Nature Commun. 5, 4705 (2014)

Credit: NPG

Superradiance refers to the collective emission of light from a group of N two-level systems at a rate that is proportional to N2, which is significantly higher than the rate predicted by classical physics. Although enhanced emission rates are closely linked to enhanced absorption rates due to time-reversal symmetry, the latter are much less likely to occur. Now, an international team from universities in the UK, Singapore and Australia have realized a number of analytical and numerical calculations to identify the conditions under which absorption dominates emission. The model system is a ring structure of N two-level systems, whose transition rates must be engineered so that the entire system's ladder of Dicke states can be considered as an effective two-level model. Enhancement of the transition rates at the effective central 'good' frequency is possible by engineering the spectral density or the occupation number of the modes. In fact, a large detuning between adjacent Dicke transitions can prove extremely useful for this purpose. As the authors propose such a scheme for applications in energy harvesting, they focus on defining an irreversible trapping process to extract only the photons at the good frequency. They also show that the system can be reset using a chirped laser pulse. The results can potentially be transposed in other systems that range from molecules to quantum dots, with interesting applications in optical or microwave sensing, solar cells and wireless power transfer.