Is the Hofstadter energy spectrum observable in far-infrared absorption?
  The far-infrared absorption of a periodically modulated two-dimensional electron gas in a perpendicular constant magnetic field is calculated self-consistently within the Hartree approximation. For vanishing modulation the magnetoplasmon dispersion shows simple anticrossings with harmonics of the cyclotron resonance, as expected. For increasing modulation we identify intra- and intersubband magnetoplasmon modes charaterized by the number of flux quanta through each unit cell of the periodic potential. 
Vidar Gudmundsson and Rolf R. Gerhardts