Enhanced Frequency-Upconverted Photoluminescence and Terahertz Emission From Graphene

Graphene is a gapless material with a linear energy–momentum dispersion relationship. Because of its unique band structure, graphene has been demonstrated as an ultra-broadband photon absorption material from the visible to terahertz frequency ranges. Here, we study the reverse process: photon emission from graphene. Using silica microsphere structures and femtosecond laser pulse excitation, photon emission enhancement at visible, near infrared, and terahertz ranges were achieved. These results help to promote graphene as a new type of light generation material, which can overcome the restriction that the emission wavelength is determined by the material bandgap. It is also found that the graphene's electrical properties, such as the nonlinear conductivity, changed significantly with the enhancement of the absorption during the ultrafast process.