Optical Control at Low Excitation Density

The earliest control experiments were performed in double- (or multiple-) pump and probe scheme on optical phonons generated via ISRS in transparent materials by Nelson and coworkers [24,25], Shortly later, similar experiments were carried out on coherent phonons generated in semiconductors via TDFS by Dekorsy and coworkers [26], and on those generated in semimetals via DECP by Hase and coworkers [27] (Fig. 2.1 in the previous chapter). These experiments demonstrated that the amplitude of the coherent oscillation can be controlled by varying the temporal separation At between the two pump pulses. At = nT leads to the maximum enhancement of the amplitude with an integer n and the phonon period T, while At = (n + 1/2)T results in complete cancelation. 

Within the ISRS framework, the first impulse force kick-starts the coherent nuclear oscillation. At t = nT or (n + 1/2)T, the nuclei pass their equilibrium position, moving in the direction of the first force or in the opposite, respectively. The second ISRS force then pushes the nuclei to accelerate (Fig. 3.10a) or pulls them to a halt. Such coherent control via ISRS was demonstrated 

In the framework of DECP, the first pump pulse establishes a new potential surface, on which the nuclei start to move toward the new equilibrium. The nuclei gain momentum and reach the classical turning points of their motion at t = nT and t = (n + l/2)T. The second pump pulse then shifts the equilibrium position, either away from (Fig. 3.10b) or to the current position of the nuclei (Fig. 3.10c). The latter leads to a halt of the nuclear motion. Because photo-excitation of additional electrons can only shift the equilibrium position further in the same direction, the vibrations can only be stopped at their maximum displacement [32]. 

If the dephasing time of the coherent phonons depend critically on the carrier density, photo-injection of carriers with the second pump pulse can annihilate them partially or completely, depending on its fluence but not on its relative timing. Such incoherent control was demonstrated for the LO phonons of GaAs [37], 

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