48

All I found in Google was very broad. From a physics models perspective, why can photons emitted from a laser cut? Does this cut mean that the photons are acting like matter?

Peter Mortensen
  • 2,352
sites
  • 591

6 Answers6

40

When lasers cut something, they're only cutting in the sense that they're making atoms be not as attracted as they once were to each other. When you get down to the nitty-gritty details, it is not really the same as mechanical cutting.

Remember that lasers shoot photons, and when photons hit atoms, they excite electrons. If you excite these electrons enough, they'll have enough energy to disassociate from the atoms they formerly "belonged" to. This makes individual atoms disassociate with whatever other atoms they were once bonded to, and in the mad scramble to go to a lower energy state, they very likely do not go into the same configuration they were before. Some atoms, like the ones directly hit by the laser beam, go to a vapor and float away. Others "choose" one side of the material to go to. Any bonds the material had with itself is then dissolved, so it is effectively cut.

This is different than, say, taking shears or scissors to the material. The methods of those things cutting are purely mechanical, and you don't have to worry about vapors as much as when cutting with a laser. (You also don't have to worry about reflections from materials, either!)

PipperChip
  • 2,208
  • 11
    Mechanical cutting is also "making atoms be not as attracted as they once were." – Russell Borogove May 19 '14 at 21:12
  • 2
    You don't normally ionize with a pair of scissors! – PipperChip May 19 '14 at 21:13
  • 11
    You're breaking a lot of (chemical!) bonds with the scissors, is my point. That's what cutting is. – Russell Borogove May 19 '14 at 21:16
  • 2
    @PipperChip: well, laser cutting is not ionizing, is it? The photon energy is not high enough. It is just thermal evaporation – Andreas H. May 20 '14 at 02:25
  • Is this a fancy way to describe burning, or is there a difference between laser "cutting" and burning (with the obvious exception of actual burning, by cutting something like paper and making it smoke and possible catch fire)? – Keavon May 20 '14 at 03:46
  • @Keavon: "Burning", or combustion, generally refers to an exothermic (heat-releasing) chemical reaction between a fuel and an oxidizer. Even though you can certainly burn flammable things with a powerful laser (by dumping enough heat into the material), it is not the mechanism by which a laser can cut metal, for instance. – JohannesD May 20 '14 at 10:15
  • Probably it is mostly heat cutting, but (at least with metals) ionizing cannot be entirely discounted (still probably not the main mechanism of the cut). Some of the photons will cause the photo electric effect (Einstein won a Nobel on the subject of photons kicking out electrons from metals) – Dan S May 20 '14 at 11:36
  • I think we're looking at too small a scale when we define "cutting." Many definitions here would not fly when manufacturing something, and we need to respect that and what "everyday" people mean when they say "cutting." – PipperChip May 20 '14 at 16:57
  • @Keavon: ... thus you are probably thinking of "melting" rather than "burning". –  May 22 '14 at 22:59
  • @PipperChip, Re "in the mad scramble to go to a lower energy state, they very likely do not go into the same configuration they were before", But what if they do? Say, I'm really lucky and they do for me, all the time. Am I then immune to laser burns? (Or have I misunderstood this?) – Pacerier Aug 16 '17 at 03:35
  • @Pacerier Well, if those bonds spontaneously re-formed... you should spend your luck on lottery tickets and other cosmically improbable tasks. :) Anyways, that energy needs to go somewhere: maybe you would just heat up due to an increasing number of phonons (packets of vibrational energy), eventually losing heat to your surroundings. Maybe you re-emit those photons, so you would fluoresce, and therefore don't burn. – PipperChip Aug 16 '17 at 04:23
  • I downvoted because the excitation of electrons is bogus. It either doesnt happen, or for metals it happens but is irrelevant. Of course i might be wrobf. Pls link a peer reviewed source. – lalala Aug 11 '18 at 06:21
  • 2
    @lalala Photoionization occurs when ultrafast lasers are used to cut. However, this is only used for specialized materials and processes. Normal, CW laser cutting is thermal, i.e. burning and melting. I agree that this answer is misleading. – user71659 Aug 11 '18 at 07:08
20

Cutting is a process when you deliver energy to break chemical bonds in material that you cut.

When you use a saw, you deliver mechanical (kinetic) energy that converts into kinetic energy of particles of the thing you cut, so they can get out of the thing.

Laser is just another way to deliver such energy, since the a photon has enough energy to break some bonds and deliver some heat for the molecules that can evaporate.

Since cutting is just breaking chemical bonds and removing particles in some specific place, laser has ability to cut. It has not much to do with newtonian "light particles" approach.

user46147
  • 3,014
9

Think of the laser process as being similar to melting a substance through a change in state.

An analogy might be putting a hot wire on top of an ice cube. It makes a 'slice' by heat, not by cutting. It turns the solid state ice into water and gas which doesn't hold together the same anymore.

Michael Durrant
  • 319
3

Photons posses momentum and are affected by gravity. So, they act as matter all the time. To use the effective momentum for laser cutting requires very high photon density which isn't practically feasible.

Industrial laser cutting uses extreme heat density to cut. There are many methods viz. vaporization cutting, melt & blow, thermal stress cracking, etc. See Wikipedia page Laser cutting, section Methods.

Peter Mortensen
  • 2,352
Earth is a Spoon
  • 4,978
3

Regarding as how the laser light can deliver such energy to cut something, it is because it is "coherent light". An old example to imagine this is, imagine a troop of soldiers over a bridge. If they march (i.e. everyone stomping synchronously at the same pace in the ground), they will add up each stomp at the same time, adding up energy as if all the troop were a single giant boot stomping at the bridge, so it could even break and fall - that´s why groups of people (should) never march over a bridge.

Each photon in the laser is synchronously coherent with each other, adding up energy to the beam instead of scattering the energy each on its own as a common lamp do. So the beam will be so intense over a small region of matter to the point of delivering energy to it so it breaks (burns) apart.

Lissandro
  • 31
1

Lasers "cut" as an interaction between matter and radiation (themselves different forms of natural substance)

A LASER (Light Amplification by Stimulated Emission of (Coherent) Radiation ) beam being coherent and high-energy (quantum-mechanically a-la Einstein-Planck relation, energy is directly analogous to laser frequency) when interacting with atoms and molecules, provides enough energy for electrons and molecules to move to higher energy-levels and underlying bonds become loose or break completely, resulting in the macroscopic result of "cutting".

As noted above only certain types of LASER "cut" certain types of matter compounds (related to frequency/energy of beam and the bonds base energy of the matter compound under study)

Nikos M.
  • 5,142