Indefinite and bidirectional near-infrared nanocrystal photoswitching

A Generalized Approach to Photon Avalanche Upconversion in Luminescent Nanocrystals

“…Here, we demonstrate how PA originating from Tm 3+ ions can be harnessed together with EMU through Gd 3+ ions to create a library of spectrally discrete ANPs with low avalanching thresholds and highly nonlinear emissions (Figure ). We posited that reducing the number of avalanching Tm 3+ ions (<500) in small NaGdF 4 core nanoparticles (<7 nm) would accelerate saturation of the 3 H 4 state and facilitate population of higher-energy levels of Tm 3+ ( 1 D 2 and 1 I 6 ). , Subsequently, energy from these excited Tm 3+ ions can be transferred to Gd 3+ ions in the host matrix and relayed to any activator lanthanide ion (A 3+ ) in a nanocrystal. Because the energy of Gd 3+ ions ( 6 P J ) is transferred from Tm 3+ ions excited by multiple stages of PA, the high nonlinearity of PA can be preserved in the emission of A 3+ ions (Figure b).…”

“…24 The PA power threshold increases when energy can be directly passed between Tm 3+ and A 3+ ions (Figure 3b) and is highly sensitive to surface quenchers. 23 Additionally, powers above 100 kW cm −2 can lead to photodarkening of Tm 3+ -based ANPs, 8 which quenches emission during measurements and shifts the PA threshold to higher powers. Notwithstanding the power threshold differences, we can generalize that EMU spectral tuning can be adopted in ANPs for generating highly nonlinear emission (s > 10) from virtually any activator ion by the Tm 3+ → Gd 3+ → A 3+ energy transfer cascade (Figure 1b; the mechanism of Tm 3+ excitation to 1 I 6 was informed by numerical simulations; see Section S9 in the Supporting Information for details).…”

“…Photon avalanching (PA) nanoparticles exhibit giant optical nonlinearities in their upconverted emission such that a 10–20% increase in near-infrared (NIR) excitation power can result in a 100–1000-fold increase in the emission intensity . PA behavior has been recently reported at room temperature in lanthanide (Ln 3+ )-doped nanocrystals of NaYF 4 :Tm 3+ , LiYF 4 :Tm 3+ , KPb 2 Cl 5 :Nd 3+ , and NaYF 4 :Yb 3+ , Pr 3+ . − Their highly nonlinear responses, equivalent in some cases to those of >30-photon processes, have been utilized for nanoscale thermometers, for molecular rulers, and, most saliently, in subdiffraction microscopy − to achieve optical resolutions finer than 100 nm using a conventional laser scanning microscope . However, these early avalanching nanoparticles (ANPs) mostly emit in the NIR region (i.e., 800 nm), which limits the ability to probe multiple species independently at different visible wavelengths.…”

Energy transfer and charge transfer between semiconducting nanocrystals and transition metal dichalcogenide monolayers

“…162,163 In another vein, new UCNPs that support the photon avalanching process have recently shown photoswitching capabilities. 164,165 By coupling to nanoparticles such as these, carbon dots 166,167 or other NCs supporting photomodulated behavior, active nanoscale photoswitchable functionality can now be added to TMD exciton emission, enabling new capabilities in van der Waals photonic circuits including neuromorphic computing and data storage. Another possibility is to explore NC hybridization with other 2D materials that possess unique characteristics.…”

Lanthanide-Doped KMgF3 Upconversion Nanoparticles for Photon Avalanche Luminescence with Giant Nonlinearities

“…Lanthanide (Ln 3+ )-doped upconversion nanoparticles (UCNPs) have attracted a tremendous amount of interest in a wide array of research fields because they can highly efficiently convert low-energy near-infrared (NIR) irradiation into high-energy radiation in the ultraviolet, visible, and NIR spectral regions. − Various mechanisms have been established for the upconversion luminescence (UCL) of lanthanides, including excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative sensitization upconversion (CSU), energy migration upconversion (EMU), and photon avalanche (PA). − Among these mechanisms, PA is considered to be the most efficient because of the positive-feedback cycles of ESA and cross-relaxation (CR), which boost the upconversion efficiency with large nonlinearities of up to tens of orders. − PA nanoparticles hold great promise in many frontier applications such as miniaturized lasers, optical computing, super-resolution bioimaging, single-molecule tracking, and quantum optics. − However, realizing PA in colloidal Ln 3+ -doped UCNPs with giant nonlinearities remains notoriously difficult due to the serious energy loss associated with nonradiative relaxation and the surface quenching effect, − despite the fact that PA of lanthanides has been frequently reported in bulk materials and at low temperatures. − …”