Pulsed Laser Deposition - first "baby steps"...

Long time ago was day-dreaming about making my own mirror for various laser applications, but this is very hard. Even, now, when have little Nd:YAG laser with respectable power, it is very hard to do something with homebuilt setup. I not think to make extremely good mirrors, just humble mirrors for my own research. New mirrors are very expensive, so I was try to make something cheaper, but...
My decision to try make mirror with pulsed laser deposition is because it not require so high vacuum (I hope) and no need high power to evaporate various materials for coating (such as heater, E-beam etc...)


So, how looks my PLD setup:. 

Very humble setup. Very small laser, which is actually not enough powerful for seriously coating, just for experimenting. Also, with my refrigerator service vacuum pump can't get vacuum better than 0.7 Torr. For this purpose will be better turbomolecular pump (very expensive), or oil diffusion pump (reasonable price), but then vacuum chamber must be large, polished stainless steel with good sealant and many windows (for laser input, for observation of process, etc...)

First result with sputtered silver promise - here is how looks plasma plume at various pressure (not enough high vacuum):
 

Vacuum pump give me vacuum up to 0.7 torr, but between 0.9 Torr and 0.7 Torr, I must wait more than half hour. Also plasma looks the same as at 0.9 Torr captured by camera, but in fact is little bigger - reason is that with the same power of laser, plasma plume become weaker due to low ion concentration, and edge of plume is just barely visible, so it appear weaker, though, actually, evaporated atoms are faster due to less gas resistance.
 

At this picture is shown area where plasma plume was very close spaced. There is the best results. All other area are very poor coated. Lower part of microscope slide is not covered with silver because it was covered with insulating tape as slide holder. At this area is clearly visible double reflection from both of glass surfaces.
 

Yellow, or maybe better said pale brown area is due to poor coating and maybe significant amount of silver was oxidized before reach glass surface (vacuum is poor, and no one "buffer" gas is applied). Because this is my first attempt to make mirror, I not believe to achieve any result, so not pay enough attention to clean glass surface before coating. Later saw dust, and try to remove it with cotton, but instantly damaged surface. Or silver is not good "attached" due to impurities on surface, or because vacuum is poor, so atoms speed are low. Or, maybe due to lack of knowledge about this method of mirror coating. Note: blue spot is normal for silver mirror, because silver has poor reflectance above ~450 nm, so for this color is almost totally transparent. Violet ring around blue spot is still mystery for me...
 

Later, tried coat glass with copper, but it is very hard job. Copper has good reflectance at 1064 nm, and good thermal conductivity, so it is very hard to make evaporation. Maybe also high melting point play important role in sputtering process?
 

Copper source for coating is thin shim attached to brass hexagonal block. At picture is shown a lot of laser spots. I was tried to make the best focus and the best place from where plasma plume will looks better, but all attempts are resulted with average results.
 

Compared to silver coating, copper coating is very poor. Not only for possible reasons mentioned above, but also because was gave up very quickly. It is very hard wait so long time until vacuum pump do it's job, and results may be disappointing poor.
 

For my curiosity, tried evaporate carbon pill to see how amazing will be plasma plume compared to plasma in air from previously experiments. Disappointment again. Maybe due to lack of enough oxygen, plume looks weak.
 

Carbon pill evaporate strongly, and has deep crater. It is caused just with about tens laser pulses. Insulating tape, which hold pill, has burned spots - it is time to add aiming laser. Without aiming laser it is very difficult predict where pulses will hit target. This is my next plan.
 

Thin cover glass for microscope slide looks dark gray (picture is adjusted to high contrast, because of poor coating). Regarding to deep craters on pill, I was expect more carbons there, but seems to me that many carbons atoms flied around inside vacuum chamber. At this way, I can make a neutral density filters (gray filters) for photographic job. :-)
 

But, this things surprise me; though looking through glass it appear gray, by looking from low angle it appear pale white. Or carbon is make some kind of oxide, or pill consist other additive which make this effect. 



Conclusion: I will continue experimenting with PLD when get time. I want to make one nice aluminum mirror. But one things is very difficult there - all metals oxidize very fast, and need protective coating, maybe SiO2 or other. But, Nd:YAG laser which I have is too weak for long use - it is without air or water cooling, and for just one coating it require hundreds and maybe thousands of pulses to finish some relatively good job. Maybe one day will make bigger one... We shall see...
 



Date: July, 20th, 2005.