Two-Photon Absorption - Revision history

Lwcamp at 19:07, 7 March 2026

2026-03-07T19:07:10Z

← Older revision		Revision as of 12:07, 7 March 2026
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	==References==		==References==

	[[Category:Lasers]][[Category:Physics]][[Category:Physics & Engineering‏‎]]		[[Category:Lasers]][[Category:Physics]][[Category:Physics & Engineering‏‎]][[Category:Physics & Math & Engineering]]

Lwcamp at 03:24, 18 January 2025

2025-01-18T03:24:18Z

← Older revision		Revision as of 20:24, 17 January 2025
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	So if <math>(4/\pi) \, (\alpha_2 \, P_0 \, R)/(S \, D)</math> is much less than 1, two photon absorption will be negligible. If it is much larger than 1, two photon absorption will mean that most of your beam never gets where you want it to go.		So if <math>(4/\pi) \, (\alpha_2 \, P_0 \, R)/(S \, D)</math> is much less than 1, two photon absorption will be negligible. If it is much larger than 1, two photon absorption will mean that most of your beam never gets where you want it to go.

	The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. ~~However~~, ~~based on~~ typical ~~two-photon cross sections~~ of ~~other atoms and~~ molecules~~, the quantity~~ <math>~~(4/\pi) \,~~ \alpha_2 </math> ~~for sea level air on earth can be expected to be~~ somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>~~(4/\pi) \,~~ \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>~~(4/\pi) \,~~ \alpha_2 </math> will be proportional to the atmospheric density.		The actual two-photon absorption cross sections of oxygen and nitrogen molecules are difficult to find. There is data for water across much of the near ultraviolet, and if this is typical of light elements in small molecules you might expect that at a molecular density similar to sea level air on Earth you might have <math>\alpha_2 </math> somewhere in the range of <math>10^{-14} \mbox{cm}/\mbox{W}</math> to <math>10^{-12} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>\alpha_2 = 10^{-13} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>\alpha_2 </math> will be proportional to the atmospheric density.

	==Credit==		==Credit==

Tshhmon at 21:55, 23 April 2024

2024-04-23T21:55:42Z

← Older revision		Revision as of 14:55, 23 April 2024
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	==References==		==References==

	[[Category:Lasers]][[Category:Physics]][[Category:Physics ~~& Math~~ & Engineering‏‎]]		[[Category:Lasers]][[Category:Physics]][[Category:Physics & Engineering‏‎]]

Lwcamp at 18:51, 26 August 2022

2022-08-26T18:51:08Z

← Older revision		Revision as of 11:51, 26 August 2022
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	==References==		==References==

	[[Category:Lasers]][[Category:Physics]]		[[Category:Lasers]][[Category:Physics]][[Category:Physics & Math & Engineering‏‎]]

Sevoris at 15:43, 26 August 2022

2022-08-26T15:43:27Z

← Older revision		Revision as of 08:43, 26 August 2022
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	==References==		==References==

	[[Category:Lasers]]		[[Category:Lasers]][[Category:Physics]]

Lwcamp at 05:04, 12 October 2021

2021-10-12T05:04:58Z

← Older revision		Revision as of 22:04, 11 October 2021
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	The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. However, based on typical two-photon cross sections of other atoms and molecules, the quantity <math>(4/\pi) \, \alpha_2 </math> for sea level air on earth can be expected to be somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>(4/\pi) \, \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>(4/\pi) \, \alpha_2 </math> will be proportional to the atmospheric density.		The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. However, based on typical two-photon cross sections of other atoms and molecules, the quantity <math>(4/\pi) \, \alpha_2 </math> for sea level air on earth can be expected to be somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>(4/\pi) \, \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>(4/\pi) \, \alpha_2 </math> will be proportional to the atmospheric density.

			==Credit==
			Author: Luke Campbell

			==References==

	[[Category:Lasers]]		[[Category:Lasers]]

Lwcamp at 15:20, 29 September 2021

2021-09-29T15:20:18Z

← Older revision		Revision as of 08:20, 29 September 2021
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	The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. However, based on typical two-photon cross sections of other atoms and molecules, the quantity <math>(4/\pi) \, \alpha_2 </math> for sea level air on earth can be expected to be somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>(4/\pi) \, \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>(4/\pi) \, \alpha_2 </math> will be proportional to the atmospheric density.		The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. However, based on typical two-photon cross sections of other atoms and molecules, the quantity <math>(4/\pi) \, \alpha_2 </math> for sea level air on earth can be expected to be somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>(4/\pi) \, \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>(4/\pi) \, \alpha_2 </math> will be proportional to the atmospheric density.

			[[Category:Lasers]]

Lwcamp: Created page with "It takes about 6.2 eV of energy to ionize an air molecule. So any laser beam that has an energy-per-photon of 6.2 eV or more will almost immediately be absorbed by air. This..."

2021-09-29T03:58:09Z

Created page with "It takes about 6.2 eV of energy to ionize an air molecule. So any laser beam that has an energy-per-photon of 6.2 eV or more will almost immediately be absorbed by air. This..."

New page

It takes about 6.2 eV of energy to ionize an air molecule. So any laser beam that has an energy-per-photon of 6.2 eV or more will almost immediately be absorbed by air. This corresponds to a wavelength of 0.2 μm.

But if you have a beam of light with a wavelength between 0.2 μm and 0.4 μm, then two photons acting together will have enough energy to ionize a molecule in the air. This is much harder than just ionizing with one photon, but if the beam is very intense it can happen. 0.4 μm happens to be just on the threshold between violet and ultraviolet light, and the 0.2 μm to 0.4 μm range spans the near ultraviolet between vacuum ultraviolet and the visible part of the spectrum, If you have an ultraviolet laser and are trying to focus it to a very intense spot at your target, two photon absorption can end up removing most of the power from your beam before it gets to your target!

If you start with a power of <math>P_0</math> in your beam, emitted from a focusing aperture of diameter <math>D</math> and focused to a spot of size <math>S</math> on the target (which can be as small as the diffraction-limited spot size, but can also be larger), and the target is at a range of <math>R</math>, and if the two-photon absorption coefficient is <math>\alpha_2</math>, the power that hits the target will be <ref name=Prabhakaran_2012>https://www.sciencedirect.com/science/article/pii/B9780444533494002077 P.Prabhakaran, T.D.Kim, and K.S.Lee, in Polymer Science: A Comprehensive Reference, 2012</ref>
<div class="center" style="width: auto; margin-left: auto; margin-right: auto;"><math>
P = \frac{P_0}{1 + (4/\pi) \, (\alpha_2 \, P_0 \, R)/(S \, D)}
</math></div>
So if <math>(4/\pi) \, (\alpha_2 \, P_0 \, R)/(S \, D)</math> is much less than 1, two photon absorption will be negligible. If it is much larger than 1, two photon absorption will mean that most of your beam never gets where you want it to go.

The actual two-photon absorption cross sections of oxygen and nitrogen are difficult to find. However, based on typical two-photon cross sections of other atoms and molecules, the quantity <math>(4/\pi) \, \alpha_2 </math> for sea level air on earth can be expected to be somewhere in the range of <math>10^{-7} \mbox{cm}/\mbox{W}</math> to <math>10^{-10} \mbox{cm}/\mbox{W}</math>. If you are building a real-life ultraviolet laser death ray, you will need to get the correct value for the wavelength you are using. However, for the purposes of fiction just choosing <math>(4/\pi) \, \alpha_2 = 10^{-9} \mbox{cm}/\mbox{W}</math> won’t be too wrong. This <math>(4/\pi) \, \alpha_2 </math> will be proportional to the atmospheric density.