As far as I know, vacuum is the only dispersion free medium for electromagnetic waves. This makes me wonder if there are any other dispersion free media for these waves? (Experimentally established or theoreticaly predicted) If there are none for electromagnetic waves, are there any for other kinds of waves?
The Alfven wave, an ion oscillation with a frequency less than the ion cyclotron frequency, found in plasma oscillation is dispersion less. So, plasma can be said to be dispersionless for this wave.