Impact of strain on electron-phonon coupling of quantum emitters
Defects in semiconductors acting as optically active spin qubits are intriguing objects of fundamental study and future technological developments. These defect-based color centers are of particular interest for detection and response to physical variations such as pressure and strain, or conversely -- as we demonstrate the possibility of herein -- pressure and strain can be utilized to manipulate quantum emitter properties. To investigate how strain can alter the fundamental electron-phonon interaction of quantum defects, we employ the negatively charged silicon vacancy ($\mathrm{V_{Si}^{-}}$) in 4H-SiC as a use-case and study its vibrational structure under applied tensile and compressive uniaxial strain using first-principles calculations. We show that the strain variations of the emission spectrum can be explained by differing responses of bulk-like and quasi-localized vibrational modes. Importantly, the $\mathrm{V_{Si}^{-}}$ defect exhibits a strain-induced enhancement of the Debye-Waller factor under uniaxial tensile strain applied along the $a$-axis of 4H-SiC, thereby improving its performance as a quantum emitter. The strain-dependent changes in the phonon sideband enable distinguishing between compressive and tensile strain, opening up the possibility of magnetic-field-free strain detection using only spin-conserving transitions of solid-state quantum emitters.