Terahertz acoustic phonon Cerenkov emission in bilayer graphene

Abstract

We present a theoretical investigation on the generation of Cerenkov emission of terahertz acoustic phonons in bilayer graphene (BLG) in the presence of a driving dc electric field. We have numerically and analytically studied the Cerenkov phonon emission spectrum, Pspectrum(ωp,θ), and phonon intensity, Pintensity(θ), dependence on the phonon frequency ωp, drift velocity vd, electron temperature Te, concentration n, and phonon emission angle θ in BLG with and without considering the chirality of the charge carriers. We find that the magnitude of Pspectrum(ωp,θ) increases at larger drift velocities and applied electric fields with the peak of the spectrum shifting toward the higher frequency side. The spectrum magnitude in BLG is found to be much enhanced as compared to conventional 2D semiconductors and transition metal dichalcogenides, which makes it viable for SASER and other practical device applications. The chiral nature of carriers strongly influences the Pspectrum(ωp,θ) behavior and sharpens the spectrum peak but with a decrease in the magnitude. The chirality favors the negative emission spectrum caused by the absorption of acoustic phonons. Pspectrum(ωp,θ) and Pintensity(θ) are found to be strongly dependent on temperature but independent of carrier concentration in the equipartition regime. The study is significant from the point of application of BLG as an acousto/optoelectronic device and high-frequency phonon spectrometers.