Direct visualization of the Gouy phase by focusing phonon polaritons

We report the generation of aberration-free cylindrical phonon-polariton wave packets in uniaxial LiTaO3 crystals by nonresonant impulsive stimulated Raman scattering. The unique properties of phonon polaritons with a typical carrier frequency in the THz regime allow direct measurement of the spatiotemporal amplitude and phase distributions. We demonstrate that under these conditions the phase anomaly (Gouy phase) may be visualized directly through spatiotemporal imaging as the cylindrical wave propagates through its focus.