Diamond offers an exceptional platform for optoelectronics owing to its ultra-wide band gap, superior thermal and photonic properties. Yet its optical response is notoriously difficult to tune, as conventional doping suffers from deep impurity levels and poor activation efficiency. Here, we show that strain-gradient engineering provides a doping-free route to modulate broadband optical emission in microfabricated diamond. By taking advantage of size-induced large elasticity, we show that controlled elastic bending of diamond nanoribbons generates spatially varying strain fields that can be resolved at nanoscale using STEM-EELS, which reveals synchronous electronic bandgap shifts and phonon spectrum broadening. Spatially mapped cathodoluminescence exhibits continuous emission shifts, accompanied by intensity variations and spectral widening. Theoretical analyses show that nonuniform strain couples electronic band restructuring with phonon mode redistribution, expanding the pathways for phonon-assisted optical transitions. These findings transform diamond from a static wide-band-gap semiconductor into a mechanically reconfigurable broadband emitter, establishing strain-gradient engineering as a general paradigm for tunable photonics in wide-band-gap semiconductors.

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• Band gap
• Electronic structure
• Phonons
• Strain
• Electron energy loss spectroscopy
• First-principles calculations
• Molecular dynamics