  Detection of local strain at the nanometer scale with high sensitivity
remains challenging. Here we report near-field infrared nano-imaging of local
strains in bilayer graphene through probing strain-induced shifts of phonon
frequency. As a non-polar crystal, intrinsic bilayer graphene possesses little
infrared response at its transverse optical (TO) phonon frequency. The reported
optical detection of local strain is enabled by applying a vertical electrical
field that breaks the symmetry of the two graphene layers and introduces finite
electrical dipole moment to graphene phonon. The activated phonon further
interacts with continuum electronic transitions, and generates a strong Fano
resonance. The resulted Fano resonance features a very sharp near-field
infrared scattering peak, which leads to an extraordinary sensitivity of
~0.002% for the strain detection. Our studies demonstrate the first nano-scale
near-field Fano resonance, provide a new way to probe local strains with high
sensitivity in non-polar crystals, and open exciting possibilities for studying
strain-induced rich phenomena.
