Ultrafast terahertz (THz) spectroscopy has emerged as a powerful tool for probing carrier dynamics in semiconductors with femtosecond temporal resolution. Recent breakthroughs have enabled the measurement of carrier mobilities exceeding 10^6 cm^2/Vs in graphene-based heterostructures at room temperature. THz time-domain spectroscopy (TDS) can resolve carrier lifetimes as short as 100 fs, providing insights into recombination mechanisms in novel materials like perovskites and transition metal dichalcogenides (TMDCs).
The application of THz spectroscopy to two-dimensional (2D) materials has revealed anisotropic carrier transport properties with directional mobilities varying by up to 300%. For example, MoS2 monolayers exhibit mobilities of ~200 cm^2/Vs along the armchair direction but only ~70 cm^2/Vs along the zigzag direction. THz emission spectroscopy has also been used to map hot carrier distributions with energy resolutions better than 10 meV, critical for optimizing photovoltaic devices.
Advanced THz imaging techniques now enable spatially resolved mapping of carrier densities across entire wafers with micron-scale resolution. A recent study achieved a detection limit of ~10^9 carriers/cm^3 over a 4-inch silicon wafer, representing a three-order-of-magnitude improvement over conventional Hall effect measurements. THz near-field microscopy has further enhanced spatial resolution to <100 nm, allowing for the study of nanoscale inhomogeneities in doped semiconductors.
The integration of THz spectroscopy with ultrafast pump-probe setups has opened new avenues for studying non-equilibrium phenomena. For instance, coherent phonon oscillations were observed in GaAs at frequencies up to 5 THz, providing direct evidence of electron-phonon coupling strengths exceeding theoretical predictions by ~20%. These capabilities are driving innovations in high-speed electronics and optoelectronics.
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