Optical phonons and polariton dispersions of congruent LiNbO₃ studied by far-infrared spectroscopic ellipsometry and Raman scattering

Abstract

IR active optical modes and phonon–polaritons with E(x) and A1(z) symmetries were studied in a ferroelectric congruent lithium niobate crystal. The real and imaginary parts of a dielectric constant along the a- and c-axes were accurately determined by far-infrared spectroscopic ellipsometry (FIRSP) from 40 to 700 cm−1. For the nine transverse optical (TO) modes with E(x) symmetry, it was difficult to observe the 5th E(TO5) mode at 361 cm−1 and the 9th E(TO9) mode at 665 cm−1 by Raman scattering owing to the very low Raman intensity, while these modes were clearly observed by FIRSP. In contrast, the 6th E(TO6) mode at 371 cm−1 was not observed by FIRSP owing to the very weak absorption, while it was clearly observed by Raman scattering. All the four TO modes with A1(z) symmetry were clearly observed independently by FIRSP and Raman scattering. The dispersion relations of phonon–polaritons including the damping of polaritons were determined using the real and imaginary parts of a polariton wavevector calculated from complex dielectric constants. The polariton dispersion of the lowest A1(z) mode at 254 cm−1 is in agreement with the previous forward Raman scattering experiment; however, any anticrossing predicted by the previous impulsive Raman scattering experiment was not observed.

Terahertz Polariton Dispersion in Uniaxial Optical Crystals

“…In ferroelectric LN crystals, there are four normal modes with A 1 (z) symmetry at room temperature. These four modes were clearly observed in both backward Raman scattering and reflection IR spectra [10]. The mode frequencies of four modes are in agreement with the values reported in literature [14].…”

“…Recently, a far-infrared spectroscopic ellipsometry (FIRSP) system has been developed by the combination of a far-infrared spectrometer and an ellipsometry [10]. The Michelson interferometer of the Fourier transform spectrometer is followed by a rotating analyzer type ellipsometer.…”

Probing the interatomic potential of solids with strong-field nonlinear phononics

“…For small amplitude excitation (0.1 MV/cm), both PR and SH measurements yielded harmonic oscillations (see Fig. 1c and d dashed lines), which were readily attributed to a combination of a 15-THz phonon-polariton and the 19-THz TO phonon of the A1 mode 17 . As shown in Fig.…”

“…Extended Data Figure 6 further shows the reflectivity fit considering only the two dominating oscillators only (blue line) and the reflectivity obtained from the FDTD simulation using these two modes (green line). Both yield a satisfactory description of the optical properties in the region of interest (12)(13)(14)(15)(16)(17)(18)(19)(20).…”

Ultrafast reversal of the ferroelectric polarization by a midinfrared pulse

“…Though, even in ferroelectric phase this coupling is expected to be not large due to the small value of ⟨ ⟩. Indeed, the effective charge of 1 (TO 3 ), which has irreducible representation 1 in paraelectric phase, is very small [15]. We will focus on a single phonon mode excitation and as a consequence only on two-phonon modes coupling, one of each is the ferroelectric soft mode and the other is an infrared-active mode IR (see [20,21] for discussions on three phonon modes interaction).…”