Linear optics studies light-matter interactions where material polarization is linearly proportional to the electric field. As intensity increases, polarization scales nonlinearly with field strength, described by a Taylor series of higher-order terms under perturbative assumptions—this defines perturbative nonlinear optics. Beyond these field strengths, where matter interacts with electromagnetic fields comparable to or exceeding the forces binding electrons to atoms or lattices, systems enter the nonperturbative regime, giving rise to strong-field optics. It was first observed in gases and plasmas and later extended to solids ^[1], enabling novel phenomena like high-harmonics generation (HHG). Subsequently, the emergence of metaphotonics opened a new regime of light-matter interaction, enabled by engineering of ultrafast fields confinement at the nanoscale—both in individual subwavelength-scale resonators and arrays of such resonators, known as metasurfaces, ^[2]. 

My research explores HHG in metaphotonic platforms and nonlinearity-induced nonreciprocal self-action. In optics, nonreciprocity refers to unequal transmission of light upon reversal of direction of illumination, thereby violating Lorentz reciprocity. Out of known nonreciprocity pathways, strong nonlinear responses offer a feasible route to nanoscale nonreciprocity ^[3]. In my works, I have demonstrated ultrafast nonreciprocal response using epsilon-near-zero materials and asymmetric HHG. I also show a tuneable HHG source by varying light-matter across a metasurface, enabling functionalities limited to bulk media. This paves the way for coherent deep-UV light sources. Finally, I study strong-field effects in chiral metasurfaces—structures that lack mirror symmetry and introduce the concept of nonreciprocal chirality in metasurfaces.

References:
^[1] S. Ghimire, A. DiChiara, E. Sistrunk et al. Observation of high-order harmonic generation in a bulk crystal. Nature Phys 7, 138–141 (2011) | ^[2] V. Zubyuk, L. Carletti, M. Shcherbakov, and S. Kruk. Resonant dielectric metasurfaces in strong optical fields. APL Mater. 9(6), 060701 (2021) | ^[3] A. Tripathi, C.F. Ugwu, V.S. Asadchy et al. Nanoscale optical nonreciprocity with nonlinear metasurfaces. Nat Commun 15, 5077 (2024)