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-based luminescent hybrid materials.

We show theoretically with the simplest possible model that the intensity of an upconversion luminescence that is excited by the sequential absorption of n photons has a dependence on absorbed pump power P, which may range from the limit of Pn down to the limit of P1 for the upper state and less than P1 for the intermediate states. The two limits are identified as the cases of infinitely small and infinitely large upconversion rates, respectively. In the latter case, the dependence of luminescence intensities from intermediate excited states on pump power changes with the underlying upconversion and decay mechanisms. In certain situations, energy transfer upconversion and excited-state absorption can be distinguished by the measured slopes. The competition between linear decay and upconversion in the individual excitation steps of sequential upconversion can be analyzed. The influence of nonuniform distributions of absorbed pump power or of a subset of ions participating in energy-transfer upconversion is investigated. These results are of importance for the interpretation of excitation mechanisms of luminescent and laser materials. We verify our theoretical results by experimental examples of multiphoton-excited luminescence in Cs3Lu2Cl9:Er3+, Ba2YCl7:Er3+, LiYF4:Nd3+, and Cs2ZrCl6:Re4+.

Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems

Erbium doped materials are of great interest in thin film integrated optoelectronic technology, due to their Er3+ intra-4f emission at 1.54 μm, a standard telecommunication wavelength. Er-doped dielectric thin films can be used to fabricate planar optical amplifiers or lasers that can be integrated with other devices on the same chip. Semiconductors, such as silicon, can also be doped with erbium. In this case the Er may be excited through optically or electrically generated charge carriers. Er-doped Si light-emitting diodes may find applications in Si-based optoelectronic circuits. In this article, the synthesis, characterization, and application of several different Er-doped thin film photonic materials is described. It focuses on oxide glasses (pure SiO2, phosphosilicate, borosilicate, and soda-lime glasses), ceramic thin films (Al2O3, Y2O3, LiNbO3), and amorphous and crystalline silicon, all doped with Er by ion implantation. MeV ion implantation is a technique that is ideally suited to dope these materials with Er as the ion range corresponds to the typical micron dimensions of these optical materials. The role of implantation defects, the effect of annealing, concentration dependent effects, and optical activation are discussed and compared for the various materials.

Erbium implanted thin film photonic materials

Lanthanide Nanoparticles: From Design toward Bioimaging and Therapy

An upconversion pumping scheme was used to produce cw laser action at 0.55 μm in YAlO3:Er3+ at temperatures up to 77 K on the 4S3/2→4I15/2 transition. Two infrared dye lasers at 792.1 and 839.8 nm were used as the pump sources for stepwise two‐photon excitation of the 4S3/2 upper laser level at 18 406 cm−1. The laser operates in the fundamental TEM00 mode, and a cw output power of ∼1 mW was achieved with ∼200 mW pump power from each infrared dye laser.

Green infrared-pumped erbium upconversion laser

Continuous-wave laser operation in the violet at 380 nm has been achieved on the 4D3/2 → 4/11//2 transition in LaF3:Nd3+ at temperatures of <90 K. Two different upconversion pumping mechanisms resulted in laser action: (a) sequential two-step absorption of an IR (790-nm) photon and a yellow (590-nm) photon by the Nd3+ ions; (b) cooperative energy transfer upconversion following the absorption of one yellow (578-nm) photon per Nd3+ ion. At temperatures near 20 K up to 12-mW power at 380 nm was obtained using pump powers of several hundred milliwatts from two cw dye lasers and conservative 1 % output coupling. At 77 K, the output power decreased to ~4 mW.

Violet cw neodymium upconversion laser

5D3→7FJ emission in terbium-doped sol–gel glasses

Avalanche upconversion in LaF3:Tm3+

Rare-earth ion distribution in sol–gel glasses co-doped with Al3+

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