# Optical constants of noble metals: the Drude model for microwave modelling

I have a question regarding the optical constants of noble metals.

According to Johnson and Christy's paper Optical Constants of Noble Metals (Phys. Rev. B 6, 4370–4379 (1972), doi:10.1103/PhysRevB.6.4370), in the Drude free electron theory the equation for the dielectric permittivity is $$\epsilon(\omega)=1-\frac{\omega_p^2}{\omega(\omega+i/\tau)}=1-\frac{\omega_p^2}{\omega(\omega+i\gamma)},$$ where $\epsilon(\omega)$ is the dielectric constant at frequency $\omega$, $\omega_p$ the plasma frequency, and $\gamma=1/\tau$ the collision frequency.

In the nanohub photonics database the constants can be calculated from different theoretical and experimental models.

However, in CST Microwave Studio I need to give the epsilon infinity value, which really comes from modified drude model: $$ε=ε(\infty)-\frac{ω_p^2}{ω^2+iγω}.$$ How do I get the value of $\epsilon(\infty)$ for gold? Is there any reference by using which we can directly convert the free electron theory Drude model to the modified Drude model?

I am specifically searching for the values of $ε(\infty)$, $ω_p$, and $γ$ for gold in the 700nm to 1100 nm range (specifically 830nm).

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