Nanomechanical method to gauge emission quantum yield applied to nitrogen-vacancy centers in nanodiamond

We present a technique to nanomechanically vary the distance between a fluorescent source and a mirror, thereby varying the local density of optical states at the source position. Our method can, therefore, serve to measure the quantum efficiency of fluorophores. Application of our technique to nitrogen-vacancy defects in diamond nanocrystals shows that their quantum yield can significantly differ from unity. Relying on a lateral scanning mechanism with shear-force probe-sample distance control our technique is straightforwardly implemented in most state-of-the-art near-field microscopes.