Graphene is a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice. It is a zero-bandgap semiconductor, meaning it has no bandgap. The absence of a bandgap in graphene results in unique optical properties that set it apart from other 2D materials.Optical properties of graphene:1. Transparency: Graphene is highly transparent, absorbing only about 2.3% of visible light. This is due to the linear dispersion relation of its electronic bands near the Dirac points, which leads to a constant optical conductivity.2. Tunable optical response: The optical properties of graphene can be tuned by applying an external electric field, which modifies the Fermi level and changes the absorption of light. This makes graphene a promising candidate for various optoelectronic applications, such as modulators and photodetectors.3. Strong nonlinear optical response: Graphene exhibits strong nonlinear optical effects, such as saturable absorption and four-wave mixing, which can be exploited for ultrafast photonics and all-optical signal processing.Comparison with other 2D materials:1. Transition metal dichalcogenides TMDCs : TMDCs, such as MoS2, WS2, and MoSe2, are layered materials with a direct bandgap in the visible range 1-2 eV . Unlike graphene, they can efficiently absorb and emit light, making them suitable for applications in photovoltaics, light-emitting diodes, and photodetectors. However, their optical properties are not as easily tunable as those of graphene.2. Phosphorene: Phosphorene is a single layer of black phosphorus with a direct bandgap of around 2 eV. It has strong light-matter interactions and a high absorption coefficient, making it a promising material for optoelectronic devices. However, its stability under ambient conditions is lower than that of graphene.3. Hexagonal boron nitride h-BN : h-BN is an insulator with a wide bandgap of around 5-6 eV. It is transparent in the visible range and has a low refractive index, making it an ideal substrate for 2D materials-based devices. However, its insulating nature limits its applications in optoelectronics.In summary, the absence of a bandgap in graphene leads to unique optical properties, such as high transparency and tunable optical response. While other 2D materials with different bandgaps offer advantages in specific optoelectronic applications, graphene's versatility and tunability make it a promising candidate for a wide range of applications in photonics and optoelectronics.