Band-structure enhancement of the indirect forbidden transitions

Abstract
We present an experimental investigation of the resonant enhancement of the indirect forbidden transitions near a direct-indirect crossover. Near-resonance conditions have been achieved in TiO2 by tuning the relative energy positions of a direct forbidden gap (3.031 eV) with respect to an indirect one (3.049 eV). We find a band-structure enhancement of the indirect absorption strength when crossing the direct-forbidden gap. The quantitative details of the experiment can be satisfactorily accounted for by a simple model which includes (i) the stress-induced breaking of the selection rules and (ii) the resonance effect. Uniaxial compressions up to 24 kbars in [100] direction have been found necessary to tune the band gaps.