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 has been attracting a significant amount of research interests in recent years. Considering the advantages of the high refractive index, silicon and germanium are two alternatives of base material in the design process of the LWIR metasurfaces. Although refractive index is higher and optical loss is lower in the LWIR band, the research reports on germanium-based metasurfaces with high aspect ratios are less than those on silicon The increased complexity of the fabrication process resulted from chemical properties of germanium may be one of the reasons for the above phenomenon.
In this work, a simple fabrication method of germanium-based metasurfaces is proposed, where the deposited Al2O3 layer with a high selectivity is chosen as the hard mask and retained after the dry etching process. Although extinction coefficient of the alumina thin film can't be neglected in LWIR band, but a thin layer of alumina would not have a significant impact on the phase modulation ability of the meta-atoms due to its high selectivity.
Fig. 1 (a) The side-view SEM image of the germanium-based meta-atoms. (b) The amplitude and phase of the transmitted light through the meta-atoms. (c) The y-z cross section view of the normalized magnetic energy density in a periodic array for different meta-atoms.
The research group is led by Prof. Jinsong Gao from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. The research results are published in Chinese Optics Letters, Vol. 22, Issue 7, 2024: Dongzhi Shan, Jinsong Gao, Nianxi Xu, Naitao Song, Hongda Wei, Qiang Li, Yi Zhao, Hai Liu, Qiao Sun, Xiaonan Jiang, Chao Li, Zhiyu Zhang. A simple fabrication method of germanium-based metasurfaces[J]. Chinese Optics Letters, 2024, 22(7): 073602.
To demonstrate the validity of the fabrication method proposed in this paper, an all-germanium LWIR metalens whose diameter and focal length are both 25 mm is fabricated and the corresponding imaging performance is characterized. The experimental study on the fabrication methods of germanium-based metasurfaces is very significant as the meta-atoms with a higher refractive index can achieve 0 to 2π transmission phase variation with a smaller period under the same thickness-to-period ratio, which is consistent with the requirement of the period miniaturization in some cases.
Fig. 2 The overview of the fabrication method proposed in this paper and images of the metalens at different steps.
Further, the research group will continue to carry out the research on the micro/nano processing techniques of some important materials for optical metasurfaces design.
