  Simultaneously enhanced reflectance and transmittance greater than 35 dB are
demonstrated for the lasing spaser (or spasing) behavior in an active fishnet
metamaterial. In mimicking a lasing cavity, an equivalent active slab with
Lorentz dispersion for the index of refraction is established to model the
spasing metamaterial through the Fabry-Perot effect. Numerical and theoretical
results show good agreement in the equal enhancement of reflectance and
transmittance, as well as the non-monotonic dependence of the spasing intensity
on the gain coefficient. In addition, directed emission of the spasing beam is
verified numerically.
