Ultrafast phonon dynamics play a crucial role in emerging thermoelectrics and photovoltaics due to their effect on heat transport in materials. We have recently developed a novel ultrafast spectroscopic method called time-resolved surface third-harmonic generation (TRSTHG) to detect the background-free, surface-enhanced, high signal-to-noise-ratio ultraviolet signal generated between the two incident near-IR pulses. This unique approach elucidates the underlying phonon modes at thermal equilibrium to conduct heat on multidimensional potential energy surface with femtosecond time resolution. Instead of measuring reflectivity or transmission changes of the probe at the sample upon interaction with the pump pulse thus limiting the detection range either to a few surface monolayers or bulk medium, we can analyze the quantum beats and unravel the transient phonon propagation dynamics at the 100—1000 nm length scale. This length scale corresponds to an intermediate regime where nanomaterials assemble and mesoscopic physics dominates, bridging quantum and classical worlds. Results in quartz crystal, polycrystalline CaF2, amorphous BK7 glass and water film will be discussed.