Directional Super- and Sub-Fluorescence in Single Upconverting Nanocrystals at Room Temperature

Superfluorescence (SF) describes a remarkable optical phenomenon that occurs when an ensemble of emitters, with their dipoles collectively coupled, emits a brief yet extraordinarily intense burst of light. In SF, an external radiative field initially excites an incoherent energy level, and then the emitting dipoles synchronize their emissions through dipole-dipole interactions, forming a macroscopic dipole and establishing a coherent quantum state. These characteristics differentiate SF from processes such as lasing, superradiance (SR), and normal fluorescence, as SF arises from a single quantum transition involving N dipoles.

Our breakthrough observations in upconversion lanthanide doped microcrystals (UCNC) of a long-lived tail in the SF decay spectra, attributed to Subfluorescence (SbF), at room temperature, validates theoretical predictions of this anti-symmetric state. Furthermore, the delayed induced SbF is a delocalized hybrid atom photon state that partially traps photons, giving rise to possibilities in quantum batteries or photon storage. The capacity to control not only the fast SF mode but also the longer lived delocalized SbF mode, for example, by designing nanostructures that slow down emission propagation in undesired directions while speeding up emission propagation in allowed directions, can potentially creating a flying or travelling qubit, that enables information transmission between quantum networks.