An optical Bragg scattering readout for nano-mechanical resonances of GaN nanowire arrays

We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in periodic selected-area growth arrays. Hexagonal 2D arrays of GaN NWs with pitch spacings of 350–1100 nm were designed and prepared to allow optical Bragg scattering of 632.8 nm laser light. The NWs studied have diameters ranging from 100 to 300 nm, lengths from 3 to 10 μm, resonance frequencies between 1 and 10 MHz, Q-values near 10 000 at 300 K, and Young's modulus of 310 ± 45 GPa. The optical system can detect the thermally induced Brownian mechanical motion of the NWs and driven NW motion and allows the simultaneous monitoring of hundreds of mechanical resonators with a single laser beam. The read-out system allows large arrays of NWs to be characterized and, upon mapping the resonance frequencies to individual array elements, to be applied as, e.g., spatially resolved te...