Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

Area of Science:

• Nonlinear Optics
• Condensed Matter Physics
• Materials Science

Background:

• Multiple optical harmonic generation is crucial for frequency conversion and comb generation in electronics and optoelectronics.
• Graphene's unique electronic band structure with massless Dirac fermions suggests high efficiency for terahertz harmonic generation, but experimental confirmation is lacking.

Purpose of the Study:

• To experimentally confirm and demonstrate efficient terahertz harmonic generation in single-layer graphene.
• To investigate the potential of graphene for terahertz frequency synthesis.

Main Methods:

• Generation of terahertz harmonics up to the seventh order in single-layer graphene.
• Utilizing low-intensity terahertz fields (tens of kV/cm) at room temperature and ambient conditions.
• Direct observation of terahertz harmonics in the time domain.

Main Results:

• Achieved terahertz harmonic generation up to the seventh order in single-layer graphene.
• Observed high field conversion efficiencies exceeding 10⁻³, 10⁻⁴, and 10⁻⁵ for third, fifth, and seventh harmonics, respectively.
• Graphene's nonlinear optical coefficients for these harmonics are 7-18 orders of magnitude higher than typical solids.

Conclusions:

• The collective thermal response of Dirac electrons in graphene is key to efficient terahertz harmonic generation.
• Graphene offers a direct pathway for highly efficient terahertz frequency synthesis using current graphene electronics.
• Demonstrated the potential of graphene for advanced optoelectronic applications requiring high-frequency signal generation.