Super-diffusive sub-picosecond extraction of hot carriers in black phosphorous (2506.14550v1)

Abstract: Harvesting hot carriers before they lose energy to the lattice is a critical route toward surpassing the conventional thermodynamic limit in optical-to-electrical (O-E) conversion. However, photocurrent from such hot carriers has remained challenging to directly detect because they equilibrate on picosecond timescales, outpacing conventional electronic measurement. Here, by employing terahertz electronics with sub-picosecond temporal resolution, we directly monitor hot-carrier-driven O-E conversion in black phosphorus (BP). Photoexcitation near the metal contact under zero source-drain bias generates an ultrafast photocurrent with a decay time of ~400 fs, orders of magnitude faster than the typical sub-nanosecond energy relaxation in BP, demonstrating a measured 3 dB bandwidth of 260 GHz with an intrinsic limit of ~600 GHz. Notably, this photocurrent flows via energetic holes toward the contact electrode, regardless of the equilibrium carrier type, revealing a super-diffusive hot-carrier extraction mechanism. Furthermore, we show that the ultrafast hot-carrier contribution can coexist with the much slower cold-carrier contribution based on the photovoltaic effect, demonstrating that hot carriers can be harvested without discarding lower-energy carriers. These findings highlight the potential of sub-picosecond hot-carrier extraction to expand the O-E conversion bandwidth without sacrificing efficiency, bridging fundamental hot-carrier physics with ultrahigh-speed technological applications.

• The paper presents a novel approach using a laser-triggered photoconductive switch to directly measure hot-carrier dynamics in black phosphorus with a 400 fs decay time.
• It reveals that super-diffusive hot carriers coexist with slower photovoltaic cold carriers, significantly enhancing optical-to-electrical conversion efficiency.
• The methodology paves the way for designing ultrafast optoelectronic devices and photodetectors by exploiting the rapid, 600 GHz intrinsic response.

Overview of Sub-Picosecond Hot Carrier Dynamics in Black Phosphorus

The paper titled "Super-diffusive sub-picosecond extraction of hot carriers in black phosphorous" presents a pioneering approach to accessing hot-carrier dynamics within black phosphorus (BP) through terahertz electronic techniques with sub-picosecond temporal resolution. Addressing the fundamental limitations of hot-carrier utilization, the paper explores mechanisms pertinent to optical-to-electrical (O-E) conversion, pertinent for advancing photovoltaic and ultrafast optoelectronic devices.

Key Findings and Methodology

Utilizing a laser-triggered photoconductive switch, the researchers directly measure the photocurrent linked with hot carriers in BP. Such a direct measurement is crucial given the ephemeral nature of hot carriers, which exhibit sub-picosecond energy relaxation that challenges traditional electronic assessment methods. The experiment demonstrated a rapid photocurrent decay occurring within approximately 400 fs, revealing a 3 dB bandwidth with an intrinsic limit nearing 600 GHz. These numerical results are orders of magnitude greater than previous values, which were constrained to around 3 GHz. The significant advancement was facilitated through the sub-picosecond capability of the employed measurement apparatus.

In the experimental setup, black phosphorus flakes of varying thickness were subjected to optical pumping, while a gate-tunable structure was utilized to accommodate variance in carrier density. A key discovery was that ultrafast hot-carrier contributions coexist with slower cold-carrier currents driven by the photovoltaic effect, indicating that extracting mobile hot carriers does not necessitate the exclusion of their less energetic counterparts. The hot-carrier extraction was characterized by super-diffusive motion, with photoexcited holes migrating swiftly toward contact electrodes due to elevated kinetic energy and diffusion constants vastly surpassing those of cold carriers.

Implications and Future Directions

This work holds implications for both the theoretical understanding and practical application of hot-carrier physics. The super-diffusion mechanism of hot carriers in BP provides a methodology for broadening the O-E conversion bandwidth, integral for designing high-speed photodetectors and efficient energy-harvesting systems. Realistically, fitting metallic contacts proximal to photoexcitation sites could exploit super-diffusion in device structures, retaining hot carriers while also benefiting from the photovoltaic effect.

The implications extend to exploring similar mechanisms across other two-dimensional materials and semiconductor systems, suggesting the potential universality of this approach. The combination of properties in BP—such as its anisotropic carrier mobility and tunable bandgap—could be exploited in a diverse array of materials through waveguide integration or other engineered structures to optimize carrier extraction and thereby significantly enhance optoelectronic device performance.

Conclusion

By elucidating the process of super-diffusive hot-carrier extraction, this paper presents a vital advancement in the field, pointing toward novel architectures for optoelectronic devices capable of ultrafast operation. The realization that both hot and cold carriers can be harvested efficiently promises substantial improvements in the functionality and efficiency of future technological applications relying on rapid O-E conversion. Exploring the integration of these principles across various materials will likely catalyze further breakthroughs in high-speed and energy-efficient electronics.