Physicists have demonstrated efficient mechanical strain-mediated coupling between the quantized microscopic vibrations (optical phonons) and the macroscopic oscillations of an atomically-thin membrane made from a graphene monolayer. This fundamental work holds promise for the development of 2D-systems with mechanically tunable light-matter interactions.
Recent progress in photonics, optoelectronics and optomechanics has been enabled by improvements in the quality of low-dimensional materials. Two-dimensional (2D) materials (semiconducting transition metal dichalcogenides (TMD) and graphene) are examples of choice. They display remarkable electronic properties and interact strongly with light. At the same time, 2D materials are lightweight, ultrasensitive nanomechanical systems that are controllable by externally applied strain. Although a variety of opto-electronic devices and nano-mechanical resonators made from 2D materials have been demonstrated, the subtle connections between the microscopic properties (e.g., excitons, phonons, interlayer coupling) of these atomically thin crystals and their macroscopic mechanical figures of merit remain unexplored.
In this work, physicists from Université de Strasbourg and CNRS, in collaboration with the University of Nottingham (UK) demonstrate efficient strain-mediated coupling between microscopic quantum degrees of freedom of a 2D resonator (here the phonons, i.e., the quantized vibrations of the 2D atomic registry) and its macroscopic, flexural modes. This work has been published in Nature Communications.
For their project, the physicists have used circular drums made from pristine graphene monolayers. These one atom thick membranes are suspended over micrometric cavities and capacitively driven by a sinusoidal voltage whose frequency is in the vicinity of a mechanical resonance, as are many “nano-electromechanical systems” (NEMS). The frequency-dependent mechanical response of these drums is monitored using an optical interferometric method, which allows us to probe displacements as small as a few picometers (see Fig. 1a). When the frequency of the actuation matches a resonant mode of the drum (typically a few 107Hz), one observes a maximal displacement, as in any driven oscillator. The specificity of this work is to perform, in operando, an optical measurement of the optical phonon frequency using inelastic light scattering spectroscopy (a technique that is more commonly known as Raman scattering spectroscopy). Optical phonons in graphene have well-defined frequencies near 40 THz, i.e. six orders of magnitude higher than the resonance frequency.