Generating coherent phonon waves in narrow-band materials: a twisted bilayer graphene phaser

Abstract

Twisted bilayer graphene (TBG) exhibits extremely low Fermi velocities for electrons, with the speed of sound surpassing the Fermi velocity. This regime enables the use of TBG for amplifying vibrational waves of the lattice through stimulated emission, following the same principles of operation of free-electron lasers. Our work proposes a lasing mechanism relying on the slow-electron bands to produce a coherent beam of acoustic phonons. We propose a device based on undulated electrons in TBG, which we dub the phaser. The device generates phonon beams in a terahertz (THz) frequency range, which can then be used to produce THz electromagnetic radiation. The ability to generate coherent phonons in solids breaks new ground in controlling quantum memories, probing quantum states, realizing non-equilibrium phases of matter, and designing new types of THz optical devices.

Surface acoustic wave induced phenomena in two-dimensional materials

“…In addition, coherent phonons in 2D materials could be induced through pumping by intense laser and magnetic fields. [192][193][194][195][196][197] The resonant optical excitation of infrared-active lattice vibrations is feasible to induce targeted structural deformations in 2D materials. 198 In 2016, the temporal dynamics of coherent lattice vibration modes in mono-and few-layer WSe 2 flakes was realized by performing time-resolved transmission measurements.…”

Optical Control of Slow Topological Electrons in Moiré Systems

“…1(a-b)). The ability to tune the Floquet steady-state results from the unique slow electron regime in TBG where phonons travel faster than-and decouple from-many flat band electronic states [36,37]. The drive strength controls electron velocities, tuning between an equilibrated electronic gas with finite σ xy at low drive amplitudes and a decoupled electronic gas with reduced electron velocities and suppressed σ xy at high drive amplitudes.…”

“…Furthermore, this strong amplitude-dependence arises only when the undriven effective electronic velocity v 0 eff is close to c ph in TBG (Fig. 1(c)), a condition unique to TBG near the "slow-electron" regime [36,37]. Electrons in TBG are also decoupled from phonons in the dielectric medium, since typical screening media such as hBN have speeds of sound much larger than v 0 eff [57].…”