Due to the ability to modulate the wavefront with subwavelength spatial resolution and the great application potentials in the thermal infrared region, the long-wavelength infrared (LWIR) metasurfaces have been attracting a significant amount of research interest in recent years^[1–4]. Considering the advantages of high refractive index^[5,6], silicon and germanium are two alternatives of base materials in the design process of the LWIR metasurfaces^[7]. Although the refractive index is higher and the optical loss is lower in the LWIR band, there are less research reports on germanium-based metasurfaces with high aspect ratios than there are on silicon^[3,8]. The increased complexity of the fabrication process resulting from the chemical properties of germanium may be one of the reasons for the above phenomenon^[7].

In this paper, we propose a simple method for fabricating the germanium-based metasurfaces with a high aspect ratio, and the LWIR metalens is chosen as the target device. First, we discuss the advantages of selecting germanium as the base material of the meta-atoms in some metasurface applications. Then, we introduce the simple fabrication method of germanium-based metasurfaces, and the feasibility is analyzed. Finally, an all-germanium LWIR metalens whose diameter and focal length are both 25 mm is fabricated, and the corresponding imaging performance is characterized to demonstrate the validity of the fabrication method. The fabrication method proposed in this paper is expected to play a role in LWIR metasurface research.

Focusing on the metasurface devices for shaping optical wavefronts, such as metalens, the discrete meta-atoms are elaborately arranged to meet the continuous phase change requirement, and the period of the meta-atoms must satisfy the Nyquist sampling criterion^[9–11]. The following equations describe the phase profiles of the diffraction-limited (DL) hyperbolic metalens and the wide field of view (WFOV) metalens, respectively^[12,13]: $${\varphi }_{\mathrm{DL}}(r)=-{\mathit{k}}_{\mathbf{0}}(\sqrt{f^2+r^2}-f),$$(1)$${\varphi }_{\mathrm{WFOV}}(r)=\frac{-{\mathit{k}}_{\mathbf{0}}{r}^2}{2f},$$(2)where ${\varphi }_{\mathrm{DL}}(r)$ and ${\varphi }_{\mathrm{WFOV}}(r)$ represent the required phase at the position $r$ of the DL metalens and the WFOV metalens to the wavelength $\lambda$, respectively; ${\mathit{k}}_{\mathbf{0}}$ is the free-space wavenumber; and $f$ is the focal length. According to the Nyquist sampling criterion^[11], the periods of the meta-atoms comprising these two metalenses need to satisfy the following equations: $$P_{\mathrm{DL}}<\frac{{\lambda }_0}{2}·\sqrt{1+F^{\#2}},$$(3)$$P_{\mathrm{WFOV}}<\frac{{\lambda }_0}{2}·\frac{1}{1+\sin \theta },$$(4)where $P_{\mathrm{DL}}$ and $P_{\mathrm{WFOV}}$ represent the period of the meta-atoms constituting the DL metalens and the WFOV metalens, respectively; ${\lambda }_0$ is the free-space wavelength; $F^{\#}$ is the F-number of the DL metalens; and $\theta$ is the maximum angle of view of the WFOV metalens. The curves shown in Fig. 1 correspond to Eqs. (3) and (4), respectively, and it can be seen that the DL metalens with a low F-number, which corresponds to a better imaging performance, and the WFOV metalens with a large field of view both require that the period of the meta-atoms can be miniaturized to reconstruct the ideal wavefront. The miniaturized meta-atoms can also simultaneously avoid excitation of unwanted diffraction orders^[6].

In the design process, the isolated cylindrical waveguide model whose phase modulation is based on the propagation phase is often chosen as the basic element model of the polarization-insensitive metalens^[14], as shown in Fig. 2(a). To the isolated cylindrical waveguide model, the meta-atoms with a higher refractive index can achieve 0 to $2\pi$ transmission phase variation with a smaller period under the same thickness-to-period ratio^[5], which is consistent with the requirement of the period miniaturization of the metalens, and Figs. 2(b) and 2(c) show the comparison between silicon-based meta-atoms and germanium-based meta-atoms. As shown in Fig. 2(c), the germanium-based meta-atoms own a larger propagation constant, which corresponds to a better phase modulation ability. In addition, compared to silicon, germanium owns a lower absorption coefficient in the LWIR band^[7].

