Metalenses with Integrated Liquid Crystals

Scheme of a liquid crystal integrated metalens. (Source: Z. Shen et al., Nanjing U.)

The development of meta­surfaces opened a horizon for the advance of planar optics. Among various metadevices, the metalens has attrac­ted widespread attention for practical appli­cations in imaging and spectro­scopy, where it allows multi­functional wavefront mani­pulations for improved focus. As part of the trend of minia­turization and inte­gration of photonic systems, metalenses are replacing the tradi­tional refractive lenses made of polished crystals or polymers. But their functions remain static. The prospect of realizing active meta­lenses has motivated the introduction of materials with special properties, such as switchable bifocals or discrete focal lengths. Until recently, dynamic mani­pulation of metalenses, especially tunable chromatic dispersion, has remained out of reach.

Metalenses are plagued by the chromatic aber­ration. Overcoming this failure is a vital concern for improved resolution in full-color and hyperspectral imaging. On the contrary, for spectro­graphic analysis and tomo­graphic appli­cations, chromatic dis­persion helps to separate focal spots for different frequencies, to avoid crosstalk. The ability to mani­pulate chromatic dispersion dynami­cally with a single metalens would promote system inte­gration and functional versa­tility.

A research team from Nanjing Uni­versity recently demons­trated active mani­pulation of chromatic dispersion, achieving achro­matic focusing within a designated broadband. The team integrated a photo-patterned liquid crystal into a dielec­tric meta­surface. In their design, the meta­surface generates a linear-resonant phase dispersion, which means that the phase front of the transmitted wave is delayed linearly by the dielec­tric metasurface. The liquid crystal is responsible for generating frequency-inde­pendent geometric phase modulation.

The team verified the chromatic aberration control of the combined lens and demons­trated a significant dynamic broadband imaging contrast effect. The design can be extended to other active meta­devices; as an example, the team presented a beam deflector with controllable dispersion. “Combining the flexibility of metadevices with the broadband electro-optical charac­teristics of liquid crystals makes the design competent for wavefront control from the visible wavelengths to the THz and micro­waves,” remarked Yanqing Lu of the College of Engi­neering and Applied Sciences at Nanjing University.

Lu’s colleague at the College of Engi­neering and Applied Sciences Wei Hu notes, “We anticipate that liquid crystal inte­grated metadevices will give birth to a variety of active planar photonic elements to enhance the functional flexi­bility of optical systems.” (Source: SPIE)

Reference: Z. Shen et al.: Liquid crystal integrated metalens with tunable chromatic aberration, Adv. Phot. 2, 036002 (2020); DOI: 10.1117/1.AP.2.3.036002

Link: Liquid Crystal and Nanophotonics Group, Nanjing University, Nanjing, China

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