AC field driven population inversion

Non-equilibrium physics can offer much richer phenomena than equilibrium physics. One such example is the population inversion: high energy states are occupied while the low energy ones become emptied, effectively a negative temperature phenomena. It is expected that, many exotic things might be explored with this. For instance, bandwidth vanishing. A new paper reports how to achieve this using non-adiabatic switching of ac field [PRL 106, 236401 (2011)].

We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We
numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.