# Creating a low pressure region inside a laser beam

• taylaron
In summary, the speaker is looking for advice on how to generate a low pressure region within a laser beam and is seeking clarification on whether or not it is possible to replicate Bernoulli's principle using laser light instead of high velocity air. The speaker is also looking for advice on how to generate a "hollow" beam which could be used to confine particles.f

#### taylaron

Gold Member
Greeting PFers,
I need to use the low pressure region inherent within the cross section of a laser beam for a project. However, I don't know how to calculate the necessary laser power to generate a sufficiently low pressure.

The topic is briefly dicussed *here*, but I see no relevant equations.

I have no idea how to approach the problem of calculating the temperature of the air inside the beam. I would appreciate any help.

Thanks,

Do you need a low pressure, or a low density? I don't see how heating would lower pressure - it will certainly lower the density, of course.
Do you have any air circulation?

The reference only says that it was "observed" - but there is an immense literature on laser cooling - even a Nobel prize! You can start here: http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascool.html

But what they observed may not have been Doppler cooling; it may have been strong absorption of laser light by the water vapor, causing it to heat up and rapidly move away. Why are you using such an old reference work?

I'm trying to determine wither or not it's possible to replicate Bernoulli's principle employed in the levitating ping-pong ball experiment using laser light instead of high velocity air. The air within the laser beam would be hotter than outside the beam, so small particulates should* stay confined to within the beam given sufficient pressure differential.

I think this would not work - the heating effects are very small and dissipate rapidly, plus you would also be heating your tiny ping pong balls. As you increase the power the laser beam becomes dangerous - class 4 - and even the scattered light could cause eye damage.

So you would have to make a video instead of a live demonstration.

The best option I can think of offhand is to generate a "hollow" beam: use an axicon to generate a Bessel beam ... thete is a lot of recent literature in this area.

There are wavelengths with significant absorption in air (or another gas), but the particle would have to be invisible for the laser beam at the same time (otherwise it gets heated up and the laser beam is disturbed or not present behind the particle). That looks like a weird combination.

The hollow beam looks more promising. That has been demonstrated, but only for microscopic particles.

instead of ping pong balls, i want to use microscopic metal particles (a stream of them). i considered using a hollow beam, but i need the particles to melt.