A HF pulsed UV light effect on metal alloy

[Chris Wallin]

I'm trying to understand the effect on a metal alloy of UV light (diode or laser) pulsing at about 1MHz.

I am assuming that the stirring of atoms will generate heat, possibly moving the alloy to its eutectic point (depending on the alloy) and in particular to atomic motion.

Can anyone confirm this assumption and/or point me to any source that explains the physics of this experiment?

 

[Nidum]

Look up the theory of laser cutting .

Laser cutting is a very mature technology and there has been extensive research into the physics involved .

 

[Chris Wallin]

Thanks Nidum. Will check that out.

 

[DrDu]

Nidum has certainly given you a good advice pointing you to laser cutting.
Here something about the basic physics: That part of the laser light absorbed will heat up the material. All the light which is not being reflected will be absorbed. The conversion of the excitation into thermal energy is ultrafast, so you don't have to bother about the mechanism.

The rise of temperature will depend on the thermal conductivity of your alloy (if you heat up only a small spot ) and the power with which you are irradiating.

 

[Nidum]

 

[Chris Wallin]

Thanks again for the replies. I will investigate laser cutting. It may shed light on my main question which is how the high frequency pulsing of light (such as UV) as opposed to steady light (e.g. UV laser beam) affects a metallic alloy. This effect would no doubt change with frequency.

 

[f95toli]

Why do you think 1 MHz is high frequency?
What is "high" is always relative to the timescales involved in the problem; and 1 MHz would only be a high frequency if there is something happening much slower than 1/1e6= 1uS in this situation.
Now, 1 microsecond is actually quite a long time in most physics problems and it is certainly much longer than any timescale you would typically encounter when it come to e.g. electron transport which would presumably be one of the main timescales involved if you are considering heating at the "atomic" level.

This doesn't mean that there can't be technical reasons for why you might need to pulse a laser (to e.g. build up enough energy in each pulse), but I get to the feeling that this is not what you are getting at.

 

[Chris Wallin]

Thanks for that f95toli. The laser cutters I came across seem to have a pulse range of about 100 kHz–1 MHz (refer http://www.spectra-physics.com/prod...t?cat=micromachining&subcat=femtosecond#specs), sometimes up to 2MHz. I originally was planning on controlling the pulse rate with a PLC but found out that the affordable ones cannot go as high as 1MHz. Instead I may need to be using a programmable function generator.

Last night I came across a study by the University of Tsukuba in Japan that seems to have proven atomic motion under pulsed light (refer http://www.eurekalert.org/pub_releases/2016-02/uot-sro022216.php) - see attached press release.

From what I can understand, the pulsing induced a change in atomic position and rotation of the target compound. This correlates with another study in 2001 wherein pulsed laser light induces motion along the laser pulse propagation direction (refer http://journals.aps.org/pra/abstract/10.1103/PhysRevA.64.013411).

There's 2 things I am trying to understand:

1. My intuition tells me that if the pulse frequency and light intensity are below those of laser cutting, the pulsed beam would excite the electrons and also cause motion of the atoms. If the alloy is eutectic (liquid), a gyroscopic motion in the plane of the light beam would be observed. But I have not found 'legible' scientific facts that confirm this… yet.

2. Moreover, I am wondering what radiation would be emitted from the gyring alloy. Again, I am assuming that the pulsed light (UV or other) would excite the electrons to higher shells and that a light particular to the alloy would be emitted. This particular light would be unrelated to the pulsed light, as the latter is only serving to cause excitation and motion, and instead, my intuition tells me that the new emitted light would be slowly pulsing with the motion of the alloy. Still only my intuition.

Does that shed any more light on the matter? - no pun intended )

 

[Nidum]

http://www.mdpi.com/2072-666X/5/4/1219/htm

Also search on " laser interaction with metal surfaces " and variations .

Edit : removed redundant search words

 

[Nidum]

Also search on " laser matter interaction theory "

 

[Nidum]

I first saw a laser in about 1970 when the local University invited an American professor over to give a series of lectures .

He just had one ruby laser and a couple of laser pointers . Excellent lecture though which mixed fun demonstrations with superb teaching .

I was in VI-ii at the time and doing A levels .

 

[Chris Wallin]

I first saw a laser in about 1970 when the local University invited an American professor over to give a series of lectures .

He just had one ruby laser and a couple of laser pointers . Excellent lecture though which mixed fun demonstrations with superb teaching .

I was in VI-ii at the time and doing A levels .

Thanks Nidum. Most appreciated and interesting. Let me see what comes out of these pointers. I can tell you know something about lasers. I could hardly spell it in 1970! :)

 

[f95toli]

I think you might have misunderstood some of the papers.
I haven't had a chance to look at all the links, but at least the first one refer to work using femtosecond laser pulses.
Now, such lasers are indeed pulsed at say a few MHz (or even slower) but for technical reasons. the actual pulse is actually only a few femtoseconds long which is why you can study atomic motion etc.
Hence, it is the length of the pulse that is important for the physics,. the repetition rate is what it is because that it how long it takes to set up the system for the next pulse,. i.e. it is just a technical limitation and has nothing to do with what it is going on in the sample.

 

[DrDu]

It would be helpful to know what size is the target, the intensity of the laser and what you are actually up to.

 

[Chris Wallin]

Thanks f95toli. Yes, you are right, I was not paying attention to the femtosecond duration. Having done more research, I simply need a heating effect of the pulsed light in addition to atomic excitation to create motion in the liquid compound, in a similar way to boiling water, i.e. substantially less than laser cutting energy.

 

[Chris Wallin]

It would be helpful to know what size is the target, the intensity of the laser and what you are actually up to.

Yes, you are right, but that's for me to determine. I'm still validating theoretically that this will work (refer post #8).