Frequency conversion arises from the nonlinear polarization response of matter when subjected to intense electromagnetic fields, with second order processes playing a particularly central role. These χ² mediated interactions, including second-harmonic and (SHG) sum-frequency generation (SFG), form basis for upconverting infrared light into the visible regime. While traditional nonlinear crystals such as lithium niobate, beta barium borate, and potassium titanyl phosphate have long enabled efficient frequency conversion, their reliance on strict phase matching and precise thermal control limits their miniaturisation and integration.

Advances in nanomaterials now provide powerful alternatives to these limits. Two dimensional semiconductors, such as transition metal dichalcogenide (TMDC) monolayers, and epsilon near zero materials like indium tin oxide (ITO) exhibit intrinsically strong nonlinear susceptibilities despite their subwavelength thickness. In the first part of this talk, I introduce the fundamental optical physics of these materials, highlighting how excitonic resonances, and near zero index behaviour collectively boost nonlinear light–matter interactions. These properties make TMDCs and ITO promising active media for infrared to visible upconversion in ultrathin, integrated photonic platforms.

Building on these material foundations, the second part of the talk focuses on optical nanoantennas and metasurfaces, engineered arrays of subwavelength “meta atoms” that manipulate electromagnetic waves. By tailoring geometry and symmetry, these resonant nanostructures provide precise control of nonlinear polarization, enabling strong field enhancement, directional emission, and phase control well beyond what bulk crystals can offer. In the second part of this talk, I investigate metasurface platforms specifically engineered for infrared upconversion, where guided mode resonances in lithium niobate nanoantennas dramatically amplify second order processes. This approach supports compact, efficient IR upconversion to the visible suitable for advanced imaging applications. We further demonstrate how structured light excitation of these metasurfaces enables edge enhanced and phase contrast imaging through nonlinear frequency conversion.

Overall, this work explores the convergence of novel nonlinear materials with resonant optical nanoantennas and metasurfaces to realise ultracompact, high efficiency upconversion platforms. We demonstrate infrared imaging through IR upconversion in a high Q lithium niobate metasurface, achieving enhanced nonlinear conversion. These results not only deepen our understanding of nonlinear light–matter interactions in low-dimensional systems but also open new pathways for integrated photonic technologies in imaging, sensing, and on chip nonlinear optics.

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