  Graphene has shown great potential for modulating infrared (IR) light in
devices as small as 350 nm. At these length scales, nanoscale features of
devices, and their interaction with light, can be expected to play a
significant role in device performance. Bubbles in van der Waals
heterostructures are one such feature, which have recently attracted
considerable attention thanks to their ability to modify the optoelectronic
properties of 2D materials through strain. Here we use scattering-type scanning
near-field optical microscopy (sSNOM) to measure the nanoscale IR response from
a network of variously shaped bubbles in hexagonal boron nitride
(hBN)-encapsulated graphene. We show that within individual bubbles there are
distinct domains with strongly enhanced IR absorption. We correlate this with
strain in the graphene, found with confocal Raman microscopy and vector
decomposition analysis. This reveals intricate and varied strain
configurations, in which bubbles of different shape induce more bi- or uniaxial
strain configurations. Ridges in the bubbles, seen by atomic force microscopy
(AFM), coincide with the domain boundaries, which leads us to attribute the
domains to nanoscale strain differences in the graphene. This reveals pathways
towards future strain-based graphene IR devices.
