Impedance Spectroscopy of Bifurcation Oscillations in S-Type Self-Oscillatory Devices

[EN] Self-sustained electronic oscillators are emerging as key building blocks for physical neuromorphic computing. Among them, S-type negative differential resistance (NDR) devices offer wide material versatility and strong tunability of their internal dynamics. While nonlinear theory predicts their stability regimes and bifurcations with high accuracy, experimental access to these regimes remains limited. Here, we combine theory and experiment to characterize the stability regimes of an S-type NDR thyristor device exhibiting self-sustained oscillations. By mapping nonlinear control-theoretic concepts onto impedance spectroscopy (IS) measurements, we show that frequency-domain characterization provides direct experimental signatures of stability, bifurcations, and oscillatory regimes, despite the intrinsically nonlinear nature of the system. The measurements further reveal the interaction between autonomous and forced oscillations, as well as entrainment and phase locking phenomena. Our results establish IS as a powerful in situ probe of nonlinear dynamical behavior in oscillatory devices, with direct relevance for neuromorphic and oscillator-based hardware.