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

TL;DR

This paper demonstrates the generation and analysis of coherent phonon polaritons in various materials using ultrafast laser pulses, employing a Cherenkov radiation framework to describe their behavior and phase-matching conditions.

Contribution

It introduces a Cherenkov radiation-based formalism to quantitatively describe polariton fields generated by ultrafast pulses of arbitrary shape, extending understanding of their wavevector selection and propagation.

Findings

Polariton fields can be modeled as Cherenkov radiation from moving dipoles.

The formalism accurately predicts phase-matching and Cherenkov angles.

Differences between superluminal and subluminal pulse group velocities are significant.

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 (difference frequency generation). 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 wavevectors 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 point-like 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.