**Abstract: **Heat is one of the most fundament
al forms of energy\, and the ability to control heat plays a critical role
in most current and future energy applications. Recently\, nanoscale engi
neering has provided new approaches to manipulate heat transport at the sc
ales of the heat carriers in solids. Despite these advances\, we still la
ck a comprehensive understanding of energy carriers in solids\, which woul
d allow us to achieve precise control of energy transport at the nanoscale
. My research interests lie in furthering our knowledge of energy carriers
\, especially electrons and phonons (quantized lattice vibrations). In the
first part of my talk\, I will give a brief introduction to ultrafast las
er spectroscopies that allow us to simultaneously characterize macroscopic
thermal properties and microscopic thermal processes of energy carriers i
n bulk crystals as well as nanometer-thick thin films. In the second part
of my talk\, I will discuss a generalized Fourier’s law derived from the B
oltzmann transport equation that is valid from diffusion to ballistic regi
mes. This generalized Fourier’s law contains two parts\, nonlocality of th
ermal conductivity\, which has been previously hypothesized\, and nonlocal
ity of boundary conditions\, which has long been ignored in literatures. W
e apply the derived generalized Fourier’s law to predict the surface tempe
rature responses of an ultrafast laser spectroscopic technique called time
-domain thermoreflanctance (TDTR) under various conditions\, demonstrating
an excellent match between the theoretical predictions and experimental r
esults. Furthermore\, by exploiting the generalized Fourier’s law in a syn
thetic TDTR experiment on a single crystal boron arsenide\, we show that i
n the non-diffusive thermal transport regime\, simply interpreting the obs
ervation using a Fourier’s law with a modified thermal conductivity\, a co
mmon practice in the community\, would lead to erroneous results. To map t
he macroscopic observations to intrinsic phonon properties\, it is crucial
to appropriately take account into the microscopic boundary conditions. O
ur work shows that in a non-diffusive regime\, the two parts of the genera
lized Fourier’s law are equally important to accurately describing the the
rmal transport\, and we can take advantage of the nonlocal nature of the b
oundary conditions as an extra knob to manipulate the heat.