Screening factor high-frequency

One, as frequency z approaches very high values, the relativistic retardation screening factor R(z) goes to zero. 

In these expressions eQ and are the static and high-frequency dielectric constants of the host, vs is a (mean) velocity of sound therein, Cpe is the piezoelectric coupling constant, CQ a numerical factor that depends on the crystal structure and kD the Debye inverse screening length. 

When we deal with the IR spectral range, the first two terms in Eq. (1.36) can be replaced to a good approximation by the value of the dielectric function at the frequency a> in the transparency region (1.2.4°), where only the valence electrons are assumed to contribute to the polarization (atomic centers are rigidly fixed). This value is called the high-frequency (optical) dielectric constant or the screening factor and denoted as oo[ oo = e((o) if o)oj In partic-... 

High-frequency dielectric constant=screening factor... 

---