  We present two types of metamaterial-based spectral bandpass filters for the
terahertz (THz) frequency range. The metamaterials are specifically designed to
operate for waves at normal incidence and to be independent of the field
polarization. The functional structures are embedded in films of
benzocyclobutene (BCB) resulting in large-area, free-standing and flexible
membranes with low intrinsic loss.
  The proposed filters are investigated by THz time-domain spectroscopy and
show a pronounced transmission peak with over 80]% amplitude transmission in
the passband and a transmission rejection down to the noise level in the
stopbands. The measurements are supported by numerical simulations which
evidence that the high transmission response is related to the excitation of
trapped modes.