As was mentioned previously, the experimental study on the fabrication methods of germanium-based meta-atoms is very significant in some cases, such as metalens. Due to the negative influence of the chromium etching solution on the optical surface of the germanium^[15] and the fabrication method of the silicon-based metalens using Cr as the hard mask, the germanium-based metalens cannot be simply replicated^[2,8]. After a series of experiments, we propose a simple fabrication method of germanium-based metalenses, and Fig. 3 schematically illustrates the key steps. The pattern of the metalens is written on the photoresist coated on the germanium substrate using the laser direct writing system (Heidelberg Instruments, DWL4000). After the resist development, a 300-nm layer of ${\mathrm{Al}}_2{\mathrm{O}}_3$ is deposited onto the patterned substrate, and the pattern is transferred into the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer through the resist removal process. The deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ acts as a hard mask whose selectivity is more than 97, and the dry etching is performed in a mixture of ${\mathrm{SF}}_6$ and ${\mathrm{C}}_4{\mathrm{F}}_8$ plasmas using an inductively coupled plasma (ICP) system. Different from most hard masks, the ${\mathrm{Al}}_2{\mathrm{O}}_3$ would not be removed after the dry etching to reduce potential damage to the optical surface of the germanium substrate, and the entire fabrication process is simplified simultaneously.

Although the extinction coefficient of the alumina thin film cannot be neglected in the LWIR band^[7], the 300-nm alumina layer would not have a significant impact on the phase modulation ability of the meta-atoms due to its small thickness^[16], as shown in Figs. 4(b) and 4(c). A scanning electron micrograph (SEM) image of the germanium-based meta-atoms, which are fabricated by the method proposed in this paper, is shown in Fig. 4(a). The surfaces of the meta-atoms remain smooth, and the sidewall angles are close to 90°.

To validate the feasibility of the fabrication method, we conduct the simulation, fabrication, and experimental characterization of the germanium-based metalens. In the simulation process, a 202.5-µm-diameter LWIR DL metalens whose F-number is 1 is designed to operate at the wavelength of 9.3 µm, and the isolated cylindrical waveguide model is chosen to work as the basic element model. The metalens is comprised of eight different meta-atoms to encode eight phase levels, and their transmission amplitude and phase shift corresponding to the normal incidence are shown in Fig. 5(a). The required and realized phase at each radial coordinate across the metalens are shown in Fig. 5(b). The simulated focusing result of the metalens at an incident angle of 0° is shown in Fig. 5(c), which corresponds to the simulated light intensity distributions on the $xz$-plane ($y=0\mathrm{\mu m}$) and $xy$-plane ($z=202.5\mathrm{\mu m}$) at the target wavelength. Figure 5(d) shows the one-dimensional diagram of the light intensity distributions of the metalenses with and without the deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$, and the focusing effect of the metalens is close to the theoretical values of the diffraction limit. According to the simulation results, the focusing efficiencies of the metalenses with and without the deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ are 36.12% and 36.80%, respectively^[6], and the high refractive index of the germanium inevitably leads to a higher reflection loss at the interface. The simulation results demonstrate that the performance of the metalens is almost unaffected when the deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ is retained.

In the fabrication process, the meta-atoms shown in Fig. 5(a) are chosen to construct a metalens with an F-number of 1, and the diameter and the working wavelength are 25 mm and 9.3 µm respectively. First, a positive photoresist (S1805) layer with a thickness of 780 nm is spin-coated on the germanium substrate and baked at 110°C for 2 min. Then, a 300-nm alumina layer is deposited on the patterned substrate after laser direct writing and resist development. Finally, the sample is etched at a ${\mathrm{SF}}_6$ flow rate of 20 sccm and a ${\mathrm{C}}_4{\mathrm{F}}_8$ flow rate of 40 sccm when the ICP power and BIAS power are 160 W and 30 W, respectively, after the resist removal process. The images of the germanium-based LWIR metalens are shown in Fig. 6.

To characterize the performance of the germanium-based metalens, the detector of a Gobi 640 GigE camera and the fabricated metalens are combined to construct a metalens camera, as shown in Figs. 7(a) and 7(b). The laser illumination indoor imaging experiment and outdoor imaging experiment are conducted, respectively. During the indoor imaging experiment, the LWIR source is a ${\mathrm{CO}}_2$ laser emitting 9.3 µm light, the target is shown in Fig. 7(c), and the corresponding image is shown in Fig. 7(d). During the outdoor imaging experiment, the chimneys built in the thermal power plant 3 km away from the metalens camera is chosen as the target, as shown in Fig. 7(e), and the corresponding image is shown in Fig. 7(f). These experimental results confirm the imaging capability of the germanium-based LWIR metalens. It should be noted that the influence of stray light on the images is resulted from the absence of the filters.

In summary, we propose a simple fabrication method of germanium-based metalenses in this paper, the deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer with a high selectivity is chosen as the hard mask, and it is still retained after the dry etching process. The alumina layer with a small thickness would not have a significant impact on the phase modulation ability of the meta-atoms, and the performances of the metalenses with and without the deposited ${\mathrm{Al}}_2{\mathrm{O}}_3$ are almost identical. The simulation and experimental characterization results verify the feasibility of the fabrication method in this paper which may have a promising potential in the field of LWIR metasurfaces.