Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene

Area of Science:

• Condensed Matter Physics
• Nonlinear Optics
• Materials Science

Background:

• Graphene possesses a high third-order nonlinear susceptibility (χ(3)) in the terahertz (THz) range, significantly exceeding conventional materials like GaAs and lithium niobate.
• This intrinsic nonlinearity, coupled with ultrafast carrier dynamics, is crucial for third harmonic generation (THG) in the sub-THz and low THz frequencies.

Purpose of the Study:

• To investigate the controlled enhancement of graphene's nonlinear optical properties in the THz range.
• To explore the potential of gating-induced Fermi energy shifts in modulating THz nonlinearity for light generation applications.

Main Methods:

• Utilized monochromatic, high peak power (1.8 W) THz signals from a quantum cascade laser.
• Employed an ionic liquid gate to tune the graphene Fermi energy (E_F) up to >1.2 eV.
• Conducted pump-and-probe experiments to analyze nonlinear absorption at 3.2 THz.

Main Results:

• Demonstrated intense absorption nonlinearity at 3.2 THz, with a significant 3rd-order contribution observed for E_F > 0.7 eV.
• Showcased the tunability of graphene's THz nonlinearity via electrostatic gating.
• Confirmed efficient THG in graphene at room temperature with moderate fields.

Conclusions:

• Graphene's tunable THz nonlinearity, controlled by ionic liquid gating, offers a promising route for novel THz light sources.
• The findings open avenues for engineering advanced optoelectronic devices operating at frequencies exceeding 9 THz.