Electrically switchable photonic diode empowered by chiral resonance

The on-chip integration of nonreciprocal optical devices remains a critical challenge for modern optoelectronics, as conventional magneto-optic approaches suffer from material incompatibility and excessive optical losses. Nonlinear photonic diodes have emerged as a promising magnet-free alternative, yet their widespread adoption has been constrained by inherent limitations in reconfigurability. Here, we present an all-silicon, electrically tunable photonic diode leveraging engineered chiral resonances in an ultra-compact microring architecture. The pronounced asymmetric modal coupling enables nonreciprocal transmission with two distinct operation modes at threshold powers down to -5 dBm. The chirality further enables unprecedented control over self-pulsation dynamics, manifesting in propagation-direction-dependent oscillation thresholds and temporal signatures. Crucially, post-fabrication electrical reconfigurability allows dynamic switching between forward, backward, and disabled states. This work represents a significant advancement in integrated nonreciprocal photonics, offering a CMOS-compatible solution with transformative potential for optical interconnects, photonic neural networks, and signal processing systems.

💡 Research Summary

The authors present an all‑silicon, electrically reconfigurable photonic diode that exploits engineered chiral resonances in an ultra‑compact spiral‑shaped microring resonator on a silicon‑on‑insulator (SOI) platform. By introducing a controlled geometric deformation (characterized by the angular spacing θ and deformation parameter ε) the device creates asymmetric back‑scattering coefficients A (clockwise to counter‑clockwise) and B (counter‑clockwise to clockwise). The resulting mode chirality α = (|A| − |B|)/(|A| + |B|) can be tuned from strongly negative to strongly positive values. A negative α yields a forward‑blocking state (high transmission from port 1, low from port 2), while a positive α reverses the direction; α≈0 balances the two directions and effectively disables the diode.

Electrical control is achieved with an integrated phase shifter that modulates the effective refractive index Δn_eff of the silicon waveguide. By applying a voltage the back‑scattering strengths A and B are altered, allowing real‑time switching of the chirality sign and thus toggling among forward, backward, and disabled states without any magnetic materials.

In the linear regime the device exhibits Lorentzian resonances with a loaded Q≈11 300 and maintains Lorentz reciprocity. However, reflection measurements reveal a pronounced asymmetry (≈13 dB contrast), confirming strong chirality (α≈‑0.6) in the fabricated device. Dual‑port interferometric measurements further extract the individual scattering coefficients, showing excellent agreement with coupled‑mode theory.

When the input power is increased, two‑photon absorption (TPA) generates free carriers, while the associated free‑carrier absorption (FCA) and thermo‑optic (TO) effects produce heating. The dominant thermal contribution red‑shifts and broadens the resonance, but the free‑carrier dispersion (FCD) partially counteracts the shift. Because the asymmetric back‑scattering creates different intracavity powers for opposite launch directions (P₁≠P₂), the temperature rise and resonance shift become direction‑dependent, leading to highly non‑reciprocal transmission.

Two distinct nonlinear operating modes are demonstrated. In the “pre‑activated” mode the laser wavelength is continuously swept upward, allowing thermal accumulation that drives the resonator into the high‑temperature branch of the bistability curve. Non‑reciprocal transmission appears at input powers as low as –7 dBm, reaches a maximum isolation of 18 dB at –3 dBm, and the non‑reciprocal bandwidth expands up to ≈2.9 nm as power increases. In the “independent” mode the laser wavelength is fixed on the cold resonance; thermal buildup is avoided, and non‑reciprocal transmission emerges at ≈‑5 dBm with a more modest isolation of ≈13 dB and a narrower bandwidth that saturates around –4 dBm. In both modes, reversing the applied voltage flips α and switches the diode between forward and backward operation.

A striking consequence of the engineered chirality is direction‑dependent self‑pulsation (SP). At high input powers (≈0 dBm) a Hopf bifurcation destabilizes the steady state, giving rise to self‑sustained oscillations in the MHz range (1–2 MHz). The SP threshold for forward launch is ≈0 dBm, whereas backward launch requires only ≈‑2 dBm, directly reflecting the intracavity power imbalance caused by chirality. Near threshold the pulse waveform consists of a ~0.1 µs dip followed by a slow decay; at higher powers the waveform sharpens to ~15 ns spikes, indicating that FCD dominates the dynamics. Importantly, SP occurs only within the thermally broadened resonance and does not degrade transmission outside the SP band, preserving near‑reflectionless propagation.

Overall, the work demonstrates a CMOS‑compatible, electrically tunable photonic diode with ultra‑low threshold (‑5 dBm), sizable non‑reciprocal bandwidth (≈3 nm), and novel chirality‑mediated dynamical features. The ability to switch the diode’s directionality on demand, combined with the observed self‑pulsation behavior, opens new avenues for integrated optical interconnects, photonic neural‑network hardware, and high‑speed signal‑processing architectures where magnet‑free, reconfigurable non‑reciprocity is essential.