Resonance fluorescence optical pumping

So, the depth of the minimum for large <5 is r /IJI while for small <5 it is of the order of rmax and we see that the effective range of 5, when the minimum is the most pronounced, is —rmax < 6 < 6cr. As a consequence, at low temperatures and under optical pumping at frequencies above the excitonic resonance excitons will accumulate in this minimum, which can be detected, for example, by pump-probe experiments. The fluorescence from these states should increase with temperature since states with small k become populated. 

If the optically pumped sample is placed inside an RF field with the frequency o)rf tuned into resonance with the transition ij) / ) between two sublevels of the lower state, the level population N in) that was depleted by optical pumping will increase again. This leads to an increased absorption of the optical pump beam, which may be monitored by the corresponding increase of the laser-induced fluorescence intensity. Measuring Imicorf) while co f is tuned yields a double-resonance signal at o)rf = (Oinj = [E in) - E ij)]/h (Fig. 5.7). 

Optical Pumping and Double-Resonance Techniques Fluorescence Dip 6 — 6,... 

In addition to LIF resonant two-photon ionization (Sect. 1.4) can also be used for the sensitive detection of collision-induced rotational transitions. This method represents an efficient alternative to LIF for those electronic states that do not emit detectable fluorescence because there are no allowed optical transitions into lower states. An illustrative example is the detailed investigation of inelastic collisions between excited N2 molecules and different collision partners [995]. A vibration-rotation level (v, J ) in the a Jig state of N2 is selectively populated by two-photon absorption (Fig. 8.10). The collision-induced transitions to other levels v + An, / + AJ) are monitored by resonant two-photon ionization (REMPI, Sect. 1.2) with a pulsed dye laser. The achievable good signal-to-noise ratio is demonstrated by the collisional satellite spectrum in Fig. 8.10b, where the optically pumped level was v = 2, J = 7). This level is ionized by the P(l) parent line in the spectrum, which has the signal height 7.25 on the scale of Fig. 8.10b. 

Although the recoil effect is very small when a single photon is absorbed, it can be used effectively for optical cooling of atoms by the cumulative effect of many absorbed photons. This can be seen as follows When atom A stays for the time T in a laser field that is in resonance with the transition j) ), the atom may absorb and emit a photon tko many times, provided the optical pumping cycle is short compared to T and the atom behaves like a true two-level system. This means that the emission of fluorescence photons fuo by the excited atom in level k) brings the atom back only to the initial state i), but never to other levels. With the saturation parameter S = Bikp (Oik)/Aik, the fraction of excited atoms becomes... 

Figure 10.7 Normalized fluorescence (a), excitation (b) and optically pumped PL spectra (c, d) taken from platelet crystals of BP1T. The spontaneous fluorescence (a), AST (c) and SRRS (d) spectra were taken at Xex = 365, 355 and 460 nm, respectively. The excitation spectrum (b) was taken for the fluorescence band at 493 nm. Reproduced from H. Yanagi, I. Sakata, A. Yoshiki, 5. Hotta and 5. Kobayashi, Polarization dependence of Stimulated resonance Raman scattering from a single crystal of bi-phenyl-capped thiophene, Jpn. J. Appl. Phys., 45, 483-487 (2006) with permission from The Institute of Pure and Applied Physics...

The term single vibronic level (SVL) fluorescence has been used to distinguish this emission from the more general case of resonance fluorescence where optical selection of a single level does not necessarily occur either because monochromatic pumping was not used or because the excitation fell in a region of overlapping absorption bands. 

RF transitions between sublevels of the upper state result in a change of the polarization and the spatial distribution of the laser-induced fluorescence. They can therefore be monitored through polarizers in front of the photomultiplier. Since the RF transitions deplete the optically pumped upper level, the RF double-resonance signal at co2 = cokmj = [E kj) — E km)yh has an opposite sign to that of the lower state at ol> = coinj (Fig. 10.6). 

Shelby et al. measured large pseudoquadrupole splittings of 33.4 and 41.6 MHz of the Pr " ground state in YAG, which were obtained by optical pumping and fluorescence detected nuclear quadrupole resonance. " ... 

Trivalent lanthanide ions are used extensively for optically-pumped solid-state lasers because they possess suitable absorption bands and numerous fluorescence lines of high quantum efficiency in the visible and near-infrared. Figure 35.11 summarizes the energy levels, transitions, and approximate wavelengths of trivalent lanthanide ion lasers. In cases where the transitions are to Stark levels of the ground /-state manifold, operation at low temperatures is usually required. The number of different crystalline hosts in which each ion has lased is indicated in fig. 35.1. For several ions, stimulated emission has been observed between more than one pair of / states. A frequency-selective element (e.g., prism, grating, filter) is usually added to the resonator cavity to accomplish this. 

Fig.l0.1a,b. Schematic level diagram of optical pumping and double-resonance transitions (a) General scheme and (b) special case where the population of level m) is achieved by fluorescence from k)... 

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