Metalens as a Hologram Generator

Schematic of multiwavelength achromatic metalens. In addition to acting as a beam and hologram generator, this metalens can also focus four different wavelengths of light at the same distance. (Source: Capasso Lab, Harvard SEAS)

Researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS have developed a flat optical component that is simul­taneously a metalens, a micro­scope objective that can resolve details smaller than a wave­length of light, and an optical vortex and holo­gram gene­rator. Each func­tionality is controlled by a different wave­length of light.

“The break­through of this new flat optical device is that it can radi­cally change its function based on the wave­length of light it reflects,” said Federico Capasso, the Robert Wallace Professor of Applied Physics. “By tying func­tionality to wave­length, we have opened up a whole range of new possi­bilities for meta­surfaces.” “In this research, we decoupled functions at different wave­lengths,” said Zhujun Shi, graduate student at SEAS. “Compared to previous flat optical devices, this device has an extra degree of freedom that you can tune at dif­ferent wave­lengths. For example, at one color, this lens behaves like a tradi­tional meta­lens but at another wave­length, it generates a vortex beam.”

The Harvard Office of Tech­nology Develop­ment has protected the intellec­tual property relating to this project and is exploring commerciali­zation oppor­tunities. The lens builds on previous tech­nology developed in the Capasso Lab, which used different pola­rized light to change the function of a lens. But since there are only two forms of circu­larly pola­rized light the researchers could embed only two different functions in the meta­surface.

“By control­ling the device function with wave­length, rather than polari­zation that is bound to two states, we have drama­tically increased the infor­mation capa­city of the lens,” said Moham­madreza Khora­saninejad, former post­doctoral fellow in the Capasso Lab. “With this tech­nology, we demon­strated an achromatic meta­lens in blue, green, yellow and red wave­lengths, two beam gene­rators, and a full-color hologram.”

While this is not the first lens to tie function to wave­length, it is the most effi­cient. Previous wave­length-dependent meta­lenses encoded different functions in different areas of the surface; for example, red light would be focused in one quadrant and blue light in another. With this tech­nology, Shi and the rest of the team engineered the indivi­dual nanoscale optical elements to embed func­tionality at the local level, across the entire lens.

“By encoding every­thing locally, in a single layer, we improved effi­ciency from 8 percent demon­strated in previous wave­length-dependent meta­surfaces to more than 30 percent,” said Yao-Wei Huang, post­doctoral fellow at SEAS. Next, the team aims to improve that effi­ciency even further and develop a trans­mitting, rather than a reflec­tive lens. (Source: Harvard SEAS)

Reference: Z. Shi et al.: Single-layer Metasurface with Controllable Multi-wavelength Functions, Nano Lett., 20 February 2018; DOI: 10.1021/acs.nanolett.7b05458

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences SEAS, Harvard University, Cambridge, USA

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