Evidence for excitonic polarons in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots

We investigate the interband transitions in several ensembles of self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots by using photoluminescence excitation spectroscopy under strong magnetic fields up to $28\mathrm{T}$. Well-defined resonances are observed in the spectra. The magnetic field dependence of the resonance energies allows an unambiguous assignment of the interband transitions which involve both discrete states of the quantum dots and wetting layer states. A strong anticrossing between two transitions is observed in all samples, which cannot be accounted for by a purely excitonic model. The coupling between the mixed exciton-longitudinal optical (LO) phonon states is calculated using the Fröhlich Hamiltonian. The excitonic polaron energies as well as the oscillator strengths of the interband transitions are determined. An anticrossing is predicted when two exciton-LO phonon states have close enough energies with phonon occupations which differ by one. A good agreement is found between the calculations and the experimental data evidencing the existence of excitonic polarons.

Figure 1

Normalized PLE spectra of the A3 sample taken at $4\mathrm{K}$ for $E_{\mathit{det}}=1194$, 1206, and $1215\mathrm{meV}$, where $\Delta E=E_{\mathit{exc}}-E_{\mathit{det}}$.

Figure 2

Magneto-PLE spectra of the A3 sample recorded at $4\mathrm{K}$ from $B=0$ to $28\mathrm{T}$ every $4\mathrm{T}$ and for a $E_{\mathit{dect}}=1215\mathrm{meV}$. Traces have been vertically offset for clarity. The dashed lines are guides for the eyes.

Figure 3

Magneto-PLE spectra of samples A1 (a), A2 (b), and A3 (c) at $4\mathrm{K}$ from $B=0$ to $28\mathrm{T}$ every $4\mathrm{T}$ and for a $E_{\mathit{dect}}=1215\mathrm{meV}$. The dashed lines are guides for the eyes.

Figure 4

Magnetic field dispersion of PLE resonances in sample A3 in open figures with the calculated energy transitions in solid lines (a) and a zoom (b) of the polaron states (1), (2), and (3), as labeled in Fig. 5. See Sec. 4 for details and parameters of the calculated fit.

Figure 5

Calculated excitonic polaron energies and intensities as a function of the magnetic field (a) and a zoom (b) of the anticrossing between polaron states (1) and (2). The solid lines in (b) correspond to noninteracting exciton-phonon states. The area of the circles is proportional to the oscillator strengths.

Figure 6

Experimental (full squares) and calculated (solid lines) spectra for different magnetic fields. The experimental data was taken for an $E_{\mathit{dect}}=1215\mathrm{meV}$ and for sample A3. The parameters used in the calculation are given in the text.