Periodic Reporting for period 1 - TeraNanoLIGHT (Nonlinear Interaction of Terahertz Light with Two-Dimensional Nanomaterials)
Reporting period: 2024-03-01 to 2025-02-28
Two-dimensional (2D) nanomaterials, such as graphene and transition metal dichalcogenides (TMDs), offer remarkable opportunities for shaping the future of nanoscale optoelectronics and photonics. Their atomically thin structure, exceptional carrier mobility, and strong light–matter coupling make them promising candidates for the development of nanodevices operating at terahertz (THz) frequencies — a spectral region critical for next-generation information processing technologies. Among their most compelling features is their potential to exhibit strong and highly tunable nonlinear optical responses under intense THz excitation, a property that could enable key functionalities such as frequency conversion, field-driven switching, and ultrafast modulation at unprecedented speed and spatial resolution. The TeraNanoLIGHT project was established to explore these nonlinear phenomena at the intersection of ultrafast optics and nanoscience. The central aim was to investigate the interaction between intense, few-cycle THz fields and quantum-confined 2D materials with nanometer spatial and femtosecond temporal resolution. This regime — where both strong-field effects and quantum confinement dominate — remains largely unexplored, yet holds the potential to reveal new light–matter interaction mechanisms that are inaccessible in either far-field or weak-field conditions. To reach the necessary field strengths, the project employed scattering-type scanning near-field optical microscopy (s-SNOM) to confine THz pulses to nanoscale hotspots, achieving local electric fields exceeding several megavolts per centimeter. Within this framework, TeraNanoLIGHT set out to study THz-induced nonlinearities in several contexts. One key objective was to observe and characterize the propagation of THz surface plasmons in graphene as a function of field strength, moving from linear to nonlinear dynamics. Another goal was to investigate how oscillating THz fields act as ultrafast AC biases, enabling non-resonant nonlinearities such as intervalley scattering or lightwave-driven transport. The project also aimed to explore the conditions for generating high-order harmonics in atomically thin materials, especially TMD monolayers and heterostructures with tailored interlayer symmetries. Finally, TeraNanoLIGHT considered whether these intense, localized interactions could themselves be used to improve the spatial resolution of near-field microscopy. By achieving these objectives, the project aimed to contribute to a deeper understanding of ultrafast, nonlinear THz interactions in low-dimensional systems.
The TeraNanoLIGHT project investigated the nonlinear interaction between intense terahertz (THz) fields and low-dimensional quantum materials with nanometer spatial and femtosecond temporal resolution. The research was carried out using a near-field optical microscopy platform capable of probing ultrafast THz dynamics at the nanoscale, allowing access to physical regimes that are inaccessible by conventional far-field techniques. One major achievement of the project was the real-time observation of THz surface plasmon polaritons (SPPs) in graphene. By resolving their formation and propagation in both space and time, the project provided direct insights into plasmon dispersion, damping, and coherence. The study also demonstrated that these propagating modes can be modulated on subcycle timescales by ultrafast optical excitation, offering a novel route for dynamic control of plasmonic signals in two-dimensional materials. Complementary to the experiments, numerical simulations were performed to characterize the enhancement of THz fields under the near-field tip. These simulations confirmed that local field strengths well above 10 MV/cm are achievable, establishing the feasibility of driving nonlinear processes such as intervalley scattering or high-harmonic generation in suitable materials. Building on this framework, the project explored the nonlinear THz response of semiconductors at the nanoscale. As a testbed material, InAs was chosen, and field-resolved measurements revealed strong modifications of the material’s dielectric response under increasing THz field strength, consistent with lightwave-driven carrier redistribution within the conduction band. The final stage of the project focused on the exploration of ultrafast excitonic dynamics in the 2D magnetic semiconductor CrSBr. Time-resolved measurements revealed that exciton properties — including binding energy and lifetime — can be tuned by magnetic ordering. This study uncovered a new mechanism for controlling Coulomb interactions in van der Waals materials and contributed to the broader understanding of correlated states in low-dimensional systems. In summary, TeraNanoLIGHT demonstrated that terahertz near-field spectroscopy provides a powerful approach to investigate ultrafast carrier dynamics and field-driven effects in quantum materials at the nanoscale. The combination of localized field enhancement, spatial resolution, and temporal precision enabled the project to probe material responses under strong-field excitation conditions. These results establish a solid foundation for future studies in strong-field nanophotonics, ultrafast control, and nonlinear phenomena in two-dimensional systems.
The TeraNanoLIGHT project has advanced the experimental capabilities for studying nonlinear light–matter interactions in low-dimensional materials at terahertz (THz) frequencies. While previous studies have explored strong-field phenomena in the far field, this project demonstrated that spatially confined, femtosecond-resolved THz spectroscopy can be used to investigate carrier dynamics and collective excitations in a range of materials. One of the key outcomes was the ability to resolve the spatio-temporal evolution of THz surface plasmons in graphene, including how these propagating modes are modulated by ultrafast optical excitation. This combination of spatial and temporal resolution enabled the direct observation of field-induced changes in plasmon propagation, offering a new approach to study the interaction between charge carriers and intense electromagnetic fields in two-dimensional systems. Additionally, the project used finite-element simulations to characterize the field enhancement under the near-field tip, confirming that the system can reach field strengths beyond 10 MV/cm. These results provide a realistic framework for accessing nonlinear carrier dynamics in solid-state materials using moderate incident field amplitudes, and are relevant for future studies on high-harmonic generation and strong-field control of electronic structure. To this end, the project demonstrated the first nanoscale observation of lightwave-driven intervalley electron transfer in n-doped InAs, resolving nonlinear carrier redistribution with femtosecond precision. Finally, the project applied ultrafast MIR spectroscopy to uncover a previously unobserved tuning mechanism in the 2D magnetic semiconductor CrSBr, where magnetic ordering was shown to reshape excitonic confinement and coherence. These findings link magnetism and excitonics at the nanoscale in a way that could not have been addressed with existing techniques. Finally, the project demonstrated the first nanoscale observation of lightwave-driven intervalley electron transfer in n-doped InAs, resolving nonlinear carrier redistribution with femtosecond precision. Altogether, TeraNanoLIGHT pushes the frontier of what can be experimentally resolved in the strong-field regime, combining spatial confinement, temporal resolution, and nonlinear field–matter interaction in a unified platform. These results move the field beyond current limitations and establish a new experimental framework for exploring high-field quantum phenomena at the nanoscale.