  Optical control of spin in semiconductors has been pioneered using
nanostructures of III-V and II-VI semiconductors, but the emergence of
two-dimensional van der Waals materials offers an alternative low-dimensional
platform for spintronic phenomena. Indium selenide (InSe), a group-III
monochalcogenide van der Waals material, has shown promise for opto-electronics
due to its high electron mobility, tunable direct bandgap, and quantum
transport. There are predictions of spin-dependent optical selection rules
suggesting potential for all-optical excitation and control of spin in a
two-dimensional layered material. Despite these predictions, layer-dependent
optical spin phenomena in InSe have yet to be explored. Here, we present
measurements of layer-dependent optical spin dynamics in few-layer and bulk
InSe. Polarized photoluminescence reveals layer-dependent optical orientation
of spin, thereby demonstrating the optical selection rules in few-layer InSe.
Spin dynamics are also studied in many-layer InSe using time-resolved Kerr
rotation spectroscopy. By applying out-of-plane and in-plane static magnetic
fields for polarized emission measurements and Kerr measurements, respectively,
the $g$-factor for InSe was extracted. Further investigations are done by
calculating precession values using a $\textbf{k} \cdot \textbf{p}$ model,
which is supported by \textit{ab-initio} density functional theory. Comparison
of predicted precession rates with experimental measurements highlights the
importance of excitonic effects in InSe for understanding spin dynamics.
Optical orientation of spin is an important prerequisite for opto-spintronic
phenomena and devices, and these first demonstrations of layer-dependent
optical excitation of spins in InSe lay the foundation for combining
layer-dependent spin properties with advantageous electronic properties found
in this material.
