Laser

[Brainee]

How does laser photons become more when each photons is consumed when producing each new laser photon?

[swansont]

22 minutes ago, Brainee said:

How does laser photons become more when each photons is consumed when producing each new laser photon?

The photon isn’t consumed. It induces another photon to be emitted from an excited atom or molecule. That’s the stimulated emission, and the cause of the amplification. One photon in, two photons out.

[Brainee]

Does the first photon continue as well as the stimulated photon and together form the laser "beam"?

[exchemist]

 

8 minutes ago, Brainee said:

Does the first photon continue as well as the stimulated photon and together form the laser "beam"?

Yes. "Stimulated emission" is emission caused by the influence on an excited atom of the electric vector of another photon. The effect is to make the atom emit its own photon, in phase with the one causing the stimulation.  

What you do is create a "population inversion", a term in statistical mechanics that means you have more atoms in excited states than you would have at thermal equilibrium. That is an unstable, non-equilibrium situation. One emitted photon, passing through this inverted population, will quickly collect more and more other photons as it passes among the excited atoms, leading to a beam of photons all in phase. This in principle will continue until an equilibrium population distribution is regained. In practice mirrors are used, so the photons bounce back and forth, collecting more and more companions with each pass.

Edited January 25, 2023 by exchemist

[Brainee]

Are atoms naturally excited or how do you excite them?

[swansont]

46 minutes ago, Brainee said:

Are atoms naturally excited or how do you excite them?

No, they aren’t. They tend to be in the ground state. And you can’t use photons of the laser’s wavelength to get the excess atoms in the excited state (the population inversion), because of the stimulated emission. So you have to use some other method.

In a HeNe laser, an electric discharge excites the He, and collisions with Ne causes Ne excitations. You have more atoms in an excited state than a lower state, so stimulated emission can give you amplification.

In diode lasers, there is an electronic excitation of the electrons to a higher band.

 

[MigL]

1 hour ago, Brainee said:

Are atoms naturally excited or how do you excite them?

Wear something sexy ...

[iNow]

Careful, bud. You might engorge someone's up quark.

[Brainee]

Sorry, I don't get it:

23 hours ago, exchemist said:

 

What you do is create a "population inversion", a term in statistical mechanics that means you have more atoms in excited states than you would have at thermal equilibrium. 

How do you create a 'population inversion'?

[exchemist]

23 minutes ago, Brainee said:

Sorry, I don't get it:

How do you create a 'population inversion'?

@swansontpost explains that. Electrical discharge in a gas laser, or electronic excitation in the solid state, are the two methods he refers to.  But don't ask me to get deep into the physics of that. (I'm only a chemist, so what I know comes from quantum chemistry.) 

[swansont]

Another method us to use light (from a flash lamp or LEDs) to excite atoms from the ground state (0) to some state (3), which then decays to another state (2), which then has a population inversion with respect to a lower state (1)

E3>E2>E1>E0

incoming light has energy E3-E0, laser transition has energy E2-E1

The transition from 3 to 2 can be non-radiative

The transition from 1 to 0 (or some other intermediate state) is usually strong, so the in little population in 1, allowing N2>N1, which is what an inversion is  (N is the population of the state)