  Terahertz (THz) metamaterials have been developed for THz sensing, detection,
imaging, and many other functions due to their unusual absorbers. However, the
unusual absorption spectra change with different incident angles. Thus, we
designed and fabricated a focal plane array with metal-insulator-metal (MIM)
structure metamaterial absorbers for further research. The absorption spectrum
with incident angles from 20 to 60 was measured using THz time-domain
spectroscopy (THz-TDS), and the experimental results reveal that the absorption
spectrum changes with incident angle variations. A basic analytical asymmetric
peak model for extracting absorption-frequency characteristics was developed in
this study to quantitatively explore this variation in the absorber behavior
with incident angles. The best result was that the frequency corresponding to
the highest absorption can be easily found using this peak model. The
experimental data was coherent with the validation of the asymmetric peak
model. Moreover, a second model to quantitatively relate parameters to the
incident angle was discovered, allowing for the prediction of absorption
spectrum shifts and changes. The absorption spectrum was predicted to have a
valley-like absorption curve at particular incident angles based on the
secondary models deduction. The proposed extraction method's essential feature
is that it can be applied to any physics-based MIM metamaterial system. Such a
model will guide the design and optimization of THz metamaterial absorbers,
sensors, imagers, and many others.
