<p>A brief review is given of lifetime and line broadening studies with the free electron laser at FOM-Rijnhuizen (FELIX), emphasising work on far infrared (FIR) modulated photoluminescence (PL) of bound-to-bound transitions in InAs/GaAs quantum dots and mid-infrared (MIR) four wave mixing experiments on localised modes of H<sup>-</sup> ions in calcium fluoride crystals. We have made new far/near infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The FIR resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots, and analysis of the FIR double resonance linewidth enables us to show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation. In addition we have made time resolved studies of local modes in ionic crystals, where nonradiative decay plays an important role in the optical pumping cycle of laser gain media. We have made pump-probe studies of the local modes created upon the introduction of a light impurity, in particular the H - ion, in CaF<sub>2</sub> in the spectral region 700 to 1200 cm<sup>-1</sup>. We have also measured the free induction decay of this mode using a noncollinear degenerate four wave mixing geometry. The observed coherent transient is striking in having large quantum beat modulations at negative time which are associated with vibrational ladder climbing.</p
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