Controlling the nonlinear optical properties of MgO by tailoring the electronic structure

The study of the non-linear response of matter to high electric fields has recently encompassed high harmonic generation (HHG) in solids at near-infrared (NIR) driving wavelengths. Interest has been driven by the prospect of ultrafast signal processing and all-optical mapping of electron wave-functions in solids. Engineering solid-state band structures to control this process has already been highlighted theoretically. Here, we show experimentally for the first time that second harmonic generation (SHG) can be enhanced by doping crystals of magnesium oxide (MgO) with chromium (Cr) atoms. We show that the degree of enhancement depends non-linearly on dopant concentration. The SHG efficiency is shown to depend on the crystal orientation relative to the laser polarisation in pure MgO. Surprisingly, this dependence is lost when Cr dopants are introduced into MgO. A physical picture of the effect of Cr dopants is aided by density functional theory (DFT) calculations of the electronic structure for pure and doped samples. This work shows unambiguous enhancement of the SHG efficiency and suppression of the angular dependence by modifying the electronic structure. The observed effects are consistent with an electronic structure that facilitates interband SHG and demonstrates a minimal angular dependence. This work highlights the potential of manipulating the electronic-structure of solids to control or test theories of their non-linear optical response.