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connection to the original photon we hitched a ride with (which emerged from the laser)!
• At some point, we will encounter a strong absorber. When this happens, our photon’s energy will be ‘taken’ by an electron and then released into the local environment (lattice) as vibrational (or heat) energy. This will make things a bit toasty there, which is usually what we are trying to achieve.
• The whole trip might last somewhere between 0.33 and 100 picoseconds (or longer), depending on how many scattering events it undergoes. This means, that, if you are using a 5 nanosecond laser for tattoo removal, then the vast majority of the photons you fire into the skin have already been absorbed or lost, before the end of the pulse!! This is still the case for most picosecond lasers too.
• Here endeth the trip!
So, in summary:
Photons are merely little ‘balls of energy’. We utilise them to transfer energy from the laser into the tissues, to generate heat.
Many are lost from the tissue via Fresnel Reflections and back-scattering.
Those that remain in the skin disperse throughout it, reducing the fluence as they move deeper.
Eventually, all photons will be absorbed by something, somewhere, after possibly many scattering events.
(By the way, if you want to know how many photons are in a ‘typical’ laser pulse, click here...)
Ciao for now, Mike.
Mike’s Blog Posts 273