Optimum power scaling on efficient generation of quasi-single- cycle terahertz pulses

Abstract

We review our progress made on the efficient conversion of the broadband terahertz (THz) pulses from ultrafast laser pulses propagating in semiconductor electro-optic materials. By investigating the behaviors of the THz output vs. the pump beam in terms of incident angle, polarization, and azithumal angle, we can precisely determine the contributions made by optical rectification and photocurrent surge. When a material is pumped below its bandgap, optical rectification is the primary mechanism for the THz generation. Above the bandgap, however, the two mechanisms mentioned above compete with each other, depending on the material characteristics and pump intensity. At a sufficiently-high pump intensity, optical rectification becomes the dominant mechanism for a second-order nonlinear material. When a material is pumped above its bandgap, second-order nonlinear coefficients are resonantly enhanced. In such a case, the THz output power and normalized conversion efficiency can be dramatically enhanced. We have also analyzed how the THz generation is affected by some competing processes including two-photon absorption.