Quasiperiodic Transitions to Chaos of Instabilities in an Electron-Hole Plasma Excited by ac Perturbations at One and at Two Frequencies

Periodic instabilities of frequency f₀ in an electron-hole plasma in Ge are excited with an external perturbation V₁ sin (2πf₁t). Varying V₁ and f₁ results in quasiperiodic, mode-locked, and chaotic states comparable to those expected theoretically. Increasing V₁ while keeping the ratio f₀f₁ fixed at the golden mean (1.6180...), we observe a universal transition to chaos, in quantitative agreement with theory. Driving the instability simultaneously at two incommensurate frequencies, f₁ and f₂, we observe transitions between three-frequency quasiperiodicity, two-frequency mode locking, and chaos.