THz intraband absorption in HgTe nanocrystals in the linear and nonlinear regimes

With their tunable intraband absorption in the THz range and carrier dynamics on the order of tens of picoseconds, large HgTe nanocrystals (NCs) hold strong potential for THz optoelectronic devices, including modulators and detectors. Their THz intraband absorption has been attributed to multiple transitions of single carriers between quantized states. However, since most investigations have been limited to room temperature, many fundamental physical questions remain open. Here, we investigate the THz intraband absorption in both linear and nonlinear regimes across a broad temperature range. Our results highlight the essential role of thermally activated carriers in the linear regime, which has been so far neglected in existing models, and show that nonlinear intraband absorption in large HgTe NCs arises from an increase in chemical potential combined with changes in electronic temperature, both influenced by saturation effects associated with the progressive filling of electronic states. Furthermore, using mid-infrared pump–THz probe experiments, we probe the carrier dynamics and demonstrate that surface traps and ligand modes have a negligible effect on hot carrier recombination. The dominant recombination pathway is attributed to non-radiative interband processes mediated by optical phonon emission. This work provides new insight into the physical mechanisms underlying the THz intraband absorption of HgTe NCs in both linear and nonlinear regimes, refines their theoretical description, and highlights the potential of large HgTe NCs for THz optoelectronic devices such as modulators and detectors.