View Full Version : Flat lens offers a perfect image

Sylvester Graham
26-Aug-2012, 12:58
Not like I understand them, but I figured the physics of optics was the last bastion in keeping top notch optical quality off of cell phones. Not anymore?


I wonder if theres depth of field?

26-Aug-2012, 19:24
This is a metamaterial, i.e. a solid with negative refractive index, which means that the phase and group velocities are in the opposite direction. It's not so much a crystalline solid like glass, but a structure with lots of internal resonant structures that need to be on the same order of size as the wavelengths that it's manipulating. Note that they say in the article "for telecom frequencies", i.e probably in the GHz (10^9 Hz) range. These materials have been around for at least 5 years for microwaves but we don't have the technology to shrink the resonant structures down small enough (100,000x smaller features than current metamaterials) to build a metamaterial that will work with light (>10^14 Hz).

So don't get your hopes up just yet. When they get there though, there is some conjecture (I don't know that this is universally agreed) that the Rayleigh limit may not apply, which means we could possibly get images through very small apertures without diffraction.

26-Aug-2012, 23:38
Note that they say in the article "for telecom frequencies", i.e probably in the GHz (10^9 Hz) range.
Later in the article it mentions "infra-red to Terahertz" frequencies, so it's getting up there.
I expect most of the application potential is in the IR laser band.

This is probably not a photographic possibility for many years, if at all.
It's a tuned device, meaning that making it work across a 2:1 frequency range (visible light) would be a challenge.

Nonetheless, interesting advance in technology. I'm sure the fiber optic wavelength multiplexing folks are excited about it.

- Leigh

Mark Sawyer
27-Aug-2012, 00:46
Capasso and his collaborators at SEAS create the flat lens by plating a very thin wafer of silicon with a nanometer-thin layer of gold. Next, they strip away parts of the gold layer to leave behind an array of V-shaped structures, evenly spaced in rows across the surface.

Sounds rather like an ultra-fine fresnel lens...

Jim Jones
27-Aug-2012, 06:45
... or the zone plate that pinhole photographers sometimes use.

Struan Gray
27-Aug-2012, 12:43
FWIW, Frederico Carpasso's group at Harvard *has* made metamaterials that work at optical frequencies. Some really impressive work.

A lens is a device for adjusting the phase of the light wave in the direction perpendicular to its direction of travel. All lenses work this way, including Fresnel lenses. A zone plate is in fact a little different because it manipulates the amplitude of the wave, but as it propagates beyond the plate it is still the phase differences between light from the different transparent rings of the plate which create the 'focussing'.

Carpasso's team are world-leaders at using phase changes induced by interactions with very small metal particles to modulate the resulting light beam. The metal particles are host to collective oscillations of their electrons which are called 'plasmons', which change the phase of any light which interact with them. [Plasmons are in general hard to detect individually, but collectively they are responsible for the shiny reflectivity of metals, and, in photography, the yellow colour of colloidal silver] In principle, using plasmons is no different from using a lens, except that in theory at least you have far more options when it comes to tuning the phase modulation, so aberration-free focussing (for a given reproduction ratio and wavelength) becomes possible and there are lots of other cool applications if you can only get the ideas to work in practice.