Fig. 3: Phonon-polariton imaging using ultrafast optical or electron microscopy.

a Experimental geometry for dynamical phonon-polariton imaging involving a pair of anisotropically-shaped pump pulses and an isotropically-shaped probe pulse. b Left panel: Snapshot of an optical micrograph of phonon-polaritons in LiTaO₃. Right panel: 2D spatiotemporal mapping of phonon-polariton propagation after photoexcitation. c Schematic of phonon-polariton generation and free electron probing of its propagation inside a h-BN flake. d Electron energy loss spectra with (left) or without (right) interactions with the phonon-polariton wavepacket. e Measurement of the energy-filtered electrons for different time delays τ_d expressed in picoseconds between the laser pump and the electron probe. The experimental signals (dots) are averaged along the edge direction (indicated by the blue arrow in the inset). Solid curves are fits to a Gaussian profile plus an exponential decay. Inset: A bright-field image of the h-BN flake; scale bar, 5 μm. f 2D mapping of phonon-polariton wavepacket propagation as a function of time and distance from the edge. Blue dots indicate the positions of Gaussian wavepacket peaks, and the orange curve represents the temporal profile of the laser excitation. The gray translucent line is a guide to the eye. The phonon-polariton group velocity undergoes a sudden acceleration followed by deceleration, which are indicated by the purple and red arrows, respectively. The measurements in e and f used a 55-nm-thick h-BN flake that was excited by a 6470-nm laser with a bandwidth of 175 nm. Panels a and b adapted from ref. ¹⁵⁹, AIP Publishing. Panels c-f adapted from ref. ¹⁴⁹, AAAS.