Subpicosecond accumulated photon echoes have been observed in 1,3,3,1',3',3'-hexamethyl-2,2'-indotricarbocyanine iodide embedded in polyvinyl alcohol at 10 K by using a cw multimode diode laser. The temporal resolution of this measurement is determined by the width of the overall spectral envelope of the diode laser. The dephasing time of 40 psec derived by the echo decay curve is in good agreement with the persistent hole width.
Using a superluminescent diode, we observed femtosecond accumulated photon echoes in 1, 1′-diethyl-4, 4′-quinodicarbocyanine iodide embedded in polyvinyl acetate. The output of the superluminescent diode directly excited the sample to produce photon echoes. By using a simple experimental system we obtained a temporal resolution as high as 100 fsec. This resolution is determined by the spectral width of the superluminescent diode and the noncollinear excitation beam configuration of the system.
Accumulated photon echoes were observed in
Nd3+-doped silicate glass by using coherent and incoherent light, and it was theoretically and experimentally verified that the time resolution is determined not by the pulse width but by the correlation time of the excitation field. Even by using a cw laser, instead of a mode-locked laser, a subpicosecond time resolution was obtained when it was operated with a broad and smooth spectrum. The broadest output spectrum, and hence the highest time resolution, was attained when the dye laser was pumped by a mode-locked
Ar+ laser, and their cavity lengths were mismatched by a few \ensuremath{\mu}m. In an experiment with a resolution time of 220 fsec, a new very fast decay was observed in the echo decay curve besides a slower decay already reported.
The time resolution of a heterodyne-detected accumulated photon echo has been improved significantly by using a dye laser whose lasing frequency is repetitively scanned. A time resolution of less than 30 fsec was obtained, even with a non-mode-locked cw dye laser. This technique is applicable provided that the absorption recovery time of the sample is longer than the repetition period of the frequency-scanning dye laser. The absorption recovery times of several dyes that were doped with polyvinyl alcohol have also been measured.
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.
Since the first observation of accumulated photon echoes by Hesslink and Wiersna [1], nearly transform-limited mode-locked pulse trains have been used as the excitation source, and the resolution times have been limited by the widths of the pulses. In this paper we show theoretically and experimentally that the time resolution of the accumulated photon echoes is determine not by the pulse width but by the correlation time of the excitation field [2]. This indicates that it is possible to obtain a sub-picosecond resolution in accumulated photon echoes Sven by using a cw laser (non-mode-locked), if the laser output has a sufficiently broad and smooth spectrum.
Accumulated photon echoes have been observed by use of a light-emitting diode. This is to our knowledge the first observation of photon echoes that are excited by a nonlaser light source. In the incoherent-light photon echo, the time resolution is equal to the inverse of the overall spectral width of the excitation light. Therefore we can easily get a high time resolution by using nonlaser light with a broad spectrum. Moreover the use of nonlaser light in the photon-echo experiment will extend the technique into wavelength regions where lasers are not currently available.