Strong Generation of Coherent Acoustic Phonons in Semiconductor Quantum Dots

We present experimental results obtained in two-color pump-probe experiments performed in semiconductor self-assembled quantum dot (QD) layers. The sample reflectivities present several acoustic contributions, among which are strong acoustic phonon wave packets. A comparison between one- and two-color experiments and a fine analysis of the echo shape attest that a high magnitude phonon pulse emerges from each single QD layer. This conclusion is supported by a numerical modeling which perfectly reproduces our experimental signals only if we introduce a strong generation in each QD layer. We explain such a strong emission thanks to an efficient capture of the carriers by the QDs.

Figure 1

(a) Schematic diagram of the experimental setup. Most experiments were carried out using an infrared pump and a blue probe. $\mathrm{BBO}=\beta \mathrm{\text{−}}{\mathrm{BaB}}_2{\mathrm{O}}_4$ crystal. The infrared pump significantly penetrates the sample, whereas the blue pulses only probe the sample surface. (b) Experimental result obtained using a two-color experiment with pulses centered at 812 nm on a two-QD layer sample. The most noticeable point is the structures localized at $T_1=70$ and $T_2=175\mathrm{ps}$.

Figure 2

(a) Experimental results obtained on samples $A$ and $B$ containing two and one QD layers, respectively, using various pump and probe colors. (b) Period of the oscillation detected at zero delay as a function of the probe wavelength. (c) Shape comparison between the three first acoustic echoes. One should note that the structures detected at $T_1$ and $3^{*}{T}_1$ can be perfectly superimposed contrary to the $2^{*}{T}_1$ one.

Figure 3

(a) Experimental result obtained on sample $A$ after substracting of the relaxation contribution. (b) Complete numerical result. (c) Numerical modeling of the acoustic contributions issued from the InP layers of the sample. The excellent agreement obtained between (a) and (b) requires a high generation process in the QD layer.

Figure 4

Experimental results obtained on sample $A$ at different laser wavelengths using a two-color pump-probe setup. For each wavelength, we also present a numerical fit.