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Lets say I want to heat air with laser,what wavelenght should I chose,ultraviolet,infrared or something in visible spectrum? To clarify,I want the laser beam to lose power and get converted to heat in shortest amount of distance possible. I am looking for maximum absorbtion, to convert the laser into heat.

When I think about it,two different wavelenght may produce equal heating of air,but the thing is one that have short range will produce heating that is more concentrated in space while other will heat air over longer distance so the energy will be spread over greater amount of air,I want that short range concetrated type heating so minimum quantity is heated to high as possible peak temperature.

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  • $\begingroup$ en.wikipedia.org/wiki/Atmosphere_of_Earth#Optical_properties $\endgroup$ – John Dvorak Jan 8 '17 at 19:57
  • $\begingroup$ If it's moist air - definitely IR. Otherwise difficult exciting $N_2$, $O_2$ molecules by themselves. There are more efficient ways of heating air other than lasers. $\endgroup$ – docscience Jan 8 '17 at 20:33
  • $\begingroup$ You may have better luck with phased array ultrasonics; acoustic frequencies modulated at a distance by one or more ultrasonic sources. How high of a temperature are you trying to heat the air to? $\endgroup$ – docscience Jan 8 '17 at 20:38
  • $\begingroup$ the other gentleman who commented is saying "short wavelenght",you say IR which is long wavelenght,now I dont know who to believe $\endgroup$ – wav scientist Jan 8 '17 at 21:22
  • $\begingroup$ @wavscientist: Both approaches make sense, but for different reasons. IR is the simple approach. UV isn't going to do much, until you hit non-linear effects (ionization). That requires much more engineering to work. Then again, how do you plan to avoid heating up your laser source? Even more engineering question, not so much physics theory. $\endgroup$ – MSalters Jan 9 '17 at 0:56
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As you need to heat air to high temperature anyway, maybe you should consider a very high-power (pulse) laser that induces breakdown in air (https://www.rp-photonics.com/laser_induced_breakdown.html ); after the breakdown, air will be ionized and will absorb more power. Ideally, you should choose shorter-wavelength laser to be able to better focus the beam, but it is often harder to get high power at shorter wavelength. There are several processes that need to be taken into account: when the air ionization is so high that the plasma frequency exceeds the radiation frequency, the plasma becomes opaque for the radiation; heated air expands, which causes decrease of temperature; some other nonlinear effects, such as self-focusing, can be significant as well.

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  • $\begingroup$ Yes! Thats exactly what I wanted to do,turn air into plasma with pulse laser.One man that commented said use IR,you say short wavelenght,so now I dont know,infrared or ultraviolet pulse laser,what is best to make air into plasma? I dont want to heat the air to extremly high temperature,if its hot enough that it turn into plasma,that is hot enough for my needs.Actualy my goal is to make the air make the "pop" noise,I wonder if it makes that noise even if I heat it to temperature lower than plasma temperature... $\endgroup$ – wav scientist Jan 8 '17 at 21:18
  • $\begingroup$ @wavscientist: You may wish to look at citeseerx.ist.psu.edu/viewdoc/… (Aspects of laser-generated acoustic shock waves in air) $\endgroup$ – akhmeteli Jan 8 '17 at 21:31

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