Researchers in the US have created nanoparticles that emit pulses of superfluorescent light at room temperature. Unusually, the emitted light is anti-Stokes shifted, meaning that it has a shorter wavelength (and thus a higher energy) than the wavelength of light that initiates the response – a phenomenon known as upconversion. The new nanoparticles, which the team discovered while looking for a different optical effect, could make it possible to create new types of timers, sensors and transistors in optical circuits.
“Such intense and rapid emissions are perfect for numerous pioneering materials and nanomedicine platforms,” team leader Shuang Fang Lim of North Carolina State University tells Physics World. “For example, upconverted nanoparticles (UCNPs) have been widely employed in biological applications ranging from background-noise-free biosensing, precision nanomedicine and deep-tissue imaging, to cell biology, visual physiology and optogenetics.”
Shielding electron orbitals
Superfluorescence occurs when multiple atoms within a material simultaneously emit a short, intense burst of light. This quantum-optical phenomenon is distinct from isotropic spontaneous emission or normal fluorescence, is difficult to achieve at room temperature and tends not to last long enough to be useful. UCNPs, however, are different, says team member Gang Han of the University of Massachusetts Chan Medical School. “In a UCNP, the light is emitted from 4f electron transitions that are protected by higher-lying electron orbitals that act as a ‘shield’, allowing for superfluorescence even at room temperature,” Han explains.
In the new work, the team observed superfluorescence in ions that couple with each other within a single nanoparticle of neodymium-ion-compacted lanthanide-doped UCNPs. Unlike superfluorescence in other materials, such as highly ordered perovskite nanocrystals or semiconductor quantum dots assemblies that use each nanoparticle as an emitter, in lanthanide-doped UCNPs, each lanthanide ion in a single nanoparticle is an individual emitter. “This emitter can then interact with other lanthanide ions to establish coherence and allow for anti-Stokes-shift superfluorescence in both random nanoparticle assemblies and in single nanocrystals, which at just 50 nm in size are the smallest-ever superfluorescence media ever created,” Lim says.
Synchronization into a cohesive macroscopic state
“The superfluorescence comes from the macroscopic coordination of the emissive phases of the excited ions in the nanoparticle after the excitation energy is deposited,” adds team member Kory Green. “A laser pulse excites the ions within the nanoparticle and those states are not coherently organized at first.
“For superfluorescence to occur, that initially disorganized set of ions have to synchronize into a cohesive macroscopic state before emission. To facilitate this coordination the structure of the nanocrystal and the density of the neodymium ions has to be carefully selected.”
Upconverting nanoparticles allow mice to see in infrared
The discovery, which the team report in Nature Photonics, was made by chance while Lim and colleagues were trying to make materials that lase – that is, materials in which light emitted by one atom stimulates another to emit more of the same light. Instead, they observed superfluorescence, in which the initially unsynchronized atoms align, then emit light together.
“When we excited the material at different laser intensities, we found that it emits three pulses of superfluorescence at regular intervals for each excitation,” Lim says. “And the pulses don’t degrade – each pulse is 2 nanoseconds long. So not only does the UCNP exhibit superfluorescence at room temperature, it does so in a way that can be controlled. This means the crystals could be used as timers, neurosensors or optical transistors on photonic integrated circuits, for example.”