  Breaking inversion symmetry in solids plays a central role in nonlinear
optics because it can change material properties, such as producing even-order
nonlinear optical (NLO) effects. Although a centrosymmetric diamond has been
developed as a photonic platform, including waveguides and light sources, the
NLO effects in bulk diamonds are limited to the third-order. Thus, exploiting
more powerful second-order NLO effects, such as second harmonic generation
(SHG), are still challenging. Here we explore symmetry-breaking-induced
second-order NLO effects in bulk diamonds using the color center,
nitrogen-vacancy center. Exciting with ultrashort laser pulses, SHG and
third-harmonic generation (THG) are simultaneously observed at the same time,
exhibiting characteristic intensity patterns, depending on both the excitation
fluence and the angle of light polarization. We uncovered that SHG serves as
the source for THG by the cascading process. Our findings will offer NLO
effect-based quantum sensing by diamond color centers, such as imaging of the
electromagnetic field by electro-optic effects on the nanofemto scale.
