Induced Directional Switching of Platicon Microcombs in Photonic Crystal Ring Resonators

Technical Summary: Induced Directional Switching of Platicon Microcombs in Photonic Crystal Ring Resonators

Problem Statement
Microcombs generated in normal-dispersion photonic crystal ring resonators (PhCRs) offer a versatile platform for integrated photonic circuits, enabling applications such as optical clocks and telecommunications. However, these systems inherently suffer from a directional bias that favors backward-propagating states. This bias arises because the photonic crystal grating creates a Bragg mirror effect, coupling counter-propagating modes. When pumping the red-shifted supermode to initiate the comb, the intracavity power is predominantly backward-propagating. Consequently, extracting the comb requires bulky, non-integrated optical circulators, creating a significant bottleneck for fully on-chip integration. While stochastic direction switching has been observed, it is unreliable, and existing methods to enforce forward operation often limit design flexibility or fail to provide deterministic control.

Methodology
The authors propose and validate a deterministic method termed Side-mode Induced Forward Forcing (SIFF). The core strategy involves engineering auxiliary mode splittings on resonances adjacent to the pumped mode ($\mu = \pm 1$), in addition to the splitting of the pumped mode itself ($\mu = 0$).

• Theoretical Framework: The dynamics are modeled using a coupled system of bidirectional Lugiato-Lefever equations (LLEs). These equations account for the forward ($\tilde{A}_\mu$) and backward ($\tilde{B}_\mu$) field evolution, incorporating mode-dependent coupling rates ($\gamma_\mu$) induced by the grating, dispersion, and nonlinear effects.
• Device Fabrication: Two devices were fabricated using a 570 nm thick tantalum pentoxide ($Ta_2O_5$) layer on silica.
  1. Standard PhCR: Features a single-period corrugation affecting only the pumped mode ($\mu=0$).
  2. SIFF PhCR: Incorporates the primary corrugation plus two additional superimposed gratings to split the adjacent modes ($\mu = \pm 1$).
• Experimental Setup: A tunable continuous-wave (CW) laser was scanned across the red-shifted supermode. Comb generation was monitored in both forward and backward directions using optical spectrum analyzers and photodiodes, with careful calibration of insertion losses and management of parasitic facet reflections.

Key Contributions and Results

1. Demonstration of SIFF: The study demonstrates that introducing engineered mode splittings on adjacent modes ($\gamma_{\pm 1}$) can deterministically reverse the comb's propagation direction. While the comb initially forms in the backward direction (as in standard PhCRs), the side-mode splitting acts as a coherent seed, steering the nonlinear dynamics toward a stable, forward-propagating platicon state.
2. Optimization Conditions: Numerical simulations reveal an optimal synchronization condition between the pump mode splitting ($\gamma_0$) and the side-mode splitting ($\gamma_{\pm 1}$). The "SIFF probability" (the parameter space where the forward comb dominates) is maximized when $\gamma_0 - \gamma_{\pm 1} \approx 9$.
   • If $\gamma_{\pm 1}$ is too small, the backward bias persists.
   • If $\gamma_{\pm 1}$ is too large (over-splitting), the system reverts to a backward-dominated state or forms a "platicon crystal" (a dual-pulse state with harmonic mode-locking), suppressing the forward-seeding mechanism.
3. Experimental Validation:
   • In the standard device, the comb remained predominantly backward-propagating across the entire existence range, exhibiting hysteresis and breathing dynamics at high detunings.
   • In the SIFF device, the comb transitioned rapidly from a brief backward transient to a stable, low-noise, forward-propagating state that dominated the entire platicon existence range. The backward power in the $\mu = \pm 1$ lines was enhanced due to the engineered coupling, confirming the mechanism.
4. Robustness: The SIFF mechanism was shown to be robust even in the presence of parasitic back-reflections (approx. 10%) from chip facets, which typically complicate directional control.

Significance
The paper claims that SIFF provides a critical pathway toward circulator-free, integrated normal-dispersion microcombs. By enabling deterministic control over the propagation direction, this approach eliminates the need for bulky external optical circulators required to extract backward-propagating combs. This offers a scalable architecture for compact telecommunications and sensing systems. The authors note that while the effect is robust, further investigation is required to analytically prove the existence range of these solutions and to fully understand the impact on power conversion efficiency. The work represents a vital building block for the next generation of fully integrated photonic sources.