Large refractive index enhancement in PbTe/Pb1−xEuxTe multiquantum-well structures

Optical interband absorption between confined carrier states is studied in PbTe/Pb1−xEuxTe multiple quantum wells (MQWs). The frequency dependence of the refractive index is calculated by the Kramers–Kronig transformation of the absorption coefficient of the MQWs. It is shown that the observed interference fringes in mid-infrared transmission can only be accounted for if the enhancement of the refractive index η(ω) at the interband transition energies which follow from the Kramers–Kronig transformation are taken into account. These changes in η are as large as 19% (at T=77 K) and are thus relevant for the design and fabrication of mid-infrared QW lasers.