Etching silicon wafers to make colorful Rugate optical filters (porous silicon)

Passing an electrical current through a silicon wafer in a special acid etchant will create a porous layer with a variable index of refraction. I describe how this process works, and how the Fourier transform relates filter design to electrical etch waveform and resulting spectral response.
Notes: 1. The Rugate filters look especially good in polarized light because the reflection from the silicon wafer is reduced, but the reflection from the filter remains strong. I noticed this while holding the wafer in front of my computer monitor. Later, when showing it to friends, the color intensity was poor. Their monitor must have been horizontally polarized, so holding the wafer low, and tilting it upward didn't work! Some monitors are horizontally polarized, and some are vertical.
2. The magnet used to hold down the PTFE cup to the wafer may have a very slight impact on the etch process. The dramatic shift in filter performance at the periphery is due to O-ring restricting the conductive etchant to a sharp edge, creating an electrical field concentration.
Outstanding visual Fourier series tutorial:
www.jezzamon.com/fourier/index...
LR-1 spectrometer: www.aseq-instruments.com/LR1.html
p-type wafers on eBay. (You have to hunt around and check the photos for info on the label attached to the box of wafers. Be sure to get wafers less than 0.01 ohm-cm) eg www.ebay.com/itm/25-silicon-w... n-type wafers are photosensitive during the etch process.
Online graphing calculator page from this video: www.desmos.com/calculator/hij...
Original Desmos page that I used: www.desmos.com/calculator/qpn...
Code for Keithley 2450 control and processing.org waveform generation:
github.com/benkrasnow/Porous_...
Fourier transform to understand optical coatings:
www.willeyoptical.com/pdfs/92_...
Porous silicon refs:
www.rp-photonics.com/rugate_f...
www.intechopen.com/books/poro...
sci-hub.se/doi.org/10...
sci-hub.se/doi.org/10...
sci-hub.se/doi.org/10...
sci-hub.se/doi.org/10...
sci-hub.se/10.1364/OE.16.015531
sci-hub.se/doi.org/10...
sci-hub.se/doi.org/10...
Silicon wafer identification flats:
www.tf.uni-kiel.de/matwis/ama...
Applied Science on Patreon:
/ appliedscience

[@arduinoversusevil2025]

Twice gobsmacked. First because of the content and second because of how consistently amazing your demos are. Thank you.

[@Mikelectric]

True that. I'll be attempting this for a job

[@JK360noscope]

Stop slandering this man you dirty canut

[@rehoboth_farm]

I'm surprised you were able to sneak in without being mobbed. LOL! Good to see you AvE.

[@austinball4343]

Right back at you AvE.

[@framegrace1]

Dental Implants use this method to "rugate" the titanium so bone can grow in the crevices and grasp it better. Rugation can make the titanum totally integrated with the bone. It also makes it change colour.

[@BRUXXUS]

Whoa, I had no idea! That's super cool!

[@Walter-Montalvo]

That is so neat! Since bone is a porous material, it does make sense that it would bind better to another porous material.

[@sudazima]

@@Walter-Montalvo its not related directly to the porousity of bone but to the fact that cells like to get into porous spaces. including cells that make bone

[@Walter-Montalvo]

@@sudazima Huh super interesting. So if there is a very smooth surface (polishes titanium) and a porous surface (porous titanium) then the cells will prefer the porous one. Also it means that joints like hip replacements perhaps need to be very smooth to prevent the cells from binding, right?

[@sudazima]

@@Walter-Montalvo its more complicated than they but yes. the hip joints have different concerns but in anycase no bone cells will grow in that space anyway. infact it may even help having a porous surface. places without cells tend to give strong immune reactions and indeed high concentrations of white blood cells are found in srtifical joint fluids, although its not well understood afaik.

[@Xiaotian_Guan]

When the first page of google results for the term 'Rugate optical filters' doesn't contain a link to wikipedia, I know I'm in trouble.

[@officer_baitlyn]

pretty weird i didnt get any results in terms of wikipedia in english but when i put Rugate optical filters into german google i got the german page on the topic but there is no english translation de.wikipedia.org/wiki/Rugate-Spiegel

[@Vaaaaadim]

We could be the first to make it

[@billeethesciencegeek]

Check Rugate-Spiegel, It's only in German, but it's there.

[@f.d.6667]

I tend to agree - however, in some regions/languages Wikipedia's half-knowledge has become a real problem as WP editor-"activists" are are increasingly twisting the content in such a way that current scientific facts are being ignored when the older (obsolete) ones support a certain political world view. Some examples as bisphenols/bisphenol-A, where the English Wikipedia article was very current and complete, while the German version was a total stub and (obviously) kept that way for years (Germany is the biggest producer ob BPA in the EU)... The same can be observed for the environmental impact of most "green" technologies or pretty much anything that has a "political" dimension. So yes, Wikipedia can be helpful but it is also used to an extreme extend to manipulate people. I for my part am trying to avoid Wikipedia (especially the German edition) as much as I can, as for my field (human factors / environmental medicine) the content is for the most part either outdated or very relevant aspects are missing.

