Charge and energy transfer processes in van der Waals heterostructures are central to their optoelectronic and photonic applications. Pump-probe spectroscopy serves as a powerful tool to resolve these ultrafast dynamics, particularly in systems where interlayer coupling dictates device performance. The timescales involved span femtoseconds to nanoseconds, depending on the nature of the interaction, band alignment, and interfacial quality. Interlayer exciton formation and hot carrier cooling are two critical phenomena that influence the efficiency of energy conversion and charge separation in these structures.

In heterostructures composed of transition metal dichalcogenides (TMDCs), such as MoS2/WSe2 or MoSe2/WS2, charge transfer typically occurs on sub-100 femtosecond timescales when the band alignment is type-II. This rapid separation of electrons and holes across layers is driven by the built-in potential at the interface. For instance, in a MoS2/WS2 bilayer, electrons transfer from the WS2 layer to MoS2 within 50 femtoseconds, while holes move in the opposite direction. The process is highly efficient due to the strong Coulomb interaction and minimal lattice mismatch, which reduces scattering losses.

Interlayer excitons, where the electron and hole reside in separate layers, exhibit longer lifetimes compared to intralayer excitons. These bound states are stabilized by the reduced overlap of electron-hole wavefunctions across the van der Waals gap, leading to recombination times extending into the nanosecond regime. In MoSe2/WSe2 heterobilayers, interlayer exciton lifetimes have been measured between 1 and 30 nanoseconds, depending on the twist angle between the layers. A twist angle of 0° or 60° often results in the longest lifetimes due to momentum-matched interlayer transitions, whereas random angles introduce disorder that accelerates non-radiative decay.

Hot carrier cooling is another key process probed by pump-probe spectroscopy. After photoexcitation, carriers initially occupy high-energy states and lose energy through electron-phonon scattering. In heterostructures, the cooling dynamics can differ significantly from monolayers due to additional pathways for energy dissipation. For example, in graphene/TMDC systems, hot electrons in graphene can cool by transferring energy to the TMDC layer via interfacial coupling. The cooling rate is influenced by the density of states and phonon modes of both materials. Cooling times in graphene/MoS2 heterostructures range from 0.5 to 2 picoseconds, faster than in isolated graphene due to the additional scattering channels provided by the TMDC.

The efficiency of energy transfer between layers is highly dependent on the band alignment and the presence of defects or impurities. Resonant energy transfer, such as Förster or Dexter mechanisms, can occur when the excitonic states of one layer overlap with the absorption spectrum of another. In TMDC heterostructures, Förster transfer typically dominates, with timescales on the order of picoseconds. For instance, in WS2/MoSe2 structures, energy transfer from WS2 to MoSe2 occurs within 3 picoseconds when the layers are closely spaced. The transfer rate decreases exponentially with interlayer distance, highlighting the importance of precise layer control in device fabrication.

The interplay between charge and energy transfer can be further modulated by external fields or strain. Applying a vertical electric field can tune the band alignment, either enhancing or suppressing interlayer exciton formation. Strain engineering alters the bandgap and exciton binding energies, thereby affecting the transfer pathways. In hBN-encapsulated TMDC heterostructures, strain-induced shifts in exciton energies have been shown to modify charge transfer times by up to a factor of two.

Pump-probe experiments with varying pump fluences reveal nonlinear effects in these processes. At high excitation densities, many-body interactions such as exciton-exciton annihilation become significant, shortening the apparent lifetimes of interlayer excitons. Biexciton states may also form, introducing additional decay channels. In MoS2/WSe2 heterostructures, exciton-exciton annihilation rates have been estimated at 0.01 cm²/s, leading to fluence-dependent decay dynamics.

Temperature also plays a crucial role in determining transfer and cooling timescales. At low temperatures, phonon populations are reduced, leading to slower hot carrier cooling and enhanced exciton lifetimes. In contrast, elevated temperatures accelerate scattering processes, often at the expense of energy transfer efficiency. Measurements on TMDC heterostructures at 10 K show interlayer exciton lifetimes up to 100 nanoseconds, while at room temperature, these values drop by an order of magnitude.

The insights gained from pump-probe spectroscopy are instrumental in designing heterostructures for specific applications. For photovoltaic devices, rapid charge separation and slow recombination are desirable, achievable through optimized band alignment and defect passivation. In contrast, light-emitting applications benefit from long-lived interlayer excitons that enhance radiative efficiency. The ability to probe these dynamics with femtosecond resolution provides a roadmap for tailoring material combinations and interfacial properties to meet performance targets.

Understanding these timescales also informs the development of novel quantum devices. Interlayer excitons, with their extended lifetimes and valley-polarized states, are promising candidates for valleytronics and quantum information processing. The manipulation of hot carrier distributions could enable high-efficiency photocatalysis or above-threshold photodetection. As fabrication techniques advance, allowing for precise control of twist angles and interlayer spacing, the versatility of van der Waals heterostructures will continue to expand, driven by a deeper comprehension of their ultrafast charge and energy transfer mechanisms.

In summary, pump-probe spectroscopy reveals the rich dynamics of charge and energy transfer in van der Waals heterostructures, from femtosecond-scale hot carrier cooling to nanosecond-lived interlayer excitons. These processes are governed by material composition, band alignment, and interfacial quality, with external parameters like strain, electric fields, and temperature providing additional tuning knobs. The quantitative understanding of these timescales is essential for harnessing the full potential of layered materials in next-generation optoelectronic and quantum technologies.