Quantum Hydrodynamic Framework of Coherent Terahertz Emission in MXenes Driven via Engineered Femtosecond Waveforms
Coherent terahertz (THz) emitters based on low dimensional quantum materials have attracted significant interest for compact broadband photonic technologies. However, theoretical descriptions that consistently connect microscopic quantum dynamics to emitted THz radiation remain limited for MXenes, whose electronic properties are governed by transition metal d orbitals and chemically tunable surface terminations. A multiscale density-matrix formalism for coherent THz generation in metallic MXenes driven by engineered three color femtosecond waveforms is developed. Material-specific electronic structures are described by parameterized multiband tight binding Hamiltonians, while ultrafast carrier dynamics are treated using a non-secular Redfield master equation that explicitly retains interband coherence, carrier populations, electron phonon interactions, impurity scattering, and environment-induced relaxation. The transient photocurrent is explicitly decomposed into population and coherence driven contributions and linked to the emitted THz field through its time derivative, establishing a direct connection between microscopic quantum dynamics and macroscopic radiation. The results show that environmental coupling does not merely attenuate THz emission, but redistributes coherent and population currents and reshapes the emitted waveform in both the time and frequency domains. Systematic investigations of optical waveform parameters, effective electronic structure, and open system relaxation identify physically grounded strategies for enhancing broadband THz generation. The proposed methodology establishes a transferable microscopic design platform for connecting material specific electronic structure, quantum coherence, open system relaxation, nonlinear carrier transport, and coherent THz emission across MXenes and related low dimensional quantum materials.