Myung‐Ki Kim

Surface plasmons in 2D materials such as graphene exhibit exceptional field confinement. However, the low electron density of majority of 2D materials, which are semiconductors or semimetals, has limited their plasmons to mid‐wave or long‐wave infrared regime. This study demonstrates that a 2D Ti 3 C 2 T x MXene with high electron density can not only support strong plasmon confinement with an acoustic plasmon mode in the short‐wave infrared region, but also provide ultrahigh nonlinear responses. The acoustic MXene plasmons (AMPs) in the MXene (Ti 3 C 2 T x )–insulator (SiO 2 )–metal (Au) nanostructure generate in the 1.5–6.0 µm wavelength range, exhibiting a two orders of magnitude reduction in wavelength compared to wavelength in free space. Furthermore, AMP resonators with patterned Au rods exhibit a record‐high nonlinear absorption coefficient of 1.37 × 10 −2 m W −1 at wavelength of 1.56 µm, ≈3 orders of magnitude greater than the highest value recorded for other 2D materials. These results indicate that MXenes can overcome fundamental plasmon wavelength limitations of previously studied 2D materials, providing groundbreaking opportunities in nonlinear optical applications, including all‐optical processing and ultrafast optical switching.