Ultra-thin subwavelength film sensing through the excitation of dark modes in THz metasurfaces

We demonstrate ultra-subwavelength thin film sensing through experimental and numerical studies of the transmission characteristics of several symmetric and asymmetric planar split ring resonator (SRR) based terahertz metasurfaces coated with thin film of analyze layer(s). A sharp narrow-band dark mode resonance excited in the asymmetric metasurfaces is seen to be particularly very interesting to be exploited in thin film sensing, which is otherwise absent in the symmetric case. For this mode, our experiments clearly have demonstrated sensing of extreme subwavelength (close to thousandth part of the wavelength) thickness film. With the increasing thickness of the analyze layer a gradual red-shift of the resonance mode is observed. The sensitivities and figure of merit (FoM) of all the metasurfaces are derived and compared. The resonant dark modes in asymmetric metasurfaces are found to display substantially better FoM as compared to the typical dipolar modes.