Mechanical force is a critical feature for most physical processes, and remote measure of mechanical signals with high force sensitivity and spatial resolution is crucial for progress in fields as diverse as robotics, biophysics, civil engineering, and medicine. Existing nanoscale remote force sensors, however, are very limited in the dynamic range of forces they can detect, and are rarely compatible with subsurface operation, restricting sensor applicability [1]. In this talk, I will describe how we leverage the extreme optical nonlinearity offered by photon-avalanche [2], and its susceptibility to steep change due to minute changes in the environment – to create nanoscale force sensors that can be addressed remotely by continuous-wave, deeply-penetrating, infrared light, and can detect picoNewton to microNewton forces with a dynamic range spanning more than four orders of magnitude [3]. Using atomic force microscopy coupled with single-nanoparticle optical spectroscopy, we characterize the mechano-optics of different Tm3+-doped avalanching upconverting nanoparticles on a single particle level, to rationally design force sensors with different modalities of force-dependent optical readout, including mechanobrightening and mechanochromism. By manipulating the interionic distances and hence energy transfer pathways within the nanosensors by application of force, we demonstrate exceptional mechanical sensitivity coupled with high single-particle brightness, over multiple scales of force. The adaptability of these nanoscale optical force sensors, along with their multiscale sensing capability, enable operation in the dynamic and versatile environments present in diverse, real-world structures spanning biological organisms to nanoelectromechanical systems.
