Theory of the terahertz radiation via excitation of the semiconductor structures above the absorption edge

Below the band gap optical excitation of the ultrashort electrical pulses in the semiconductors due to the optical rectification have been studied by numerous groups. The situation when the excitation pulse is above the band gap have not been studied up until recently, since, it had been assumed that (a) the response time is determined by the recombination time (i.e. it is slow) and (b) the screening effects will severely attenuate the effect However, in recent results strong THz radiation had been observed. We have developed a simple theory that explains how high intensity terahertz radiation is obtained in zinc-blende materials and in the two-dimensional structures despite the constraints mentioned above. Our theory uses the combination of Kane k.p theory and bond charge theory of the nonlinear susceptibilities. We have shown that the optical rectification tensor has two different components. The first ultrafast (virtual) component has the “refractive-index-like” dispersion and relatively small magnitude. The second component is real and is associated with the absorption of electron from the bonding orbital of the valence band into the antibonding orbital of the conduction band