Anharmonic phonon-polariton dynamics in ferroelectric LiNbO3 studied with single-shot pump-probe imaging spectroscopy

Abstract

We demonstrate that single-shot pump-probe imaging spectroscopy with an echelon mirror enables us to disclose the ferroelectric phonon-polariton dynamics across a wide temperature range from 10 K to 375 K while avoiding the photorefractive effects that appear prominently at low temperatures. The E-mode phonon-polaritons corresponding to the two transverse optical modes, TO 1 and TO 3 , up to $7 THz were induced in LiNbO 3 through an impulsive stimulated Raman scattering process. Subsequently, using single-shot pump-probe imaging spectroscopy over a minimal cumulative time, we successfully visualized the phonon-polariton dynamics in time-wavelength space even at low temperatures. We found that the phase-matching condition significantly affected the observed temperature-dependent phonon-polariton frequency shift. The anharmonicity of the TO 1 and TO 3 modes was then evaluated based on an anharmonic model involving higher-order interactions with acoustic phonons while eliminating the influence of the frequency shift due to the phase-matching condition. The observed wavenumber-dependent damping rate was analyzed by considering the bilinear coupling of the TO 1 or TO 3 modes with the thermally activated relaxation mode. We found that the phonon-polariton with a higher frequency and wavenumber had a higher damping rate at high temperatures because of its frequent interaction with the thermally activated relaxation mode and acoustic phonons. The TO 3 mode displayed greater bilinear coupling than the TO 1 mode, which may also have contributed to the observed high damping rate. Thus, using our unique single-shot spectroscopy technique, we could reveal the overall anharmonic characteristics of the E-mode phonon-polaritons arising from both the acoustic phonons and the relaxation mode.

Probing Ultrafast Dynamics of Ferroelectrics by Time‐Resolved Pump‐Probe Spectroscopy

“…The pump-probe techniques even give rise to an additional degree of freedom, and in turn provide potential approaches to nondestructive readout in nonvolatile ferroelectric memory. The pump-probe techniques are being developed for higher temporal resolution, higher [ 42,52,72,86,88] BSTO, [ 89,90] BTO, [38] THz emission spectroscopy 𝛼-(ET) 2 I 3 , [73] TPOP TTF-CA, [51,92] LNO, [37] TPXP croconic acid, [93] OPSP THz pump SHG probe OPXP Lattice dynamics lattice vibration/ optical phonon excitation, acoustic phonon, acousto-optic conversion, ultrafast photoinduced strain, phase transition, photovoltaic response, electron-lattice coupling, electron-phonon coupling, phonon-polariton coupling, lattice-polarization coupling THz time domain spectroscopy BTO, [95,112] PZT, [ 102,103] STO, [ 40,43,68,113,114] BFO [ 53,71,82,100,101,104] OPOP LNO, [55,101] PTO, [ 94,96,115] TTF-CA, [41] Sn 2 P 2 S 6 [97] OPSP THz pump SHG probe OPXP TPOP THz emission spectroscopy Spin dynamics magnetic/spin excitation, electric-magnetic coupling, magnon-phonon coupling, electromagnon OPOP BFO, [ 81,83,105,107,108] BSTO/LCMO,…”

“…[ 44 , 45 , 46 , 47 , 48 , 49 ] Amazingly, the ultrafast pump‐probe spectroscopy also provide photo‐physical and structural‐functional properties of ferroelectrics. [ 50 ] Until now, a greater number of dynamical processes in ferroelectrics probed by time‐resolved pump‐probe spectroscopy have been reported, including polarization dynamics, [ 42 ] ultrafast polarization modulation, [ 51 ] light induced/photoassisted polarization, [ 52 ] photoinduced mechanical strain, [ 53 ] phase transition, [ 41 ] electron‐phonon/phonon‐polariton/electric‐magnetic coupling, [ 54 ] electronic/coherent phonon/lattice vibration excitation, [ 55 , 56 ] carrier dynamics and radiative recombination, [ 57 ] among others. While the ultrafast dynamics of ferroelectrics is an important research topic and have fostered many fascinating studies, there is still lack of a systematic review.…”

Selective imaging of the terahertz electric field of the phonon-polariton in LiNbO3

“…. We note that excitation of the phonon-polariton via ISRS was not likely to occur because the pump polarization contained only the ordinary wave [7,8,40], as discussed in the next section.…”

“…The phonon-polariton, which is a coupling mode of a terahertz (THz) electric field with a phonon in a medium [1][2][3][4], has been studied extensively and observed in noncentrosymmetric materials owing to its relevance to nonlinear optics and THz-wave emissions. The coherent phonon-polariton can be excited by an ultrashort optical pulse via impulsive stimulated Raman scattering (ISRS) [5][6][7][8] or optical rectification [9][10][11][12][13][14], and the phase-matching condition results in Cherenkov radiation. In particular, this Cherenkov-type phonon-polariton has been used for efficient THz-wave emission and can be clearly observed in LiNbO 3 [7,8,[15][16][17][18][19], which is a typical ferroelectric material.…”

Selective imaging of terahertz electric field of phonon-polariton in LiNbO$_3$

“…The phonon-polariton, which is a coupling mode of a terahertz (THz) electric field with a phonon in a medium [1][2][3][4], has been studied extensively and observed in non-centrosymmetric materials owing to its relevance to nonlinear optics and THz-wave emissions. The coherent phononpolariton can be excited by an ultrashort optical pulse via impulsive stimulated Raman scattering [5][6][7][8] or optical rectification [9][10][11][12][13][14], and the phase-matching condition results in Cherenkov radiation. In particular, this Cherenkov-type phonon-polariton has been used for efficient THz-wave emission and can be clearly observed in LiNbO 3 [7,8,[15][16][17][18][19], which is a typical ferroelectric material.…”