Coherent Phonon-Polaritons and Subluminal Čerenkov Radiation

Impulsive stimulated Raman scattering can be used to generate a wide range of coherent low-lying excitations [1] and, in particular, phonon-polaritons which are hybrid modes resulting from the coupling of infrared-active transverse optical phonons to infrared photons. In most cases reported in the literature, phase matching is achieved by using the so-called traveling grating geometry involving two pump beams [2]. Here, we show that coherent polaritons can also be generated using a single pump beam, provided the group velocity of the optical pulse is below a certain threshold defined by the infrared phase velocity at zero frequency, c0 [3]. We also show that the condition for phase matching is identical to that for producing Čerenkov radiation at subluminal speeds and, more generally, that phase-matched excitation of phononpolaritons by ultrashort pulses and dipolar Čerenkov emission are the same physical phenomenon [3]. Our experiments were performed using the standard pump-probe setup on a single crystal of ZnSe at 10K. We used a laser system that produces 90 fs pulses in the range 1.8-2.3 eV by pumping an optical parametric amplifier with a Ti:sapphire regenerative amplifier at a repetition rate of 200 kHz and heterodyne detection methods. Time-domain results are shown in the accompanying figure. At central energies hωL > 1.97 eV, the group velocity of the optical pulse is less than c0 and the differential transmission reveal oscillations associated with the generation of coherent polaritons at a frequency that increases with ωL. Such a behavior is consistent with a Čerenkov field created by a quasi-one-dimensional distribution of dipoles. 1. R. Merlin, Solid State Commun. 102, 207-220 (1997). 2. See, e.g., H. J. Bakker, S. Hunsche and H. Kurz, Rev. Mod. Phys. 70, 523 (1998), and references therein. 3. T. E. Stevens, J. K. Wahlstrand, J. Kuhl and R. Merlin, Science 291, 627 (2001). Normalized differential transmission as a function of pump-probe delay. The top and bottom traces were shifted vertically.