A Single Charged Quantum Dot in a Strong Optical Field: Absorption,   Gain, and the AC Stark Effect
  We investigate a singly-charged quantum dot under a strong optical driving field by probing the system with a weak optical field. When the driving field is detuned from the trion transition, the probe absorption spectrum is shifted from the trion resonance as a consequence of the dynamic Stark effect. Simultaneously, a gain sideband is created, resulting from the coherent energy transfer between the optical fields through the quantum dot nonlinearity. As the pump detuning is moved from red to blue, we map out the anticrossing of these two spectral lines. The optical Bloch equations for a stationary two-level atom can be used to describe the numerous spectral features seen in this nano solid state system. 
Xiaodong Xu, Bo Sun, Erik D. Kim, Katherine Smirl, P. R. Berman, D. G.   Steel, A. S. Bracker, D. Gammon, and L. J. Sham