The real part xx
1 , yy
1 , zz
1 and average
1 of the corresponding principal complex
tensor components are obtained from the imaginary part of these principal complex tensor
components by means of Kramers-Kronig transformation [29] as shown in Fig. 5 c and d . The values
of 0 1
xx , 0 1
yy , 0 1
zz and 0 1
average were calculated and presented in Table 3. We note that a smaller
energy gap yields a larger 0 1 value. This could be explained on the basis of the Penn model [30].
Penn proposed a relation between 0and g E , 2 0 1 P g E . g E is some kind of averaged
energy gap which could be related to the real energy gap. It is clear that 0 is inversely proportional
with g E . Hence a smaller g E yields a larger 0. Knowing the imaginary and real parts of the
frequency dependent dielectric function we can obtain the reflectivity spectrum and the absorption
coefficients.