Sensing the Mechanical Properties of AlN Thin Films Using Micromechanical Membranes in Combination with Finite-element Simulations

The current interest in quantum technologies calls for the development of novel materials and hybrid structures. Understanding the mechanical properties of a material can be a challenge, especially at the nanoscale. We use the eigenfrequencies of in-house fabricated silicon nitride membranes in combination with finite-element simulations to extract the stress in a film that is deposited on top. The high stress results in sharp resonances that can be located precisely so that the mechanical properties of the top layer can be determined accurately. We highlight this approach using aluminum nitride – an important material for on-chip quantum optics and optomechanics – grown onto these micromechanical membranes. The detection is done optomechanically by exciting the modes using a piezo actuation and detecting the vibrations in the reflected laser light. For this, different lasers are at our disposal. The resonances of a wide variety of highly stressed membranes are measured. The frequencies follow the expected inverse length dependence of a stressed membrane and depend on the thickness of the top layer. To connect the experimental observations to the material properties, finite-element simulations are used. It is shown that full simulations of the membranes are only possible for simplified geometries. When simulating the actual geometry, this, however, becomes infeasible. It is shown that simulations of a single unit cell – in particular band structure calculations – can be used to accurately model the actual structure of the membrane. Although this approach is strictly speaking only valid for infinitely large membranes, it is shown that edge effects are negligible. With the simulations, the stress in the bilayer is determined. A cross-over between compressive and tensile stress is observed as a function of the AlN thickness

Photonic Cavity Synchronization of Nanomechanical Oscillators

Synchronization in oscillatory systems is a frequent natural phenomenon and is becoming an important concept in modern physics. Nanomechanical resonators are ideal systems for studying synchronization due to their controllable oscillation properties and engineerable nonlinearities. Here we demonstrate synchronization of two nanomechanical oscillators via a photonic resonator, enabling optomechanical synchronization between mechanically isolated nanomechanical resonators. Optical backaction gives rise to both reactive and dissipative coupling of the mechanical resonators, leading to coherent oscillation and mutual locking of resonators with dynamics beyond the widely accepted phase oscillator (Kuramoto) model. Besides the phase difference between the oscillators, also their amplitudes are coupled, resulting in the emergence of sidebands around the synchronized carrier signal.Comment: 23 pages including supplementary materia