  Understanding carrier creation and evolution in materials initiated by pulsed
optical excitation is central to developing ultrafast optoelectronics. We
demonstrate herein that the dynamic response of a system can be drastically
modified when its physical dimension is reduced to the atomic scale, the
ultimate limit of device miniaturization. A comparative study of single-layer
(SL) tungsten diselenide(WSe2) relative to bulk WSe2 shows substantial
differences in the transient response as measured by time- and angle-resolved
photoemission spectroscopy (TRARPES). The conduction-band minimum in bulk WSe2,
populated by optical pumping, decays promptly. The corresponding decay for SL
WSe2 is much slower and exhibits two time constants. The results indicate the
presence of two distinct decay channels in the SL that are correlated with the
breaking of space inversion symmetry in the two-dimensional limit. This
symmetry breaking lifts the spin degeneracy of the bands, which in turn causes
the blockage of decay for one spin channel. The stark contrast between the
single layer and the bulk illustrates the basic carrier scattering processes
operating at different timescales that can be substantially modified by
dimensional and symmetry-reduction effects.
