Nonlinear optics provides a powerful approach to probing emergent phenomena in

condensed matter, including symmetry breaking, electronic dynamics, and collective

excitations. However, conventional measurements of nonlinear optical intensity generally

provide limited access to the phase information encoded in the nonlinear response. In this talk,

I discuss an approach to accessing phase information in nonlinear optical responses through

coherent interference between distinct optical response pathways. I theoretically and

experimentally demonstrate this concept through second-harmonic generation in oxide thin

films and low-dimensional materials, showing how phase-sensitive measurements can reveal

information beyond conventional intensity-based characterization. I then extend the approach

to higher-order nonlinear processes, from third-harmonic generation to optical frequency downconversion,

where interference between different nonlinear pathways may provide access to

phase information in quantum optical processes. These results establish a general route toward

phase-resolved nonlinear optics, opening new opportunities for connecting the characterization

of complex material properties with emerging directions in quantum optical science.