Simple silicon coating solves long-standing optical challenge | Hacker News Reader

Simple silicon coating solves long-standing optical challenge

phys.org

38 pointsthedday18 comments

It looks like that this solution corrects for dispersion in group velocity [1], but it's unclear that it can correct for dispersion in phase velocity at the same time [2]. My intuition that the latter is not possible this way, but optics is not my field.

Correcting for dispersion in group velocity seem to have applications in pulsed-laser experimental setups (as presented), but correcting for dispersion in phase velocity would be required for correcting for the more conventional chromatic aberration people are familiar with in photography.

[1] https://en.wikipedia.org/wiki/Group_velocity [2] https://en.wikipedia.org/wiki/Phase_velocity

Isn’t group velocity (transmission speed) what you want to fix for chromatic aberration issues?

> For light, refraction follows Snell's law, which states that, for a given pair of media, the ratio of the sines of the angle of incidence θ1 and angle of refraction θ2 is equal to the ratio of phase velocities (v1 / v2) in the two media, or equivalently, to the indices of refraction (n2 / n1) of the two media.

[1] https://en.wikipedia.org/wiki/Refraction

Huh, ok. I stand corrected.

The "long-standing optical challenge" is chromatic dispersion[1], which causes the chromatic aberration[2] familiar to anyone that has a strong eyeglasses prescription.

[1] : https://en.wikipedia.org/wiki/Dispersion_(optics) [2] : https://en.wikipedia.org/wiki/Chromatic_aberration

Chromatic aberration is a long-standing issue with VR too. Hopefully this new material works as described and can be produced cheaply.

Interesting. The apparently little I’ve ever read about lasers said they are a single color and all of the waves are in-phase - monochromatic and coherent. But that’s an over simplification 1). And I suspect working with extremely short pulses makes it much less so, because the laser has the widest spectrum and least coherency when it turns on (the pulse starts).

1) https://physics.stackexchange.com/questions/564727/why-is-a-...

158nm high aspect ratio silicon pillars across an optical surface — not sure why they refer to it as “simple”? For sure it’s a great idea that is not super complex but yea would be interesting to understand how it would be made on different optical surfaces. Super interesting.

I think "simple" refers to the user's perspective:

"Now, anyone can buy a lens, put the coating on and use the lens without worrying about dispersion," said Ossiander.

I'd also wonder how durable the coating is, and whether or not it can be cleaned. That could seriously limit its applications.

Assuming that I was looking at the correct paper, it looks like they are depositing a substrate and the pillars on top, and that's it. Internet disclaimer that I'm not an expert, I don't know how you protect the pillars without having the same dispersive effects that they are fighting. For example, image sensors have glass in front of the sensor (for both protection and low pass), and the pixels are at least an order of magnitude larger, and 100x the area. But, maybe you can just compensate for that as well.

Looks like the pillars are 164nm tall, 610nm high, which is pretty big all things considered. So normal semiconductor cleaning methods are probably fine. But the ways you would clean a camera sensor for instance would be very not fine.

https://www.nature.com/articles/s41467-021-26920-6.pdf

Durability would hardly limit the applications, the vast majority of optical coatings are inside things like camera lenses. Glass is a sensitive thing after all anyway. They don't put any of the sensitive coatings on the front element, it's essentially a glass covering these days.

Eyeglasses are the only optics I can even think of that are regularly damaged and in contact with various hazards that can't really easily be shielded.

I think it is simple because it is uniform over the surface of optical element. In optics things get "complex" when they are changing over the area of the optical element. These tend to be a real PITA because it is difficult to produce anything that is not uniform to astonishingly exact specification as they tend to be in optics. They also tend to require separate design for every optical system.

Everything is relative...

The nanopillar silicon coating was made using the same commercial lithography tools as industrial semiconductors, making it easy to quickly apply these coatings to existing optical components and expand the applicability of femtosecond laser pulses.

Using existing manufacturing equipment from a well-established field is surely a lot simpler than developing ground-breaking new manufacturing devices.

Capasso's [1] group has been working on projects like this for a couple of decades. It falls within the realm of work on metasurfaces. At a high level you can consider the silicon working in a sub wavelength regime, so that the optics interacts with an effective material which is engineered by changing the geometry of the sub wavelength features. They recently launched a startup [2] to commercialise the developed capabilities.

[1] https://scholar.google.com/citations?user=CmpEzW8AAAAJ

[2] https://www.metalenz.com/

Could this be a way to cheaply improve low-cost camera lenses? Aberration at wide apertures is oft a criteria that people avoid them due to (other than lack of sharpness).

Is the explanation in the article relevant to ultrashort laser pulses which are more like wavelets with smeared frequency spectrum (uncertainty principle) than something where "red light" to be "speed-bumped" exists?

Did not find before/after pictures for comparison in the article :(

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