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 or multiple pulses 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. a 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.

We report a group of unusually big molecular orbitals in the C60/pentacene complex. Our firstprinciples density functional calculation shows that these orbitals are very delocalized and cover both C60 and pentacene, which we call superintermolecular orbitals or SIMOs. Their spatial extension can reach 1 nm or larger. Optically, SIMOs are dark. Different from ordinary unoccupied molecular orbitals, SIMOs have a very weak Coulomb and exchange interaction. Their energy levels are very similar to the native superatomic molecular orbitals in C60, and can be approximately characterized by orbital angular momentum quantum numbers. They have a distinctive spatial preference. These features fit the key characters of charge-generation states that channel initially-bound electrons and holes into free charge carriers. Thus, our finding is important for C60/pentacene photovoltaics.