# Photon Absorption and Emission: Conductors v. Semiconductors

I'm having a hard time understanding how photon absorption and emission in metals (conductors) compares to semiconductors. Obviously, in SCs, absorbed photons lead to electron-hole pairs and emitted photons correspond to recombinations. I don't exactly understand how this process works in metals, since we don't even consider holes in the first place (and have essentially no band gap).

Do we just consider absorbed photons as promoting electrons to excited states, rather than pair generation?

Why exactly do metals emit absorbed light rapidly? I've read that light induces alternating currents on the metal surface and that ACs rapidly emit light, but I have no idea why. I'm not sure what alternating currents have to do with this (I'm sure there is an obvious explanation I should be familiar with).

I'm more familiar with band structure in SCs and therefore I'm having a hard time comparing radiative processes in metals to SCs.

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About alternating currents, in a metal the electrons are so weakly bound that when you drive the system with an AC voltage source, the electrons oscillate with almost the same frequency. We can then think of the electrons as time-dependent dipoles which emit radiation at the frequency of oscillation. The power radiated is proportional to $\omega^4$ Dipole Radiation. –  Kevin Driscoll Apr 3 '13 at 14:45

## 1 Answer

Probably, it is useful to imaging absorption/emission in metals as an excitation or relaxation of the electron plasma. In the case of absorption, the energy of light is spent on oscillating of electron plasma supplemented by accelerating of electrons. For beginning, understanding of Drude theory of metals will be useful.

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If this is the case, why does the electron plasma relax so quickly (if I'm putting that correctly)? Why do metals re-emit absorbed light so much faster than bandgap materials? –  SMPAT May 2 '13 at 13:13
Probably, because electron plasma in metals is more dense than in semiconductors. –  freude May 2 '13 at 13:23
OK, let me see if I've got this straight. I've done a little reading into plasmons. How about this: We think of a metal as a plasma. For (certain) doped semiconductors, we think of plasmons as we do in metals. In metals, the significance of the plasmon frequency ($\omega_p$) is that for $\omega_p>\omega_\text{photon}$, the metal is reflective, and for $\omega_p<\omega_\text{photon}$, the metal is transparent. Semiconductors have a lower $\omega_p$ than metals, and are therefore less reflective (metals have a higher tendency to re-emit photons). Am I getting warm? –  SMPAT May 3 '13 at 7:02
Close enough. The main difference of plasmas in metals and semiconductor is that the metals contain electron plasma, and semiconductors contain electron-hole plasma, i.e. plasma which consist of particles charged negatively as well as positively. Therefore, in semiconductors, different excitations (oscillations) of such plasma are possible, i.e. there are not only plasmons, but also excitons that is correlation between positive and negative charges. –  freude May 3 '13 at 7:12
Also, electron and hole plasmas can annihilate or be generated giving contribution into optical response. Therefor, physics of light-matter interactions in semiconductor and metals is a little bit different for the frequency range being close to the band gap. –  freude May 3 '13 at 7:25