Polariton propagation in shallow-confinement heterostructures: Microscopic theory and experiment showing the breakdown of the dead-layer concept

Polariton effects due to the interplay of the excitonic polarization of a semiconductor and a propagating light field are studied for transmission spectra of $\mathrm{Zn}\mathrm{Se}∕\mathrm{Zn}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ heterostructures. Calculations in terms of microscopic boundary conditions for the exciton motion within a finite-height confinement potential can explain the measured transmission spectra. These calculations also show the absence of polarization-free regions near the sample interfaces. Macroscopic models based on Pekar’s additional boundary conditions can only reproduce the spectra if the band alignment at the $\mathrm{Zn}\mathrm{Se}∕\mathrm{Zn}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ interfaces is modified in comparison to the microscopic calculation and if a sample thickness is used that exceeds the independently determined experimental value. Our findings demonstrate the breakdown of the dead-layer concept for shallow confinement potentials.

Figure 1

Illustration of the considered semiconductor heterostructure in a slab geometry. Conduction band (cb) and heavy-hole valence band (vb) alignments are visualized. The system homogeneously extends in the $x\text{−}y$ plane and has a finite thickness $\mid z_2-z_1\mid$ in the $z$ direction. For more details see text.

Figure 2

Transmission spectra for the (a) $20\mathrm{nm}$, (b) $28\mathrm{nm}$, (c) $40\mathrm{nm}$ sample. Dashed line: Experiment. Solid line: Microscopic theory.

Figure 3

Transmission spectra for the (a) $20\mathrm{nm}$, (b) $28\mathrm{nm}$, and (c) $40\mathrm{nm}$ sample without Fabry-Perot effects. Solid line: Microscopic theory, $d_{eh}∕e_0=3.7\mathrm{\AA }$. Dotted line: Pekar’s ABCs, $d_{eh}∕e_0=2.89\mathrm{\AA }$. Dashed-dotted line: Pekar’s ABCs, $d_{eh}∕e_0=3.7\mathrm{\AA }$.

Figure 4

Steady-state polarization for monochromatic, resonant excitation of the three lowest polariton modes in the microscopic theory without Fabry-Perot effects for the (a) $20\mathrm{nm}$, (b) $28\mathrm{nm}$, and (c) $40\mathrm{nm}$ sample. The excitation frequency is tuned to the first (dotted line), second (dashed-dotted line), or third (dashed line) resonance, respectively. The solid lines illustrate the confinement potential for carriers in the ZnSe layer.