Excitation power dependence

First, the excitation-power dependence of the emission count rate of single DMPBI nanocrystals was examined (Figure 12.9). The emission count rate of the single... 

Figure 12.9 The excitation-power dependence of the emission count rate of single DMPBI nanocrystals (dots), and a saturation curve calculated from a two-level model (solid line). One count rate value to one laser power was calculated as an average of 30 nanocrystals. S. Masuo, A. Masuhara, T. Akashi, M. Muranushi,...

The excitation intensity was also varied in order to determine the effect this had on the PL spectrum. PL1 and PL3 bands had a blueshift per decade of 3.7 meV and 5.5 meV, respectively, with an increase in excitation intensity. The blueshifts were attributed to donor-acceptor pair (DAP) recombination.68,69 PL2 did not show any excitation power dependency, whereas the analysis of the PL4 band was not attempted because of the uncertainty in its precise location. The effect of increasing excitation intensity can be clearly observed in Fig. 6.29. 

Fig. 1.21. (a) Light-induced ESR intensity as a function of the 3-factor in an MDMO-PPV/PCBM blend, = 9.5 GHz, T = 100 K, Aexc = 488 nm, P = 20 lW, 20 mW, and 200 mW. (b) A doubly integrated LESR signal of the prompt contribution as a function of the excitation power dependence. Squares correspond to the positive polaron signal and circles to Cg0... 

Whereas many examples of the photon avalanche phenomenon exist in the literature, only one study has been made for elpasolite systems [347], for Cs2NaGdCl6 Tm3+, where the blue upconverted emission is due to the 1G4 3H6 transition. However, the situation is rather more complex than in Fig. 24j because several other processes can occur, which also lead to emission from D2. Three features related to the 64 emission are highlighted here. First, Fig. 27a shows that a quadratic emission intensity-excitation power dependence is obtained at low excitation intensities for samples of Cs2NaGdCl6 Tm3+ doped with between 6-15 mol% Tm3+. However, a dramatic increase of the emission intensity appears above the excitation threshold value, ca. 9 kW cm 2. In Fig. 27a, the slope increases to 6 for the 10 mol% Tm3+-doped sample. Second, the time-dependence of the upconverted emission exhibits different behaviour at different excitation powers. A notable difference from other systems is that, at the threshold excitation power, Pthres, the blue emission has an almost linear rise-time which is followed by a further slower rise over several seconds. Third, at high excitation powers, the establishment of the stationary state is quicker, and the 3F4—>3G4 ESA decreases the transmitted laser light by several percent, Fig. 27b. 

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

Fig. 3.15. Left reflectivity change of GeTe showing the coherent /l i, phonon under repetitive excitation with a pulse train. The intervals between the pump pulses, Af 12, At23, and Af,34 are 290, 320, and 345 fs, respectively. Right pump power dependence of the frequency of the Aig mode for excitation with a single pulse and the pulse train. From [43]...

Fig. 18.8 SERS detection in LC ARROW, (a) Top view of experimental beam geometry excitat ing rhodamine 6G molecules bound to silver nanoparticles (leyLC) excitation beam, 1R Raman signals) (b) R6G concentration dependent SERS power for three representative Raman peaks PI P3, inset spectra at various excitation powers...

S2 - Sq fluorescence in condensed media has so far been found in several types of molecules. However, metalloporphyrins are contrasted with these compounds by another arresting feature such that the S2 fluorescence can be observed even upon photoexcitation to the state. Stelmakh and Tsvirko have first noticed the anomalous S2 - Sq fluorescence in metalloporphyrins (15,16). Figure 1(a) shows the fluorescence spectra of ZnTPP in EPA taken by the 540 nm excitation of a nitrogen pumped dye laser. The fluorescence band at around 430 nm observed by visible excitation is safely assigned to the S2 state fluorescence. The laser power dependence of the fluorescence intensity is quadratic at low power density of excitation (<5 x 10 photons cm"2 pulse ) but shows typical saturation effect with increasing the laser intensity. It should be emphasized here that the S2 fluorescence of ZnTPP can be observed without focusing of the laser beeim. 

Due to the difference in the number of probe photons required to ionize the clusters from each state, careful study of the ion signal probe power dependence made it possible to determine the origin of each of the observed components. The X, X2 component is the result of the two photon excitation of the S02 F band. The X3 decay component is due to the one photon excitation of the coupled 1A2, Bi states. The plateau is believed to be due to ion-state fragmentation of larger clusters and does not seem to influence the values of the time constants obtained from the fitting procedure. 

To gain better understanding of the mechanism of anomalous thermalization in Er3"1" Y2O2 S nanocrystals, the temperature and power dependences of the luminescence intensity in excitation spectra were quantitatively studied. Integrated intensities of excitation spectra at 491.8 and 492.9 nm versus temperature (from 2.2 to 66 K) are shown in fig. 9, where each experimental point represents the integrated intensity of each main peak. The intensity of saturated hot bands in nanocrystals decreases rapidly to zero as temperature increases from 6 to 8 K, and... 

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