Nonlinear optoelectronic phenomena in two-dimensional materials have garnered significant attention due to their unique light-matter interactions and potential applications in next-generation photonic and optoelectronic devices. Unlike bulk materials, 2D systems exhibit strong nonlinear optical responses at atomic-scale thicknesses, enabling compact, efficient, and high-speed devices for signal processing, telecommunications, and optical computing. Key phenomena include harmonic generation, saturable absorption, and Kerr effects, each offering distinct advantages for manipulating light at ultrafast timescales.

Harmonic generation in 2D materials arises from the nonlinear polarization response under intense optical excitation. Second-harmonic generation (SHG) and third-harmonic generation (THG) are particularly prominent in non-centrosymmetric crystals such as monolayer transition metal dichalcogenides (TMDCs). MoS2, for instance, exhibits strong SHG due to its broken inversion symmetry in the single-layer form, with reported nonlinear susceptibility values comparable to traditional nonlinear crystals like LiNbO3 but at a fraction of the thickness. Similarly, graphene demonstrates efficient THG owing to its massless Dirac fermions, enabling broadband frequency conversion. The efficiency of these processes depends on material thickness, stacking order, and excitation wavelength, with heterostructures of graphene and TMDCs showing enhanced nonlinear responses due to interlayer charge transfer.

Saturable absorption is another critical nonlinear phenomenon where the absorption coefficient decreases with increasing light intensity. This effect is exploited in mode-locked lasers and ultrafast optical switches. Monolayer graphene exhibits universal saturable absorption across a wide spectral range due to Pauli blocking, where photoexcited carriers fill available states, preventing further absorption. WS2 and MoSe2 also demonstrate strong saturable absorption, with modulation depths exceeding those of conventional semiconductor saturable absorber mirrors. The recovery time of these materials is ultrafast, often in the sub-picosecond regime, making them ideal for high-repetition-rate pulse generation.

The optical Kerr effect, characterized by an intensity-dependent refractive index, enables all-optical phase modulation and self-focusing. Graphene’s giant Kerr nonlinearity, orders of magnitude larger than that of silica, allows for significant phase shifts even at low pump powers. TMDCs like WSe2 exhibit layer-dependent Kerr responses, with thicker flakes showing stronger nonlinear phase modulation due to increased light-matter interaction. These properties are harnessed in nonlinear waveguides and resonators for all-optical signal processing, where 2D materials are integrated onto photonic circuits to achieve compact, high-speed modulators.

Material selection for nonlinear optoelectronic devices depends on the target application. Graphene is favored for broadband operation, while TMDCs offer strong wavelength-specific responses. Heterostructures combining multiple 2D materials can further enhance nonlinear effects through tailored band alignment and charge transfer. For instance, graphene-MoS2 stacks exhibit hybridized nonlinearities, enabling both saturable absorption and harmonic generation in a single device. Black phosphorus, with its anisotropic optical properties, provides polarization-dependent nonlinearities, useful for polarization-sensitive photonics.

Device configurations for leveraging these phenomena include integrated waveguides, microresonators, and plasmonic structures. Edge-coupled graphene waveguides demonstrate efficient nonlinear interaction over micrometer-scale propagation lengths, enabling on-chip frequency converters. TMDC-coated silicon nitride ring resonators enhance harmonic generation through resonant field enhancement, achieving conversion efficiencies competitive with bulk nonlinear crystals. Plasmonic nanostructures coupled with 2D materials further concentrate light, boosting nonlinear effects at nanoscale dimensions.

Applications in optical computing and telecommunications benefit from the ultrafast response and compact footprint of 2D nonlinear devices. All-optical logic gates based on graphene’s Kerr effect can perform binary operations at terahertz speeds, bypassing the latency of electronic circuits. TMDC-based frequency converters enable wavelength division multiplexing in fiber-optic networks, increasing data transmission capacity. Saturable absorbers in mode-locked lasers generate ultrashort pulses for high-capacity optical communication systems. However, trade-offs exist between bandwidth and efficiency. Graphene’s broadband response comes with relatively low conversion efficiency per layer, necessitating multilayer stacks or hybrid designs. TMDCs offer higher efficiency but are limited to specific spectral ranges.

Thermal management and material stability also influence device performance. High optical intensities can induce heating, degrading nonlinear responses over time. Hexagonal boron nitride (hBN) encapsulation improves thermal dissipation and protects against environmental degradation, extending device lifetimes. Scalable synthesis techniques, such as chemical vapor deposition, are critical for producing uniform 2D films with consistent nonlinear properties.

Future advancements may explore twisted 2D heterostructures, where moiré superlattices introduce tailored nonlinearities through engineered band structures. Additionally, integrating 2D materials with metasurfaces could enable unprecedented control over nonlinear wavefront shaping. As fabrication techniques mature, 2D nonlinear optoelectronic devices will play a pivotal role in enabling ultrafast, energy-efficient photonic systems for communication, computing, and sensing.

The interplay between material properties, device engineering, and application requirements defines the roadmap for 2D nonlinear optoelectronics. By addressing efficiency, bandwidth, and scalability challenges, these materials will unlock new paradigms in light-based technologies.