  Speckle is maybe the most fundamental interference effect of light in
disordered media, giving rise to fascinating physical phenomena and enabling
applications in imaging, spectroscopy or cryptography, to name a few. While
speckle formed outside a sample is easily measured and analysed, true bulk
speckle, as formed inside random media, is difficult to investigate directly
due to the obvious issue of physical access. Furthermore, its proper
theoretical description poses enormous challenges. Here we report on the first
direct measurements of intensity correlations of light inside a disordered
medium, using embedded DNA strings decorated with emitters separated by a
controlled nanometric distance. Our method provides in situ access to
fundamental properties of bulk speckles as their size and polarization degrees
of freedom, both of which are found to deviate significantly from theoretical
predictions. The deviations are explained, by comparison with rigorous
numerical calculations, in terms of correlations among polarization components
and non-universal near-field contributions at the nanoscale.