[@Erd_Geschoss]

@@f.d.6667 Except your statement is untrue (see: "narrative"). About two thirds of the german article - being about the same overall lengt as the english one even including about double the amount of sources - go exactly into the dangers of bisphenol A and the necessary elimination from products.

[@TechIngredients]

Wow, simply beautiful!

[@TomKappeln]

Visiting the family ...

[@fjs1111]

Trump 2024!!

[@flirkami]

Seriously I don't even have words for your work anymore. You understand and explain highly difficult concepts in physics and chemistry so intuitively that it's just an awesome change in perspective for me as a student. Thank you so much!

[@rinner2801]

Honestly one of the best teachers I've ever had the pleasure to open up my brain to.

[@AidanDavisXD]

Every video could be a PhD or an entire career. And you never do the same topic twice.

[@letsgocamping88]

He’s a one man R&D department

[@Anar10n]

Im pretty sure they were someone's PhD or patents

[@TheTablet314]

To be fair, he's presenting already done research and his implemetation of it. I guess that's more like the 'recreation' part of an bachelor/undergraduate thesis instead of anything higher that paves it's own way.

[@-danR]

How can someone so nerdy be so un-nerdy?

[@gelo1238]

@@-danR just pure genius

[@6Diego1Diego9]

i can't believe you do this in your free time off work

[@letsgocamping88]

What is his work. That’s the question!

[@dr.zarkhov9753]

@@letsgocamping88 when the desire for knowledge is passionate enough the work becomes secondary. I wish I had more people like him in my life. I've had a few but they are mostly gone now.

[@max_kl]

@@letsgocamping88 He works in the research department at Verily (Google/Alphabets life sciences division) where he prototypes new products

[@alonsovm2880]

@@max_kl that explains a lot

[@KnowledgePerformance7]

He also spoke of working on VR at Valve

[@Erhannis]

Holy HECK. I'd previously thought to myself, like, "It'd be neat if you could, like, build a material to have a specific arbitrary absorption spectrum. While I'm wishing for magic, I'd like a wand and a time machine, please." To discover that it's not only _possible_ but _fairly straightforward_ positively boggles my mind. I have not been so astonished in YEARS.

[@msd2000]

I was truly excited to try all this myself until we got into "everyone's favorite" - hydrofluoric acid. But wow.. that was a very good education on several subjects.

[@TimmyBlumberg]

I feel like he just thoughtlessly reaches across the boundaries of scientific disciplines

[@f.d.6667]

Yup... HF is the one stuff I DON'T want in my house... UO2 glass - pfff, thorinated electrodes - pfff, americium 241 - meh... HF? Nope.

[@rinner2801]

Hey I taught myself the wonders of hydrofluoric acid while learning to etch PCBs when I was 11 years old. I was even able to buy everything I needed from local Maplin in UK at the time. I assume it's considered a bit more hazardous today?

[@GRBtutorials]

@@rinner2801 HF to etch PCBs?! Are you sure you didn’t confuse hydrofluoric acid with hydrochloric acid? The latter is commonly available in many places and is used in combination with hydrogen peroxide to etch PCBs. I don’t see why you’d want to use HF to etch a PCB...

[@canadajim]

@@rinner2801 Considering the consequences of even a small exposure, yes.

[@BlackbodyEconomics]

You've gotta be the closest thing to a true, modern-day "renaissance man" there is. I don't know of anybody else who can regularly demonstrate an intricate knowledge of such a wide variety of fields and topics as you do. I am always very impressed with your videos. Simply fascinating!

[@nunyabusiness8538]

This man describes things in ways that nobody else on this site can even comes close to. Like his brain is a real gem

[@TimPerfetto]

How dare you compare his brain to a rock? Why are you ruining his life?

[@electronicsNmore]

Never a bad video on this channel. A++

[@Nighthawkinlight]

This is awesome, and way over my head. I figured anti reflective coatings on lenses were some sort of metalized vapor deposition. Are they simply an etched surface in the glass itself? Or a thin layer of etched silicon on the glass? I'm curious how this is done on non conductive glass (and plastic?). Are metalized looking sunglasses made with a similar process? Can you make a filter for sodium emission, with a similar result to didymium glassblowing lenses? Super cool. Thanks for another video that pushes me to learn something new.

[@UpcycleElectronics]

I'm pretty sure metalized sunglasses are done with iridium vapor deposition. I vaguely recall Ben has done uploads on this in the past too. I know the cheap iridium coated lenses often have adhesion issues. The coatings on the cheap glasses can be washed off with regular dish soap, unlike a good pair of Oakley's or Smith's, but I'm no expert. I just learned the hard way - buy my cheap cycling sunglasses with black lenses.

