Spectral analysis of high-harmonic coherent acoustic phonons in piezoelectric semiconductor multiple quantum wells

Multiple harmonic components have been observed in the Fourier spectra of the coherent acoustic phonon oscillations in InGaN/GaN multiple quantum wells (MQW's). The dependence of the relative magnitudes of the first two Fourier components on the sample geometry was analyzed by conducting time-resolved pump-probe measurements on MQW samples with different well-to-barrier ratios. The results were also compared with the recently proposed loaded-string model [Phys. Rev. B 64, 235316 (2001)], and an overall agreement is obtained which validates the picture of string oscillation with inhomogeneous loadings. We also demonstrated manipulation of the spectral components using the coherent optical control technique. Our demonstration can be regarded as a first step to the realization of wave-form synthesis of nanoacoustic waves.