  Mechanical force is an essential feature for many physical and biological
processes.1-12 Remote measurement of mechanical signals with high sensitivity
and spatial resolution is needed for diverse applications, including
robotics,13 biophysics,14-20 energy storage,21-24 and medicine.25-27 Nanoscale
luminescent force sensors excel at measuring piconewton forces,28-32 while
larger sensors have proven powerful in probing micronewton forces.33,34
However, large gaps remain in the force magnitudes that can be probed remotely
from subsurface or interfacial sites, and no individual, non-invasive sensor is
capable of measuring over the large dynamic range needed to understand many
systems.35,36 Here, we demonstrate Tm3+-doped avalanching nanoparticle37 force
sensors that can be addressed remotely by deeply penetrating near-infrared
(NIR) light and can detect piconewton to micronewton forces with a dynamic
range spanning more than four orders of magnitude. Using atomic force
microscopy coupled with single-nanoparticle optical spectroscopy, we
characterize the mechanical sensitivity of the photon avalanching process and
reveal its exceptional force responsiveness. By manipulating the Tm3+
concentrations and energy transfer within the nanosensors, we demonstrate
different optical force-sensing modalities, including mechanobrightening and
mechanochromism. The adaptability of these nanoscale optical force sensors,
along with their multiscale sensing capability, enable operation in the dynamic
and versatile environments present in real-world, complex structures spanning
biological organisms to nanoelectromechanical systems (NEMS).
