  In spintronic devices, the two main approaches to actively control the
electrons' spin degree of freedom involve either static magnetic or electric
fields. An alternative avenue relies on the application of optical fields to
generate spin currents, which promises to bolster spin-device performance
allowing for significantly faster and more efficient spin logic. To date,
research has mainly focused on the optical injection of spin currents through
the photogalvanic effect, and little is known about the direct optical control
of the intrinsic spin splitting. Here, to explore the all-optical manipulation
of a material's spin properties, we consider the Rashba effect at a
semiconductor interface. The Rashba effect has long been a staple in the field
of spintronics owing to its superior tunability, which allows the observation
of fully spin-dependent phenomena, such as the spin-Hall effect, spin-charge
conversion, and spin-torque in semiconductor devices. In this work, by means of
time and angle-resolved photoemission spectroscopy (TR-ARPES), we demonstrate
that an ultrafast optical excitation can be used to manipulate the
Rashba-induced spin splitting of a two-dimensional electron gas (2DEG)
engineered at the surface of the topological insulator Bi$_{2}$Se$_{3}$. We
establish that light-induced photovoltage and charge carrier redistribution --
which in concert modulate the spin-orbit coupling strength on a sub-picosecond
timescale -- can offer an unprecedented platform for achieving all
optically-driven THz spin logic devices.
