Metamaterials (MMs) have attracted considerable interest owing to their exotic electromagnetic properties and functionalities that do not exist in naturally occurring materials [1–5]. The desired electromagnetic response in MMs can be achieved by properly tailoring their meta-atoms or molecules by a few key multipole excitations, such as electric and magnetic dipoles. Recently, toroidal multipoles have attracted great attention because they are necessary for the complete multipole representation of an arbitrary radiating source along with the familiar electric and magnetic multipoles [6,7]. The toroidal dipole response in artificial MMs is usually weak and masked by more dominant electric and magnetic multipoles. Therefore, toroidal MMs are initially designed to amplify toroidal moments and suppress the competing electric and magnetic multipoles [8,9]. Fortunately, since the first experimental demonstration of toroidal dipole response in MMs in 2010 [10], there has been great progress in metallic or dielectric metamaterials from the microwave to visible wavelengths [11–18]. Toroidal dipoles offer a deeper insight into the multipolar response of subwavelength optical structures to many optical phenomena such as electromagnetically induced transparency (EIT) [16,19], unconventional optical activity [9,20], negative index of refraction [21], and bound states in the continuum (BIC) [22–25].

Recently, the anapole mode as a radiationless state with nontrivial oscillating current configuration has gained widespread attention, and it can be achieved by the destructive interference between the electrical and toroidal dipoles [26–30]. The nonradiating response of the anapole mode accompanied by enhanced near fields in single nanoparticles has found numerous applications such as in cloaking [31–33], harmonic generation [34–36], nanoscale lasers [37], and Raman scattering enhancement [38]. The anapole mode was first proposed and experimentally demonstrated in microwaves using metallic MMs with three-dimensional (3D) dumbbell apertures in 2013 [39]. By adjusting the quantity and rotation angle of metallic plates, anapole resonant electromagnetic transparency with a high Q factor was achieved. Recently, a quasiplanar plasmonic MM [40] from a combination of dumbbell apertures and vertical split-ring resonators (SRRs) was designed and fabricated, which exhibited an anapole response in the near-infrared (IR) spectra. However, the fabrication of 3D or quasiplanar structures of MMs is complicated, especially for the visible and terahertz (THz) frequency ranges. Therefore, designing planar anapole metamaterials is expected for realistic applications. Although there are some works discussing the anapole excitation in planar metamaterials in the microwave [41], THz [42–45], and optics [46–49] frequency ranges, the anapole-induced resonant transparency has not been observed. In addition, the tunability of anapole frequency is poor in previous works, and the design and realization of anapole resonance over a wide frequency range remains challenging.

In this paper, we describe the quantitative and experimental demonstration of anapole-induced total resonant transparency in a dumbbell apertured planar metamaterial at THz frequencies. The near-Lorentz transmission line of resonant electromagnetic transparency was achieved through destructive interference between the electric dipole and toroidal dipole as well as effective suppression of the magnetic quadrupole. By optimizing the structure parameters, robust anapole resonance with high transmittance was achieved over the frequency range of 0.15–0.93 THz by adjusting one geometric parameter. The proposed planar anapole metamaterials were fabricated on stainless-steel sheets using a laser micromachining technique, and the measured transmissions of the resonant transparency were in good agreement with the simulations.

It is worth mentioning that an optical anapole was achieved in a 3D dumbbell-SRR metallic MM [40]. The destructive interference between P and ikT can occur when the MM is made of single dumbbell apertures without SRRs, but the scattered power of the QM is close to that of the P, which greatly increases the radiative loss of the resonance. In order to suppress the excitation of the QM, an SRR positioned vertically to the dumbbell aperture was introduced. As a result, the scattered power of the QM was reduced to approximately 1/3 of the P; however, the resonant transparency was not achieved, which may be due to the large material loss. For the simple dumbbell metamaterial we designed for the THz band, the scattered power of QM is effectively suppressed to only 1/10 of P, much smaller than that in the quasiplanar structure at the optical band, leading to the enhancement of anapole resonant transparency. This result can be explained by the large difference in the intrinsic ohmic resistance of metal at the optic and THz bands. As the ohmic resistance of the metal in the THz band is much smaller than that in the optic regime, the incident electric field induced pair of counter-rotating surface currents along the two circular cuts of the dumbbell aperture shown in Fig. 2 is greatly enlarged, resulting in the enhancement of the excitation of the T, i.e., relative suppression of the QM.

Figures 4(c) and 4(d) plot the transmission spectra and scattered powers for the metamaterial at select values of W, which are similar to the results shown for select R values. As W increases from 200 to 300 μm, the transmission peak of the resonance increases quickly and reaches a maximum when anapole resonance is achieved at W=300μm, while the resonance frequency decreases steadily.

However, the resonance characteristics of the metamaterial with respect to G are quite different when R and W are fixed at 100 and 300 μm. As shown in Figs. 4(e) and 4(f), the resonance frequency shifts from 0.15 to 0.31 THz when G increases from 15 to 150 μm, but the resonance peak remains higher than 0.95. It is interesting to note that P and ikT not only increase quickly with the increase of G but also remain equal in strength and opposite in phase at all times [see Fig. 4(f)]; hence, complete destructive interference occurs in the entire G range, leading to anapole-induced total resonant transparency. That is, the anapole-induced resonant transparency can be tuned in the range of 0.15–0.31 THz by simply changing the width of the dumbbell aperture G.

In conclusion, we quantitatively and experimentally demonstrated the total resonant transparency in a planar anapole metamaterial. The metamaterial consists of dumbbell apertures on a stainless-steel sheet, which was easily fabricated using a laser micromachining technique. The near-Lorentz transmission line of resonant electromagnetic transparency was achieved by destructive interference between the electric dipole and the toroidal dipole, with concurrent suppression of the magnetic quadrupole. Thus, a simple method for achieving robust anapole-induced resonant transparency with high transmittance in the frequency range of 0.15–0.93 THz is provided. The measured transmission results were in good agreement with the simulations. Such planar anapole metamaterials with low ohmic loss at THz frequencies can be potentially used in filters, sensors, or other photonic devices.