Localization of two-dimensional exciton polaritons

The grating-coupler-induced emission and excitation of quantum-well-exciton polaritons (QWEP's) in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells (QW's) and quantum-well wires (QWW's) has been studied by photoluminescence (PL) and reflection spectroscopy. The QWEP excitations propagating perpendicular to the grating coupler are observed in PL on the high-energy side of the ground-state heavy-hole exciton transition as an additional strongly polarized signal with the electric field perpendicular to the grating direction. In the QWW the QWEP's show, down to wire widths of ${\mathit{L}}_{\mathit{x}}$=150 nm, a nearly free behavior, i.e., very similar to QWEP's in QW systems. When decreasing the wire width below ${\mathit{L}}_{\mathit{x}}$\ensuremath{\approxeq}150 nm the polariton-specific features disappear in the spectra, which we attribute to the localization of the QWEP's. Below ${\mathit{L}}_{\mathit{x}}$=100 nm, the concept of quasi-one-dimensional excitons is an appropriate model to describe the behavior of the observed transitions.