Activators, upconversion

Figure 6c, d shows the results simulated for a case in which Wetu 0 but E = 0, i.e., only GSA/ETU is active. The pulsed experiment. Fig. 6c, shows the characteristic delayed maximum observed in Fig. 4b. When E = 0, N2 has a value of exactly zero at time zero, and so the rise of the upconversion transient truly begins at zero. Comparison to Fig. 6 a, b shows that this rise derives from the decay rate constant of the upper state, k2 = A 2a + A 2b- Since N2 is proportional to Nf in ETU (Eqs. 7 and 10), the decay of the transient N2 population lasts substantially longer than the natural decay of the upper state, and has a rate constant exactly twice that of Ni under these low-power conditions when all of the above assumptions are met. Figure 6d shows the corresponding data following a square pulse. The decay again proceeds with a rate constant exactly twice that of Nj under the assumed conditions, with a small deviation at short times where k2 is still consequential. Based on this comparison, it is clear that ESA and ETU mechanisms are readily distinguishable using either square-wave or pulsed excitation modes under these conditions (see below for k2 < ki).

This example illustrates a very interesting new upconversion process that is induced by exchange interactions between two active ions. The observed upconversion process is not consistent with the properties of either ion, but is truly a property of the new chromophore obtained with these exchange interactions. Analogous upconversion mechanisms are now imaginable for a variety of previously unexplored combinations of ions. These results will undoubtedly lead to the discovery of new upconversion materials involving ions previously thought unsuitable for upconversion. 

In this chapter we have focused on recent developments in the exploration of novel upconversion phenomena, ranging in scope from the discovery of new upconversion activators and mechanisms to the application of spectroscopy in providing detailed mechanistic information about a variety of unusual and exciting upconversion processes. The discovery of new upconversion ions, combinations of ions, and host materials are all areas of intense research. These areas will continue to provide interesting photophysical systems to study and potentially employ as luminescent materials. 

Three-photon active (SPA) materials have been studied extensively over the last few years owing to their potential applications in the fields of telecommunications and biophotonics [26-28, 30], Two major advantages of these materials—longer excitation wavelengths and much better spatial confinement—make them attractive in comparison with two-photon absorption (2PA) based materials [29], One of the most important applications of SPA materials is three-photon pumped frequency-upconversion cavity-less lasing [26, 30], Short infra-red (IR) pulses induce the ASE process via 3-photon absorption followed by fast non-radiative decay to a long-lived state which collects population. Conventional experiments with a pulsed longitudinal pump [27, 28, 30] show that stimulated emission occurs in both forward and backward directions with respect to the pump pulse. 

The concept of upconversion 32) in which higher-lying states of an activator are excited by successive energy transfers from a less energetic sensitizer has also been applied to lanthanide lasers (33). 

Dyad 4 illustrates that both pathways can be active. In initial experiments on dyad 4 the attached tetrapyrrole was found to quench the carotenoid S3 state of6 from 95 to 28 fs and its S, level from 12 to 9 ps. The rise of the 8, level of tetrapyrrole 5 as measured by fluorescence upconversion required a major exponential component (74%) of 41 fs- and a minor component (26%) of4 ps-. While the match between the 9 ps decay of the carotenoid 8, and the 4 ps rise component of the tetrapyrrole 8, is only qualitative, these prelimenary experiments do provide evidence that both states can be energy donors. 

S3/2— I15/2) of erbium ion have a laige emission section and are easy to achieve upconversion, so erbium ion is a good active ion as upconversion phosphors materials. The energy level transition( l]3/2— I15/2) can emit out 1.5 pm eye-safe laser radiation and the ion is also a well active ion for the eye-safe laser material. Gd203 powder with a cubic structure is a good host material for its excellent photics and thermal properties. It not only can be used as upconversion phosphors material but also can be used to prepare transparent ceramics as a host material. 

In Table 10.2 the efficiency of the green emission intensity of Yb, Er -codoped host lattices under infrared excitation is given [4J. The excitation density is the same, as are the activator concentrations. It is seen that the efficiency depends. strongly on the choice of the host lattice. That of a-NaYF4 yields very efficient upconversion materials [3]. Oxides are less suitable than fluorides, since lifetimes in oxides are shorter than in fluorides due to a stronger interaction between the luminescent ion and its surroundings (Sect. 2.2). If the lifetime of the intermediary I11/2 level is decreased, the total efficiency of the upconversion process will also decrease. 

The positive role of upconversion in the field of materials has already been outlined above. There are also negative aspects of upconversion, viz. saturation effects. This is due to the fact that upconversion implies a transition from an excited state upwards. If we are interested in the emission from this specific excited state, cither in view of its luminescence or its stimulated emission, we have to consider the fact that upconversion will decrease the population of this level, so that the intensity of the emission of interest decreases. This will especially be the case for higher activator concentrations and/or high excitation densities (saturation effect). 

A UC phosphor consists of a host and dopant (activator). The dopant acts as luminescent centers, and the host provides a matrix to bring these centers into optimal position. A large number of suitable hosts doped with actinide [36, 37] and transition-metal ions—such as Cm ", U ", Mo ", Os ", Ni ", Ti ", and Re" —have been reported to show upconversion luminescence [38]. However, this occurs mainly in the RE elements due to its special inner shell configurations,... 

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