Terahertz emission from ultrathin bismuth layers

Area of Science:

• Solid State Physics
• Materials Science
• Terahertz Spectroscopy

Background:

• Bismuth (Bi) is a semimetallic element with unique electronic properties.
• Thin film growth techniques like molecular beam epitaxy (MBE) allow precise control over material properties.
• Terahertz (THz) spectroscopy is a powerful tool for probing electronic transitions in materials.

Purpose of the Study:

• To investigate the terahertz (THz) emission properties of ultrathin bismuth (Bi) layers grown on silicon (Si) substrates.
• To determine the critical thickness and optical excitation conditions for THz emission from Bi films.
• To analyze the components of the THz signal and their origins.

Main Methods:

• Growth of bismuth (Bi) layers thinner than 10 nm on (111) Si substrates using molecular beam epitaxy (MBE).
• Excitation of Bi layers with tunable wavelength femtosecond optical pulses.
• Measurement of terahertz (THz) radiation pulses emitted from the Bi layers.

Main Results:

• Terahertz (THz) emission was observed when the excitation photon energy exceeded 0.45 eV.
• This threshold energy corresponds to a semimetal-to-semiconductor transition in the ultrathin Bi layers.
• The measured THz signal comprised both isotropic and anisotropic components.

Conclusions:

• Ultrathin bismuth (Bi) films exhibit a thickness-dependent semimetal-to-semiconductor transition.
• The observed THz emission is directly linked to this electronic phase transition.
• The anisotropic THz signal components suggest contributions from lateral photocurrents and shift currents.