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 $f_0$ in an electron-hole plasma in Ge are excited with an external perturbation $V_1$ sin ($2\pi f_{1}t$). Varying $V_1$ and $f_1$ results in quasiperiodic, mode-locked, and chaotic states comparable to those expected theoretically. Increasing $V_1$ while keeping the ratio $\frac{f_0}{f_1}$ 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, $f_1$ and $f_2$, we observe transitions between three-frequency quasiperiodicity, two-frequency mode locking, and chaos.