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We report on the generation of coherent phonon polaritons in ZnTe, GaP andLiTaO$_{3}$ using ultrafast optical pulses. These polaritons are coupled modesconsisting of mostly far-infrared radiation and a small phonon component, whichare excited through nonlinear optical processes involving the Raman and thesecond-order susceptibilities (difference frequency generation). We probe theirassociated hybrid vibrational-electric field, in the THz range, byelectro-optic sampling methods. The measured field patterns agree very wellwith calculations for the field due to a distribution of dipoles that followsthe shape and moves with the group velocity of the optical pulses. For atightly focused pulse, the pattern is identical to that of classical Cherenkovradiation by a moving dipole. Results for other shapes and, in particular, forthe planar and transient-grating geometries, are accounted for by a convolutionof the Cherenkov field due to a point dipole with the function describing theslowly-varying intensity of the pulse. Hence, polariton fields resulting frompulses of arbitrary shape can be described quantitatively in terms ofexpressions for the Cherenkov radiation emitted by an extended source. Usingthe Cherenkov approach, we recover the phase-matching conditions that lead tothe selection of specific polariton wavevectors in the planar and transientgrating geometry as well as the Cherenkov angle itself. The formalism can beeasily extended to media exhibiting dispersion in the THz range. Calculationsand experimental data for point-like and planar sources reveal significantdifferences between the so-called superluminal and subluminal cases where thegroup velocity of the optical pulses is, respectively, above and below thehighest phase velocity in the infrared.Comment: 13 pages, 11 figure

Cherenkov radiation emitted by ultrafast laser pulses and the generation of coherent polaritons