[@turun_ambartanen]

en.wikipedia.org/wiki/Anti-reflective_coating MgF2 with PVD or TiN and NbN with Sputter deposition

[@sudazima]

for expensive optical lenses you would deposit silicon on top and do the chemical etching after.

[@6alecapristrudel]

@@UpcycleElectronics Iridium? You sure? If I wanted to make cheap sunglasses, expensive and stupid high-melting Iridium won't be my first choice. Even if it takes a really small amount of it.

[@GRBtutorials]

@@6alecapristrudel You don’t have to melt it for vapor deposition, and only minute quantities would be used, so it’s not that expensive.

[@BRUXXUS]

High-tech glitter that's bursting with flavor! Also, I would love to see some more into the light emitting side of this that you mentioned at the end. Sounds super interesting.

[@tanmay______]

Wow 😮 I never thought Fourier transform could manifest itself in such a form. Great video!

[@alystair]

I wonder if this means you could turn an MP3 into a series of colour filters... although it'd get muddled all together unless you change the physical location of the different fouriers...

[@BIGWUNuvDbunch]

You see, the thing you have to realize, Morty *hic* is that.. is that Fourier transforms are everywhere Morty. Everything is a Fourier transform Morty, and *burp* it took your tiny brain this *burp* long to realize it

[@volvo09]

@@alystair "I can see the music man!"

[@Walter-Montalvo]

I am still trying to wrap my head around the fact that FT shows itself in the spectrum. That's why I love these videos, no dumbing down and plenty of material to challenge you at many levels. Gold!

[@danv8718]

I just can't understand how it's possible you don't have millions of subscribers. Hands down the best science youtuber out there. Not even close.

[@wades623]

Because a lot of people are stupid and subscribe to stupid celebrities and anyone that makes videos with no real value

[@pyromen321]

The scientific name for hydrofluoric acid is “bone hurting juice”

[@minecraftermad]

yeah wouldn't drink that

[@ZoonCrypticon]

"Flesh-Eating Hydrofluoric Acid - Periodic Table of Videos" ( kzbin.info/www/bejne/pZrTnKaIna6GnK8 )

[@dustinfisher5463]

Thought I was going to bed but apparently I’m busy for the next half hour

[@garethbaus5471]

I am busy figuring out how to get in line for the DMV they won't let me get in line in person and just sent me back to try again another day.

[@robertbarnett6879]

@@garethbaus5471 lmfaoo samee, its 11:50pm rn

[@garethbaus5471]

@@robertbarnett6879 it is a bit after 2:00 am here(I normally work night shift so this isn't much later than I usually am up)

[@frogz]

......its 3 am and i had to watch this twice...

[@DIYBiotech]

I want to be this man when I grow up

[@AliHSyed]

Awww

[@timothykelley4495]

@@AliHSyed I'm all grown up and I STILL want to be this man!!!!!

[@Walter-Montalvo]

@@timothykelley4495 Perfectly said, you took words out my mouth!

[@ryanhebron4287]

That was a very fast 30 minutes! I would love to see the filters as light emitters. This stuff is so interesting.

[@TimmyBlumberg]

I’m just at the beginning of the episode, but how excited he is explaining the concept is magnificent. Can’t wait to see what arcane scientific processes are in store

[@frollard]

This is easily the most education I've gleaned in 30 minutes ever. Seriously...lots of potential for stuff to go over one's head - between the circuit diagrams, doped silicon chemistry, optics and optoelectronics, fourier math...the list goes on. Thank you once again for being simply awesome.

[@psionski]

Don’t forget the photography and quantum mechanics... this video contained references to, like, 15 topics I’m interested in, and 25 more that I didn’t know I was interested in... it’s insane how it all comes together...

[@feelx92ger]

I remember the Feynman lecture about means of reflection and never really bothered to think of practical implications. To see this princible come to use was almost cathartic for me. Excellent work Ben!

[@jhonbus]

11:05 - seeing those graphs overlay was the point when I worked out what was going on here, and figured out how the process works - you're effectively building up multiple coating layers to stack several of these graphs, which is just like stacking sine waves to produce an arbitrary function. I love it when you get an intuitive feeling for something like this! Great job at explaining!

[@MandrakeFernflower]

1 year later: so I started my own chip fab company

[@Maclman1]

kzbin.info/www/bejne/iXOcZaWHqbCfgas Check out Sam Zeloof

[@elvishfiend]

Have you seen any of Jeri Ellsworth's old videos? She made a mosfet or two from silicon wafers

[@gu4xinim]

You're looking for Sam kzbin.infovideos

[@QuantumFluxable]

@@gu4xinim yea jeri did similar stuff until she got too busy with work at valve i think

[@gu4xinim]

@@QuantumFluxable jerry is absolutely amazing, I hope one day to be half as good as her at something. I commented directly to the one on top, didnt even knew she did this, nice to know.

