Ultrahigh-Resolution Spectroscopy: Photon Echoes in YAl${\mathrm{O}}_3$: ${\mathrm{Pr}}^{3+}$ and La${\mathrm{F}}_3$: ${\mathrm{Pr}}^{3+}$

By using a new, delayed-heterodyne photon-echo technique, we have obtained a spectral resolving power of 1.2×${10}^{11}$, measuring a 2.0-kHz linewidth (half width at half maximum) for the 6105-Å ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\leftrightarrow {}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ transition of YAl${\mathrm{O}}_3$: ${\mathrm{Pr}}^{3+}$ 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 La${\mathrm{F}}_3$: ${\mathrm{Pr}}^{3+}$ giving a linewidth of 5 kHz. Optical free induction decay gave incorrect results (i.e., significantly broader linewidths).