Phase-amplitude spectroscopy of GaAs polariton

Summary form only given. In the absence of strong damping, the coupling between the GaAs exciton and the electromagnetic field leads to the creation of the exciton-polariton. The polariton dispersion curve has been studied by techniques such as resonant Brillouin scattering. But the longitudinal-transverse splitting (/spl Delta//sub LT/) 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 /spl Delta//sub LT/, and the exciton damping /spl gamma/, cannot be read off in a simple way from the absorption (or phase) spectrum.