Phonon-polariton coupling

One of the applications of TRXRD is to study complex systems where electric fields couple to multiple degrees of freedom. Though femtosecond laser pulses can generate THz radiation from ferroelectric LiTa03, the corresponding lattice motion remained undetected by optical measurements. Cavalleri and coworkers demonstrated the coherent modulation of the X-ray intensity at 1.5 THz [10], and assigned it as phonon-polariton mode of A symmetry (Fig. 3.3). Nakamura and coworkers detected the coherent LO phonon of CdTe... 

The elementary excitations mentioned so far are not related in any special way to the solid state and will therefore not be treated in this article. We will discuss here the following low-lying quantized excitations or quasi-particles which have been investigated by Raman spectroscopic methods phonons, polaritons, plasmons and coupled plasmon-phonon states, plasmaritons, mag-nons, and Landau levels. Finally, phase transitions were also studied by light scattering experiments however, they cannot be dealt with in this article. 

A phonon-polariton is a boson quasi-particle that couples an infrared photon with an optical phonon. 

The coupled phonon-polariton oscillations can be detected by measurement of oscillatory birefringence with a variably delayed probe pulse. (The transit time and the spectral content of the probe pulse also should show oscillatory time dependences.) As in forward ISRS, this pulse surfs along a crest or null of the polariton wave. Since the polariton radiates outward from the excitation beam, the probe pulse need not be overlapped spatially with the excitation pulse. By varying the spatial separation between the two parallel-propagating beams, the polariton group velocity and dispersion can be determined. Phonon-polariton dynamics in LiTaOj crystals were determined in this manner [36, 59]. An example of data is shown in Figure 9. 

Phonon-polaritons arise from the coupling between electromagnetic radiation and mechanical vibrational provided by the infrared activity of the mode being considered, and their propagation characteristics are mainly governed by their disjiersion relation at small wave vector k. 

Depending on the substrate excitations which are coupled with light into the SP mode, one distinguishes surface plasmon polaritons, surface phonon polaritons, surface exciton polaritons, etc. In this section we shall consider surface plasmon polaritons in some detail. This type of electromagnetic wave was first discussed by Sommerfeld in connection with the propagation of radiowaves along the Earth s surface (Sommerfeld 1909). 

Plasmon-phonon coupling represents mixing of two quasi-particles. The coupling of three quasi-particles has also been observed. The term plasmariton was used by Alfano 45) for a coupled state of a TO phonon and a dressed photon , namely, a photon surrounded by an electron cloud (a coupled state of a plasmon and a photon). The quasi-particle dressed photon is also called a transverse plasmon. Because the coupled state of a photon and a TO phonon has been termed polariton, a plasmariton can also be regarded as coupled state of a plasmon and a polariton. Earlier the term plasmariton was used in a more restricted sense, namely, when a partly transverse character of the plasmon is induced by an external magnetic field. 

In Section I, the spectra of e"(ai) consist of Dirac 5 peaks (1.79). In a real crystal these peaks are broadened by static disorder, thermal fluctuations, and excitation-relaxation processes. Discarding for the moment the static disorder, we focus our attention on broadening processes due to lattice phonons, which may be described alternatively in terms of fluctuations of the local energies of the sites, or in terms of exciton relaxation by emission and absorption of phonons. These two complementary aspects of the fluctuation-dissipation theorem64 will allow us to treat the exciton-phonon coupling in the so-called strong and weak cases. The extraordinary (polariton) 0-0 transition of the anthracene crystal will be analyzed on the basis of these theoretical considerations and the semiexperimental data of the Kramers-Kronig analysis. 

Let us now consider the case of strong exciton-photon coupling, which is that in the singlet state of the anthracene crystal. As shown in Section I, we have to consider as zero-order hamiltonian H that of the polaritons, and the transition between polaritons will be induced by the exciton-phonon coupling Hep. Let us denote by Jf and the hamiltonian of the total system and of its... 

Unlike ISS, the electro-optic effect (or its inverse) can occur only in noncentrosymmetric media and in general does not lead to any real material excitation. However, if there are low-frequency IR-active modes in the crystal, they may be excited impulsively [36, 59]. Such phonons couple strongly to IR radiation to form mixed modes called polaritons. Impulsive stimulated polariton scattering can be described approximately by coupled equations of motion for the polarization contributions P, and due to ionic motions (i.e., phonons) and electronic motions, respectively [9, 60] ... 

Polariton - A quantum associated with the coupled modes of photons and optical phonons in an ionic crystal. 

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