A phonon-laser has been a tantalising prospect since the inception of a laser. Although an optical transition in an atom can be easily selected and amplified by a photon-cavity in a laser, the lack of discrete phonon transitions makes their selection and amplification by an equivalent phonon-cavity a formidable challenge.
To address this, we have developed an electromechanical resonator, shown in Fig. 1(a), which harbours an atom-like spectrum of discrete mechanical vibrations namely localised phonon modes. An analysis of this spectrum reveals the electromechanical atom can host a 3-mode system, which mimics a 3-level laser scheme, where the energy difference of 2 higher (ωH and ωM) modes is resonant with a long-lived lower mode (ωL) as schematically depicted in Fig. 1(b).
In this configuration pumping the higher mode, via the piezoelectric transducers incorporated into the mechanical element, results in an output signal i.e. spontaneous phonon emission or a mechanical vibration in both the lower and middle modes from just a single input into the higher mode. An analysis of the phonon emission observed in the lower mode reveals that it exhibits all the hallmarks of a laser including (i) an onset to the emission (ii) a spectrally sharp line width of 80 mHz as shown in Fig. 1(c) (iii) and gain narrowing. Even more remarkably, when the higher mode is pumped with broadband incoherent noise, it still results in spectrally pure thus coherent emission, i.e. lasing in the lowest mode .
These observations confirm that all-mechanical phonon-lasing can occur in a process resembling stimulated Brillouin scattering and it suggests that concepts from photonics can be readily applied to phononics in the electromechanical resonator platform. This in turn paves the way towards a new class of technology utilising ultra-pure mechanical vibrations .