  Studying thermal transport at the nanoscale poses formidable experimental
challenges due both to the physics of the measurement process and to the issues
of accuracy and reproducibility. The laser-induced transient thermal grating
(TTG) technique permits non-contact measurements on nanostructured samples
without a need for metal heaters or any other extraneous structures, offering
the advantage of inherently high absolute accuracy. We present a review of
recent studies of thermal transport in nanoscale silicon membranes using the
TTG technique. An overview of the methodology, including an analysis of
measurements errors, is followed by a discussion of new findings obtained from
measurements on both solid and nanopatterned membranes. The most important
results have been a direct observation of non-diffusive phonon-mediated
transport at room temperature and measurements of thickness-dependent thermal
conductivity of suspended membranes across a wide thickness range, showing good
agreement with first-principles-based theory assuming diffuse scattering at the
boundaries. Measurements on a membrane with a periodic pattern of nanosized
holes indicated fully diffusive transport and yielded thermal diffusivity
values in agreement with Monte Carlo simulations. Based on the results obtained
to-date, we conclude that room-temperature thermal transport in membranebased
silicon nanostructures is now reasonably well understood.
