  Cyclotron resonance, along with its second and third harmonics, has been
observed in the quasi-two-dimensional organic superconductor
$\beta^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The harmonic
content is richly angle-dependent, and is interpreted in terms of the
interlayer warping of the Fermi surface giving rise to the cyclotron resonance.
In a departure from Kohn's theorem, but in agreement with recent theoretical
predictions, the effective mass deduced from the cyclotron resonance
measurements is greater than that determined from magnetic quantum
oscillations.
