Fig. 1. OPTP system. (a) Typical OPTP optical path^[40]; (b) OPTP optical path with a dual lock-in amplifier^[52]

Fig. 2. Carrier dynamics in WSe₂^[56]. (a) Terahertz time-domain spectroscopy reference signal E(t) and photogenerated changes ΔE in the THz waveform at pump delay time of 10 ps and 400 ps; (b) delay line are fixed at the peak and zero-crossing points to obtain the real and imaginary parts of the conductivity

Fig. 3. Impact of exciton dynamics on photoconductivity. (a) Variation of complex conductivity in MoS₂ at different delay time and (b) terahertz dynamics in MoS₂ with the same incident pump energy but different pump photon energies (inset: slow relaxation τ₂from a double-exponential fit as a function of photon energy)^[78]; (c) relaxation processes in monolayer and multilayer MoS₂^[79]; (d) relaxation processes in MoS₂ under different pump photon energies^[80]

Fig. 4. Phonon-assisted relaxation processes. (a) Left image shows the relationship between electron mobility and temperature, determined by the measured momentum scattering rate, while the right image illustrates the relationship between the measured momentum scattering rate and temperature^[86];(b) dependence of the carrier relaxation time in MoS₂ on pump power^[87]; (c) relaxation time versus pump power dependence of the 6.8 nm and 20 nm PtTe₂ films are indicated on the left and right, respectively and (d) temperature response during laser irradiation^[88]

Fig. 5. Manifestation and mechanism of Auger effect in relaxation processes. (a) Real part of the conductivity of WS₂ and (b) relaxation time dependence on pump power^[93]; (c) pump dependence of A-exciton relaxation time in monolayer WS₂^[94]; (d) different processes of Auger-assisted electron and hole capture at defect sites^[95]

Fig. 6. Influence of defect-assisted relaxation processes. (a)(b) Relaxation time of the real and imaginary parts of the conductivity in WSe₂, respectively^[56];(c) ultrafast carrier dynamics process in MoS₂^[105]; (d) frequency dependent photoconductivity of MoS₂^[98]; (e)(f) dynamics of 2H-MoTe₂ after photoexcitation at different carrier densities, along with the dependence of the fitted relaxation time on carrier density^[106]

Fig. 7. Variation of charge transfer in TMDs heterostructures under excitation with different wavelengths. (a) MoTe₂/WTe₂ heterojunction and the individual MoTe₂ and WTe₂ layers, excited by light with wavelengths of 1500 nm and 800 nm, respectively ^[108]; (b) schematic of the energy band alignment at the graphene/WS₂ heterojunction interface and photoemission^[109]; (c) dependence of interface hot electron extraction time on pump power for the PtSe₂/graphene heterostructure, along with a schematic illustrating the transfer of hot electrons across the interface barrier into the PtSe₂ layer^[110]; (d) processes related to interlayer charge transfer and exciton formation in the MoSe₂/MoS₂ heterojunction^[111]

Fig. 8. Influence of different stacking orders in heterojunctions on charge transfer. (a) Terahertz photoconductivity and effective electric fields at the interface of graphene/WSe₂ and WSe₂/graphene heterostructures^[113]; (b) schematic diagram of charge transfer in PtSe₂/graphene (left) and graphene/PtSe₂ (right) heterostructures under optical excitation, along with the impact of the effective field introduced by the substrate^[114]; (c) conductivity in ReSe₂, ReSe₂/MoS₂, and MoS₂/ReSe₂ heterostructures^[115]; (d) (e) schematic diagram of charge transfer in WS₂/MoS₂ and MoS₂/WS₂ heterojunctions under optical excitation with 532 nm green light and 635 nm red light^[116]