  We measure the isothermal rejuvenation of stable glass films of poly(styrene)
and poly(methylmethacrylate). We demonstrate that the propagation of the front
responsible for the transformation to a supercooled-liquid state can serve as a
highly localized probe of the local supercooled dynamics. We use this
connection to probe the depth-dependent relaxation rate with nanometric
precision for a series of polystyrene films over a range of temperatures near
the bulk glass transition temperature. The analysis shows the spatial extent of
enhanced surface mobility and reveals the existence of an unexpected large
dynamical length scale in the system. The results are compared with the
cooperative-string model for glassy dynamics. The data reveals that the
film-thickness dependence of whole film properties arises only from the volume
fraction of the near-surface region. While the dynamics at the middle of the
samples shows the expected bulk-like temperature dependence, the near-surface
region shows very little dependence on temperature.
