Crossover from coherent to incoherent excitation of two-dimensional plasmons in $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ single quantum wells by femtosecond laser pulses

We have investigated the dynamics of two-dimensional (2D) free carrier plasma oscillations in selectively doped $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ single quantum wells by time-domain terahertz (THz) emission spectroscopy. Considering both excitation energy and power dependence of the 2D plasmon radiation intensity, we have found two distinct excitation mechanisms of the 2D plasmon; coherent impulsive stimulated Raman process and incoherent sudden heating of the electron system by photoexcited carriers. By monitoring the phase of the emitted THz radiation, a clear crossover from one mechanism to the other is observed as the pump photon energy goes through the absorption edge of the electron system.