Wed. Sep 28th, 2022
Image of a traditional optical lens.

I try not to write too much about optics because, well, I love optics, but nobody else does. Unfortunately for you, I can’t resist the subject.

If you turn your cell phone over, you will find the lens on your camera. This allows the camera to create a beautiful image. But for light in the 1-100nm wavelength range (called extreme ultraviolet), lenses don’t work. Or rather, that was the hitherto accepted wisdom. But that wisdom will change, as researchers have shown how to make a lens for extreme ultraviolet radiation.

Lenses not working? You need to get your eyes checked

Lenses are basically a curved surface of a material in which light moves at different speeds. For example, when light hits the material of a glass lens, it slows down (by about 30 percent) — this is the effect of the material’s refractive index. Because the lens is curved, the light at the edge of the lens can travel just a little farther before slowing down, putting it in front of the light near the center of the lens.

But light is a wave and must change fluidly in space. To maintain that smooth profile, the light at the edge of the lens has to bend toward the center. As a result, the light focuses some distance outside the lens.

But what if the light doesn’t slow down on penetrating a material? Well, then nothing would happen; the wave continues as if nothing stands in its way. This is almost what happens with extreme ultraviolet light. For these wavelengths, most materials have an index of refraction that is just under one. This means that the light accelerates by about less than one percent when entering material. The difference is just too small to make good optics.

But that’s not the only problem. All materials strongly absorb extreme ultraviolet light. You would lose most of the light even if a material focused it nicely.

To get around this problem, people use mirrors to focus and otherwise manipulate extreme ultraviolet light. Mirrors make life very difficult when designing a high quality optical system. Not only that, these mirrors aren’t as reflective, so you’re still losing a lot of light.

Find Refractive Index

It turns out that there are ways to get something with an index of refraction at extreme ultraviolet wavelengths. The refractive index of a material is generated by the electrons moving back and forth in response to the incoming light field. An electron’s willingness to wobble is highly dependent on the combination of its environment and the wavelength of light that causes it to wobble. when it’s light about the right wavelength to cause the electron to jump from one energetic state to another, then the electron can indeed wobble a lot.

A group of researchers has made use of this. They shone a beam of extreme ultraviolet light through a beam of helium gas. The wavelengths close to the energy needed to drive helium’s electrons from their normal energy state to a higher energy state were strongly bent. Indeed, the results fit the theory very well. Wavelengths slightly shorter than required for excitation are defocused by a gas beam (because the index of refraction is just less than unity), while wavelengths slightly longer than required are focused.

By luck, creating a jet using a circular opening produces a gas with a parabolic density profile. That means the gas jet is a very good lens.

That said, as cool as this is, it will be of limited use. You need the right gas to make a lens that works at a specific wavelength. You’re really stuck with what nature gives you – if there’s no gas for the wavelength you want, you’re out of luck.

As with glass optics, the shape and density of the optic really matter. But you can’t grind a gas into the shape you want. Instead, the gas jet nozzle (or nozzles) must be shaped very carefully to give just the right density profile. This is possible, but it is not a trivial exercise and is very sensitive to small changes in environmental conditions.

Still, I’ve been mistaken about the usefulness of this sort of thing before, and I hope I am again.

Nature2018: DOI: 10.1038/s41586-018-0737-3 (About DOIs)

By akfire1

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