THz Sensing With Anomalous Extraordinary Optical Transmission Hole Arrays

Abstract

Subwavelength hole array (HA) metasurfaces support the so-called extraordinary optical transmission (EOT) resonance that has already been exploited for sensing. In this work, we demonstrate the superior performance of a different resonant regime of HA metasurfaces called anomalous EOT, by doing a thorough numerical and experimental study of its ability in thin-film label-free sensing applications in the terahertz (THz) band. A comprehensive analysis using both the regular and anomalous EOT resonances is done by depositing thin layers of dielectric analyte slabs of different thicknesses on the structures in different scenarios. We carry out a detailed comparison and demonstrate that the best sensing performance is achieved when the structure operates in the anomalous EOT resonance and the analyte is deposited on the non-patterned side of the metasurface, improving by a factor between 2 and 3 the results of the EOT resonance in any of the considered scenarios. This can be explained by the comparatively narrower linewidth of the anomalous EOT resonance. The results presented expand the reach of subwavelength HAs for sensing applications by considering the anomalous EOT regime that is usually overlooked in the literature.

Terahertz Sensing Based on Metasurfaces

“…[61] The resonance associated with the polarization parallel to the large hole array periodicity was identified as the normal extraordinary transmission resonance, and thus it was termed as regular extraordinary transmission. [56] with a substrate thickness of 50 µm, the anomalous extraordinary transmission resonance is deeply in cutoff, whereas with 75 µm it is also below but near the cutoff condition. This came as a surprise because for this polarization the periodicity of the hole array would not be compatible with extraordinary transmission and, in fact, it did not appear in previous reports.…”

“…[55,56] In ref. Different solutions have been presented in the last years in which it is possible to use both the patterned and unpatterned surfaces of the structures, leading to a dual surface-sensing.…”

“…However, the interest in rectangular hole arrays spurred some years later while studying the frequency response of doubly periodic single-layer hole arrays supported by a polypropylene slab. [56]. [61] The resonance associated with the polarization parallel to the large hole array periodicity was identified as the normal extraordinary transmission resonance, and thus it was termed as regular extraordinary transmission.…”

“…Interestingly, two different resonances were identified, each one of them corresponding to orthogonal polarizations along the two principal unit cell lattice vectors. [56] consisted of a periodic rectangular array of circular holes of diameter 105 µm and periodicity 115.5 × 350 µm etched on an aluminum layer of thickness 0.4 µm laying on a polypropylene substrate of two different thicknesses, 50 and 75 µm. On the other hand, when the polarization was parallel to the short hole periodicity, an unexpected resonance peak was noticed in the spectrum.…”

“…As said above, to excite it, one needs a minimum substrate thickness and in the case considered, 75 µm is near but not sufficiently thick enough. [56] for two different substrate thicknesses: 75 µm (left), and 50 µm (right) under normal incidence for different analyte thicknesses. On the other hand, when the analyte is deposited on the metallization side, the frequency shift of the anomalous extraordinary transmission resonance is negligible, as shown in the second column panels of Figure 3b, making this configuration ineffective for thin-film sensing purposes because the analyte Adv.…”