One of the primary lessons in optics is that optical gain in materials requires population inversion. This means that the number of electrons in the excited state should be greater than that in the ground state. Application of quantum interference effects in the matter–light interaction (quantum optics), however, has shown otherwise. That is, gain can also be generated in the absence of any population inversion. Such a process happens when specific coherent conditions are met, usually by coupling two electron levels to a third one using a strong laser beam.

A recent study conducted in the Nanophotonics and Quantum Devices group at the University of Alabama in Huntsville, US, combined quantum optics with plasmonics to investigate a novel way to generate gain in quantum dots (QDs). As the first step, the response of an isolated single QD to a probe field was studied when the QD was interacting with a laser beam. The results were basically the well known Mollow spectra (shown by the solid line in figure a).

In the next step, a metallic nanoparticle (MNP) was brought into the vicinity of such a system. As the distance between the QD and MNP was reduced, the initial effect was the enhancement of the field experienced by the QD (shown by the dashed line in figure a). When this distance reached a critical value the spectrum changed dramatically, forming a relatively strong gain peak (shown in figure b).

The physics behind these results has been related to the way the quantum interference effects in the QD are influenced by the plasmons and to the "molecular" nature of the QD-MNP system. In fact, when a QD and a MNP interact with a laser field the quantum coherence generated in the QD allows their combined system to act like a molecule (meta-molecule). This molecule has two main characteristic states (meta-states).

In one state, the QD feels significant amount of the electromagnetic field and its spectrum is as that in figure a. When such a molecule is transferred into the other state, the QD is obscured from the field and the spectrum becomes as that seen in figure b. Under this condition, no population inversion is formed in the QD.

One of the main aspects of this research is combining quantum optics with plasmonics. In terms of applications, the results suggest that QD-MNP systems can be considered as responsive optical nano-materials, wherein variation of the separating distance, refractive index of the environment and light intensity can lead to different optical responses via collective properties of such systems. These results may have impacts in lasers, amplifiers, switches and other applications of QDs.

More information can be found in the journal Nanotechnology.