demands. The need for high-speed, highresponsivity detection is traditionally met by p-type-intrinsic-n-type (PIN) photodiodes, [1] avalanche photodiodes (APD), [2] metal-semiconductor-metal (MSM), [3,4] or metal-graphene-metal [5][6][7] photodetectors.Of these, top-illuminated, MSM devices are planar, relatively easy to fabricate, and are more readily integratable with field effect transistor (FET) technology as they exploit the same Schottky contacts used for the FET gate, and do not require the

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and n = 1, 2, 3 or 4, and T z denotes surface terminations such as OH, O, and/ or F. MXenes are so-called for being principally derived by selectively etching the A-group (mostly Al) layers from the MAX-phases. They are distinct among 2D materials in that they are intrinsically metallic, easily processable in water, and have, in their short twelve-year history, displayed an array of unique properties such as record volumetric capacitances and conductivities, optical nonlinearities, and high laser damage thresholds. [1][2][3][4][5][6] While their potential to revolutionize energy storage and act as flexible electrodes rightfully attracted much attention, MXenes also hold promise as candidates in photonic and optoelectronic devices such as electromagnetic interference, EMI, shields, optical and terahertz (THz) switches, modulators, wavelength converters, and detectors. [7][8][9][10][11][12][13][14][15] The optical properties of MXenes are diverse [16] and can be tailored by chemical composition, structure, and/or morphology. [17,18] Their optical propertie scarise from a complicated interplay between free carriers, interband transitions as well as plasmon modes that occur in the visible and near-IR spectral ranges. [16,18,19] MXene plasmonic properties in particular are intriguing; while there is still much to be learned about the nature and properties of plasmons in different MXenes, localized surface plasmon resonances (LSPR) have already been hypothesized to underly efficient conversion of light to heat, saturable absorption in the visible and at telecommunication wavelengths, and high-performance photodetectors. [13,[20][21][22][23] In this work, we investigate the nature and dynamics of photoexcitations in three different members of the MXene family, viz. Ti 3 C 2 , Mo 2 Ti 2 C 3 , and Nb 2 C. We use two complementary pump-probe techniques to uncover the effects of optical excitations. The first is transient optical absorption, TA, that characterizes the effects on the optical properties in the visible and near-IR range, where LSPR features are dominant. The second, time-resolved THz spectroscopy (TRTS) measurements provide information about the dynamics of free charge carriers. The MXenes chosen here represent a broad range of electronic and optical properties. [16] As recent studies have uncovered, the most well-studied MXene, Ti 3 C 2 , is metallic with a high free carrier density (≈10 22 cm −3 ) and exhibits a pronounced LSPR extinction peak in the near-IR, in 750-800 nm range. [16,18,24] On 2D MXenes have diverse and chemically tunable optical properties that arise from an interplay between free carriers, interband transitions, and plasmon resonances. The nature of photoexcitations and their dynamics in three different members of the MXene family, Ti 3 C 2 , Mo 2 Ti 2 C 3 , and Nb 2 C, are investigated using two complementary pump-probe techniques, transient optical absorption, and time-resolved terahertz (THz) spectroscopy. Measurements reveal pronounced plasmonic effects in the visible and near-IR in...

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Figure 7. a,c,e) Simulated ER and b,d,f) IL at 1 THz as a function of a,b) fill factor, c,d) period, and e,f) thickness. Parameters that were fixed in a simulation are given in the top panels (a,d,e).