Single Nanocrystal Anti-Stokes Shifted Superfluorescence

Superfluorescence (SF) describes a unique optical phenomenon of an ensemble of emitters, when the dipoles of emitters can couple collectively to produce a short but extremely intense burst of light. This quantum optical phenomenon is distinct from the traditional spontaneous photon emission from a single emitter. Such unusual coherent and cooperative spontaneous emission was predicted theoretically by Dicke in atoms confined in a volume of dimensions smaller than a wavelength, without the need for a laser cavity. In SF, an external radiative field excites an initially incoherent energy level. After this, the emitting dipoles establish coherence via dipole-dipole interactions to form a macroscopic dipole. The ensuing radiation combines normal fluorescent emission and an intense burst of coherent radiation, known as an SF pulse. In addition to being typically observed in rarefied impurities in existing materials, SF currently must take place at an ultra-low temperature ~ 6 K. Therefore, discovering and developing a new SF system under ambient conditions is both essential and something that has long been sought. Herein, we propose to investigate lanthanide-doped upconversion nanoparticles (UCNPs) as the media to realize SF under ambient conditions. In lanthanide ions, the emission stems from the electronic transitions in the 4f orbitals, which is shielded by the outer-lying fully-occupied 5s and 5p orbits, enabling the f orbital states to maintain an atomic-like character, and highly robust emissions that are not affected by the environment. Due to the aforementioned properties of the lanthanide ions, each can be considered as an individual emitter that can interact with each other through the radiation field to establish coherence and to enable this SF to take place. In our preliminary study, we demonstrated a paradigm-shifting upconverted cavity free SF in single nanocrystals under single wavelength NIR excitation and under ambient conditions. In stark contrast to previous reported SF, this is the first time that SF has been discovered in a single nanocrystal regime and it is the smallest-ever SF media. More importantly, UCNPs were excited by near-infrared (NIR) light and emit visible light; such SF is photon-upconverted compared to the excitation light. We observed UCNP SF with an extremely narrow spectral width (full-width at half-maximum, FWHM = 2 nm), and significantly shortened lifetime (t = 46 ns, 10000-fold accelerated radiative decay). The intrinsic nature of the rapid and intense, narrow spectral peaks in SF, are ideal for a wide variety of potential photonic and electronic applications, including lasing, on-chip integration in nanoelectronics and optical computing. We aim to investigate the effect of ion-clustering and tailored epitaxial nanostructure and dopant concentration on SF by combining single nanocrystal correlated structural and optical characterization. The major goal of this proposal is to optimize the performance of upconverted superfluorescence via systematically exploring upconverted SF at the single-particle level. This is based on our expertise in nanocrystal engineering, single-nanocrystal correlated high-resolution fluorescence spectroscopy with AFM/SEM, and plasmonic coupling. This can elucidate the concentration and temperature effects on photophysical mechanisms. The following three specific aims are proposed: (1) we will study the dopant concentration in diverse levels of ion-compacted crystal lattices, thereby gaining an understanding of the influence of ion-ion spacing on subsequent dipole self-organization, and on SF outcomes; 2) We will investigate the influence of temperature in controlling non-radiative pathways via ion-phonon dephasing processes, with single-particle, time-resolved SF measurements; and 3) We will investigate the distribution of Nd3+ ions and the nanocrystal structures present, and evidence of their selfassembly, with Raman spectrosco