  Using an ultrafast laser pulse to manipulate the spin degree of freedom has
broad technological appeal. It allows one to control the spin dynamics on a
femtosecond time scale. The discipline, commonly called femtomagnetism, started
with the pioneering experiment by Beaurepaire and coworkers in 1996, who showed
subpicosecond demagnetization occurs in magnetic Ni thin films. This finding
has motivated extensive research worldwide. All-optical helicity-dependent spin
switching (AOS) represents a new frontier in femtomagnetism, where a single
ultrafast laser pulse can permanently switch spin without any assistance from a
magnetic field. This review summarizes some of the crucial aspects of this new
discipline: key experimental findings, leading mechanisms, controversial
issues, and possible future directions. The emphasis is on our latest
investigation. We first develop the all-optical spin switching rule that
determines how the switchability depends on the light helicity. This rule
allows one to understand microscopically how the spin is reversed and why the
circularly polarized light appears more powerful than the linearly polarized
light. Then we invoke our latest spin-orbit coupled harmonic oscillator model
to simulate single spin reversal. We consider both cw excitation and pulsed
laser excitation. The results are in a good agreement with the experimental
result. We then extend the code to include the exchange interaction among
different spin sites. We show where the "inverse Faraday field" comes from and
how the laser affects the spin reversal nonlinearly. Our hope is that this
review will motivate new experimental and theoretical investigations and
discussions.
