  We present octave-wide bandpass filters in the terahertz (THz) region based
on bilayer-metamaterial (BLMM) structures. The passband region has a
super-Gaussian shape with a maximum transmittance approaching 70% and a typical
stopband rejection of 20 dB. The design is based on a metasurface consisting of
a metallic square-hole array deposited on a transparent polymer, which is
stacked on top of an identical metasurface with a sub-wavelength separation.
The superimposed metasurface structures were designed using finite-difference
time-domain (FDTD) simulations and fabricated using a photolithography process.
Experimental characterization of these structures between 0.3 to 5.8 THz is
performed with a time-domain THz spectroscopy system. Good agreement between
experiment and simulation results is observed. We also demonstrate that two
superimposed BLMM (2BLMM) devices increase the steepness of the roll-offs to
more than 85 dB/octave and enable a superior stopband rejection approaching 40
dB while the maximum transmittance remains above 64%. This work paves the way
toward new THz applications, including the detection of THz pulses centered at
specific frequencies, and an enhanced time-resolved detection sensitivity
towards molecular vibrations that are noise dominated by a strong,
off-resonant, driving field.
