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BlackHole213
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I originally posted the following question on physics.stackexchange, but no one was able to answer it. I did find this answer on PhysicsForums, but I was already aware of the oscillation of electrons in response to an external electric field.
What is the physical cause behind a material having a negative real part of its dielectric function? Given the complex permittivity, [itex]\epsilon(\omega)=\epsilon(\omega)'+i\epsilon(\omega)''[/itex], the Drude model gives
\begin{align}
\epsilon'=1-\frac{\omega_{P}^2}{\omega^2+\omega_{\tau}^2}
\end{align}
where [itex]\omega[/itex] is the frequency of the incoming light, [itex]\omega_{P}=\sqrt{\frac{Ne^2}{m\epsilon_0}}[/itex]is the plasma frequency, [itex]N[/itex] is the electron density, [itex]m[/itex] is the electron's mass, [itex]e[/itex] is the electronic charge, and [itex]\omega_{\tau}[/itex] is the frequency of collisions between conduction electrons and the ion lattice.
If [itex]\omega[/itex] is small enough, then [itex]\epsilon'<0[/itex]. But this is just the mathematics behind negative permittivity. How does it physically happen?
What is the physical cause behind a material having a negative real part of its dielectric function? Given the complex permittivity, [itex]\epsilon(\omega)=\epsilon(\omega)'+i\epsilon(\omega)''[/itex], the Drude model gives
\begin{align}
\epsilon'=1-\frac{\omega_{P}^2}{\omega^2+\omega_{\tau}^2}
\end{align}
where [itex]\omega[/itex] is the frequency of the incoming light, [itex]\omega_{P}=\sqrt{\frac{Ne^2}{m\epsilon_0}}[/itex]is the plasma frequency, [itex]N[/itex] is the electron density, [itex]m[/itex] is the electron's mass, [itex]e[/itex] is the electronic charge, and [itex]\omega_{\tau}[/itex] is the frequency of collisions between conduction electrons and the ion lattice.
If [itex]\omega[/itex] is small enough, then [itex]\epsilon'<0[/itex]. But this is just the mathematics behind negative permittivity. How does it physically happen?