Up-conversion

In general, low concentrations of the active ion, of the order of 1 %, are used. At these concentrations, the ions form point defects well isolated from each other. At higher concentrations, dopant ions tend to cluster and other energy loss mechanisms interfere with up-conversion. 

The oxide CeC 2 doped with approximately 1% Er3+ exhibits up-conversion involving only one active ion. The Er3+ ions substitute for Ce4+ to form a low concentration of Erte defects randomly distributed within the oxide matrix. Irradiation with near-infrared photons with a wavelength of 785 nm excites the Er3+ ions from the 4Ii5/2 ground state to the 4I9/2 level, that is, a GSA mechanism  

These ions loose energy internally to phonons (lattice vibrations), a process called decay or relaxation, to reach the 4In/2 and 4Ii3/2 energy levels  

These states subsequently decay by internal energy loss to the 2Hi 1/2,4S3/2, and 4F9/2 energy levels  

The energy levels of the infrared radiation suited to the energy-transfer (ET) process matches the Yb3+ ion energy transition from the ground-state 2F7/2 level to the 2F5/2 level, and lasers with an output of 975 nm are usually employed. This energy also matches the 4I15/2 to 4In/2 ground-state absorption (GSA) transition 

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The sum-frequency case of co = co -t CO2 is called up-conversion, tire difference-frequency co = co - CO2 down-conversion, reflecting tire increase or decrease of tire generated optical frequency co from tire input frequencies co and 052-... 

In these equations, = 0 is the bottom of the catalyst bed and Xx is the conversion in the flow direction from bottom to top, while X2 is the conversion in the opposite flow direction. Bunimovich et al. (1990) suggest using Eqs. (52) to (54) for an initial estimate of the temperature profiles in order to speed up conversion on integration of the full model equations in Table X. This step would only be taken if it were the stationary cyclic state profiles that are wanted. 

It has been shown that photoexcitation of the guanine-cytosine (G-C) base pair leads to proton transfer [231], Watson-Crick (WC) base pairs have excited state lifetimes much shorter than other non-WC base pairs indicating once again that the natural occurring WC base pairs are more photostable than other alternative configurations [115, 118, 232-235], Much work has been done in the gas phase where many different base pair isomers exist. The ultrafast relaxation of the WC base pair has also been confirmed in solution using fluorescence up-conversion measurements [117]. 

Peon J, Zewail AH (2001) DNA/RNA nucleotides and nucleosides direct measurement of excited-state lifetimes by femtosecond fluorescence up-conversion. Chem Phys Lett 348 255... 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

Possible future applications of up-converting phosphors include (i) three-dimensional displays 249-251 (ii) fiber optic amplifiers (referred to above) that operate at 1.55, 1.46, and 1.31 pm,, 2 1-255 (iii) up-conversion lasers 250 and (iv) remote sensing thermometers for high-temperature applications (utilizing the temperature dependence of optical properties of, for example, cubic Y203 Er3+).256-258... 

Enhancement of x2 will lead to improvement (in terms of efficiency per interaction volume) in the following applications up-conversion in the visible or near U.V. of powerful I.R. laser radiation, frequency modulation of a laser carrier beam, optical parametric oscillation and amplification for solid state infrared tunable coherent devices. 

Figure 9.14 Energy transitions in defects (a) simple excitation and release of energy (.b) up-conversion of two low-energy photons to one high-energy photon (c) typical fluorescence in which some energy is lost as heat to the solid (dotted arrow) before transition to the ground state and (d) up-conversion of two low-energy photons to a photon of intermediate...

Figure 9.17 Simplified energy level diagram for Er33 showing the important up-conversion transitions (a) GSA and relaxation, (b) ESA and relaxation, (c) ESA and relaxation, and (d) emission of green and red. [GSA = ground-state absorption ESA = excited-state absorption. Spectroscopist s energy units (cm-1) are converted thus 25,000 cm-1 = 3.1 eV = 5.0 x 10-19J.]...

Fig. 9.lld). The up-conversion spectrum consists of three major peaks (Fig. 9.18). [All up-conversion spectra from Er3+ (including those using energy transfer, below) are similar, but the relative intensities of the three peaks vary with concentration of defects and the host matrix.]... 

Figure 9.18 Up-conversion emission spectrum from Er3+ ions.

Figure 9.19 Simplified energy level diagram for the Er3+/Yb3+ couple showing the important up-conversion and energy transfer transitions a) GSA, ET, and relaxation (/>) CR, ESA, ET, and relaxation...

The up-conversion efficiency is low and varies with the concentration of the activator and sensitisor ions. A maximum efficiency is observed with concentrations of about 1-3% of the active center. Above this value increasing back transfer from Er3+ to Yb3+ and increasing interactions between both lanthanide ions, leading to cluster formation and Yb3+-Yb3+ energy transfer, limits the efficiency. 

A number of other up-conversion processes are known. The blue emission from a Yb3+/Tm3+ couple in which the active emitters are defect Tm3+ centers is mainly due to the efficient excitation ET process from Yb3+ centers. Two-frequency up-conversion has been investigated using Pr3+ defects in a fluoride glass matrix. Illumination with one pump wavelength results in GSA, while simultaneous irradiation with a second pump wavelength further excites the GSA centers via ESA. The doubly excited defects emit red light. Up-conversion and visible output only takes place at the intersection of the two beams. 

The intensity of the upconverted beam as a function of the intensity of the exciting radiation for Er3+ doped into CeC>2 is given in the following table. Determine whether the up-conversion is likely to be a two- or three-photon absorption process. 

Qian HS, Li ZQ, Zhang Y (2008) Multicolor polystyrene nanospheres tagged with up-conversion fluorescent nanocrystals. Nanotechnology 19(255601)4... 

The instruments that provide the best time resolution (about 100 femtoseconds) are based on fluorescence up-conversion. This very sophisticated and expensive technique will be described in Chapter 11. 

The technique of fluorescence up-conversion (see Chapter 11), allowing observations at the time-scale of picoseconds and femtoseconds, prompted a number of fundamental investigations on solvation dynamics that turned out to be quite complex (Barbara and Jarzeba, 1990) (see Box 7.1). 

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