  Information technology depends on how one can control and manipulate signals
accurately and quickly. Transistors are at the core of modern technology and
are based on electron charges. But as the device dimension shrinks, heating
becomes a major problem. The spintronics explores the spin degree of electrons
and thus bypasses the heat, at least in principle. For this reason, spin-based
technology offers a possible solution. In this review, we survey some of latest
developments in all-optical switching (AOS), where ultrafast laser pulses are
able to reverse spins from one direction to the other deterministically. But
AOS only occurs in a special group of magnetic samples and within a narrow
window of laser parameters. Some samples need multiple pulses to switch spins,
while others need a single-shot pulse. To this end, there are several models
available, but the underlying mechanism is still under debate. This review is
different from other prior reviews in two aspects. First, we sacrifice the
completeness of reviewing existing studies, while focusing on a limited set of
experimental results that are highly reproducible in different labs and provide
actual switched magnetic domain images. Second, we extract the common features
from existing experiments that are critical to AOS, without favoring a
particular switching mechanism. We emphasize that given the limited
experimental data, it is really premature to identify a unified mechanism. We
compare these features with our own model prediction, without resorting to a
phenomenological scheme. We hope that this review serves the broad readership
well.
