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

Abstract

We report on the generation of coherent phonon polaritons in ZnTe, GaP, and LiTaO 3 using ultrafast optical pulses. These polaritons are coupled modes consisting of mostly far-infrared radiation and a small phonon component, which are excited through nonlinear optical processes involving the Raman and the second-order susceptibilities ͑respectively, impulsive stimulated Raman scattering and difference frequency generation͒. We probe their associated hybrid vibrational-electric field, in the THz range, by electro-optic sampling methods. The measured field patterns agree very well with calculations for the field due to a distribution of dipoles that follows the shape and moves with the group velocity of the optical pulses. For a tightly focused pulse, the pattern is identical to that of classical Cherenkov radiation by a moving dipole. Results for other shapes and, in particular, for the planar and transient-grating geometries are accounted for by a convolution of the Cherenkov field due to a point dipole with the function describing the slowly varying intensity of the pulse. Hence, polariton fields resulting from pulses of arbitrary shape can be described quantitatively in terms of expressions for the Cherenkov radiation emitted by an extended source. Using the Cherenkov approach, we recover the phase-matching conditions that lead to the selection of specific polariton wave vectors in the planar and transient grating geometry as well as the Cherenkov angle itself. The formalism can be easily extended to media exhibiting dispersion in the THz range. Calculations and experimental data for pointlike and planar sources reveal significant differences between the so-called superluminal and subluminal cases where the group velocity of the optical pulses is, respectively, above and below the highest phase velocity in the infrared. Using the Cherenkov radiation formalism, the fields generated by a spatiotemporally shaped pulse in a thick dispersive medium can be calculated analytically.

High-Resolution Terahertz Spectrometer

“…This expresses that both contributions subtract from each other for frequencies below the TO phonon frequency. In the case of GaP, the electronic and ionic part cancel out in the frequency range around 7 THz [19], (21), (22). While the response of GaP recovers beyond the minimum around 7 THz, the spectral coverage of the spectrometer is still limited to frequencies below 8 THz.…”

“…Fast oscillations present on the signal after the main THz pulse [see Fig. 3(a) inset) arise from the reduced propagation velocity of phonon-polaritons in the GaAs emitter substrate at frequencies close to the transverseoptical (TO) phonon [IS], [19], (20) .…”

Conversion of short optical pulses to terahertz radiation in a nonlinear medium: Experiment and theory

“…This blocks efficient coupling of this type of EMR into free space. The Vavilov-Cherenkov type of terahertz DFG so far has been identified 20,29,30 and detected in the same crystal [31][32][33] by mapping the Cherenkov cone formed by converted field, but it encounters serious problems of coupling into free space. 34,35 According to another proposition of Ref.…”

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

“…Surface-emitting THz generation can overcome these problems [5]; absorption is minimized because the THz-frequency wave is generated at the crystal surface: the Cherenkov-type radiation is generated as surface emission [6][7][8][9][10][11]. We have previously demonstrated a Cherenkov phase-matching method for monochromatic THz generation via difference frequency generation (DFG) using lithium niobate crystals [12][13][14][15].…”