  We propose and verify a method for controlling the frequency dependence of
the group delay of electromagnetic waves over a broad frequency range using the
Brewster effect in single-layer metamaterials with finite thickness, here
referred to as metafilms. When the metafilm's reflectance vanishes regardless
of the incident frequency, the group delay can be large near its resonance
frequency while maintaining the transmittance close to unity regardless of the
incident frequency. Furthermore, when several reflectionless metafilms are
stacked together, the total group delay should be given as the sum of the
individual group delays. In this study, we realize reflectionless metafilms by
arranging the meta-atoms so that the Brewster effect occurs regardless of the
incident frequency. We evaluate in numerical simulations and experiments the
frequency dependence of the transmittance and of the group delay of a
three-layer metamaterial composed of reflectionless metafilms with different
resonance frequencies, and find that the total transmittance and group delay of
this metamaterial agree respectively with the product of the transmittances and
the sum of the group delays of the constituent metafilms.
