The negative thermo-optic effect in KTaO3: an ellipsometry study

Abstract

We used optical ellipsometry to study and elucidate the nature of the negative thermo-optic effect in KTaO3 single crystals, which consists of an increasing refraction index magnitude in the visible region upon cooling (dn/dT < 0). The optical constant functions were determined within the 1–6 eV region. The energies of the band edge indirect and direct optical transitions and their temperature evolutions in the region of 80–300 K were carefully investigated. The absorption edge in the region of the lowest indirect R → Γ optical transitions, 3.7–4.2 eV, was observed to shift to higher energies upon cooling. Simultaneously, an increase and ‘red shift’ of the optical absorption in the region of the direct X5'upper → X3 transitions (4.3–4.8 eV) were observed, which should be responsible for the nature of the observed negative thermo-optics in KTaO3.