MIT "achromatic metasurface" flat lens

Started Sep 16, 2015 | Discussions thread
alanr0 Senior Member • Posts: 2,073
Re: Achromatic Metalens over 60 nm Bandwidth
1

J A C S wrote:

nigbat wrote:

And a wider yet bandwidth:

https://www.nature.com/articles/s41565-017-0034-6

They have actual images there. This is well into microscope territory (magnification 50x) and they go down to the diffraction limit. They talk about two lenses, one with NA=0.2 and another one with NA=0.02 and diameter 220 microns but I did not get all the details. They have very good images of targets about 1mm wide all illuminated by single lasers of various wavelengths.

As far as I can tell, their achromatic design (n=0) implements a continuous quadratic change of phase delay with radius by varying the design of the nanostructured titania fins across the lens.  The fins act as waveguides whose dispersive properties are adjustable to minimise chromatic dispersion.

This approach avoids the phase discontinuities and focal length changes inherent to diffractive operation used in previous nanostructured lenses.   At present their achievable phase delay is limited to a few microns, so will not scale readily to larger lenses suitable for more conventional photography - a 50 mm f/1.4 lens needs 3.2 mm of path delay in the centre of the lens.

Potentially more interesting for mainstream photography is to combine a nanostructured coating with a conventional glass lens.  They showed in computer simulation that a low cost spherical lens (5.4 mm f/5) combined with their material could give diffraction-limited imaging.

Note also that the current design appears to be polarisation sensitive.  Their illumination is left circularly polarised.  The lens transform 20% of the incident light to right circularly polarised light, and only the cross-polarised component is focussed.   For good image contrast crossed circular polarisers reject the unwanted components.

Cheers,

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Alan Robinson

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