The Physics of Nonlinear Absorption and Ultrafast Carrier Relaxation in Semiconductors

Abstract

The absorption of light quanta of energy greater than the band gap in a semiconductor induces an electron to make a transition from the valence band to a state high in the conduction band, leaving behind a hole in the valence band (Fig. 1). After such an absorption process, the photoexcited electron is left with an excess energy ΔEe that is given by $${E_e} = (h\nu -{E_g}){(1 + {m_e}/{m_h})^{ -1}},$$ (1) where me is the electron effective mass and mh is the hole effective mass. The excess energy of the photogenerated hole is $${E_h} = (h\nu -{{\rm E}_g}) -{E_e}.$$ (2) These energetic electrons (holes) will quickly relax through various collisional processes to the bottom (top) of the conduction (valence) band, where eventually they will recombine. It is well established that if the photoexcitation is sufficiently intense this relaxation process results in the generation of hot electron and phonon distributions.