Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures

Abstract: The possibility of hybridizing collective electronic motion with mid-infrared (mid-IR) light to form surface polaritons has made van der Waals layered materials a versatile platform for extreme light confinement and tailored nanophotonics. Graphene and its heterostructures have attracted particular attention because the absence of an energy gap allows for plasmon polaritons to be continuously tuned. Here, we introduce black phosphorus (BP) as a promising new material in surface polaritonics that features key advantages for ultrafast switching. Unlike graphene, BP is a van der Waals bonded semiconductor, which enables high-contrast interband excitation of electron-hole pairs by ultrashort near-infrared (near-IR) pulses. We design a SiO$_2$/BP/SiO$_2$ heterostructure in which the surface phonon modes of the SiO$_2$ layers hybridize with surface plasmon modes in BP that can be activated by photo-induced interband excitation. Within the Reststrahlen band of SiO$_2$, the hybrid interface polariton assumes surface-phonon-like properties, with a well-defined frequency and momentum and excellent coherence. During the lifetime of the photogenerated electron-hole plasma, coherent polariton waves can be launched by a broadband mid-IR pulse coupled to the tip of a scattering-type scanning near-field optical microscopy (s-SNOM) setup. The scattered radiation allows us to trace the new hybrid mode in time, energy, and space. We find that the surface mode can be activated within ~50 fs and disappears within 5 ps, as the electron-hole pairs in BP recombine. The excellent switching contrast and switching speed, the coherence properties, and the constant wavelength of this transient mode make it a promising candidate for ultrafast nanophotonic devices.

Femtosecond Photo-Switching of Interface Polaritons in Black Phosphorus Heterostructures

The study "Femtosecond photo-switching of interface polaritons in black phosphorus heterostructures" by Huber et al. presents an innovative exploration of ultrafast photo-induced phenomena in van der Waals layered materials, with a focus on black phosphorus (BP) heterostructures. By leveraging the semiconductor nature of BP in a designed SiO(_2)/BP/SiO(_2) heterostructure, the researchers investigate the dynamics of hybrid interface polaritons, revealing potential applications for ultrafast nanophotonic devices.

Overview of the Findings

The introduction of BP as an alternative to graphene is a pivotal aspect of this study. Unlike graphene's continuous tunability of plasmon polaritons due to its gapless electronic structure, BP offers high-contrast interband excitation thanks to its semiconducting nature. This property allows for efficient electron-hole pair generation via near-infrared (near-IR) pulses, facilitating the creation and manipulation of surface plasmon oscillations.

The heterostructure's configuration enables the coupling between the surface phonon modes of SiO(_2) and the surface plasmon modes in BP, culminating in a hybrid mode with phonon-like attributes within the Reststrahlen band of SiO(_2). The interaction is further characterized by s-SNOM measurements, which demonstrate the capability to track the temporal, spectroscopic, and spatial profiles of the hybrid mode with remarkable precision. Notably, the polaritonic mode is shown to activate within approximately 50 femtoseconds (fs) and diminishes within 5 picoseconds (ps) due to carrier recombination, indicating its transient nature and fast switching capability.

Key Experimental Insights

The heterostructures are manufactured by mechanical exfoliation of BP flakes onto a silicon wafer, with a protective 5 nm SiO(_2) layer sputtered to mitigate degradation under ambient conditions. The ultrafast s-SNOM technique utilized in this research employs femtosecond near-IR pump and mid-IR probe pulses to examine the dynamic processes at play. In this setup, ultrafast interband excitations in BP generate free carriers, leading to a shift in the plasma frequency to mid-IR ranges and impacting the heterostructure’s optical response, as evidenced by significant modulation of the scattered field intensity.

Implications and Future Prospects

The results highlight the potential of BP-based heterostructures in developing ultrafast optical switches and modulators, offering stable operation over sub-picosecond timescales. Such attributes position these systems as compelling candidates for integration into nano-optoelectronic devices where speed and miniaturization are crucial. The inherent properties of black phosphorus, including its adjustable bandgap and anisotropy, afford additional degrees of freedom for device engineering, paving the way for directional mode splitting and resonant excitation conditions.

Moreover, this study opens avenues for further exploration of composite materials utilizing BP in conjunction with other two-dimensional materials such as graphene and transition metal dichalcogenides. These complexes could potentially enhance the functionality and performance of photonic and plasmonic devices, especially in the mid-infrared range where applications in sensing, imaging, and communication are continually expanding.

In conclusion, the comprehensive analysis and novel insights provided in this paper lay a robust foundation for advancing mid-infrared optoelectronics through the design of next-generation heterostructures exploiting the unique optoelectronic properties of black phosphorus.