Coherent Control of Picosecond Acoustic Waves in a SESAM Structure
Résumé
Picosecond acoustic waves at ~365 GHz in a semiconductor saturable absorber mirror (SESAM) structure are optically controlled by femtosecond time-resolved spectroscopy. Picosecond acoustic waves are excited and detected in the In<sub>0.27</sub>Ga<sub>0.73</sub>As multiple quantum wells of the SESAM structure by femtosecond lasers operating at around ~1 �m wavelength. A second pump pulse with an adjustable time delay and intensity with respect to the first one, is used to exert control over the acoustic waves, which yields various acoustic patterns. The coherent acoustic spectral components can be manipulated in terms of enhancement, selective suppression, comb-spacing doubling, and tunable profiles. By the aid of a simplified model with identical generation and detection sensitivity functions in the quantum well stacks, the periodicity of modulation, the phase jump, the selection of suppressed modes, and the upper boundary in the time-delay or intensity-ratio dependent variations are well interpreted. The modelling agrees well with experimental results in a wide tunable range of the time delay and the intensity ratio. The results show promising applications in arbitrary acoustic wave synthesis and acoustic field filters. Additionally, a prism is inserted in the pump path to induce a subharmonic-like frequency component. By controlling the rotating angle of the prism, the amplitude of subharmonic components can be adjusted. The potential explanation of the induced acoustic change can be the excitation of subharmonic frequencies or the interference of acoustic waves induced by different pump spectral components with varied time delays.