An analytic theory and experimental studies of the accumulated photon echo is presented. We define a temporal correlation function
G~ of the excitation field E ( t ) as
G~(τ,t)=(1/T1)∫0∞dt′E(t−t′)E(t−t′−τ)⋅exp(−t′/T1), where T 1 is the population relaxation time of the sample, and call the width of
G~ with respect to τ a temporal correlation time
τ~c(t). It is proved that the echo intensity is written by the temporal correlation functions when (1) T 1 and the duration of the excitation light τ p are much longer than
τ~c(t) in order to make the function
G~((τ,t) well-defined, (2) T 1 and τ p are much longer than the homogeneous dephasing time T 2 , and (3) one can find a time interval Δ t such that
G~(τ,t) does not change much between t and t + Δ t , where
Δt≫τ~c. Then we can show that the time resolution of the accumulated photon echo is determined by the averaged
τ~c(t) over an interval of the integration of the echo signal. This discussion is valid for any excitation source if the above conditions are satisfied, and applies to almost all cases of the practical accumulated photon echo experiment. A discussion is also made on the spurious spike on the echo decay curve near zero delay time that appeared in an experiment with a high time resolution in Nd 3+ -doped silicate glass. It is shown that the most probable cause for this spike is uncorrelated inhomogeneous broadenings of the resonant lines of neodymium ions.
By using stochastic theory, we analyze the non-Markovian temporal behavior in four types of photon echoes, including the two-pulse echo, stimulated echo, accumulated echo, and heterodyne-detected accumulated echo. In particular, the origin of the exp(-
ct3) dependence of the two-pulse echo in the limit of slow modulation is discussed in detail. It is also pointed out that, at the slow-modulation limit, the temporal behavior of the stimulated echo changes from exp(-
ct3) dependence to exp(-
ct2) dependence as the waiting time (the time interval between the second and the third excitation pulses) is varied.
Photon-echo-relaxation measurements made on the
3H4${\mathrm{\ensuremath{-}}}^{3}$
P0 transition of 0.01 at. %
Pr3+:YAG (where YAG represents yttrium aluminum garnet),
3H4${\mathrm{\ensuremath{-}}}^{1}$
D2 transition in 0.1 at. %
Pr3+:
YAlO3, and
7F0${\mathrm{\ensuremath{-}}}^{5}$
D0 transition in 0.25 at. %
Eu3+:
YAlO3 show that the photon-echo relaxation rate increases when the intensities of the excitation pulses are increased. Although a part of the relaxation-rate increase in
Pr3+:YAG may be attributed to an instantaneous spectral diffusion (ISD) in which the presence of excited neighboring
Pr3+ ions change the local field and the absorption frequency of the rare-earth ions, our data deviate significantly from the ISD-model predictions. An additional intensity-dependent relaxation mechanism is required to explain the results.
It has been generally accepted that the off-resonance heterodyne optical field in Stark-switching and frequency-switching techniques does not have any measurable influence in photon echo and other related experiments. In this work we present theoretical and experimental evidence that through a previously unknown memory effect the heterodyne field produces important undulations in the decay curves of photon echo. This effect appears to be also responsible for other experimental anomalies observed in optical coherent transient effects.1