[@starlite528]

It can not be overstated just how nasty and dangerous HF is, even as low at 0.5% concentration

[@devrim-oguz]

It would eat your hand and you wouldn't even know it.

[@grinchyface]

It's not that bad. I am not sure how it got so overblown but it's certainly not some bone eating, life ruining thing that it's been made to be in pop culture.

[@atrumblood]

Shhhhh. Let natural selection do its thing.

[@sudazima]

heavily depends on what your used to, ill take some HF over boiling pirhana

[@crackwitz]

I'll take the boiled piranha please, and may I see your wine menu

[@DEtchells]

This was so great! I’d never understood how you could design multi-coatings to basically be band-reject filters, without getting all tangled up with the reflections between the layers. When you said it was just a Fourier Transform of the transmission/reflection spectral curve, the light suddenly dawned! ==> I’d love to see you delve into anti-reflective nano-coatings. They were a bit of a breakthrough in the world of photographic optics a few years back, but the tech is ultra-proprietary. They’re kinda amazing; the idea is you have some substance with a refractive index close to that of the glass itself (or maybe one of the outer layers of a multi-coating stack), and then deposit it in a such a way that it becomes more and more porous as you move from the glass surface to the air. If the particles or nano-structures of the material are well shorter than a wavelength of light, you basically create an optical material where the refractive index varies smoothly from a high value to a very, very low one. The smoothly-varying refractive index, ending up close to 1 at the lens/air interface can almost completely eliminate reflections. Anti-reflection tech in general opened up new horizons for optical engineers because it let them use large numbers of elements in a lens design (sometimes 20 or more) without having to worry about flare and poor local contrast. I get the impression that there are different methods used to create these, at least from the illustrations various companies have used to portray them. Most mfrs show little spheres piled up densely at the interface with the glass and very sparsely on the air side. OTOH, Canon has illustrated their technology as tall pyramidal spikes. I don’t know if this represents a material difference or is just obfuscation. I wonder if they’re using some kind of PVD or sputtering? I could imagine sputtering from a target composed of nano-spheres, varying the rate to adjust the density. Or maybe it’s some more complicated second-order factor, adjusting the balance between tending to deposit or erode the material being deposited on the substrate? It might be too challenging to figure out from scratch, but it’d sure be interesting if you could do it!

[@UpcycleElectronics]

This is the most intuitive application of FFT I've ever seen. I wish the Computer History Museum could get Ben to do an occasional interview with a few chip fab guys and hardware engineers. Heck, do a few interviews in Ben's shed with a more in-situ vibe instead of the typical CHM legal deposition format. This could feed Ben's curiosity, and be a great benefit to all by documenting more of the science and engineering history in an approachable way for the average layperson. Thanks for the upload. -Jake

[@Alexagrigorieff]

Application of Fourier Transform in general, not Fast Fourier Transform (FFT). FFT Is just an algorithm for fast calculation of discrete Fourier Transform (DFT).

[@UpcycleElectronics]

@@Alexagrigorieff Thanks

[@zenithparsec]

So basically, you play the Fourier transform of the filter into the etching solution, which modulates the intensity of the etching at the depth it's reached at that time, which causes the same modulation to occur in the porosity, resulting in the actual filter. Nice. Wait... does modulating the intensity change only the porosity or does it also affect the etching depth rate? Like, how does the already porous layer know it's finished? Is there some kind of passivating at the surface which is independent of the porosity level being applied?

[@Nichoalsziv]

I think he said the already prous layer usesup whatever is allowing it to transmit electricity, so it no longer etches. Was in the part with the foam as an example.

[@pizzablender]

@@Nichoalsziv Yeah, but the etching speed could still be dependent on the current. If that is the case, high-current parts of the waveform must be shortened a bit to compensate and crate a better filter.

[@turun_ambartanen]

I hope he explains it in more detail. My guess is that the etch rate is directly correlated to the current and you have control over the etched depth by simply varying the time the wafer is etched with that current. However, 1:24 and 1:50 show an oscillating voltage?

[@AppliedScience]

Yes, very good observation. The higher electrical currents increase both the porosity and the etch rate. Because of this, to produce a true sinusoidal variation in index through the thickness, the electrical waveform would have to be modified from a true sine wave. I realized this, and even suspected one of the academic papers compensated for this because their waveform looked modified (the peaks are sharper/narrow at higher currents, and more rounded at lower currents). However, I couldn't find anyone actually discussing this etch rate correction in the text. It should be pretty linear, so not too difficult. Despite this, making Rugate filters works just fine with sine wave electrical currents, and this detail didn't make it into the final video :)

[@commanderbutterbars3910]

@@AppliedScience If I read this correctly would it be possible to assign values to colors like binary , etch multiple layers and using each layer as more or less a bit, and create a very low capacity, but persistent, read only optical storage...thingie... that could then be interpreted by the spectroscope readout?

