Excited state absorption , upconversion

Upconversion lanthanide-containing nanophosphors, which emit higher-energy photons when excited by lower-energy photons have stirred increasing research interest in recent years. The predominant mechanisms of upconversion in nanophosphors are excited-state absorption (ESA), energy-transfer upconversion (ETU) and photon avalanche (PA) (Prasad, 2004 Auzel, 2005). In the ESA process, two photons are sequentially absorbed by the same ion,... 

A wide variety of upconversion mechanisms have been proposed and observed. The majority of these involve some combination of absorption and nonradiative energy-transfer (ET) steps. Absorption may come in two basic forms. Ground-state absorption (GSA, Fig. 2 a) results in promotion of an ion from its ground state to an excited state. Excited-state absorption (ESA, Fig. 2 b) involves absorption of a photon by an excited ion, and results in promotion of that ion to a higher excited state. 

Part of the saturation effects in projection-television phosphors (see Sect. 7.3.4) can be ascribed to an upconversion process of the type shown in Fig. 10.7. This is very similar to the Auger processes mentioned for semiconductors (see Sect. 4.6). Often this type of upconversion prevents a material from becoming a good laser matmal. If the stimulated emission radiation is reabsorbed by ions which are still in the excited state, the laser efficiency drops. From Fig. 10.7, it becomes clear that the upconversion process, which is, in this case, usually called excited state absorption, influences the population inversion in a negative way considering the two ions in Fig. 10.7, the population inversion is complete before the upconversion occurs, but after upconversion and nonradiative decay to the emitting state, the population... 

Schematic illustration of UC processes for ions (a) excited state absorption (ESA) upconversion, (b) energy transfer upconversion (ETU), (c) photon avalanche (PA) upconversion, and (d) energy migration-mediated upconversion. The dotted, dashed-dotted, and full arrows represent energy transfer, nonradiative relaxation, and photon absorption/emission processes, respectively.

Upconversion mechanism (a) ground state absorption followed by excited state absorption (GSA/ESA) (b) ground state absorption followed by energy transfer upconversion (GSA/ ETU) (c) Cross relaxation (CR). 

The solid phase lends itself to the preparation of lanthanide arrays. Indeed, lanthanide doped yttrium aluminium garnets are weU known as laser materials, while doped materials containing more than one kind of lanthanide have proved very effective at upconversion of energy - the sequential absorption of two photons giving rise to anti-Stokes emission [9]. In such systems, excited state absorption by the intermediate state gives rise to formation of a high energy... 

Fig. 9 Some upconversion processes SHG second harmonic generation, TP A two-photon absorption, GSAj ESA ground state absorption/ excited state absorption, PA photon avalanche, ETU energy transfer upconversion. Refer to the text for explanation...

Two-Photon Absorption, Ground State/Excited State Absorption, and Energy Transfer Upconversion... 

The other upconversion pathways are foremost relevant in doped inorganic materials. Multiionic mechanisms can be developed based on those fimdamental principles. The most frequent lanthanide ions for upconversion in inorganic materials are Pr +, Er +, and Tm +. Most of those inorganic materials use f- f transitions to achieve excited state absorptions. Lanthanide ions having both NIR and visible emission are thus needed. This limitation does not occur for two-photon absorption, which may use any lanthanide ion. For more details, the reader can refer to specific reviews. ... 

ATPE = addition de photons par transfert d energie EDTA = ethylenediamine tetraacetic acid EG = ethylene glycol ESA = excited state absorption ETU = energy transfer upconversion GSA = ground-state absorption HRTEM = high-resolution transmission electron microscopy ... 

Oscillations of fluorescence, stimulated emission and excited-state absorption have been studied by pump-probe techniques and fluorescence upconversion, and have been seen in numerous small molecules in solution (Fig. 11.7A [120, 122-124]), and also in photosynthetic bacterial reaction centers [27, 125, 126]. They typically damp out over the course of several picoseconds as a result of vibrational relaxations and dephasing. Vibrational coherences generally decay more slowly than electronic coherences because the energies of vibrational states are not coupled as strongly to fluctuating interactions with the surroundings. Vibrational dephasing also tends to be less dependent on the temperature. 

T1O2 sol-gel layers, 27 Er-Zr second shell coordinations, 27 ESA. See excited state absorption etching technique, 442 ETU. See energy transfer upconversion Euler s critical stress, 226 europium-activated photonic crystal, 379 europium-doped systems. See also erbium-doped systems aluminum-codoping, 378 electric dipole aspects, 373 FLN spectra, 376 luminescence spectra, 374 magnetic dipole aspects, 373 non-radiative lifetimes, 375 non-radiative transitions, 375 radiative lifetimes, 375 room temperature spectra, 376 silica gels, 376... 

In upconversion systems absorption takes place in two stages. An ion absorbs a photon of the incident radiation and goes to an excited state. It then transfers most of the energy either to another state of that ion or to the excited state of another ion. If this second excited state is metastable, it has time to absorb another photon before it spontaneously... 

Precise measurements of the excited state lifetimes of the DNA constituents were not available till very recently, mainly due to the limited time resolution of conventional spectroscopic techniques. Studying the DNA nucleosides by transient absorption spectroscopy, Kohler and co-workers observed a very short-lived induced absorption in the visible which they assigned to the first excited state [5,6]. The lifetimes observed were all well below 1 picosecond. The first femtosecond fluorescence studies of DNA constituents were performed using the fluorescence upconversion technique. Peon and Zewail [7] reported that the excited state lifetimes of DNA/RNA nucleosides and nucleotides all fall in the subpicosecond time, thus corroborating the results obtained by transient absorption. 

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