Magnetic-field-dependent energy levels in a highly anisotropic electronic material

We report the direct observation of magnetic-field-dependent energy levels in an anisotropic two-dimensional electron gas. The experiment was carried out by far-infrared-reflectivity measurements on a well studied member of the Bechgaard salts, (TMTSF${)}_2$ ${\mathrm{ClO}}_4$, at low temperatures. We find two distinct bands above a threshold field and a number of unusual features which we interpret in terms of the underlying quantized field-induced spin-density-wave subphases. We also observe in the optical data details associated with anomalies in this material in high magnetic fields.