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 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

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Lanthanide pairs, which can upconvert low energy photons into higher energy photons, are promising for efficient upconversion emission. A typical system with Yb3+ as a sensitizer can convert short NIR into visible/ultraviolet light via energy transfer between lanthanide ions. Such upconverting nanocrystals doped with lanthanide ions have found significant potential in bioimaging, photochemical reactions and energy conversion. This review presents a fundamental understanding of energy transfer in lanthanide-supported photon upconversion. We introduce the emerging progress in excitation selection based on the energy transfer within lanthanide ions or activation from antennae, with an outlook in the development and applications of the lanthanide upconversion emissions.

Energy transfer in lanthanide upconversion studies for extended optical applications

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest due to their superior physicochemical features, such as large anti-Stokes shifts, low autofluorescence background, low toxicity and high penetration depth, which make them extremely suitable for use as alternatives to conventional downshifting luminescence bioprobes like organic dyes and quantum dots for various biological applications. A fundamental understanding of the photophysics of lanthanide-doped UCNPs is of vital importance for discovering novel optical properties and exploring their new applications. In this review, we focus on the most recent advances in the development of lanthanide-doped UCNPs as potential luminescent nano-bioprobes by means of our customized lanthanide photophysics measurement platforms specially designed for upconversion luminescence, which covers from their fundamental photophysics to bioapplications, including electronic structures (energy levels and local site symmetry of emitters), excited-state dynamics, optical property designing, and their promising applications for in vitro biodetection of tumor markers. Some future prospects and efforts towards this rapidly growing field are also envisioned.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

Studies of laser‐pumped Pr3+ infrared quantum counters have led to the discovery of a new photon avalanche region, occurring when pump flux exceeds a certain critical value. Infrared output near 4.5 μm is associated with the large visible output, and pump power is strongly absorbed. Critical pump power density is in the range 1.2–12.2 W/cm2.

The photon avalanche: A new phenomenon in Pr3+‐based infrared quantum counters

Infrared holography at 10.6 μm has been accomplished by using a thermochromic material, cuprous mercuric iodide, to record an on‐axis interference pattern. The pattern is subsequently reduced in a two‐step photographic process, and reconstruction is accomplished in the visible with He–Ne laser light.

INFRARED HOLOGRAPHY AT 10.6μm

Vanadium dioxide undergoes a semiconductor-metal phase transi­ tion at temperature of 340 K. When prepared as a thin film on a suitable substrate, the transition exhibits hysteresis, i.e. the curve describing the state of the film versus temperature is double valued. Here the material is capable of existing in equilibrium in either of two states at the same temperature. Both high writing speed and high spatial resolution are possible, contrary to what one might normally expect from a thermal process. The writing speed is limited by the time taken to deliver the latent heat of transition, and the resolution is limited by the diffusion time during which written and unwritten adjacent areas are out of thermal equilibrium. Both of these times are connected, and they along with the latent heat dictate the exposure. Experiments thus far have produced writing times as small as 30 nanoseconds and stored spots of a few microns in size. They by no means represent limits, and writing times of a few nanoseconds and spot sizes down to a few thousand angstroms appear feasi­ ble. A discussion will be given detailing the material as a high density recording medium including laser beam writing requirements, spot sizes, stored contrast, and signal-to-noise ratio. In addition, information on optical constants, latent heat and other physical parameters will be given along with some past results concerning long term storage, cycling, lifetime, and reciprocity.

Vanadium dioxide storage material

Disclosed are methods and apparatus for recording and reproducing optical information in thermochromic materials; exemplary materials are the ternary halides, ternary chalcogenides, and certain transition metal oxides. Methods and apparatus are also disclosed for recording holographic images with infrared radiation and reconstructing infrared holograms in the visible portion of the spectrum.

Holography with thermochromic recording materials

Reflectance vs temperature measurements on thermochromic Cu(2)HgI(4) show phase transition hysteresis, which forms the basis for recording applications. Long term reflectance relaxation at room temperature has been noted, together with the ability to invert reflectance from the high temperature phase to the low temperature phase by further addition of energy. Specific heat measurements also show the phase transition hysteresis. Image recording sensitivity measurements indicate that 27 mJ/cm(2) are needed to switch a particular Cu(2)HgI(4) paint sample at its transition. Spatial resolution is shown to be at least as good as 4 microm.

Dayton D. Eden

Papers

The photon avalanche: A new phenomenon in Pr3+‐based infrared quantum counters

INFRARED HOLOGRAPHY AT 10.6μm

Vanadium dioxide storage material

Holography with thermochromic recording materials

Infrared recording with thermochromic cu(2)hgl(4).