**3.4 Broadband achromatic metalens**

In order to address another major challenge as the dispersion, we give the most popular example on an ultra-broadband, achromatic terahertz metalens which can operate at 0.3THz to 0.8THz. The phase profile of the theoretical design introduced in the Section 2, which agrees well with the phase profile transmitted by the metalens in full wave simulation. Owing to the extremely large etching aspect ratio of 1:25, a large phase compensation are achieved from the library of the unit cell, which obtains a relatively high NA value as 0.385 at the large metalens diameter of D = 10 mm. Moreover, the C-shaped unit elements employed in the design exhibit a more robust phase accumulation than the rectangular structures. The metalens is fabricated on a silicon substrate with several hundred microns thickness, which is highly desirable for integration and miniaturization. The design significantly promotes the development of achromatic meta-devices in terahertz hyperspectral imaging and can be used to investigate the robustness of functional metasurface designs [11, 12] (**Figure 10**).

A metalens device consisting of C-shaped unit cells with NA = 0.385 is fabricated with focal length of 12 mm in a diameter of 10 mm. The transmitted field

#### **Figure 10.**

*Schematic of achromatic metalens. (a) Schematic of C-shaped (or rectangular) unit element-based achromatic metalens. (b) Phase profile for achromatic metalens at the wavelength range of* λ∈*{*λ*min,* λ*max}, where* Δφ *is a certain positive value.*

#### **Figure 11.**

*The focusing demonstration of the achromatic terahertz metalens with C-shaped antenna array. (a)–(c) the intensity profiles at horizontal and vertical cross-sections on the focal plane at the wavelength of 0.3THz, 0.6THz and 0.8THz. (d)–(f) the focal spots profiles obtained from the simulation and experimental results. (g)–(i) simulated results at wavelength of 0.3THz, 0.6THz and 0.8THz. (j) Efficiency trend of the metalens focusing. (k)–(m) incident field, LCP focusing profile and RCP defocusing profile at the wavelength of 0.6THz.*

of the horizontal polarization (Ey) on the focal plane is present in **Figure 11a–c**. The experimental results show the high consistence with the simulated results in **Figure 11g–i**, which demonstrates that the focal length at 12 mm shows pretty stable across a broad frequency range. All the measured focus profiles show the full-width at half-maximum (FWHM) close to the diffraction limitation as k/(2NA) shown in **Figure 11(d–f )**. **Figure 11d** shows the horizontal intensity profile at the focal spot
