  We consider the waveguiding by thin patterned slabs embedded in a homogeneous
medium. In the longwave limit, the wave spectra of slabs are found to be well
described by a single frequency-independent parameter, which we call the
"guiding power". The guiding power can be evaluated in an effective medium
approximation, similar to the Maxwell Garnett theory, but modified for the
local field corrections specific to the two-dimensional geometry. The guiding
power is different for the transverse magnetic (TM) and transverse electric
(TE) polarizations. We show that the confinement factor of TM waves in a porous
layer with high index ratio can exceed that for a homogeneous layer. Similarly
enhanced confinement of TM waves is demonstrated for a layer of elongated
cylinders or elliptic inclusion with a high axis length ratio. The effect
originates from the suppression of local field effects and the increasing
internal field in the inclusion. It may be useful in the design of far-infrared
or THz quantum cascade lasers.
