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I am not a physicist but I have to make an air plasma sparkle in the air for a light show I am planning to do.

I have been reading about various types of lasers and laser energies, I am very confused. I need to know what type of laser I need to be able to create a sparkling spot in the air.

Specifically:

  • what laser model from which manufacturer?
  • what optical lens to focus the spot about 2 meters from the source. I know I need a 1064nm laser with a pulse duration around 10ns.
  • But how do I know which ones have enough peak pulse power to do this? Please help
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With a 1064 nm (i.e. infrared) laser you cannot ionize air molecules. The ionization energy of oxygen and nitrogen molecules is around 14 eV. Therefore you need photons with an energy of at least 14 eV, i.e. a wavelength shorter than 80 nm. This is already ultraviolet light.

See also this question: How is it possible to ionize air with wavelengths longer than UV ?

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Just wanted to say that the answer provided by Thomas Fritsch is definitely not correct. I have produced laser-induced plasma in the laboratory with a ~5 nanosecond pulsed Nd:YAG laser, frequency-doubled at 532nm. It is possible; there is plenty of literature evidence that it is also possible with 1064nm.

See, for example, "Laser Spark Ignition: Laser Development and Engine Testing" (McMillian et al, 2008) http://dx.doi.org/10.1115/ICEF2004-0917. They provide a chart with the energy density values required to ignite a plasma in mid-air, typically on the order of 10^12-10^13 W/cm2. When I was producing these in the lab, I found the threshold was also about 3*10^12 W/cm2 at 532nm (though I must admit, I wasn't sure my lenses were diffraction-limited).

If you don't believe me, there is even more compelling evidence as a video by youtuber Styropyro (https://www.youtube.com/watch?v=-BeTq99LqUo), see timestamp 9:11, where he ignites a plasma in mid-air with a tatoo-removal passively Q-switched Nd:YAG laser, also frequency-doubled (532nm). A Youtube search on "laser induced plasma" provides several more videos with people doing it at various frequencies in visible and IR. You most definitely don't need an extreme-UV laser to ionize air.

Now answering the author's question, you probably don't want to use this technique in a light show where the public is present. (1) you need insane beam powers that will easily blind a bystander if the laser is not shot straight up in the sky; (2) Even if you guarantee you shoot into the sky, you'll face problems with the FAA (airplanes!!).

I think there are demonstrations of this technology for volumetric displays (say, https://www.youtube.com/watch?v=GNoOiXkXmYQ), but I'm quite sure they are not ready for public viewing for the reasons I cited above. Maybe I'm mistaken; as if the laser pulse is sufficiently short (say, femtoseconds), then it may be possible to produce laser-induced plasmas at tolerable power levels. I'll be glad to hear comments!

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  • $\begingroup$ (+1) Good answer. My old Questek excimer laser, at 308 nm output wavelength, had the grunt to do it. Neat demo in the lab, with good eye protection. No way it would be reasonable outside the lab. $\endgroup$
    – Ed V
    Commented Dec 9, 2023 at 0:51

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