[@jeremypatrickdahan]

While you have wafers and HF, you can use them to etch hole-like nano structures that could make great molds for your gecko tape project. You can get very good results when combining with silver nano particles, which is what I used for some PV cells.

[@seditt5146]

Dude I grew up tinkering in my garage as you do here but never would I ever think some of the stuff you do is even possible in those settings. Granted yes you have much nicer equipment than I ever had but then again I have watched you build much of that over the years out of scrap parts and its flat out amazing

[@hsiaokueisen]

We have been running that kind of porous silicon research for quite long time; however, you did better job than we did! We need your skill!

[@stevenbarber3902]

I would just like to say that I love your videos and that you have inspired me to try making things for myself, Thank You

[@elilivezey7884]

26:48 finally something on his desk I can afford.... morton salt

[@LeoMakes]

I always wondered why my camera's lenses reflected either greenish or purplish light from the front element. I knew there was an anti-reflective coating on there, but I didn't know about the colors. Fantastic video!

[@royvanrijn]

This is super cool, my mind is buzzing with the things you could do with these filters. Thanks for making these videos!

[@Hotmedal]

Heavily doped is not a Cheech and Chong reference, ok Applied Science.

[@petergamache5368]

Except now it is, by virtue of calling it out... :)

[@FarhanMasood92]

so nice to bump into you here

[@LazerLord10]

I wonder if the chemical-sensetive ones have a high porocity on the top surface.

[@AlexWhittemore]

Same. Green is a relatively longer wavelength ie larger pores ie higher allowable surface tension of a chemical penetrant? But then I’d expect red to work too, unless the filter BECOMES red by the alcohol. I.e. it works on a red filter, except the original index is about the same as the alcohol impregnated index?

[@stalex91]

Do not mix porosity and pores size. You can have the same porosity (fraction of etched Si) and different pores size. Color depends on optical length which is product of layer thickness and refraction coefficient. The reason behind visibly low-changes with alcohol for certain structures is that spectrum moves over almost whole number of periods and with changed medium refraction coefficient same spectrum picture can be observed.

[@Dank_Giraffe]

The one thing I don't understand ist how this one man is aquiring and testing all this knowledge. There is no end. I mean I'm studying and I find it overwhelming finding all the time new stuff that's amazing and learning everything about it. The fact that you read all the papers and publish a video every week is something else. But I guess getting good at something today really is a rabbithole. You lose time and headspace trying to learn stuff that's not fitting to your classes. So nowadays I know that this stuff here is somewhat relevant for me someday, but it would be more efficient to learn exectly what I need for my classes right now. This is a fucking dilemma. But I'll see you again when this stuff is actually relevent to me;)

[@Dank_Giraffe]

Wait I'm a fucking idiot.... I'm having polarization microscopy as a class.... I bet I'll use the fact about germaniums refractive index

[@MarionMakarewicz]

I've been watching a lot of videos on quantum mechanics, light spectra in stars and galaxies, and light in general. And then you pop up with something like this. It really helps bring it all together.

[@Kariudosan]

Me: Hmm, having studied Electrical/Electronics Engineering and knowing a little about etchants and crystal orientation... Me: I wonder if crystal orientation is a thing here Applied Science: ...and crystallographic orientation doesn't seem to be a factor... This is why I subscribed long ago.

[@techsupport2173]

This is incredible... this is why I'm subscribed!

[@JohnnyDee62]

My formal training is in spectroscopy and this utterly fascinating!! Thank you and Ian (for suggesting)!

[@BRUXXUS]

I was JUST thinking about you today! I actually thought about reaching out to make sure everything's okay. So happy to see this! 😊

[@GregerOlofsson]

Mind blow. But not in an explodey way, more like this guy pulling my brain out like an NES cartridge and blowing new life into it.

[@C134B]

I really enjoy your videos and I wish I could support you on patreon & youtube, for now all I can do is watch your videos with ads 3 - 8 times.

[@apt40444]

Ben you're the best! Your excitement's both palpable and contagious and I love it. One of the most exciting vids I've seen in a long while. Pleeeeeease more vids on this theme.

[@longshot789]

These videos just get cooler and cooler. And that's saying something, they started out cool as hell.

[@davidliddelow5704]

This is basically capacitor decoupling but for light, pretty cool.

[@wimwiddershins]

Was thinking this. I wonder about testing the electrical properties of the silicon in a treated area?

[@gus473]

Great observation! Could be quite handy.....

[@Walter-Montalvo]

Wait what? Heh looks like I am going have start learning about capacitor decoupling. Thanks for the challenge!

[@alanclarke4646]

Could be useful in a photon computer...

