Configuration interaction in a tunable wavelength shifter

It has recently been demonstrated how hybridized plasmon-phonon collective excitations in GaAs can be blue shifted by about 20 wavenumbers (0.6 THz) relative to the unperturbed longitudonal optical lattice vibration frequency as a function of excitation beam intensity [Fluegel et al., Nat. Phot. 1, 701 (2007)]. At beam intensities greater than 10 mW/cm$^{-2}$ the wavelengh shifted mode broadens and begins to exhibit a double-peak structure. We attribute this line-shape modification to configuration interaction of the named mode with the edge of the continuous background and discuss potential implications for coupled plasmon-phonon modes generated in semiconductor hetero-structures.