Optical nonlinearity in low-dimensional materials refers to the unique behavior exhibited by materials like quantum dots (0D), nanowires and nanotubes (1D), and atomically thin sheets (2D), e.g., graphene, transition metal dichalcogenides (TMDs) and related heterostructures, when excited with intense light. Unlike their 3D counterpart, electrons in low-dimensional materials are spatially confined to atomic length scales, giving rise to strongly nonlinear effects. Their large nonlinearity enables strong parametric nonlinear processes, like harmonic generation, parametric amplification, and frequency mixing, including the generation of entangled photons via spontaneous parametric down-conversion. Low-dimensional materials are emerging as promising candidates for various applications in optoelectronics, photonics, and quantum information due to their tunable and enhanced nonlinear optical responses at the nanoscale.

Low-dimensional materials have recently emerged as ultracompact nonlinear systems with giant optical nonlinearities, i.e., up to 1000x stronger than conventional bulk nonlinear crystals. Understanding and manipulating optical nonlinearity in low-dimensional materials has thus important implications for developing novel optoelectronic devices, such as lasers, ultrafast modulators, and quantum information processing components. For example, layered semiconductors, like atomically thin TMDs, have been exploited for innumerable photonic applications at the nanoscale, such as frequency conversion, light amplification across ultrabroad bandwidths, and the generation of non-classical states of light. Very recently, nanoengineering has also enabled periodically poled TMDs for quasi-phase-matched nonlinear interactions. With their huge optical nonlinearities, TMDs achieve similar efficiencies of standard bulk nonlinear crystals but over thicknesses which are 10-100x thinner at telecom wavelengths.
Coupling such materials with light-confining microcavities, nanostructures, nanowaveguides and metasurfaces, can boost even more their effective nonlinearity, also by leveraging on strong coupling regimes between excitons and photons. In this context, highly nonlinear optical effects occurring in the tens of femtoseconds timescale, or even faster, can lead to intense and rapid modulations of the device’s transmittance, reflectance or luminescence, for efficient and ultrafast optical logic gates, photonic circuits and neural networks. This Research Topic focuses on the most recent advances in the field of nonlinear optical processes of low-dimensional materials, also highlighting future perspectives in this rapidly expanding field.

We invite the submission of Original Research, Review, Mini Review, Perspective articles on themes including, but not limited to:

• Nonlinear optical properties in 0D (quantum dots), 1D (nanotubes, nanowires) and 2D materials (graphene, transition metal dichalcogenides, etc.) and related heterostructures
• Generation of unconventional classical and quantum states of light using low-dimensional materials
• Exciton nonlinear interactions in atomically thin semiconductors and their heterostructures
• Enhanced optical nonlinearities of low dimensional materials coupled to microcavities, photonic and plasmonic nanostructures, waveguides and metasurfaces
• Ultrafast processes in low-dimensional materials
• Nonlinear properties of polaritons in low-dimensional structures
• Optical sensing based on nonlinear processes
• Optical computing and neural networks based on nonlinear micro and nanodevices
• Nonlinear topological photonics
• Few-photons nonlinearities in low dimensional materials
• Optical nonlinearities in low-dimensional perovskites and nanocrystals

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