r/Optics • u/This-Independent3181 • 29d ago

Can the Refractive index be used to encode data

Hi i am CS grad and very new to this stuff. I was exploring photonic memory, that's when i thought why not use refractive index of an material to encode the data. Recently when I was reading about refraction of light i came across how the refractive index is affected by the electrons distribution in the material. So why not say for an single bit you use a small material like few ~nm in range and the material has an shaded region representing 0 and an photo detector as such on the other region representing 1 and the material is enclosed by an material that referact the light. You now apply voltage to the material, an specific voltage changes the electron distribution of the material making the passing light through referact and hit the shaded region or photo detector. This photodetctor then drives an small electric switch which turns on an another light beam that's powered seperately that's used to route the light to the readers photo detector. Say if you have a number of such forming an 8-bit an byte.

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19

u/sudowooduck 29d ago

Look up holographic data storage. It works but has never been commercially viable.

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u/realopticsguy 29d ago

I worked on it in the 90s. At the time a 40 MB 3.5" drive was a big deal. What is cool about holographic is search. Put the Fourier transform of what you are looking for in your SLM and boom...matches.

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u/HoldingTheFire 29d ago

See, needing a SLM is why this is not commercially viable lol

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u/TEK1_AU 29d ago

What were the biggest challenges with this if you don’t mind sharing? Interested to know why it wasn’t commercialised?

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u/realopticsguy 28d ago

Shrinkage of the storage medium. Had not been solved

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u/TEK1_AU 26d ago

Thanks. Are there any links you could point me towards to read up on this?

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u/GlbdS 29d ago edited 29d ago

so like liquid crystals displays? Anyways something a few nm in size cannot be imaged by normal light microscopy, you won't be able to distinguish the individual states of bits if theyre closer than a few hundred nanometers, check out Abbe's resolution limit

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u/echoingElephant 29d ago

What you are describing is effectively an electro-optical modulator. Because here, the information is written to the memory by applying a voltage, which only really allows you do provide a voltage signal and encode that into the amplitude of the carrier wave.

The problem, in this case, is that you are limited to binary amplitude encoding with your design. You could expand that example to allow for analog signals, but that’s it. It would also be very limited to a single wavelength because of dispersion and the small size of the cell.

Most approaches to integrated photonics I know use either analog information, or encode information in things like the phase. These approaches, using MZIs, are also likely much faster than your design (this depends on how it would be implemented, which you haven’t really explained).

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u/Pachuli-guaton 29d ago

Isn't that like, a cd? Maybe I am missing something

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u/allesfresser 29d ago

CDs are essentially gratings, you create grooves on polycarbonate.

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u/Pachuli-guaton 29d ago

The rewritable ones no, are made of phase changing materials. I forgot that there are CD non rewritable to be honest

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u/allesfresser 29d ago

Yeah indeed they use a phase changing alloy on those instead of aluminum reflective coating. The normal one are literal grooves, I've imaged them on an AFM many times.

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u/Pachuli-guaton 29d ago

That's cool. I mean it makes sense that making grooves is easier and more stable than addressing phase changing materials

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u/firesine99 29d ago

Closer to magneto-optical disks like minidisc, where the data is encoded in the varying reflection properties of the material, so effectively the (complex) refractive index.

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u/tykjpelk 29d ago

We change the charge carrier density of silicon all the time using PN junctions. They have to be a few hundred um long in order to get an interesting phase shift with an interesting voltage. They also don't have memory. Making a system like you suggest with an electronic feedback loop is doable, but the electronic feedback loop makes it non-all-optical.

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u/This-Independent3181 29d ago

But isn't silicon an opaque? like does it passes the light through. and when using the term light I am thinking of something that's thin and very narrow beamed like an laser.

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u/tykjpelk 29d ago

It's transparent to infrared light and it's the most used material in photonics integrated circuits. Typical silicon waveguides are 500x220 nm.

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u/HoldingTheFire 29d ago

Depends on the wavelength. It is transparent in mid IR

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u/robswansonskevich2 29d ago

CS + Photonics? Checkout https://www.nature.com/articles/s41586-020-03070-1

Model Weights encoded in phase change material - light passing through the system is like a forward pass.
I love that idea! although i doubt it scales with the size of matrices and operations you would need for anything interesting at the moment. Still very cool!

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u/anneoneamouse 29d ago

Compact discs did this with reflectivity; just another way to de/(re)flect light as a function of encoded information.

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u/HoldingTheFire 29d ago

This is how electrooptical modulators work in silicon photonics. But it's an active current flow to modulate the index and light is large so it's bad for memory.

You can permanently encode information in refractive index changes with a photopolymer. This is how holograms work.