Purely electronic zero phonon lines. Recent developments

The usefulness of zero-phonon lines (ZPL) in science and applications is pointed out. Two recent developments are considered: (1) Interference of a single photon emitted by a single impurity in solid host with (a) the excitation light; (b) itself. (2) The exciting consequencies for ZPLs in science and applications, following from the discovery of very low lying (about 3.5 eV) excited γ-level in isomer of 229Th. Quite possible that transition from it may produce luminescence (“nuclear light”) in blue. The possibility to create broad-band absorption–emission by means of Doppler shift and also burning of spectral holes is shown.