Re: The sound of a laser.

From: AES/newspost (siegman_at_stanford.edu)
Date: 08/20/04 

Date: Fri, 20 Aug 2004 12:36:59 -0700

One aspect of the physics of laser-induced gas breakdown that I've not
seen mentioned yet in this thread (maybe I missed some of the messages)
is that it's very much a "runaway" process.

A small amount of initial heating and/or ionization of the atoms in a
tiny volume of gas can result from even rather weak direct absorption by
the atoms, or from laser-induced multiphoton ionization of the gas
atoms, or the presence of a small amount of random ionization from other
causes, laser absorption by dust particles or impurity gases, possibly
other causes.

Once heating starts or some initial ionization occurs, the initial
volume can rapidly become much more absorbing to the laser light as a
result of excited state absorption in heated neutral atoms, additional
absorption lines in the ions that have been produced as well as in the
heated atoms, and free-electron and plasma absorption processes, with
relative strengths depending in complex fashion on wavelength, optical
intensity, and presence of impurities. Basic point is that more heating
produces more absorption produces more heating produces . . .

Rates of further heating and ionization can then increase rapidly in a
runaway fashion, leading to intense heating and ionization, formation of
a visible spark, expanding shock wave, and generation of an audible
"snap".

Most of the same things also happen in the pulsed laser-induced damage
spots produced when a laser beam is focused inside a "transparent"
dielectric medium to do subsurface laser marking.

Relevant Pages

  • Re: The sound of a laser.
    ... tiny volume of gas can result from even rather weak direct absorption by ... the atoms, or from laser-induced multiphoton ionization of the gas ... Once heating starts or some initial ionization occurs, ...
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  • Re: The sound of a laser.
    ... > tiny volume of gas can result from even rather weak direct absorption by ... > the atoms, or from laser-induced multiphoton ionization of the gas ... > Once heating starts or some initial ionization occurs, ...
    (sci.physics)
  • Re: The sound of a laser.
    ... > tiny volume of gas can result from even rather weak direct absorption by ... > the atoms, or from laser-induced multiphoton ionization of the gas ... > Once heating starts or some initial ionization occurs, ...
    (sci.optics)
  • Re: The sound of a laser.
    ... >> tiny volume of gas can result from even rather weak direct absorption by ... or the presence of a small amount of random ionization from other ... >> causes, laser absorption by dust particles or impurity gases, possibly ... Basic point is that more heating ...
    (sci.physics)
  • Re: The sound of a laser.
    ... >> tiny volume of gas can result from even rather weak direct absorption by ... or the presence of a small amount of random ionization from other ... >> causes, laser absorption by dust particles or impurity gases, possibly ... Basic point is that more heating ...
    (sci.optics)