An ultra-thin “metalens” with autofocus: the “holy grail for optical engineers” that MIT pursues with this experiment
In the last two years we have seen a race to achieve the best zoom on mobiles, but in parallel (and more by way of tests) there is also the one to achieve the perfect autofocus lens. We have seen some attempts, such as Hitachi or that of researchers from the University of Utah, and now it is those of MIT who propose a new alternative.
Nowadays, in most photographic systems we see (or, rather, we intuit) a system of several lenses, perhaps moving parts, etc. to focus. In cameras proper we even notice it by the sound and the faint internal movement if we pull autofocus, or directly we look for it manually by turning the lens. And the idea with these systems is, above all, make focus less space-intensive.
A glass-free lens that focuses almost single-handedly
Nowadays it is very striking what is achieved with mobiles at the level of component integration: engineers manage to devise systems that take advantage of the available volume as it is, sometimes speaking of place telephoto lenses at about 8 millimeters. Integrating an advanced photography system, with lenses that encompass having a larger sensor, a larger aperture or increasingly precise focus is an ongoing challenge.
Hence, we are not surprised that there are attempts to lighten the thickness of all this machinery, including that of these MIT researchers. As they claim they have managed to build *an ultra-thin “metalens” that does not need any added system to focus thanks to the material and structure they have used.
The material in question is based on that of compact discs (something that may not sound familiar to many, but that marked an era). A combination of germanium, antimony and tellurium (GST, for an acronym for their chemical symbols) to which they have added selenium.
What they have seen is that, by applying heat to said material (in a layer 1 micrometer thick), ** the atomic structure is rearranged and the refractive power is altered, without affecting transparency (it does not burn or darkens). This is due to the pattern etched on the surface, so that depending on how the heat is applied the infrared light is focused at one point or another.
It is an approach very similar to the one we saw in that team at the University of Utah, this time, according to researcher Tian Gu, achieving “images without aberrations of overlapping objects positioned at different distances.” The MIT team believes that it can be manufactured together with micro heating systems, so that “they can heat the material with pulses of milliseconds”, so that the precise temperature for the focus is obtained.
The application of this could be mobile cameras as we have already mentioned, but also night vision goggles, as the team expects. Mikhail Shalaginov, another of the researchers, puffed out his chest saying that this is “a kind of holy grail for optical engineers”, although it is not yet clear how many milliseconds the approach requires (and would have to compete with the extreme speed of the PDAF approach , for example, increasingly popular on mobile).
For now, it is one more approximation, although taking into account that this line of research is committed, perhaps we can see more advances in order to see it work on a device. It is complicated: the same researchers remember the difficulty of manufacturing this type of lens en masse, so it won’t be a short-term thing.