I Want to Remove a KTP Crystal's Coating

[Strange_matter]

TL;DR

I want to remove a coating on a KTP crystal that filters 1064nm light for a makeshift optical parametric amplifier.

I want to make an optical parametric amplifier and am considering using a KTP crystal from a laser pointer. The problem is the KTP seems to have a coating that filters 1064nm light which I want the OPA to produce. I've tried to remove the coating using nail polish remover (acetone), but that didn't seem to work. Do you have any advice? I'm thinking I may try to cut the crystal with the coating off with a small blade, but I'm concerned that may have negative effects on the experiment: I want to generate position-momentum entangled light, and I'm a little concerned cutting it at an angle might influence that; although I may be worried about nothing.

 

[sophiecentaur]

Abrasive, followed by a good polishing?

 

[Strange_matter]

Abrasive, followed by a good polishing?

Would silver polish work for the polishing? I have Tarn-X. Here is a link to its components:
https://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=10006010
It also supposedly contains thiourea according to the bottle. It also seems like it could etch the crystal would that be a problem? Alternatively, do you have any tips for polishing it? Thanks.

 

[sophiecentaur]

Alternatively, do you have any tips for polishing it?

Silver polish has a chemical as well as a physical polishing action and that may not be good. Best to use a powder of a block with a polishing compound set in wax.
There are a host of grades of polish, used for metals (which is what I know a bit about), with a range of coarseness. I would try Tripoli (because I already have some) on a felt wheel, followed by rouge. KTP has a Moh hardness of 5 so Cerium Oxide powder would probably work to start with.
How optically flat does the final surface need to be? Mom 5 is not very hard so I think you could end up with a curved face (?). I suggest you read about lapidary (gemstone) polishing to get some specific ideas. Your problem will be that the minimum quantity you can buy of these materials would leave you with a lot left over for the rest of your experimenting lifetime.

 

[Strange_matter]

Silver polish has a chemical as well as a physical polishing action and that may not be good. Best to use a powder of a block with a polishing compound set in wax.
There are a host of grades of polish, used for metals (which is what I know a bit about), with a range of coarseness. I would try Tripoli (because I already have some) on a felt wheel, followed by rouge. KTP has a Moh hardness of 5 so Cerium Oxide powder would probably work to start with.
How optically flat does the final surface need to be? Mom 5 is not very hard so I think you could end up with a curved face (?). I suggest you read about lapidary (gemstone) polishing to get some specific ideas. Your problem will be that the minimum quantity you can buy of these materials would leave you with a lot left over for the rest of your experimenting lifetime.

In terms, of flatness, it may not need to be perfect. The main point of this is to create an OPA to generate entangled photons, so as long as they are position-momentum entangled leaving the crystal, that should hopefully be sufficient.

 

[sophiecentaur]

In terms, of flatness, it may not need to be perfect. The main point of this is to create an OPA to generate entangled photons, so as long as they are position-momentum entangled leaving the crystal, that should hopefully be sufficient.

I don't know how that requirement affects the requirement for coherence (aka optical flatness). The term 'entangled' suggests to me an exotic situation but perhaps coherence may not matter so much. My proposed solution takes no account of stuff like that but it would certainly remove a thin coating. Perhaps the KTP crystals are high optical spec - but you could find out about that, I guess. Is there another possible alternative source of KTP?

 

[Strange_matter]

I don't know how that requirement affects the requirement for coherence (aka optical flatness). The term 'entangled' suggests to me an exotic situation but perhaps coherence may not matter so much. My proposed solution takes no account of stuff like that but it would certainly remove a thin coating. Perhaps the KTP crystals are high optical spec - but you could find out about that, I guess. Is there another possible alternative source of KTP?

From what I've seen KTP can be somewhat expensive. I've seen a 1 cm wide crystal for $80, and it may still have the coating I'm trying to remove. I don't think a slightly uneven surface would prevent entangled photo generation; what I'm a little concerned about is an uneven surface harming the entanglement after the photons leave by causing slight differences in the positions where they exit. I may just vigorously rub the crystal on a piece of felt with the proper materials, since I don't have a wheel and that may help keep it flat. The crystal is only about 1 mm wide, which may make things a little more difficult.

 

[sophiecentaur]

A bit expensive to try random experiments perhaps but something like that deserves to be looked after. A felt wheel for a Dremmel type drill would be cheap enough and would fit in a pillar drill or hand held pillar drill. The crystal could be mounted to polish the end. You can drill a 1mm hole in a small hardwood block (scraps are easy to find) to hold it and avoid losing it whilst working on it.
That’s a small consideration in the context of your whole experiment.