Enhancement of coherent longitudinal optical phonon oscillations in a $\mathrm{GaAs}∕\mathrm{AlAs}$ multiple quantum well due to intersubband energy tuning under an electric field

We have demonstrated the generation of intense coherent longitudinal optical (LO) phonon oscillations with use of intersubband energy tuning by application of electric field. The coherent LO phonon in a GaAs/AlAs multiple quantum well was investigated in detail as a function of electric field by using a reflection-type pump-probe technique. In the case that the intersubband energy is tuned to the LO phonon energy of GaAs by the electric field owing to a quantum confined Stark effect, the amplitude of the coherent LO phonon is intensively enhanced by a factor of 23 in comparison with the amplitude in a low electric field regime. This enhancement is caused by the coupling between the coherent LO phonon and the longitudinal polarization due to the impulsive interference between excitons with different subbands.

Figure 1

Image map of the PC spectra of the $\mathrm{Ga}\mathrm{As}\left(15.3\mathrm{nm}\right)∕\mathrm{Al}\mathrm{As}\left(4.5\mathrm{nm}\right)$ MQW at 10 K as a function of electric field, where the brightness is proportional to the PC intensity. The dashed curves show the transition energies calculated by using the TM method.

Figure 2

Time-resolved reflectivity changes of the $\mathrm{Ga}\mathrm{As}\left(15.3\mathrm{nm}\right)∕\mathrm{Al}\mathrm{As}\left(4.5\mathrm{nm}\right)$ MQW at various electric fields.

Figure 3

Fourier-transform spectra of the time-domain signals shown in Fig. 2 in the time range from 1.0 to 4.0 ps at various electric fields.

Figure 4

Fourier-transform intensity of the coherent GaAs-like LO phonon and $\Delta E_{\mathrm{HH}1-\mathrm{HH}2}$, which are indicated by open and closed circles, respectively, as a function of electric field. The dashed line indicates $E_{\mathrm{LO}}$ of GaAs.

Figure 5

Fourier-transform intensity of the coherent LO phonon at $155\mathrm{k}\mathrm{V}∕\mathrm{cm}$ as a function of pump energy. The dashed lines indicate the energies of the E2HH1 and E2HH2 excitons obtained from the PC spectra. The dotted curve indicates the pump-laser spectrum with the center energy of 1.573 eV.