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I Material Heating by Light

  1. May 8, 2017 #1
    After taking a course on Solid State electronics, (and doing a bit of my own research into the matter) I've gained a little bit of knowledge in the area of band structures, and phonons/photons.

    My question is as follows:

    I know that photons with an energy greater than the band gap can cause electron excitations. Prior to decaying straight to the valence band, these electrons make their way back down to the bottom of the conduction band and release heat in the form of phonons before finally emitting a photon in their decay to the valence band (for the case of a direct gap semiconductor).

    However, is this the only mechanism by which light heats material? I've read in certain locations on this forum and other places that frequencies smaller than the band gap of photon energy are capable of directly creating phonons by absorption by the lattice (for example photon energies in the infrared). I'm not sure if this is true or not.

    However, I've also read that materials are generally transparent to light that is below their band gap. For example, glass is transparent to visible light since it's band gap exceeds the visible frequencies. However, if it's true that frequencies smaller than the band gap can still cause heating through the creation of phonons, then this light that is supposed to be in transparent is in fact absorbed?

    Essentially, do photons only cause heat indirectly in a solid by exciting electrons and having those electrons giving up some of their energy to photons before decaying? Or is there absorption of photons that immediately become phonons (though this seems to violate the concept of "transparency", at least for photons less than the band gap, I don't know if photons greater than band gap can create phonons in a direct manner like I mentioned)..

    Thank you.
    Last edited: May 8, 2017
  2. jcsd
  3. May 9, 2017 #2


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  4. May 9, 2017 #3
    So is this just another process then? Are some phonons created from electrons lowering in energy, and others are created directly via this process? For example, are acoustic phonons generated from the electrons lowering in energy? By lowering, I mean "thermalization" of electrons in the conduction band (and also emission of phonons as necessary in indirect gap transitions).

    And if this optical phonon process does occur, how does it fit into the framework of the band gap? For example, IR light is said to be transparent in Silicon since it is smaller then the band gap of silicon (ideally assuming no defects). Does IR light still produce optical phonons somehow in Silicon? But then it wouldn't seem to be transparent. Does only light with energy E > Eg produce any kind of transition (optical phonon or electronic ) ?

    In addition, do phonons also directly go in the reverse and have a probability of directly producing photons? I know phonons definitely can excite electrons which can then recombination and emit photons this way.
    Last edited: May 9, 2017
  5. May 9, 2017 #4
    I'm not sure if this question would have been better answered in the Condensed Matter section.
  6. May 20, 2017 #5
    I'm just bumping to hopefully get a response. Thank you.
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