Abstract
In the absence of strong damping, the coupling between the GaAs exciton and the electromagnetic field leads to the creation of the exciton-polariton.1 The polariton dispersion curve has been studied by techniques such as resonant Brillouin scattering.2 But the longitudinal- transverse splitting (ΔLT) being very small in GaAs, so far the polariton had not been observed directly in the linear absorption spectrum; the fundamental resonance remaining essentially exciton-like even in high quality samples. We present here the first direct observation of the polariton signature in both absorption and phase spectra of very high quality bulk GaAs. Our experimental findings are explained by a full solution of the Maxell/ Semiconductor-Bloch Equations (M-SBE), which in particular shows that ΔLT and the exciton damping γ cannot be read off in a simple way from the absorption (or phase) spectrum. The sample studied is a high quality 0.25 µm thick GaAs layer clad between two AlGaAs layers. To allow transmission experiments, a GaAs substrate has been etched and the sample has been AR-coated before to be glued on a sapphire substrate. The strain induced by the sapphire substrate lifts the degeneracy between the light-hole (Ih) and heavy-hole (hh) exciton (Figure 1). In order to resolve the small, but important, features observed in the spectra, the sapphire substrate, which was creating small Fabry-Perot fringes, had to be AR-coated too. The system was immersed in superfluid He. The combined absorption- phase transmission measurements are performed using a spectral interferometry technique,3 which measures the interference between the transmitted beam and a reference 100 fs laser pulse.
© 1999 Optical Society of America
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