Efficient generation of Raman echo and time-domain optical data storage by electromagnetically induced transparency

We have observed excitation of spin echoes and spin free induction decay (FID) by electromagnetically induced transparency (EIT) in an optically dense solid sample. The experiments are done in a double-lambda system of 605.7 nm 3H4 - 1D2 transition of Pr3+:Y2SiO5, where the 10.2 MHz ground state spin coherence is excited by low-power resonant Raman pulses. It has been shown that the spin coherence, including spin echo, is equivalent to the transparent state of EIT, and therefore a high efficiency is expected for such resonant Raman-excited spin echo. The observed efficiency of spin echo is as high as 75% of the FID signal at 5 K. A background-free detection scheme is used based on EIT and enhanced nondegenerate four-wave mixing. The technique is applied in the frequency-selective time-domain optical data storage, that utilizes the spin as well as the optical inhomogeneous spectral widths. The data storage scheme is analogous to the stimulated spin echo with resonant Raman excitation of the spin coherence. We verify that the write window is determined by the spin T₂ which is much longer than the optical T₂, especially at higher temperature. We find that the spin dephasing time T₂ is almost constant at approximately 500 μs in the range of 2 to approximately 6 K, whereas the optical T₂ decreases rapidly, by a factor of approximately 50, above 4 K. These results will be useful in the development of high capacity time-domain optical data storage operating at higher temperature.