  We utilize terahertz time domain spectroscopy to investigate thin films of
the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature
dependent terahertz (THz) conductivity measurements reveal a rapid decrease in
the Drude weight and carrier scattering rate at ~T*=20 K, well below the
hybridization gap onset temperature (100 K). Moreover, a low-temperature
conductivity plateau (below 20K) indicates the emergence of a surface state
with an effective electron mass of 0.1me. Conductivity dynamics following
optical excitation are also measured and interpreted using Rothwarf-Taylor
(R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However,
R-T modeling of the conductivity dynamics reveals a deviation from the expected
thermally excited quasiparticle density at temperatures below 20K, indicative
of another channel opening up in the low energy electrodynamics. Taken
together, these results suggest the onset of a surface state well below the
crossover temperature (100K) after long-range coherence of the f-electron Kondo
lattice is established.
