Two‐Photon Absorption in Monolayer MXenes

Abstract

an NLO effect where an electron absorbs two photons simultaneously and is excited from the ground state via a virtual state. 2PA generally occurs when the gap between the ground and excited states is larger than the energy of one photon but smaller than the energy of two photons. Hence, one-photon absorption (1PA) dominates when light interacts with a small bandgap semiconductor or gapless metal. Interestingly, 2PA instead of 1PA can be the dominant optical transition process in narrow bandgap materials under the right circumstances, but it is a challenge to experimentally observe this unusual optical phenomenon in bulk materials. This phenomenon, however, becomes possible in 2D materials thanks to their interesting electronic structures. Giant 2PA was reported in gapless bilayer graphene with four bands under the irradiation of 780 and 1100 nm photons. [2a] This 2PA could happen because the two-photon transitions are enhanced due to the additional bands arising from the interlayer interactions in the bilayer system, which are absent from monolayer graphene. To date, bilayer graphene is the only gapless semimetal in which the unconventional 2PA has been observed experimentally. The question remains whether or not it is possible to observe 2PA in a material, which does not have a similar electronic structure to that of bilayer graphene or conventional semiconductors.Two-photon absorption (2PA) is a nonlinear optical (NLO) effect that typically occurs in a conventional semiconductor whose bandgap is larger than the energy of one excited photon but smaller than the energy of two photons. In this work, the experimental observation of strong 2PA in gapless MXene monolayers is reported. This phenomenon is verified by a nonlinear transmission method and ultrafast dynamic studies based on a wavelengthdegenerate, non-collinear pump-probe technique. The unconventional 2PA is attributed to the structures of the MXene energy bands near the Fermi level, which favors two-photon transitions and suppresses one-photon transitions due to parallel band absorption effects. These results, as well as the measured NLO parameters including the 2PA coefficient, absorption crosssection, excited carrier lifetimes, and third-order NLO susceptibility, provide a systematic understanding of the unusual NLO effect in MXenes and may be applied in advanced photonic and optoelectronic devices.Light-matter interactions in 2D materials have been in the spotlight of recent optics research because of the novel applications in nanoelectronics, valleytronics, photovoltaics, and nanophotonics that they will potentially enable. [1] In the field of nonlinear optics (NLO), some 2D nanosheets, such as graphene and monolayer MoS 2 , have been experimentally observed to exhibit strong two-photon absorption (2PA). [2] This interesting NLO phenomenon has a wide range of applications in optical limiting, all-optical switching, logic, [3] storage memory, [4] and 3D lithography and printing. [5] 2PA isThe ORCID identification number(s) for the author(s) of thi...

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