Methods for Obtaining the Optical Constants of a Material

There are various methods to experimentally evaluate the optical constants of a material. An optical constant is experimentally obtained by analyzing optical spectra, such as the reectance R(ω) and transmittance T(ω), measured for the material of interest. Assuming for simplicity normal incidence in the Fresnel’s equations discussed in Chapter 1, R(ω) and T(ω) of a material are expressed in terms of the complex refractive index nˆ(ω) = n1(ω) + in2(ω) asRn nn n T Rnn n =− ++ + = − =+ +( ) ( ), ( )1 11 4 1(4.1)(ω dependencies are omitted.) If a material is transparent at ω and if both R(ω) and T(ω) can be measured, one can derive n1(ω) and n2(ω) by solving the above two equations for them (two equations for two unknowns). One can then obtain other complex optical constants such as εˆ(ω) and σˆ(ω) as discussed in Chapter 1. (In practice, multiple internal reections at the sample surfaces may need to be considered.) This method is not applicable in an opaque spectral range since T(ω) cannot be measured there. Therefore, here we will mainly discuss reection-based methods for deriving the optical constants. These methods can derive both n1(ω) and n2(ω) based on reection measurements only and are more generally applicable to various materials.