Stimulated Level-Crossing Spectroscopy

So far we have considered level crossing monitored through the spontaneous emission. A level-crossing resonance can manifest itself also as a change in absorption of an intense monochromatic laser wave tuned to the molecular transition. The physical origin of this stimulated level-crossing spectroscopy is based on saturation effects and may be illustrated by a simple example [10.93]. 

Consider a molecular transition between two levels a and b with angular momentum J = 1 and J = 0 (Fig.10.61). We denote the center frequencies of 

If an external electric or magnetic field is applied in the z direction, the laser wave, polarized in the y direction, induces transitions aM = 1 because it can be composed of + a contributions (see previous section). If the level splitting )fi(u) - the absorption coefficient is now the 

A number of stimulated level-crossing experiments have been performed on the active medium of gas lasers where the gain of the laser transition is changed when sublevels of the upper or lower laser level cross each other. 

The whole gain tube is for instance placed in a longitudinal magnetic field and the laser output is observed as a function of the magnetic field. One example is the observation of stimulated hyperfine level crossings in a Xe laser [10.96], where accurate hyperfine splittings could be determined. 

E = Eo cos(cot — kx), linearly polarized in the y-direction, induces transitions with AM = 0 without an external field. The saturated absorption of the laser beam is then, according to (2.30), 

Two-photon induced level crossing [865], which relies on the OODR scheme of Raman-type transitions (Fig. 7.8), has been performed with the two neon transitions 

Consider a molecular transition between the two levels a) and b) with the angular momenta 7 = 0 and 7 = 1 (Fig. 12.6). We denote the center frequencies of the AM = +1, 0, — 1 transitions by u +, coq, and co- and the corresponding matrix elements by /zq, and /x, respectively. Without an external field the M sublevels are degenerate and co. = co- = coq. The monochromatic wave E = Eocos(o)t— kx), linearly polarized in the y-direction, induces transitions with AM = 0 without an external field. The saturated absorption of the laser beam is then, according to (7.24), 

Two-photon-induced level crossing [12.30], which relies on the OODR scheme of Raman-type transitions (Fig. 12.7) has been performed with the two neon transitions at = 632.8 nm and 3-39 nm, which have the common upper 3Sj level. A HeNe laser is simultaneously oscillating on both transitions. The Hanle signal S(B) is monitored via the fluorescence from the 2P4 level at A = 667.8 nm. 

There is one important point to note. The width AB /j of the levelcrossing signal reflects the average width 7 = i (71+72) of the two crossing levels. If these levels have a smaller width than the other level of the optical 

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Fig. 7.7 Stimulated level-crossing spectroscopy with a common lower level (a) level scheme and (b) saturation holes in the Doppler-broadened population distribution with and without magnetic field...

Although saturation effects may influence the line shape of the level-crossing signal, for small saturation it may still be essentially Lorentzian. Stimulated level-crossing spectroscopy has been used to measure Lande factors of atomic laser levels with high precision. One example is the determination of gi P ) = 1.3005 0.1% in neon by HERMANN et al. [10.97]. 

Optical spectroscopy of Er " doped into bulk AIN ceramics has been reported [296]. The material was prepared by using hot press sintering of AIN with Et203 and (NH4)(ErE4), which yielded fully dense, translucent, hexagonal AIN. The Er concentration was a small fraction of a percent, and resided in multiple sites, with one type of center dominant. A number of the energy levels of Er " were identified for this center. The temperature dependent fluorescence lifetime was probably radiative, with which the stimulated emission and absorption cross section spectra were derived for the " I... 

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