Giant oscillator strength

Giant oscillator strength is inherent in excitons that are weakly bound to impurities or defects in crystals.

The spectrum of fundamental absorption of direct-gap semiconductors such as gallium arsenide (GaAs) and cadmium sulfide (CdS) is continuous and corresponds to band-to-band transitions. It begins with transitions at the center of the Brillouin zone, ${\displaystyle {\boldsymbol {k}}=0}$. In a perfect crystal, this spectrum is preceded by a hydrogen-like series of the transitions to s-states of Wannier-Mott excitons.^[1] In addition to the exciton lines, there are surprisingly strong additional absorption lines in the same spectral region.^[2] They belong to excitons weakly bound to impurities and defects and are termed 'impurity excitons'. Anomalously high intensity of the impurity-exciton lines indicate their giant oscillator strength of about ${\displaystyle f_{i}\sim 10}$ per impurity center while the oscillator strength of free excitons is only of about ${\displaystyle f_{\rm {ex}}\sim 10^{-4}}$ per unit cell. Shallow impurity-exciton states are working as antennas borrowing their giant oscillator strength from vast areas of the crystal around them. They were predicted by Emmanuel Rashba first for molecular excitons^[3] and afterwards for excitons in semiconductors.^[4] Giant oscillator strengths of impurity excitons endow them with ultra-short radiational life-times ${\displaystyle \tau _{i}\sim 1}$ ns.