Due to their narrow homogeneous linewidths, rare-earth ions in inorganic crystals at low temperatures have recently been given considerable attention as test materials for experiments in coherent quantum optics. Because these narrow linewidth transitions have been buried in a wide inhomogeneous line, the scope of experiments that could be carried out in these materials has been limited. However, here we present spectroscopic techniques, based on spectral hole burning and optical pumping, which allow hyperfine transitions that are initially buried within an inhomogeneously broadened absorption line to be studied with no background absorption from other transitions. A sequence of hole-burning pulses is used to isolate selected transitions between hyperfine levels, which makes it possible to directly study properties of the transitions, e.g., transition strengths, and gives access to information that is difficult to obtain in standard hole-burning spectroscopy, such as the ordering of hyperfine levels. The techniques introduced are applicable to absorbers in a solid with long-lived sublevels in the ground state and where the homogeneous linewidth and sublevel separations are smaller than the inhomogeneous broadening of the optical transition. In particular, this includes rare-earth ions doped into inorganic crystals and in the present work the techniques are demonstrated in spectroscopy of
Pr3+ in
Y2SiO5. Information on the hyperfine structure and relative transition strengths of the $^{3}H_{4}\text{\ensuremath{-}}^{1}D_{2}$ hyperfine transitions in
Pr3+:Y2SiO5 has been obtained from frequency-resolved absorption measurements, in combination with coherent and incoherent driving of the transitions.
4fn ⇒ 4fn transitions of trivalent rare earth ions doped in crystals frequently give sharp spectral lines. The transitions associated with the higher levels of a multiplet can be relatively sharp, 10-1000 GHz, where the width is determined by the decay rate of the levels by spontaneous phonon emission to lower levels. The transitions and their linewidth can be determined using conventional absorption or excitation techniques. On the other hand the transitions associated with the meta-stable lower levels typically give much sharper features with linewidths of 1 - 100 GHz and the widths are determined by the inhomogeneities of the crystal field. These inhomogeneous linewdths can be obtained by conventional spectroscopic techniques but the homogeneous linewidth requires more innovative techniques such as optical holeburning or photon echoes. The time domain photon echo measurements do not requiring a high resolution source and have frequently been used to establish that the homogeneous linewidth of such transitions. The lines can be very narrow down to the kHz regime [1,2]. Should complimentary measurements be attempted in the frequency domain utilising holeburning a special frequency stabilised lasers is required and one has been developed in association with the current work [3].