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Team Looks to the LWIR to Fabricate Achromatic Metalens

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CHANGCHUN, China, May 9, 2022 — Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences demonstrated a broadband achromatic metalens with a numerical aperture of 0.32. The demonstration stemmed from a general method, proposed by the same team, to implement a broadband achromatic metalens in the longwave infrared (LWIR) band based on the combination of the dynamic and geometric phases.

The researchers said that compared to state-of-the-art chromatic-aberration-restricted LWIR metasurfaces, the achromatic metasurfaces yielded in their work bring the field a step closer to practical applications.

The LWIR band is essential for applications such as thermal imaging and optical communications. However, conventional LWIR optics are bulky and expensive, which hinders the development of integrated LWIR optics. Metasurfaces, which are composed of subwavelength meta-atoms, have powerful light manipulation capabilities and are promising platforms for integrated optics. However, conventional metasurfaces are highly chromatic, despite comprising weakly dispersive materials.

Generally, the dynamic phase can be tuned by manipulating the resonance mode supported by the meta-atom through tuning the in-plane geometric parameters. The geometric phase is only related to local rotation angle of the meta-atom and does not change the dispersion characteristics of the meta-atom.

Therefore, by combining dynamic phase and geometric phase, the phase and dispersion can be decoupled, the researchers explained. They chose germanium as the base material of the meta-atom in the work; germanium has a refractive index and low intrinsic loss properties in the LWIR band. For the archetypes of the meta-atoms, the researchers chose combinations of nanofins. This is due to the coupled waveguide mode, supported by the adjacent nanofins, allowing users to control the dispersion more precisely.


Tests showed that the focal length of the fabricated achromatic metalens shifted only 0.65% relative to mean focal length from 9.6 to 11.6 μm. The focal length of a chromatic metalens shifted 20% relatively. Further, the Strehl ratio of the achromatic metalens is above 0.96 for the entire operating band, indicating that the broadband diffraction-limited focusing is achieved. The average efficiency of the broadband achromatic metalens was 31%, which is comparable to the efficiency of monochromatic metalens in LWIR.

The Strehl ratio is the ratio of peak diffraction intensities of an aberrated versus perfect wavefront. This measure, of the quality of an image formation, indicates the level of image quality in the presence of wavefront aberrations and can define the highest acceptable level of wavefront aberration for general observation in or with an optic. 

According to the researchers, their proposed method worked equally well with other types of achromatic metasurfaces. They demonstrated a broadband achromatic metasurface grating with constant deflection angle of 30° from 9.6 to 11.6 μm.

The research was published in Nanomaterials (www.doi.org/10.3390/nano11102760).

Published: May 2022
Glossary
metasurfaces
Metasurfaces are two-dimensional arrays of subwavelength-scale artificial structures, often referred to as meta-atoms or meta-elements, arranged in a specific pattern to manipulate the propagation of light or other electromagnetic waves at subwavelength scales. These structures can control the phase, amplitude, and polarization of incident light across a planar surface, enabling unprecedented control over the wavefront of light. Key features and characteristics of metasurfaces include: ...
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
phase
In optics and photonics, "phase" refers to a property of electromagnetic waves, such as light, that describes the position of a wave at a given point in time within its oscillation cycle. More specifically, it indicates the position of a wave relative to a reference point, typically the starting point of a cycle. When discussing phase in optics, it's often described in terms of the phase difference between two waves or the phase of a single wave. The phase difference between two waves is the...
wavefront
In considering a field of electromagnetic energy emanating from a source, the wavefront is a surface connecting all field points that are equidistant from the source.
metalens
A metalens, short for "metasurface lens," is a type of optical lens that uses nanostructured materials to manipulate light at a subwavelength scale. Unlike traditional lenses made of glass or other transparent materials, metalenses do not rely on the curvature of their surface to refract or focus light. Instead, they use carefully engineered patterns of nanostructures, such as nanoscale antennas or dielectric structures, to control the phase and amplitude of light across the lens's surface....
achromatic
Color correcting; chromatic aberration corrected.
achromatic lens
A lens consisting of two or more elements, usually of crown and flint glass, that has been corrected for chromatic aberration with respect to two selected wavelengths. Also known as achromat.
OpticslensesImagingoptical componentsLWIRmetasurfacessurfacesnanophasewavefrontwavefront aberrationwavefront aberration measurementmetalensmetasurface gratinggratingsachromaticachromatic lensesAchromatic Lensachromatic metalensAsia PacificChangchun Institute of Optics Fine Mechanics and PhysicsChinese Academy of SciencesResearch & Technologyeducation

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