text: Other techniques for writing data in three-dimensions have also been examined, including:
  Persistent spectral hole burning (PSHB), which also allows the possibility of spectral multiplexing to increase data density. However, PSHB media currently requires extremely low temperatures to be maintained in order to avoid data loss.
  Void formation, where microscopic bubbles are introduced into a media by high intensity laser irradiation.
  Chromophore poling, where the laser-induced reorientation of chromophores in the media structure leads to readable changes.