  The photo-induced band structure variation of a rare-earth-based
semiconductor, samarium monosulfide (SmS), was investigated using
high-harmonic-generation laser-based time-resolved photoelectron spectroscopy.
A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The
first one is a shift to the high-binding-energy side due to a large surface
photovoltage (SPV) effect of approximately 93 meV, comparable to the size of
the bulk band gap, with a much longer relaxation time than 0.1 ms. The second
one is an ultrafast band shift to the low binding energy side, which is in the
opposite direction to the SPV shift, suggesting an ultrafast valence transition
from divalent to trivalent Sm ions due to photo-excitation. The latter energy
shift was approximately 58 meV, which is consistent with the energy gap shift
from ambient pressure to the boundary between the black insulator and golden
metallic phase with the application of pressure. This suggests that the
photo-induced valence transition can reach the phase boundary, but other
effects are necessary to realize the golden metallic phase.
