Phase-Controlled Asymmetric Resonances for Enhanced Spectral Slope in a Ge-OI Microring Resonator at 4.2 μm

Microring resonators (MRRs) are key components of photonic integrated circuits for filtering, modulation, and spectroscopic sensing. In the mid-infrared (mid-IR), asymmetric resonances are attractive because their steep local spectral transitions can enhance wavelength discrimination in compact resonant devices. Here, we demonstrate geometry-controlled asymmetric resonances in a germanium-on-insulator (Ge-OI) MRR at 4.2 μm by introducing a single subwavelength air hole into the bus-waveguide coupling region as a localized phase perturbation. Effective-index analysis shows that the hole geometry estimates a controllable phase shift Δϕ of up to 0.82π rad, enabling systematic evolution from a nearly symmetric Lorentzian resonance to a strongly asymmetric Fano-like response. The results are explained by a phase-modified transmission model and are described using a weighted Fano-Lorentz formalism, which explains the resonance evolution in the (δ, η) parameter space. Among our devices, the optimum point is achieved at an intermediate perturbation rather than at the maximum Δϕ, yielding a loaded quality factor Q of 2.15 × 10⁵ and a slope rate of 435 dB/nm. These results show that compact air-hole perturbation can tune high-Q asymmetric resonances in Ge-OI mid-IR photonics, where the useful high-slope regime is determined by a balance among phase-induced asymmetry, linewidth preservation, coupling condition, and local transmission contrast.