MXenes is an emerging class of 2D transition metal carbides, nitrides and carbonitrides which exhibit large conductivity, ultrahigh volumetric capacitance, high threshold for light-induced damage and nonlinear optical transmittance, making them attractive candidates for a variety of optoelectronic and electrochemical applications. Here, we report on equilibrium and non-equilibrium free carrier dynamics of Ti ₃ C ₂ T _x gleaned from THz spectroscopic studies for the first time. Ti ₃ C ₂ T _x showed high (~2 × 10 ²¹ cm ⁻³ ) intrinsic charge carrier density and relatively high (~34 cm ² V ⁻¹ s ⁻¹ ) mobility of carriers with an exceptionally large, ~46 000 cm ⁻¹ absorption in the THz range, which suggests that Ti ₃ C ₂ T _x is well suited for THz detection. We also demonstrate that Ti ₃ C ₂ T _x conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared, 800 nm pulses is found to cause transient suppression of the conductivity that recovers over hundreds of picoseconds. The possibility of control over THz transmission and conductivity by photoexcitation suggests the promise for application of Ti ₃ C ₂ T _x Mxenes in THz modulation devices and variable electromagnetic shielding.

A $250\hspace{0.17em}\text{GHz}$ quasi-optical ring resonator consisting of an input coupler and three mirrors has been designed and tested. A low-loss silicon wafer in the ring provides output coupling of the stored power when irradiated by a pulse from a $532\hspace{0.17em}\text{nm}$ laser. The ring created $5.8\hspace{0.17em}\text{ns},\hspace{0.17em}268\hspace{0.17em}\text{mW}$ output power pulses when excited by a $17\hspace{0.17em}\text{mW},\hspace{0.17em}250\hspace{0.17em}\text{GHz}$ continuously operating input source, achieving a power gain of 16. In a fully tuned ring, higher gain is achievable. If the ring was used with a pulsed input source having a pulse length of several times the fill time, the ring could be used as an efficient pulse compressor with similar high gain. The resonator has a wide range of applications, including, at low power, spectroscopy and, at high power, testing of accelerator structures and materials.

First synthesis of CuBiW₂O₈ with direct bandgap of 1.9−2.0 eV and characterization of photoexcited charge dynamics from picoseconds to nanoseconds.