Exciton bleaching

Transient bleaching and recovery rates of CdS excitonic absorption, determined by picosecond pump-probe spectroscopy, depended on [H20]/[A0T] ratio and micellar surface. Fluorescence spectra and lifetimes depended on [Cd2+]/[S2 ] ratios... 

The 3.5- and 8-ntn nanoparticles show well-resolved peaks at 362 and 473 nm, respectively, as well as other features at higher energies. The 4.5-nm particles show a well-resolved peak at 400 nm and a shoulder at 450 nm. It is tempting to assume that in each case, the lowest energy absorption corresponds to the lowest allowed transition (the A exciton) in bulk M0S2. Polarization spectroscopy can be used to determine if this is the case. The lowest allowed transitions in bulk material, the A and B excitons, are polarized perpendicular to the crystallographic c axis. If the lowest allowed transition correlates to the A exciton, then it would be expected to also be a planar (xy polarized) oscillator. However, tire results of polarization studies reveal that the actual situation is more complicated. A combination of time-resolved polarized emission and one-color time-resolved polarized absorption (transient bleach) studies facillitate assignment of the polarizations of the observed nanoparticle transitions. The 3.5-nm particles are emissive and the polarization of the several of the lowest transitions may be determined... 

Therefore the lack of an observable bleach can only be explained by the cancellation of all contributions to the pump-probe signal, which is the case for a perfect harmonic state. It can be shown that the anharmonicity of a vibrational exciton is a direct measure of its degree of delocalization [5]. Thus, we conclude that the free exciton state is almost perfectly delocalized at 90 K. As temperature increases, a bleach signal starts to be observed, pointing to a non-complete cancellation of the different contributions of the total pump-probe signal. Apparently, thermally induced disorder (Anderson localization) starts to localize the free exciton. The anharmonicity of the self-trapped state (1650 cm 1), on the other hand, originates from nonlinear interaction between the amide I mode and the phonon system of the crystal. It... 

In a second experiment, narrow band pump pulses (spectral width 30 cm 1, pulse duration 250 fs FWHM) were used to selectively excite individual sub-levels of the NH band (Fig. 2e, g) [4]. On the sub-picosecond time scale, the free-exciton and the lower lying self-trapped states behave distinctly differently. When exciting the free-exciton (Fig 2e), a strong bleach and stimulated emission signal is observed which recovers on a 400 fs time scale. Simultaneously, population is transferred into lower lying self-trapped states. On the other hand, when pumping one of the self-trapped states directly (Fig. 2g), population within all self-trapped states equilibrates essentially instantaneously, but the free exciton peak is not back-populated. This is the direct observation of ultrafast self-trapping Excitation of the free-exciton leads to an irreversible population of self-trapped states, but not vice versa. 

The IR-2D spectroscopic technique applied in Section IV.C (30,42) utilized the frequency domain after selectively bleaching individual one-excitonic states using a narrowband intense pump pulse, a broadband probe pulse... 

This model predicts that a trapped electron is more efficient than a trapped hole in bleaching the exciton absorption in the case of CdS. This is because a trapped hole is not capable of localizing an electron with small effective mass, and therefore, is inefficient in reducing the electron-hole overlap. This interesting prediction was proven by a pulse radiolysis experiment [90] where the electron and hole can be separately injected into the cluster and their effects probed separately. It was found that the electron is much more efficient than the hole in bleaching the exciton absorption of CdS clusters. One can also use sensitized photoinduced electron transfer to inject either an electron or a hole into the clusters and study their effects separately. Such experiments should be very informative. 

With the basic mechanism understood, the resonant nonlinearity of semiconductor clusters can now be quantitatively analyzed. Since one trapped electron-hole pair can bleach the exciton absorption of the whole cluster, the bleaching efficiency per absorbed photon of a nanocluster is the same as that of a molecule, as described by Eq. (20). For a given rp and r, the resonant third-order optical nonlinearity of a nanocluster is simply determined by the (a - ax) term. 

In RCs where Bg is exchanged for [3-vinyl]-13 -hydroxy-bacteriochlorophyll a and then spheroidene-reconstituted, a MIA band shifts from 813 to 776 nm. This bleaching which has also been observed by MODS spectroscopy (Lous and Hoff, 1989) may be explained by a partial delocalization of the triplet excitation between spheroidene and Bg and allows the unequivocal determination of the ground state absorption of Bg (812 nm) which is obscured by the absorption of Ba (804 mn) and tbe upper exciton component of PV70 around 807 mn (Hartwich et al., 1995). The conclusion from these observations are that ... 

Figure 14.35 Intensity of the excitonic absorption band during a complete coloration/bleaching cycle in pH 3 solution after 3000 cycles- 550 mV.

The optical bleaching by stored electrons is the basis for the occurrence of strong optical nonlinearity observed in Q-particles [64]. The physical reason for this optical bleaching is still not discussed conclusively in literature. The most obvious explanation comes from a state filling model. The stored electrons occupy the lowest electronic levels in the conduction band and, consequently, the optical transition has to occur to higher electronic levels (i.e., at shorter wavelength). This effect is known in solid-state semiconductor physics as the Burstein shift [65]. Other theoretical models describe the optical bleaching as a consequence of the polarization of the exciton in the electric field of the stored electron, which is then... 

Fig. la demonstrates the differential absorption spectrum of PbS QDs in the phosphate glass pumped by 15-ps pulse at X= 1.08 pm in comparison with their linear absorption spectrum. The bleaching of the whole first excitonic absorption band takes place. This is the sign that the first excitonic absorption band is mostly homogeneously broaden. Relaxation of bleaching effect is doubleexponential and can be described as - ADD = + A2e z/r2 (Fig. lb). Ab... 

The ground state bleach signal decayed with the same lifetime as the singlet exciton population in the dilute case, but showed a doubling in magnitude over the first 10 ns in the concentrated solution, indicative of exciton fission from a localized state. 

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