A perfect flat lens with metamaterials

A new Science article demonstrates an ultra flat lens made of metamaterials that is as sharp as physically possible, has a large aperture, and high light efficiency. The work is authored by Mohammadreza Khorasaninejad, Wei Ting Chen, Robert C. Devlin, Jaewon Oh, Alexander Y. Zhu, and Federico Capasso from Harvard and Waterloo. They demonstrate three lenses for wavelength 405, 532, and 660 nm that are diffraction-limited, have high numerical aperture (NA=0.8)  and efficiency of  86, 73, and 66% respectively.

My understanding is that the biggest novelty of this work is in the fabrication itself and the use of titanium dioxide, which enables high efficiency for metamaterials at visible wavelength. Previous metamaterial approaches suffered from significant light loss in the optical range.  They needed a material that does not absorb or scatter visible light and decided to use titanium dioxide.

The optical property of a material derives from the way electromagnetic waves interact with its atoms, and in particular how electrons get pushed and react, which in particular affects the overall speed of light in the medium (index of refraction). Since there are only so many types of atoms, the behavior of optical materials is limited. The idea of metamaterials is to create composite materials made of tiny structures much smaller than the wavelength of the radiation of interest to finely engineer how EM waves get propagated. They have beenmade famous by attempts and success in creating materials with a negative index of refraction. Early attempts focused on mm wavelengths such as microwaves, and the microstructure of the metamaterial were easier to manufacture. They used microstructures such as split ring resonators to induce rotating currents and control the resonant frequency of the material. A good introduction can be found in Pendry and Smith’s Scientific American’s article about their negative index of refraction material. The present paper uses a simpler metamaterial structure made of waveguides created by tall compared to their width) “nanofins” made of titanium dioxide deposited using electron-beam lithography

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SEM micrograph of the metalensis showing the arrangement of tall nanofins. The white scale bar on the bottom right is 300nm. 

This new flat lens should not be confused with Fresnel lenses (similar to lighthouses), which make a traditional refractive lens flatter by cutting it into rings and removing the bulk of the material that would have been at the center. It’s also different from Fresnel zone plates, which leverage diffraction by alternating opaque and transparent rings to promote positive interference at the focal point.

The present lens presents no spherical aberration and is essentially perfect at a given wavelength. It does suffer, though, from chromatic aberration, which is probably the leading cause of optics complexity, which makes it slightly unfair to compare it in size to a Nikon microscope which does have to fight chromatic aberration. This does not make the results any less remarkable: for monochromatic imaging, these lenses are incredibly flat and as perfect as it gets (they mention a spot size 1.5 smaller than the Nikon). The authors also mention that chromatic aberration could be dealt with, citing work on “dispersive phase compensation” I have not read. This would be the full game changer for photography, if both spherical and chromatic aberrations could be controlled in a single flat optical element.

FocusingLensGifforWeb_0

Propagation of a plane wave in the metalens. 

Metamaterials are exciting because they vastly expand the degrees of freedom when designing optics, enabling physical properties that would be impossible with traditional materials, or properties that would require vastly larger optical systems. Traditional spherical lenses can only vary the index of refraction and the curvature of each side of a lens, and the propagation of light inside the lens is simple and isotropic. This makes it impossible to focus light rays (or a spherical wave for physical optics folks) and results in well-known spherical aberrations. Aspheric elements enable the control of the orientation at each point on the lens, making it possible to control each light ray or part of a wave, although propagation inside the lens remains standard. GRIN (GRadient INdex optics) varies the index of refraction within an optical element and provides further control. Diffractive optics controls light propagation only through flat binary opaque-transparent surfaces. In contrast, metamaterials finely control the EM properties of the material at a scale much smaller than the wavelength, resulting in tremendous levels of control over the propagation of a wave. Holographic and 3D optics are another type of technology that enables richer control over light propagation

The world of optics is dramatically changing with new (meta) materials, computation, and fabrication . Glass grinding is losing its dominance. Most cell phone optics are injection-molded plastic, which offers worse tolerance but makes aspheric elements as easy as spherical ones. The new metamaterial lens can be manufactured with semi-conductor technology. 3D printer can now print optical materials. Computation is playing a central role to optimize the large degrees of freedom enabled by thee new technologies, simulate light propagation, and reconstruct visual information from

Good coverage at phys.org and the BBC.

 

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