Terahertz-induced hot electron emission

Matej Sebek, Tobias O. Buchman, Simon Jappe Lange, Peter Uhd Jepsen

Department of Electrical and Photonics Engineering, Technical University of Denmark, Ørsteds Plads 343, DK-2800 Kongens Lyngby, Denmark

A strong terahertz (THz) field can induce electron emission via tunneling, opening new possibilities for advanced applications such as photomultiplier tubes (PMTs) [1], sensitive spectroscopy [2,3], and sub-nanometer thin film sensing [4]. This study focuses on the critical role of electronic temperature in these emissions. Our findings highlight the necessity of incorporating electronic temperature in understanding and predicting THz-induced electron emission phenomena.

Figure 1 illustrates the interaction of the THz beam with ultrathin gold films and the resulting electron emission mechanisms. In these films, THz-induced electron emission includes both thermionic-like and field-like thermal-assisted field emission processes. When THz pulses interact with the films, the film thickness significantly influences the emission intensity and energy distribution. Notably, the 2 nm gold films exhibit a high-energy tail in the electron energy spectrum due to the higher electronic temperature achieved under THz excitation. This effect, analyzed using the Murphy-Good equation, highlights the impact of electronic temperature on emission probabilities. As the film thickness decreases, surface scattering increases, enhancing electron emission due to higher localized temperatures.

Graphene also exhibits significant electron emission responses to THz radiation. Upon excitation with ultrafast THz pulses, graphene undergoes a process where electron-electron scattering rapidly increases electronic temperature. This elevated temperature facilitates the emission of hot electrons from the material. The experimental results show that the electron emission is a field-driven process and significantly enhanced by the increased electronic temperature, making graphene a highly effective emitter under THz excitation. This dual influence of field and temperature underscores the complex dynamics of electron emission in graphene and highlights its potential for advanced THz applications.

Figure 1: Schematic and physical principle of THz-induced electron emission in ultrathin gold films. (a) THz beam interaction with a gold film on a CuO seed layer. (c) Change in Fermi-Dirac distribution with electronic temperature, leading to electron emission.

In conclusion, the study demonstrates that electronic temperature plays an important role in the THz-induced electron emission from both ultrathin gold films and graphene. Significant advancements can be made in THz technology and its applications by understanding and integrating these effects into emission models.

References
[1] S. J. Lange et al., “Lightwave‐Driven Long‐Wavelength Photomultipliers,” Laser & Photonics Reviews 18(1), 2300417 (2024)
[2] H. Takahashi et al., “Fourier-transform THz spectroscopy based on electric-field interferometry using THz-PMT,” Optics Express 32(7), 12774-12782 (2024)
[3] N. Kawai et al., “High Sensitivity Spectroscopic Measurement with a Highly Nonlinear THz-PMT and an is-TPG,” 2023 48th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), IEEE (2023)
[4] M. Sebek et al., “Terahertz Sensing of Å‐scale Thin Dielectric Film Via Electron Tunnelling.” Advanced Optical Materials 2302841 (2024)