[@bigpimpin9010]

Light: exists Lens: get bent nerd

[@thepjup4507]

10 years later: Light found success at a high level engineering firm, has a 10/10 beautiful wife and 3 kids, 2 boys and a girl, who also are well on their way to success. Light drives their Ferrari on the weekends for fun and has multiple accolades in their field. One day, light is running late for work and goes through a drive through to get breakfast. Light pulls up to the 2nd window, and there he sees it; Lens. Lens is fat, balding, dirty ripped clothes, and has the unmistakable look of a broken soul in their eyes. Lens looks up, squints for a moment and realizes who they're looking at. Lens: "Light! It's so great to see you, haha wow do you work at a car dealership? Awesome Ferrari, that's cool you get to drive them around like that haha so how's life treating you?" Light: "Heh, uh, no... it's mine...." Lens, looking shocked and defeated: "Uh... oh. Wow, t-that's... that's great... Hey, listen... I-I just... I just wanted to say...." Light, looking down at the floor of his car, steaming up with rage: "What, you're sorry? Sorry for abusing, tormenting me for years, making fun of me because I was a nerd? Is that it??". A deep rage is building within light, remembering all the bullying, beatings, insults. Lens: "....yes. I'm sorry. When I knew you, my life was in shambles. My parents were going through a divorce. I was trying so hard to be cool and gain the acceptance of every one, but all it was was me projecting my deep pain unto others. It took me years of therapy after I became an alcoholic and homeless to realize what I was doing, and I never stopped thinking of you. In all truth, I always envied you...." Light, shocked at what they're hearing, all the pain they had tucked inside seemed to fade away: "L-Lens.... I never knew. I'm so sorry to hear that and I am amazed at how self aware you've become. That's really beautiful. I'm so glad you found peace, and thank you for such a genuine, heart felt apology. That's all I wanted to hear all these years.". Light's eyes start to well up with tears. Lens, a warm glow of pride and content washing over their face smiles: "I always hoped I'd see you again so I could tell you. I-I just wanted to tell you, I did always think you were... ᵇʳᶦᵍʰᵗ"

[@MrPzyt]

@@thepjup4507 Ok... and I thought I am weird. Silly me!

[@fjs1111]

Ben, thank you so much for posting this along with all the papers.. I am going to look into doing this in the near future. appreciate it.

[@iam4godru2]

this is fascinating! Thank you Ben for your work.

[@turun_ambartanen]

I would love to see some SEM images of the etched parts!

[@Pilot_Pirx]

"Let's move into the electro-chemical side of the workshop" ...as you do.

[@pdrg]

This is an experiment I know I'll never do, but boy am I glad you're doing it and wow is it fascinating. Always top quality content, thank you so much!

[@maximilianlindner]

Ben, the kind of guy who *actually* blows your mind after announcing a mind blow!

[@EdwardTriesToScience]

thats intresting, especially as a chemical sensor

[@Walter-Montalvo]

Absolutely, it does make you wonder what sort of sensor applications are possible!

[@alanclarke4646]

New type of breathalyser? Or maybe an "electronic nose"?

[@Nighthawkinlight]

Hey Ben, I got to thinking about this video again over the last few days and had a question about it's applicability to making a radiative sky cooling emitter. Do you think it would be possible to make a surface by this method that selectively reflects the scattered visible blue light from the sky, and then also rejects the bands of IR that don't fall within the sky's infrared windows between 3-5 and 8-14 μm? I don't know how far this method can be pushed.

[@AppliedScience]

Hey, I've been watching your emissive paint videos, and can see the application here. In principle, you could make a rugate filter with these three bands, but the spread between them will be a challenge. Luckily, silicon is one of the few materials that has some transmission out to 10um. Let me do some reading about long wave filters.

[@BLUYES422]

This is an awesome idea!

[@raphrath8561]

these videos are so detailed and full of information i struggle to watch them after like 3 pm.

[@bertholtappels1081]

This is incredibly engaging content. It teaches science from an engineering perspective, and engineering with a science angle. Both become much, much more interesting than they ever could be on their own. I can’t believe you manage to keep on coming up with all this stuff.

[@AtlasReburdened]

Best channel on KZbin. Every video is amazing in the most strictly literal sense. So are those wafers just factory rejects from simiconductor production?

[@lemonsqueezy8549]

Fun fact: Porous Silicon (a special kind thereof) soaked in liquid oxygen is one of the most powerful explosive mixtures we know.

[@billsmathers7787]

Do you have a reference for this? I'd love to learn more.

[@robertlapointe4093]

@@billsmathers7787 A quick google search on porous silicon liquid oxygen explosives gives lots of references, such as www.newscientist.com/article/dn1103-superpowerful-explosive-arrives-with-a-bang/, for further reading

[@user255]

@@billsmathers7787 I had to look this up to believe: doi.org/10.1002/prep.201300053 It depends on how you measure whether it is one of the most powerful mixtures. For detonation velocity it loses to many compounds (and thus to their mixtures), but energy wise it is quite high.

