Before the 1960s, all anti-Stokes emissions, which were known to exist, involved emission energies in excess of excitation energies by only a few kT. They were linked to thermal population of energy states above excitation states by such an energy amount. It was the well-known case of anti-Stokes emission for the so-called thermal bands or in the Raman effect for the well-known anti-Stokes sidebands. Thermoluminescence, where traps are emptied by excitation energies of the order of kT, also constituted a field of anti-Stokes emission of its own. Superexcitation, i.e., raising an already excited electron to an even higher level by excited-state absorption (ESA), was also known but with very weak emissions. These types of well-known anti-Stokes processes have been reviewed in classical textbooks on luminescence.1 All fluorescence light emitters usually follow the well-known principle of the Stokes law which simply states that excitation photons are at a higher energy than emitted ones or, in other words, that output photon energy is weaker than input photon energy. This, in a sense, is an indirect statement that efficiency cannot be larger than 1. This principle is

Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Lanthanide ions possess fascinating optical properties and their discovery, first industrial uses and present high technological applications are largely governed by their interaction with light. Lighting devices (economical luminescent lamps, light emitting diodes), television and computer displays, optical fibres, optical amplifiers, lasers, as well as responsive luminescent stains for biomedical analysis, medical diagnosis, and cell imaging rely heavily on lanthanide ions. This critical review has been tailored for a broad audience of chemists, biochemists and materials scientists; the basics of lanthanide photophysics are highlighted together with the synthetic strategies used to insert these ions into mono- and polymetallic molecular edifices. Recent advances in NIR-emitting materials, including liquid crystals, and in the control of luminescent properties in polymetallic assemblies are also presented. (210 references.)

Taking advantage of luminescent lanthanide ions

Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) “soft” luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).

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Lanthanide luminescence for functional materials and bio-sciences

Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.

Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping

Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.

Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals

Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion

The expected excitation power dependencies for any upconversion emission band of an acceptor ion is investigated theoretically when the excitation takes place on a sensitizer ion and subsequent energy transfer upconversion from the sensitizer to the acceptor ion is exclusively responsible for the excitation of the acceptor ion. Under these limitations it is shown that emission from a state that requires $k$ energy transfer upconversion steps will have a slope of $k$ in the low-power regime when the luminescence intensity is plotted in a double-logarithmic representation versus absorbed pump intensity. In the high-power regime, any emission band will show a slope of 1, irrespective of the number of energy transfer steps from the sensitizer to the acceptor ions that are involved. The theoretical results are verified experimentally by data on three different inorganic systems with different types of sensitizer and acceptor ions: rare earth (RE) ions as well as transition metal (TM) ions. The active ions in the systems that are studied experimentally are RE/RE, RE/TM, and TM/TM, where the first dopant indicates the sensitizer ion and the second dopant indicates the upconverting ion. These different classes of sensitizer and upconverter ions all agree with the theoretical predictions put forward by the model. Thus providing confidence in the applicability (within the boundary conditions put forward here) of the model described.

Anomalous power dependence of sensitized upconversion luminescence

Broad-band Cr5+-sensitized Er3+ luminescence in YVO4

Tm2+ doped in CsCaI3 displays unusual optical properties that are characterized by the existence of two metastable 4f−5d excited states in the near-infrared and visible spectral region, respectively. For the first time, a photon upconversion process based on sequential absorption of light by 4f−5d states is reported. The large absorption cross-section of the involved transitions allows highly efficient pumping in the NIR. An efficiency of 11% for the green upconversion luminescence is reached at 10 K, and the upconversion luminescence remains visible by eye up to room temperature. The energy positions of the relevant 4f−5d states and thus the photophysical and light emission properties can be tuned by chemical variation, such as placing the Tm2+ ion into the isostructural CsCaBr3 and CsCaCl3 lattices.

New type of near-infrared to visible photon upconversion in Tm2+-doped CsCaI3.

Direct near-IR excitation of Yb(3+) 2F(7/2)-->(2)F(5/2) levels at 10126, 10138, and 10596 cm(-1) in CsMnBr3:0.5%Yb(3+) leads to three types of luminescence at cryogenic temperatures: near-IR Yb(3+) emission and green and red upconverted luminescence. The green luminescence around 20 000 cm(-1) is identified as cooperative Yb(3+) pair upconversion. The broad red upconversion luminescence band centered at 14 700 cm(-1) is ascribed to the 4T(1g)-->6A(1g) transition of Mn(2+). Pulsed measurements indicate a sequence of ground-state absorption and excited-state absorption steps for the red upconversion process. One- and two-color excitation experiments support this, and we conclude that the red upconversion occurs by an exchange mechanism involving Yb(3+) and Mn(2+). The Yb(3+) 2F(5/2)-->(2)F(7/2) near-IR emission around 10 000 cm(-1) is also observed after Mn(2+) excitation at 21 838 cm(-1). This is indicative of a Mn(2+) 4T(1g)--> Yb(3+) 2F(5/2) relaxation process, which is a potential loss process for upconversion efficiency.

Pascal Gerner

Papers

Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion

Anomalous power dependence of sensitized upconversion luminescence

Broad-band Cr5+-sensitized Er3+ luminescence in YVO4

New type of near-infrared to visible photon upconversion in Tm2+-doped CsCaI3.

Green and red light emission by upconversion from the near-IR in Yb(3+) doped CsMnBr3.