  We investigated transient optical responses in an optimally-doped high-Tc
superconductor La2-xSrxCuO4 (x=0.15) by using 800-nm optical pump and terahertz
probe spectroscopy. With increasing the photoexcitation intensities, the
Josephson plasma resonance shows a gradual redshift, indicating the suppression
of superconductivity by the photoexcitation. With further increasing the
photoexcitation intensities, a new longitudinal mode in the loss function
spectrum appears and grows from the high energy side, accompanied by a new
transverse mode as manifested in the conductivity spectrum. The observed
spectra are described by the multilayer model with alternating interlayer
Josephson couplings. The new longitudinal and transverse modes sustain much
longer than several hundred picoseconds after the photoexcitation, indicating
that the new metastable phase with possessing alternating interlayer Josephson
couplings is induced by the strong photoexcitation.