[@lemonsqueezy8549]

doi: 10.1103/PhysRevLett.87.068301 It's true there are a few more powerful examples out there, even more proposed substances which have yet to be made in lab scale. But i think we can safely say this activated silicon does pack quite the punch :D

[@FLYGTRVIC]

Easily the best science content on KZbin, thanks. You make complex concepts easy to follow.

[@arjunbais1]

This is absolutely brilliant and what I was searching for! Was wanting to know how they make h-alpha filters and different wavelength filters.. thank you for sharing and for all the hard work. I truely appriciate it.

[@DanielSMatthews]

If you use a needle electrode and raster scan it while varying the waveform could you print a coloured image? Does the method also work on titanium, or more likely the oxide layer on the titanium?

[@kylejacobs1247]

To some extent yes. In my PhD I shot electron beams in a raster pattern at a special glass where the electrons hitting the surface caused a similar change in refractive index. By calibrating how long the beam stayed at each point, you could choose the color there. Here's a photo I took with a microscope. onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201570220 The biggest challenge in this silicon method is that it's hard to "contain" an electric field. As Ben mentioned in some silicon UV light can generate the charge carriers needed for the reaction, so a laser rastering the surface could be used to make a picture.

[@hexd0t]

This is probably where the etchant being able to etch the waver even without the current becomes a problem, because repeating the process for each pixel will take longer

[@devrim-oguz]

@@kylejacobs1247 so cool dude

[@vincenzomartorana2625]

@@kylejacobs1247 How cool is that

[@whuzzzup]

@@kylejacobs1247 What were the dimensions of that picture?

[@MedoKojiZiviOvde]

I love how he handles HF outside of a fumehood. That stuff is nasty and I can tell you since I worked 2 years with it.

[@ZoonCrypticon]

"Flesh-Eating Hydrofluoric Acid - Periodic Table of Videos" ( kzbin.info/www/bejne/pZrTnKaIna6GnK8 )

[@MedoKojiZiviOvde]

@@ZoonCrypticon can also cause an heart attack and spread easily through the body as it has great afinity with the calcium in the plasma

[@fd1053]

Thank you very much for your videos! I thoroughly enjoy them!

[@sharedknowledge6640]

This is yet another amazing video. You’re basically a one man research lab across an incredibly broad spectrum of disciplines and areas.

[@macro820]

This is so interesting thanks for sharing. I'd love to see those pores under the elec microscope

[@HorochovPL]

"Really high-tech glitter"

[@charlesashurst1816]

That is neat. The etching process is a kind of analog computer that does a Fourier transform to create a certain kind of optical filter from a particular current v time waveform.

[@bf0189]

I don't have anything to add but I wanted to thank you for putting so much effort into these videos. They are extremely valuable especially to an autodidact like me. Hope you're staying safe Ben!

[@adamwiess]

regarding the sensitivity to alcohol of just one filter, is it possible its the porosity (pore size) behaving like a molecular sieve?

[@febobartoli]

Came here to suggest this. It also means different filter can be specific for different chemicals based on molecular size, if this is true. It would be interesting to test with different solutions. Water despite its small size may not work due to its polar properties.

[@Galeere2]

Is it a stretch to call anti reflective coatings "optical impedance matching"?

[@alanclarke4646]

Ooh. Not so much of a stretch, thinking about it. If there are unmatched impedances in an AC circuit, then some of the signal IS reflected back ( eg in an unmatched transmission line). Nice one.

[@ZacharyParrott]

Not at all. My E&M textbook mentions verbatim impedance matching when discussing reflection.

[@Alexagrigorieff]

It's exactly what they are. Similar to impedance transformers on thin film UHF circuits, consisting of several quarter-wavelength pieces of different width (impedance).

[@randfee]

well done. Please do the light emitting porous silicon... as most people wouldn't expect silicon to be able to emit light outside of it's luminescense wavelengths ... also not a direct semiconductor. So... basically a silicon based LED... can't wait!

[@TomKappeln]

I love how the light "doesn't know whats ahead ...." It's sometimes soooooooooo hard to describe some things we have crystal clear in our mind ...

[@markfergerson2145]

So, it's a frequency-dependent rectifier for light frequencies. A conductive open celled foam with edge length of a few mm to a few cm should work for radar stealth. Can this be done for sound? Does applying voltage to the layer change its response? How about applying pressure via a liquid bath? Obviously adding alcohol into the sponge changes the sponge's refractive index, so you could use it to differentiate liquids by what color they turn the filter. It should reveal concentrations of solutes as well, when present. Can you etch a coarse sponge all the way through the slab? Can you select the pore size to pass molecules.particles by size?

[@Erhannis]

Ooh, many good ideas

[@Erhannis]

Re: liquids, I wonder if the surface tension problem he mentions could be overcome by pulling a vacuum and releasing it to pull the air out of the sponge and then push the liquid in

[@noobnerdpride]

So you can make an LED? Or what other light-emitting structure are you thinking of? RGB LEDs appear to have different dies inside for the different colors. Could you instead spatially control a doping process like this?

