Lasers, photo-electric effects and ionization of atoms

[Nerd]

I get the photo-electric effect but I am just wandering, is that the only principal on which lasers function? I mean do lasers "cut" materials by ionizing the atoms?

 

[dst]

Lasers typically cut metals by heating them up to melting point. When applied, they typically vapourize metal at just that region, and heat is dissipated very fast which is why they're used - to give very decent cuts where CNC milling wouldn't be the greatest option.

No need for ionisation, just pure energy transfer. Also, I don't think the photoelectric effect is the most appropriate way to describe light interacting with solids. I don't know what is though, but plenty on here do.

 

[Nerd]

What you say makes sense, however if lasers did cut metal by heating it to melting point, I would think that this happens by exciting the electrons in the metal, which would lead to more kinetic energy and thus increase heat. If this is true than why do mirrors reflect lasers beams, afterall the same that happens to the metal's atoms should happen to the mirror's atoms?

 

[dst]

It depends vastly on the wavelength. Speaking from a generalised point of view, every material has a characteristic absorption spectrum - for each wavelength of electromagnetic radiation, we can say it'll absorb x% of it. For metal-cutting, you would use a laser tuned to maximum absorption (and hence maximum heating) which I think is in the infrared range. It doesn't matter if it's transparent to us or reflective to us as long as absorption takes place in whatever range of wavelengths that is viable.

I have no idea why reflection occurs to tell you the truth. It doesn't seem to be actually explained anywhere (at least pre-undergraduate level).

 

[Nerd]

Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?

 

[Integral]

What you say makes sense, however if lasers did cut metal by heating it to melting point, I would think that this happens by exciting the electrons in the metal, which would lead to more kinetic energy and thus increase heat. If this is true than why do mirrors reflect lasers beams, afterall the same that happens to the metal's atoms should happen to the mirror's atoms?

Fact is high power laser systems do not use metalized mirrors. They use what is called a dielectric mirror, it consists of multiple layers of a dielectric material, the thickness of a layer is determined by the wavelength of the laser and the angle of reflection needed. These mirrors are transparent to all but the wavelength designed for.

 

[dst]

Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?

It depends on the absorption spectrum of oxygen. I would imagine that solid oxygen has a slightly different spectrum from gaseous oxygen (because these things mostly come from the bulk properties of a material and not the individual atoms).

As for x-ray reflecting mirrors, again, I have no idea. Considering that mirrors are simply just a very convenient way to execute a quantum mechanical trick, then surely an x-ray reflector should be possible. In fact there is research going on into antennas (the sort that you use to pick up TV and radio) being used to pick up and transmit visible light signals. This of course is not the same thing as an x-ray reflector but EM waves are just that, there is nothing special about any particular wavelength or range of wavelengths, other than the arbitrary significance we associate with them.

As I said, I honestly have zero idea as to why reflection occurs! Here's what Wikipedia says on the matter:

All interactions between photons and matter are described as series of absorptions and emissions of photons. When an arriving photon strikes a single molecule at the surface of a material, is absorbed and almost immediately reemitted, the ‘new’ photon may be emitted in any direction, thus causing diffuse reflection.[citation needed]

Specular reflection (following Hero's equi-angular reflection law) is a quantum mechanical effect explained as the sum of the most likely paths the photons can take. Light–matter interaction is a topic in quantum electrodynamics, and is described in detail by Richard Feynman in his book QED: The Strange Theory of Light and Matter.

The energy of the incoming photon may match the energy required to change the molecule from one state to another, causing a transition in kinetic, rotational, electronic or vibrational energy. When this occurs, the photon may not be reemitted or alternatively may be reemitted with a loss of energy. These effects are known as Raman, Brillouin and Compton scattering.

In fact I did read the book cited and came out knowing less .Oh, I might add - air has very little absorption mostly because it's far less dense than a solid. In my early education, that corresponds to a very very low decay/absorption constant (look up the exponential law of absorption). Whereas, with a solid there is a much higher chance of absorption.

 

[Nerd]

Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?

 

[dst]

Fact is high power laser systems do not use metalized mirrors. They use what is called a dielectric mirror, it consists of multiple layers of a dielectric material, the thickness of a layer is determined by the wavelength of the laser and the angle of reflection needed. These mirrors are transparent to all but the wavelength designed for.

I think he was talking about mirrors outside lasers :). But hey, I didn't actually know that snippet.

Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?

That's refraction, not diffusion. I think it's covered in one of the FAQ threads here. Other than that, I honestly have no idea (I'm actually trying to find out why myself).

 

[Integral]

I think he was talking about mirrors outside lasers :). But hey, I didn't actually know that snippet.

So was I.
http://en.wikipedia.org/wiki/Dielectric_mirror"

 

[Nerd]

Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?

Sorry, I meant refraction. It's a translation error on my part.

 

[EEPHD]

Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?

The whole electromagnetic spectrum is really really large (really infinite)

At any rate, both molecular nitrogen N2 and molecular oxygen O2 absorb very strongly in the ultraviolet. Almost 100% of the incident light from the sun below 190 nm is absorbed. Modtran is a wonderful program for simulating transmittance through the atmosphere

google Modtran and Hitran

 

[Redbelly98]

Do you know if a mirror can reflect X-rays?

x-rays can be reflected at large (close to 90 degrees) angles of incidence. This principle has been used by astronomers to make x-ray telescopes.

See http://en.wikipedia.org/wiki/Wolter_telescope