Fourier-transform spectroscopy using the technique of femtosecond accumulated photon echo with incoherent light has been performed in dye-doped polymers. Several types of interference effect or quantum beat between the zero-phonon line and other spectral components including the phonon sideband, the vibronic lines, and the photoproduct state have been observed. Based on the density matrix incorporating the transition-frequency modulation, it is pointed out that the factorization approximation for the relaxation function holds rigorously in the accumulated photon echo. We also clarified that the Fourier cosine transform of the heterodyne-detected echo signal provides the hole-burning spectrum.
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.
The effect of optical density on the heterodyne-detected accumulated photon echo is investigated both experimentally and theoretically. The cause of the distortion of the echo temporal behavior observed in the
4G25 manifold of Nd:YAG (where YAG is yttrium aluminum garnet) is attributed to the optical-density effect, which is closely related to the change of the field correlation function as the excitation pulse propagates through the sample.
A heterodyne-detected accumulated photon-echo spectroscopy has been applied for the first time to the study of the excitonic dephasing dynamics in CuCl spherical quantum dots embedded in a glass matrix; this spectroscopy has the advantage that a hole-burning spectrum can be extracted from the Fourier-cosine transformation of the signal profile. Owing to this advantage, we reveal that an anomalously slow rise of the time-integrated echo signal at low temperatures, which was also observed in the previous ordinary photon-echo spectroscopy but the origin was left unknown, is caused by a spectral antihole. From the decay (single exponential one) part of the signal which is found to be not influenced by this anomaly, we have obtained rich unambiguous information about the dephasing time T 2 such as the temperature dependence, which is found to be essentially the same as the previous one that one might cast some doubt on.
The stimulated photon echo memory using persistent hole-burning materials has been attracting much attention as a possible candidate for high-density data storage. Most studies have been related to the writing and reading processes and little attention has been paid for a partial erasing. We demonstrate for the first time that partial erasing of photon echo memory can be accomplished by simply employing a half-wave plate. In a photon echo memory, the write and data pulses with a time separation /spl tau/ excite a hole-burning material. These pulses create a periodic absorption modulation (periodic hole) with a frequency of l//spl tau/ in the absorption spectrum. When the read pulse excites the sample, a stimulated echo pulse is emitted at /spl tau/ after the read pulse, We note that a bit of data is stored as a periodic hole in the sample. A rotation of a half-wave plate inserted in the data pulse path (which induce a retardation of /spl lambda//2) creates two periodic holes that have the same frequency and opposite phase. Therefore, erasing a periodic hole can be done by rotating the wave plate. A dye (Sulforhodamin 640) doped polymer film (polyvinyl alcohol) cooled at 2 K was used as a persistent hole-burning material.