[@titter3648]

Or you can make optical circuits. Like they have been researching for a long time to replace electrical transistors in CPU's so you can make even smaller transistors than electronics can do.

[@Alexagrigorieff]

Silicon doesn't have the band gap wide enough for visible light LED

[@JlerchTampa]

9:40 Mind Blown! I 'thought' I knew a thing or two about thin film optical coatings having built a vacuum chamber and coated a rather large number of optical surfaces (mostly mirrors, but I played with SIO2 AR coatings) . Amazing content, thank you for sharing and providing such a detailed description of your experiments!

[@aaronschen9896]

Watching Applied science is a perfect antidote for over estimation of personal intelligence. Fascinating and well done as always.

[@sellicott]

What is the relationship between the optical frequency of the filter and the frequency of the current waveform used to produce it? It seems like there should be some connection, but it cannot be one-to-one as producing a THz waveform it definitely not in the range of a Teensy's abilities.

[@Ytqn]

I also expected him to tell something about it. He only mentions a period of 2-3 seconds, so this must be a very low frequency current below 1 Hz. Looks like some weird chemical frequency multiplier.

[@Alexagrigorieff]

The relationship is between thickness of layers and wavelength of passbands/stopbands. Thickness of layers depends on time the etching current was applied.

[@tomaszsmoka4519]

could you try to flow a changing concentration of alkohol and measure the spectrum of reflection? This could be a nice flow sensor

[@Walter-Montalvo]

Yes, I want to see ideas for sensor applications.

[@anchorbait6662]

"what's really going to bake your noodle later on is, would you still have broken it if I hadn't said anything?" mind blown, thanks Ben.

[@FrainFreeze]

Thanks for putting out these great videos Ben!

[@kswiorek]

Why didn't you put on some gloves for the heavy math in the beginning?

[@RicoElectrico]

1:35 Neuralink mug ;)

[@aurath]

Hey that's my favorite macro lens. Good taste Ben.

[@dr.zarkhov9753]

Thanks for this video. You've made this science a lot easier to understand. When I watch your videos I can't help but think of James Burke's science documentary "Connections"and anyone that has seen this should understand the sentiment.

[@Gantics-Antics]

Is the hydrofluoric acid absolutely necessary? I was super on board trying this out but HF makes me nervous lol

[@bhanubisht3385]

use KOH

[@BaldBozo]

Sometimes I wonder if this guy had to "dump a chunk of long term memory", like Johnny Mnemonic, just to fit all the stuff he knows into his brain.

[@AnianBrosig-yv3jn]

just wow, thank you for producing those videos

[@HarperChisari]

I’m in Quantum 1 and we were just calculating reflection and transmission coefficients, and this is a perfect example of its application!

[@Asdayasman]

"Where's your homework?" "It's 100% porous, teach."

[@Tferdz]

If I remember correctly my PhD, MoS2 has an index of almost 6

[@myshootinggallery3703]

Awesome demonstration on how to make optical filters with the same frequencies that make the light itself. But I would never even think about using HF without all the proper precautions in place: fume hood, PPE face shield etc. Don't use it unless you know exactly what you are doing, and you have all proper equipment to do so, period!

[@bambergazzzcoin]

I love you Ben. Never change.

[@jomac2046]

I just looked around, no I don't have a Spectrometer in my home work shed.

[@alanstarkie2001]

Too damp in there...

[@Change3D]

Whats with the neuralink mug? Any association?

[@BRUXXUS]

Oh my god... I just realized that's very likely where he's working! I think he's mentioned the kind of work he's done in a few previous videos and he did mention some wild stuff dealing with biomed devices. I know he was at Valve for a while, working on some of the Vive's VR tracking tech. Any company that Ben works for should feel super honored.

[@Change3D]

@@BRUXXUS So from LinkedIn I found that his current position is listed as Sr. Staff Hardware Engineer at Verily (Google Life Sciences). That said a "people also looked for" person linked form his page was the Director of Brain Interface Technology at UCSF. Unfortunately I don't think he works directly for Neuralink, their loss, but likely has connections. I am fascinated by this stuff and Neuralink so any info I can get would be awesome, especial if it was presented by Ben.

[@BRUXXUS]

@@Change3D Oh cool! I remember hearing BigClive mention that he worked for Google. I can't imagine what kind of stuff he's involved with considering the stuff he can make in his garage! I think Google is a pretty great company to work for in the right departments, Neuralink may still be a bit of a gamble, and I'm not sure they're operating out of the state where he lives.

[@toxicore1190]

really interesting stuff, I wonder why I did not stumble upon it earlier… thank you!

[@yosmith1]

I love it when I do some obscure search and the first on the list is Applied Science...one of my favorite youtube contributors