Enhanced absorption by linewidth narrowing in optically excited type-II semiconductor heterostructures

M. Stein, C. Lammers, M.J. Drexler, C. Fuchs, W. Stolz, M. Koch
Physical Review Letters 121 (2018) 017401

Following optical excitation excitonic lines can show reduced absorption, shifts and broadening associated with optical nonlinearities arising from the many-particle nature of the system. Yet, experimental results for the first time show the opposite effect, i.e., a surprising linewidth narrowing of the direct exitonic 1s heavy-hole transition in a type-II quantum well system. The excitonic resonance in the linear absorption is spectrally sharper in the excited sample than in the unexcited sample.

Quantum well (QW) based semiconductor heterostructures are ideal model systems to study the properties of low-dimensional carrier systems, explaining why the optical properties of these nanostructures have been extensively investigated over the last 30 years. At low temperatures excitonic resonances dominate the absorption spectrum. When optically excited these resonances typically broaden, shift and show a reduced absorption due to several many-particle effects including phase-space filling, band gap renormalisation, screening and excitation induced dephasing.

Differential absorption spectra from a type-II sample (right) and from a reference sample with a type-I band alignment (left), illustrating the temporal behavior of the pump-induced changes of the optical absorption. Blue color shades indicate an excitation-induced increase of the linear absorption.

The linear absorption (black line) of a type-II heterostructure with a 6 nm thick GaAs intermediate barrier together with its differential absorption (dark red line) and the sum of linear and differential absorption (red line). The unexcited sample already shows a distinct 1 s excitonic resonance in the optical absorption at 1.245 eV. Nevertheless, an optical excitation can lead to an even sharper and more pronounced excitonic absorption (in type-II heterostructures). The differential absorption signature shown here occurs 5 ns after the optical excitation. The inset contains a schematic of the sample’s band structure with the respective wave functions. Reprinted figure with permission.

New experiments, however, reveal a surprising linewidth narrowing of the direct exitonic 1s heavy-hole (hh) transition in a type-II quantum well system. Type-II semiconductor heterostructures are characterized by two adjacent but different QWs, often with a barrier between them. The energy levels in the conduction and valence band are designed such that for a particular photon energy only one QW is excited and that subsequently either electrons or holes find energetically more favorable states in the other QW and undergo a spatial charge transfer.

The linewidth narrowing, which does not occur in a type-I reference sample, is observed in an optical pump-optical probe experiment with femtosecond pulses and builds up on a pico- to nanosecond timescale, with the actual speed depending on experimental parameters such as excitation density, temperature and barrier width. A direct attribution of this effect to known physical mechanisms did not become evident. This lays the case for further experimental and theoretical investigations on other charge-transfer systems, such as donor-acceptor systems or heterojunction solar cells, where the spatial charge transfer through an interface is essential for their operation.