Abstract
Remarkably narrow optical homogeneous linewidths of the order of 1 kHz have now
been observed in low temperature zero-phonon transitions of dilute impurity ion
crystals, such as Pr3+ in LaF3. Novel nonlinear optical resonance techniques have been
devised for this purpose using ultrastable phase locked cw dye lasers where the
measurements are performed either in the frequency domain (hole burning) or in the
time domain (coherent optical transients). These studies effectively bring the Mossbauer
effect into the optical region. Hence, the observed linewidths are no longer limited by
inhomogeneous strain broadening (∼5 GHz) or even by static local fields due to
neighboring spins (∼100 kHz). However, weak magnetic field fluctuations from local
spins are readily detected. As an example, spin decoupling and line narrowing, which
are well known in NMR, are observed in an optical transition of Pr3+: LaF3 at 2 K
where the