Aug 24 2020

A Controllable Metalens

This is a (sort of) follow up to a previous post I wrote back in March about extreme depth-of-focus tiny flat nanolenses. The big ideas was that researchers are rapidly developing the technology to build a lens out of metamaterial that is structured on the nanoscale. Instead of using a large piece of curved glass to control light, these metalenses use the nanostructure on thin flat lenses. What impressed me at the time was the incredible potential applications of such tiny lenses, from cameras to medical applications. In fact, this tech was deemed one of the top ten emerging technologies in the 2019 World Economic Forum.

Now researchers have taken this technology one important step further – a method for possible dynamic control, meaning that these lenses can be focused and zoomed.

What the researchers did was infiltrate a metalens with “nematic liquid crystals”. This is the same technology used in liquid crystal displays – LCD monitors. They are basically transparent to translucent liquids that refract light like a lens. But the key with liquid crystals is that their properties can be modified by an external electric field (and also magnetically, thermally, or optically). They showed that they were able to “nontrivially” infiltrate the metalens with liquid crystals, and that this changed the optical properties of the metalens. They conclude:

By harnessing the tunability inherent in the orientation dependent refractive index of the infiltrated liquid crystal, the metalens system considered here has the potential to enable dynamic reconfigurability in metasurfaces.

In other words – they made the metalens tunable – this is a lens that cannot only be focused, but also can increase or decrease its magnification. In camera speak – this is a zoom lens. (“Telephoto lens” is a high magnification narrow focus lens, while a “zoom” lens can change its magnification.)

As photographers know, quality zoom lenses are bulky and expensive. Glass lenses are solid fixed things, so to change the magnification you need multiple lenses that you then move with respect to each other. This is literally 500 year old technology, but of course the quality has improved significantly. Expensive lenses correct for aberrations introduced by the lenses, such as spherical aberration and color distortion.

The ability to have a single tiny flat lens that can be focused and zoomed is incredible – if the technology ultimately works and can produce high quality images. The ability to control the zoom electrically, rather than physically, also opens up a lot of potential. To be clear, this would not be the same as “digital zoom” on many digital cameras. Digital zoom is essentially worthless – it’s a gimmick. Optical zoom actually magnifies the image, bringing it closer. Digital zoom just creates the illusion that this is happening by essentially cropping the image, so a smaller portion of the image fills the monitor. You are therefore sacrificing resolution. And of course, you can just take the picture without digital zoom, and then crop the full image any way you like in post.

What we are talking about with this liquid crystal infused metalens is actual zoom, but electrically controlled. And you don’t need actuators to physically move lenses, which is how electronically controlled focusing would work with regular lenses. You would just need the electrical current, without any physical device. Again, this allows for small cheap devices.

The most obvious application of this technology would be the next generation of smartphone cameras. They are already getting damn good, although they are still no substitute for a prosumer or professional camera. Part of the reason we can now have such tiny cameras on our phones is the miniaturization of lenses. Another reason is that digital photography allows for software to improve the quality of the image and alter its characteristics. Smart software is doing all the heavy lifting in terms of image quality and features, so you can get away with a small lens.  The primary advantage of a smart phone camera is that you have it on you at all times. The best camera in the world is no use to you if you don’t have it on you.

The primary limitation of smart phone cameras is that they lack significant optical zoom. You can get external lenses that you clip onto your phone to partially make up for this deficiency, and this is not a bad compromise between the convenience of a smart phone camera and the quality of a dedicated camera (depending on your needs). But still these small clip-on lenses are no substitute for a large piece of quality glass.

But now imagine when the tuneable metalens technology matures – having a smart phone camera with a quality lens with significant optical zoom. At some point it’s possible this technology will become so good that there is little to no advantage in terms of quality of a bulky glass lens. In fact, it’s possible that that quality advantage may eventually go to the metalens. Because the properties of these lenses can be adjusted externally, you can have AI-guided dynamic control that could actively eliminate any aberrations, adjust for lighting conditions, and produce effects optically that can now only be simulated digitally.

One possible feature is extreme telephoto – the kind of magnification that can now only be achieved with ridiculously large lenses. And speaking of which – I wonder how well this technology will apply to telescopes. I am a backyard amateur astronomer, so anything that can produce cheap good telescopes is of interest to me. When looking at objects at extreme distance, however, magnification is not enough – you need to gather a lot of light. To some extent, there is no substitute for size. But still, it seems there is a lot of headroom when it comes to the quality of consumer-level telescopes (of course, depending on how much you can spend). Perhaps this technology would work best in the eyepiece, rather than the primary light-collecting mirror or lens. It may make it feasible to bring high-end quality enhancement features, like adjusting for atmospheric aberration, to the consumer.

In any case – it seems like this technology is advancing rapidly, and the potential is stunning. I am just scratching the surface, focusing on my own hobbies. This is one of those technologies, however, that is so useful it will produce applications we won’t immediately think of. Stay tuned.

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