Simultaneous observation of coherent GaSb-like and AlSb-like longitudinal optical phonons in GaSb/AlSb superlattices

We have investigated coherent, longitudinal optical (LO) phonons; in GaSb/A lSb superlattices. The coherent, AlSb-like LO phonon in the barrier la,er a nd the coherent GaSb-like LO phonon in the well layer have been simultaneou sly, observed. The GaSb-like LO phonon exhibits a cosine-type oscillation, which is the dominant signal. We have analyzed the initial phase of the AlS b-like LO phonon by plotting the imaginary part of the Fourier-transform co mponents as a function of the real part, the so-called Cole-Cole's plot. We have found from this plot that the, difference in the initial phase betwee n the GaSb-like and AlSb-like LO phonons is about 90 degrees: The AISb-like LO phonon has a sine-type oscillation. This difference in the initial phas e indicates the difference in the generation mechanism between the coherent GaSb-like and AlSb-like LO phonons. We discuss the generation mechanism ba sed on some proposed models.