Acoustically control of integrated optical microrings: from photonic molecule to Mobius strip

Abstract

Microring resonators (MRRs) are fundamental building blocks of photonic integrated circuits, yet their dynamic reconfiguration has been limited to tuning refractive index or absorption. Here, we demonstrate acoustic control over optical path topology on a lithium niobate on sapphire platform. By launching gigahertz acoustic waves into a hybrid phononic-photonic waveguide, a dynamic Bragg mirror (DBM) is created within the optical path, coupling forward and backward propagating light. Employing a pair of coupled MRRs, we achieve strong coupling between supermodes of the photonic molecule with only milliwatt-level drive power, yielding a cooperativity of 2.46 per milliwatt. At higher power, DBM reflectivity up to 24% is achieved, revealing breakdowns of both the photonic molecule picture and perturbative coupled mode theory, indicating the transformation toward Mobius strip topology. Our work establishes a new dimension for controlling photonic devices, opening pathways toward fully reconfigurable photonic circuits through acoustic drive.

Figures (4)

• Figure 1: Acoustically driven dynamic Bragg mirror. (A) The scanning electron microscope (SEM) image of the device. Optical components, including the gratings, waveguides, and hybrid microring resonator, are shown in orange. The interdigital transducers (IDTs) for acoustic wave generation are shown in blue. (B) The photon-phonon multiplexer. The optical mode (orange arrow) in the bending waveguide couples to the straight section of the hybrid microring resonator. The acoustic wave (blue arrow) is confined to and travels along this straight section, forming acousto-optic hybrid microring. (C) Measured optical transmission spectrum with the acoustic pump off. (D) Schematic of the dynamic Bragg mirror (DBM) and phase-matching condition for the Brillouin interaction. An acoustic pump (blue lines, $\Omega$) mediates the coupling between two optical modes ($\omega_1$, orange and $\omega_2$, yellow). This non-reciprocal process causes a frequency up-conversion ($\omega_{\text{probe}} + \Omega$) or down-conversion ($\omega_{\text{probe}} - \Omega$) of the probe, depending on the relative propagation directions. (E) Experimental demonstration of strong coupling. With the acoustic pump around 0.6 mW activated, the transmission spectra are measured for a contra-directional probe (top panel, CCW) and a co-directional probe (bottom panel, CW). The spectra reveal clear phonon-induced mode splitting and frequency conversion. Highlighted areas distinguish the off-resonance conversion (I, III) and the on-resonance strong coupling (II) regimes.
• Figure 2: DBM induced reconfiguration of the microring and optical strong coupling. (A) Measured transmission spectra (light blue lines) for increasing acoustic pump power, from top to bottom. Thin black lines represent the theoretical fits, which are used to extract key coupling parameters. (B) Normalized mode splitting, $\Delta f_2 / \kappa_2$ (where $\kappa_2= 0.90\,\text{GHz}$ is the linewidth), as a function of acoustic pump power. The data (blue squares) clearly follows the characteristic square-root dependence ($\propto \sqrt{P}$) predicted by theory (black line). (C) Normalized mode splitting $\Delta f_2 / \kappa_2$ versus acoustic detuning. (D) Extinction ratio (ER) of the newly generated mode (region III) versus acoustic pump power. (E) Relative frequency $\Delta f_{13}$ (between the reference mode I and the new mode III) versus acoustic pump power. Mode I is used as a reference to compensate for thermal drift. In (B-E), squares represent the data derived from the experimental fits in (A), while the dashed lines are theoretical predictions calculated using the parameters extracted from these fits. Error bars represent fitting uncertainties.
• Figure 3: Multi-degree-of-freedom control of optical microring. (A) Schematic of the experimental setup. A tunable laser probe is routed via a polarization controller (PC) and an optical switch, which selects the probe direction (CW or CCW). The transmission is measured by a photodetector (PD). Two independent radio-frequency (RF) sources (1 and 2) are amplified and connected to the two on-chip interdigital transducers (IDTs), allowing for independent and directional control of the acoustic pump (CCW, CW, or bidirectional). (B-D) Experimental demonstration of chiral control. The top (blue) and bottom (gray) spectra in each panel represent the transmission for CW and CCW optical probes, respectively. The three columns correspond to different acoustic pump configurations: (B) A single counter-clockwise (CCW) acoustic pump. (D) A single clockwise (CW) acoustic pump. (C) A bidirectional acoustic pump. In all cases, the applied acoustic power is 1 mW. For the bidirectional case (C), the two acoustic pumps are detuned by 1 MHz to avoid the formation of an acoustic Fabry-Perot (F-P) cavity.
• Figure 4: Light path topology: from a photonic molecule to a Möbius band. (A) Schematic of the system's optical path topology. Top: The "photonic molecule" path, where the outer large ring and the inner small ring are coupled, forming hybrid supermodes. Bottom: The "Möbius strip" path. The optical field must complete two round trips to form a cloesd resonant path due to the DBM with high reflectivity. (B) Transmission spectra under strong acoustic pump. The top three traces are experimental measurements (solid blue lines) and predictions using CMT (black lines), showing the complex spectral response. The difference between the two indicates the gradual breakdown of CMT. The bottom two traces are theoretical predictions using CMT (black lines) and TMT (red lines) . (C) Theoretically calculated transmission spectra for a hypothetical system, assuming the outer and inner rings are completely decoupled. The background colors in (B) and (C) cover the same wavelength range.