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 $\left(\mathrm{WS}{\mathrm{e}}_2\right)$ relative to bulk $\mathrm{WS}{\mathrm{e}}_2$ shows substantial differences in the transient response as measured by time- and angle-resolved photoemission spectroscopy (TRARPES). The conduction-band minimum in bulk $\mathrm{WS}{\mathrm{e}}_2$, populated by optical pumping, decays promptly. The corresponding decay for SL $\mathrm{WS}{\mathrm{e}}_2$ 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.

• Received 15 February 2019
• Revised 9 November 2019

DOI:https://doi.org/10.1103/PhysRevB.100.214309