimentally measured temperatures, while the dotted lines specify $V_1$, $V_2,$ and $V_1+V_2$. Due to the predominant localization of both modes in the nanobeam cavity, both $V_{+}$ and $V_{-}$ coalesce at a value less than 5 times the mode volume of the isolated ring-resonator mode.