Effects of strain on second harmonic generation in alloyed transition metal dichalcogenide monolayers

First principles time-dependent density functional theory is used to calculate the second harmonic generation (SHG) spectra (χ⁽²⁾) of a variety of TMDs and their alloys. It is shown by comparison of calculations with and without the inclusion of the Bethe-Salpeter Equation (BSE) that when studying the effects of strain on SHG it is important to take excitonic effects into account. This can be attributed to the fact that χ⁽²⁾ is highly dependent on the dipole moments in the system which are significantly altered by the inclusion of electron-hole interactions. Changes in SHG spectra as a function of biaxial and uniaxial strain are calculated with a BSE implementation for the semiconducting TMDs, as well as several chalcogen and transition metal alloys with varying stoichiometric compositions. For all structures, significant changes in χ⁽²⁾ occur when strain is applied to the system, with the largest changes occurring in MoSe₂, WSe₂, and related alloys. These findings open new avenues of exploration for potential applications of these materials in strain-sensitive devices.