Optically excited ruby as saser: Experiment and theory
L. G. Tilstra, A. F. M. Arts, and H. W. de Wijn
Department of Physics and Astronomy, and Debye Institute, Utrecht
University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
Avalanches of phonons are generated following population inversion of the
Zeeman-split E(2E) Kramers doublet in a single crystal of dilute
ruby (Al2O3:Cr3+) at 1.4 K. The inversion
is achieved in a narrow zone of limited extent by selective pulsed optical
pumping, and the phonons are detected via luminescence. The associated
acoustic wave emanating from the zone propagates through the crystal,
reverberates in the acoustic cavity formed by the crystalline end faces,
and engages in further amplification each time it passes through the zone
until the inversion becomes exhausted. The systems thus acts as a saser
(sound amplification by stimulated emission of radiation). The avalanche
is adequately described by coherent Bloch equations governing the acoustic
wave and the interacting spin polarization, the losses being due to surface
scattering and divergence. This conclusion holds good upon reducing the
resonance length of the zone by a gradient magnetic field. Finally, the
frequency and angular distributions of the avalanche are measured by using
two disjunct zones located at opposite ends of the crystal, one serving as
generator and the other as detector, in conjunction with a gradient field
and lateral displacement. The frequency spread is found to be compatible
with the inhomogeneously broadened transition within E(2E),
while the angular divergence is determined by the geometry of the zone.