Fig. 1. (a) General schematic of the bilayer-metamaterial (BLMM) bandpass filter. Two plasmonic metasurfaces are placed adjacent to each other separated by a dielectric coupling layer of thickness d. Each metasurface consists of an array of a metallic square-loop hole structure deposited on Zeonor, a THz transparent substrate. The lattice pitch (P), length of the outer square-loop hole (L), and length of the inner square-loop hole (w) of the array are labeled with black arrows. (b) 3D atomic force microscope (AFM) image and (c) optical microscope image of the device S2, and (d) corresponding depth profile indicating a metal depth of ∼220  nm. (e) THz power transmittance of the commercial double-sided adhesive tapes from Nitto and 3TC used in the fabrication of the BLMM and 2BLMM devices. In this experiment, the double-sided tapes are laminated on a Zeonor substrate and the spectral transmission measured through that substrate is also displayed (black line).

Fig. 2. THz measured (circles) and simulated (solid lines) transmittance spectrum of the BLMM-based broad bandpass filters in (a) linear scale and (b) semi-logarithmic scale. Experiments are performed with a time-domain THz spectroscopy system and are in good agreement with FDTD simulation results. (c) Measured maximum transmission (Tmax) of the structures over center frequency in semilogarithmic scale, in which error bars present the corresponding octave-spanning FWHM linewidth. (d) Calculated fractional bandwidth (in percentage) of all BLMM devices over frequency for the experiment results.

Fig. 3. Normalized electric field amplitude distribution of the BLMM structure S5. The arrows indicate surface current distribution within one period of the array. (a) Top view distribution in x−y plane at the surface of the bottom layer plasmonic structure (z=0.2  μm) at the frequency of 2.9 THz. (b) and (c) Side view of the electrical field distribution and direction in the x−z plane, corresponding to a 2D cross section taken at y=−8  μm at a frequency of 2.9 THz and 3.6 THz, respectively. The gray 3D schematics in (b) and (c) illustrate the simulated unit cell of the stacked metasurfaces. The incident THz wave propagates in the z direction and is polarized along the x direction.

Fig. 4. (a) Schematic of the 2BLMM devices fabricated from two BLMM structures bound together with double-sided tape (separated by a length D). Comparison of the measured THz transmission of the BLMM (blue curve) and 2BLMM (orange curve) devices for (b) S2 and (c) S4. Dotted lines show the averaged attenuation floor.

Fig. 5. Measured THz transmittance spectrum of two batches (lines and circles) of the BLMM-based broad bandpass filters in (a) linear scale and (b) semi-logarithmic scale. There is a good repeatability between the pairs of S2, S3, S4, and S6 devices fabricated in two batches and characterized on two different days.

Table 1. Geometrical Parameters of the Broad Bandpass Device^a

Table 2. Performance Comparison of the Filters Presented in This Work to Similar Structures Based on Layered Metasurfaces Reported in the Literature