Ultrahigh-resolution spectroscopy: Photon echoes in YAlO3: Pr3+ and LaF3: Pr3+

By using a new, delayed-heterodyne photon-echo technique, we have obtained a spectral resolving power of 1.2×1011, measuring a 2.0-kHz linewidth (half width at half maximum) for the 6105- D21H43 transition of YAlO3: Pr3+ in an external magnetic field. This appears to be the highest resolution ever obtained in optical spectroscopy. Measurements were also made on the corresponding transition of LaF3: Pr3+ giving a linewidth of 5 kHz. Optical free induction decay gave incorrect results (i.e., significantly broader linewidths). © 1979 The American Physical Society.