Benefitting from the strong intrinsic nonlinear optical
(NLO) property
of the individual porphyrin molecule, the integration of porphyrin
molecules into tightly aligned arrays may lead to intuitively promising
high-performance materials of tailorable NLO effect. In order to verify
this speculation, we prepare crystalline and highly oriented porphyrin-based
surface-supported metal–organic framework nanofilms (SURMOFs)
and then characterize their NLO performance. Results reveal that porphyrin-based
SURMOFs exhibit the highest saturable absorption (SA) yet recorded
with a third-order NLO absorption coefficient up to −10–3 cm/W, about 7 orders stronger than porphyrin solvents
in which the porphyrin molecules are disordered, under a certain excitation
strength. Further increasing the excitation strength shows that the
NLO absorption property of the porphyrin-based SURMOFs can be effectively
modulated from SA to reverse saturable absorption, followed by a reemerging
SA. The multiple-stage NLO switching is assigned to the interplay
of simultaneous one-photon SA, two-photon absorption, and two-photon
SA effects. The superior and modulatable NLO property as well as the
designable and ordered crystalline structure suggest that porphyrin-based
SURMOFs might be employed as a new class of high-performance NLO materials
with potential applications in novel optical switches or logic gates
to realize the all-optical information process.
Femtosecond hard X-ray pulses generated by laser-driven plasma sources are eminently suitable to probe structural dynamics due to the angstrom spatial resolution and sub-picosecond time resolution. However, the insufficient flux of X-ray photons and high pulse-to-pulse instability compared with the large-scale ultrashort X-ray source, such as X-ray free-electron laser and synchrotrons, largely restricts its applications. In this work, we have optimized automation control and mechanical designs to significantly enhance the reliability and photon flux in our femtosecond laser plasma-induced X-ray source. Specifically, the optimized source provides a reliable pulse-to-pulse stability with a fluctuation of less than 1% (root-mean-square) and a total flux of Cu-Kα X-ray photons above 1011 photons/s. To confirm its functionality, ultrafast X-ray diffraction experiments are conducted on two different samples and the high consistency with previous results verifies the system’s superior performance.
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