Since the discovery of graphene in 2004, two-dimensional (2D) materials, owing to their atomic-scale thickness, absence of surface dangling bonds, and quantum confinement effects, have provided a revolutionary platform for the design of optoelectronic and spintronic devices. Semiconducting transition metal dichalcogenides (TMDs) represented by MoS₂ can achieve tunable bandgaps (1.2?1.9 eV) through layer-number modulation. However, their inherently low carrier mobility and environmental sensitivity limit their applications in high-frequency optoelectronics. Moreover, most 1T-phase TMDs are prone to oxidation and instability in air, and their high phase-transition energy barriers pose challenges to controllable preparation. As a member of TMDs, MoTe₂ has a 1T′ phase (semi-metal) and a 2H phase (semiconductor) that stably exist at room temperature, as well as a T_d phase (semi-metal) that exists only at low temperatures (<240 K), endowing it with rich phase-transition conditions. We focus on the 1T′-MoTe₂ semimetallic thin film at room temperature to deeply analyze its ultrafast carrier dynamics mechanism and obtain key material parameters, laying a solid theoretical foundation for the design of ultrafast optoelectronic devices based on MoTe₂.

We utilize a self-built optical pump-terahertz probe (OPTP) spectroscopy system. An ultrafast pulsed laser output from a titanium-doped sapphire regenerative amplifier is employed as the light source. The laser has a central wavelength of 780 nm, a pulse width of 120 fs, a repetition rate of 1 kHz, and a single-pulse energy of 3 mJ. In this system, the laser is split into generation light, pump light, and probe light by beam splitters. The generation light is focused on a 1-mm-thick ZnTe crystal with a 110 orientation to generate terahertz waves. These waves are then collimated and focused onto the detection crystal ZnTe. By using the Pockels effect induced by the terahertz wave electric field, combined with a balanced photodetector and a lock-in amplifier, the synchronous detection of terahertz signals is achieved. By moving the delay line of the pump light path, the change in the terahertz instantaneous transmittance induced by light is accurately measured, thereby obtaining the kinetic information of non-equilibrium carriers. Simultaneously moving the delay lines of both the pump and probe light paths allows for the measurement of terahertz transmission spectra at different pump delay time. Combined with relevant formulas, the change in the transient conductivity of the photo-excited sample is calculated, providing data support for the study of carrier dynamics.

A series of innovative results are achieved in the experiment. Under 780-nm light excitation, the 1T′-MoTe₂ thin film exhibits positive terahertz photoconductivity, and its decay process shows obvious biexponential characteristics: a sub-picosecond fast process and a hundred-picosecond slow process. Through in-depth analysis, it is determined that the fast process originates from electron-phonon coupling, during which hot electrons rapidly transfers energy to optical phonons. The slow process is dominated by phonon-phonon interactions, facilitating the diffusion of heat in the lattice until thermal equilibrium with the environment is reached. By fitting the pump-dependent fast process using the two-temperature model (TTM), the electron-phonon coupling coefficient g∞ of 1T′-MoTe? is accurately obtained as 7.7×10¹⁵ W·m^-3·K^-1, and the electron specific heat coefficient γ is 2.1 J·m^-3·K^-2, indicating a relatively high electron-phonon coupling strength in this semimetallic phase. When we study the complex photoconductivity of 1T′-MoTe₂, fitting with the Drude?Smith model reveals that the enhanced localization trend of the fitting parameter c with the increase in the delay time before 0.6 ps might imply the rapid formation and dissociation of certain quasiparticles (such as large polarons). Although this phenomenon still requires further experimental verification, it provides a new perspective for studying carrier behavior.

Based on the comprehensive research results, it can be concluded that the relaxation of non-equilibrium carriers in the 1T′-MoTe₂ semimetallic thin film under photoexcitation is mainly dominated by the electron?phonon coupling and phonon?phonon interactions. It is the first to comprehensively explore the ultrafast carrier dynamics of 1T′-MoTe₂ using the optical pump-terahertz probe ultrafast spectroscopy among the massive literature. We clarify the physical mechanism of non-equilibrium carrier relaxation, which is derived from electron?phonon coupling and phonon?phonon interactions. Moreover, the relationship between the time constant of the fast process of non-equilibrium carrier relaxation and the excitation power density can be effectively described by the two-temperature model. These results not only deepen the understanding of the carrier dynamics of 2D semimetallic materials but also provide a crucial theoretical basis and accurate experimental data for the design and development of ultrafast optoelectronic devices based on 1T′-MoTe₂, strongly promoting the applied research of two-dimensional materials in the field of terahertz optoelectronics.