Ellipsometric study of the Electronic Structure of GaMnAs and LT-GaAs
  We have measured the optical constants of GaMnAs from 0.62 eV to 6 eV, using spectroscopic ellipsometry. The second derivatives of the dielectric function are examined through a critical point analysis. The E1 critical point shifts to higher energies with increased doping of Mn, while all other critical points appear unaffected. The evolution of the critical points results from the interplay between band gap renormalization from ionized impurities and sp-d hybridization of the Mn induced impurity band and GaAs valence and conductions bands. 
K.S. Burch, J. Stephens, R.K. Kawakami, D.D. Awschalom, D.N. Basov