Three-level spaser for next-generation luminescent nanoprobe

The development of modern biological and medical science highly depends on advanced luminescent probes. Current probes typically have wide emission spectra of 30 to 100 nm, which limits the number of resolvable colors that are simultaneously labeled on samples. Spasers, the abbreviation for surface plasmon lasers, have ultranarrow lasing spectra by stimulated light amplification in the plasmon nanocavity. However, high threshold (>10² mJ cm^-2) and short lasing lifetime (approximately picoseconds to nanoseconds) still remain obstacles for current two-level spaser systems. We demonstrated a new type of a three-level spaser using triplet-state electrons. By prolonging the upper state lifetime and controlling the energy transfer, high gain compensation was generated. This probe, named delayed spasing dots (dsDs), about 50 to 60 nm in size, exhibited a spectral linewidth of ~3 nm, an ultralow threshold of ~1 mJ cm^-2, and a delayed lasing lifetime of ~10² μs. As the first experimental realization of the three-level spaser system, our results suggested a general strategy to tune the spasing threshold and dynamics by engineering the energy level of the gain medium and the energy transfer process. These dsDs have the potential to become new-generation luminescent probes for super-multiplex biological analysis without disturbance from short lifetime background emission.