Optical multiple pulse sequences for multiphoton selective excitation and enhancement of forbidden transitions
In this paper we present novel and simple pulse sequences for enhancing the intensity of forbidden or highly nonresonant optical transitions. These sequences provide a straightforward approach to circumventing the most serious limitation of optical coherent transientspectroscopy: Available laser intensities are often insufficient to excite a significant fraction of the ground state population into desired excited states, either because of large inhomogeneous broadening or, in the case of multiphoton absorption, large anharmonicities. Optical phase modulation (which can be produced by an acousto‐optic modulator) or amplitude modulation (which can be produced by an interferometer) with carefully chosen pulse flip angles and delays can effectively remove even very large energy mismatches, thus permitting essentially complete population inversions arbitrarily far from resonance. Coherent averaging theory and computer calculations are used to derive particularly valuable sequences. Pumping enhancement of 10^3–10^5 (depending on individual molecular parameters) for these modulated pulse sequences are predicted. Specific applications to multiphoton pumping of local vibrational modes are discussed.
Additional Information© 1983 American Institute of Physics. Received 1 September 1982; accepted 9 September 1982. We wish to thank Luciano Mueller for useful discussion about NMR composite pulse sequences in three level systems. W. S. W. held a National Science Foundation Postdoctoral Fellowship at Caltech during the initial stages of this work. This work was supported in part by the National Science Foundation under Grant No. DMR 81-05034 to the California Institute of Technology.
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