As electronic devices become smaller, bulkiness of traditional spherical lenses has increasingly become a limiting factor for miniaturizing modern technology. Metalenses can break the limitations of traditional spherical lenses, allowing for the development of ultra-thin planar lenses. Recent work with metalenses has charted a new path for the technology for use in chemical sensing and disease diagnostics.

Wang et al. have demonstrated a new type of metalens in the mid-infrared spectral range. Patterned on a germanium wafer using standard nanofabrication processes, the 6 mm by 6 mm planar lenses could act on radiation at wavelengths of 3µm, 5µm and 8µm. The resulting metalens is as thin as a piece of paper and provides one of the first successful experimental attempts at focusing mid-infrared radiation in such a way.

Unlike spherical lenses, metalenses use an array of rectangular nanofins to change the phase of circularly polarized electromagnetic waves to ensure beams of light are all directed to one focal point.

The group’s lens reached a focusing efficiency of 80 percent and a numerical aperture as high as the designed theoretical value of about 0.8, a challenge when constructing their spherical counterparts. To demonstrate the new lens’s efficacy, the team imaged the infrared signals of a burning lighter with quality on par with traditional spherical lenses.

While the metalens array exhibited relatively strong dispersion and polarization dependence, the authors noted these issues can be solved in the future by utilizing new structures for the nanofins.

The group hopes to expand their work and next make a broadband metalens that can be used for spectrometry.

Source: “Planar metalenses in the mid-infrared,” by Ang Wang, Zhemin Chen, and Yaping Dan, AIP Advances (2019). The article can be accessed at https://doi.org/10.1063/1.5124074.