page_title: Upconverting nanoparticles
section_title: Physics
hierarchical_section_title: Upconverting nanoparticles / Physics
caption_attribution_description: English: Optical luminescence emission spectra of up-conversion nano-particles illuminated at 980 nm. Nanoparticles are a cubic NaYF4 lattice, with 20% Yb, 2% Er dopants.
context_page_description: Upconverting nanoparticles are nanoscale particles that exhibit photon upconversion. In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy. Generally, absorption occurs in the infrared, while emission occurs in the visible or ultraviolet regions of the electromagnetic spectrum. UCNPs are usually composed of rare-earth based lanthanide- or actinide-doped transition metals and are of particular interest for their applications in in vivo bio-imaging, bio-sensing, and nanomedicine because of their highly efficient cellular uptake and high optical penetrating power with little background noise in the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials.
  Before 1959, the anti-Stokes shift was believed to describe all situations in which emitted photons have higher energies than the corresponding incident photons. An anti-Stokes shift occurs when a thermally excited ground state is electronically excited, leading to a shift of only a few kBT, where kB is the Boltzmann constant, and T is temperature.
context_section_description: Photon upconversion belongs to a larger class of processes by which light incident on a material induces anti-Stokes emission. Multiple quanta of energy such as photons or phonons are absorbed, and a single photon with the summed energy is emitted. It is important to make the distinction between photon upconversion, where real metastable excited states allow for sequential absorption, and other nonlinear processes like second-harmonic generation or two-photon excited fluorescence which involve virtual intermediate states such as the "simultaneous" absorption of two or more photons. It is also distinct from more weakly anti-Stokes processes like thermoluminescence or anti-Stokes Raman emission, which are due to initial thermal population of low-lying excited states and consequently show emission energies only a few kBT above the excitation. Photon upconversion is distinctly characterized by emission-excitation differences of 10–100 kBT.
  Photon upconversion relies on metastable states to facilitate sequential energy absorption.  Therefore,  a necessary condition for upconverting systems is the existence of optically active long-lived excited states. This role is traditionally filled by lanthanide metal ions embedded in an insulating host lattice. Generally in the +3 oxidation state, these ions have 4fn electronic configurations and typically exhibit f-f transitions. These 4f orbitals allow for complex electronic structures and a large number of possible electronic excited states with similar energies. When embedded in bulk crystals or nanostructures, the energies of these excited states will further split under crystal field, generating a series of states with many closely spaced energies. The 4f shell is localized near the core of the ion and is therefore non-bonding, while the 5s and 5p shells provide further shielding from the exterior crystal field. Thus, the coupling of electronic excited states to the surrounding lattice is weak, leading to long excited state lifetimes and sharp optical lineshapes.
  The physical processes responsible for upconversion in nanoparticles are the same as those in bulk crystals on the microscopic level, although total efficiency and other ensemble effects will have unique considerations in the nanoparticle case. The processes contributing to upconversion may be grouped according to the number of ions involved. The two most common processes by which upconversion can occur in lanthanide-doped nanoscale materials are excited state absorption (ESA) and energy transfer upconversion (ETU).
  A single ion in the lattice sequentially absorbs two photons and emits a photon of higher energy as it returns to the ground state. ESA is most common when dopant concentrations are low and energy-transfer is not probable. Since ESA is a process where two photons must be absorbed at a single lattice site, coherent pumping and high intensity are much more important (but not necessarily required) than for ETU. Because of its single-ion nature, ESA does not depend on the lanthanide ion concentration. 
  Two-ion processes are usually dominated by energy transfer upconversion (ETU). This is characterized by the successive transfer of energy from singly excited ions (sensitizers/donors), to the ion which eventually emits (activators/acceptors). This process is commonly portrayed as the optical excitation of the activator followed by further excitation to the final fluorescing state due to energy transfer from a sensitizer. While this depiction is valid, the more strongly contributing process is the sequential excitation of the activator by two or more different sensitizer ions.
  The upconversion process is said to be cooperative when there are one or more elementary steps (sensitization or luminescence) in the process which involve multiple lanthanide ions. In cooperative sensitization process, two ions in their excited state simultaneously decay to their ground states, generating a higher energy photon. Similarly, in cooperative luminescence, two excited state ions transfer their energy to a neighboring ion in one elem
caption: Luminescence spectra of cubic NaYF4:Yb,Er upconversion nanoparticles illuminated at 980 nm.
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