Maser Oscillation From Electronic Spin Resonance in a Cryogenic Sapphire Frequency Standard

We report the first observation of an Fe 3+ maser oscillation at zero magnetic field inside a whispering gallery (WG) sapphire resonator. The described maser is new in that it operates at zero-field and with low ion concentration. At zero-field, the Fe 3+ ion shows a 3-level structure related to the electron spin resonance (ESR). By applying a 31 GHz pump (|1/2〉 → |5/2〉), the ion operates as a maser at 12 GHz (|5/2〉 → |3/2〉). The maser effect is made possible by the high Q-factor (several 10 8 ) of the cryogenic whispering gallery resonator. Additionnaly, the sharp cavity resonance provides short term stability. Preliminary measurements indicate a frequency stability of parts in 10 -14 (Allan deviation at 100 s), still limited by the instrument. The ultimate maser stability is still unknown.