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Despite its important role in understanding ultrafast spin dynamics and revealing novel spin/orbit effects, the mechanism of the terahertz (THz) emission from a single ferromagnetic nanofilm upon a femtosecond laser pump still remains elusive. Recent experiments have shown exotic symmetry, which is not expected from the routinely adopted mechanism of ultrafast demagnetization. Here, by developing a bidirectional pump-THz emission spectroscopy and associated symmetry analysis method, we set a benchmark for the experimental distinction of the THz emission induced by various mechanisms. Our results unambiguously unveil a new mechanismâanomalous Nernst effect (ANE) induced THz emission due to the ultrafast temperature gradient created by a femtosecond laser. Quantitative analysis shows that the THz emission exhibits interesting thickness dependence where different mechanisms dominate at different thickness ranges. Our work not only clarifies the origin of the ferromagnetic-based THz emission but also offers a fertile platform for investigating the ultrafast optomagnetism and THz spintronics.
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[This corrects the article DOI: 10.1021/acsphotonics.7b01402.].
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Magnon-polaritons are shown to play a dominant role in the propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in the vicinity of the quasi-antiferromagnetic spin resonance mode. Both time-domain THz transmission and emission spectroscopies reveal clear beatings between two modes with frequencies slightly above and slightly below this resonance, respectively. Rigorous modeling of the interaction between the spins of TmFeO3 and the THz light shows that the frequencies correspond to the upper and lower magnon-polariton branches. Our findings reveal the previously ignored importance of propagation effects and polaritons in such heavily debated areas as THz magnonics and THz spectroscopy of electromagnons. It also shows that future progress in these areas calls for an interdisciplinary approach at the interface between magnetism and photonics.
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The ultrafast energy transfer process, which takes place in femtosecond time range, in bacterial photosynthetic reaction center RS601 was investigated using femtosecond pump-probe technique with selective excitation. Upon 755 nmexcitation, the excited state of bacteriopheophytin H decayed to bacteriochlorophyll B with a time constant of about 130 fs, while the excited state of B transported the energy to its energy acceptor, the dimeric bacteriochlorophyll P, in about 240 fs with the 800 nm excitation. The internal conversion process between the upper and lower exciton levels of special pair P might exist upon the excitation of 850 nm pulses. In addition, from the results obtained in our experiments, the charge separation and electron transfer from P to the acceptor H was also observed via the real intermediate B within a few picoseconds.
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The charge transfer (CT) process in PAN-C60 copolymer films was studied by absorption and picosecond time-resolved photoluminescence (TRPL) spectra. The CT process can be confirmed from the appearing of new absorption bands and fluorescence quenching of pure PAN in the copolymer films. The TRPL result revealed that excitation state relaxation obeyed biexponential decay process, which were 160 ps and 1,500 ps for the fast and slow decay process, respectively. The fast decay process results from the interaction of chains in the polymer, and a slow decay process comes from the singlet excition recombination. In the copolymer, the singlet excition state lifetime of PAN becomes shorter due to CT process between PAN and C60, which lead to PL quenching of PAN. Additionally, C60 molecules also affect the interchain interaction of PAN by its spacial shape, which closely relates with the fast decay process.