Coupling between Light and Terahertz-Frequency Acoustic Phonons in Ferroelectric BaTiO 3 /SrTiO 3 Superlattices (original) (raw)
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Physical Review Letters, 2008
We report a UV-Raman study of folded acoustic vibrations in epitaxial ferroelectric BaTiO 3 =SrTiO 3 superlattices. The folded acoustic doublets show an anomalous temperature dependence disappearing above the ferroelectric transition, which is tuned by varying the thickness of the BaTiO 3 and SrTiO 3 layers. A mechanism involving the acoustic phonon modulation of the spatially periodic ferroelectric polarization explains the observed temperature dependence. These results demonstrate the strong coupling between sound, charge, and light in these multifunctional nanoscale ferroelectrics.
Acoustic properties of nanoscale oxide heterostructures probed by UV Raman spectroscopy
Journal of Physics: Conference Series, 2007
We study high quality molecular-beam epitaxy grown BaTiO3/SrTiO3 superlattices using ultraviolet Raman spectroscopy. In the low energy spectral region, acoustic phonon doublets are observed. These are due to the artificial superlattice periodicity and consequent folding of the acoustic phonon dispersion. From the study of samples with different BaTiO3/SrTiO3 layer thicknesses the effective sound velocities within each of the layers are obtained.
Broadband Terahertz Spectroscopy of Phonon-Polariton Dispersion in Ferroelectrics
Photonics, 2018
Electromagnetic waves at frequencies below the X-ray region strongly couple to the optical vibrational modes in a solid. These coupled excitations have been called phonon polaritons. The relationship of the polariton frequency versus the polariton wavevector shows a remarkable dispersion, especially in the vicinity of the transverse and longitudinal optical mode frequencies. The significant frequency dependence enables valuable applications such as a tunable terahertz radiation source. The polariton dispersion relations of technologically important dielectric and ferroelectric crystals were reviewed in the broad terahertz range using terahertz time-domain spectroscopy, far-infrared spectroscopy, and Raman scattering spectroscopy.
2010
The complex interaction between transverse acoustic (TA) phonon, transverse optic (TO) phonon and polar nano-domains (PND) in the relaxor ferroelectric KTa1-xNbxO3 (KTN) is studied by means of high resolution diffuse and inelastic neutron scattering. The experimental results and a comparison with lead relaxors, suggest a new scattering mechanism of the TA phonon by localized modes in PNDs. A theoretical model is developed, which accurately predicts the evolution of the TA damping with temperature and wavevector. Such a mechanism suggests the possible use of high frequency acoustic modes for the study of nanocomposite materials.
Physical Review B, 2016
The damping (Γa) of the transverse acoustic (TA) phonon in single crystals of the relaxor KTa 1-x Nb x O 3 with x=0.15-0.17 was studied by means of high resolution inelastic cold neutron scattering near the (200) B.Z. point where diffuse scattering is absent, although it is present near (110). In a wide range of temperatures centered on the phase transition, T=195K÷108K, the transverse acoustic (TA) phonon width (damping) exhibits a step increase around momentum q=0.07, goes through a shallow maximum at q=0.09-0.12 and remains high above and up to the highest momentum studied of q=0.16. These experimental results are explained in terms of a resonant interaction between the TA phonon and the collective or correlated reorientation through tunneling of the off-center Nb +5 ions. The observed TA damping is successfully reproduced in a simple model that includes an interaction between the TA phonon and a dispersionless localized mode (LM) with frequency ω L and damping Γ L (Γ L < ω L), itself coupled to the transverse optic (TO) mode. Maximum damping of the TA phonon occurs when its frequency ωa ≈ ω L. The values of ω L and Γ L are moderately dependent on temperature but the oscillator strength, M 2 , of the resonant damping exhibits a strong maximum in the range T~120 K÷150 K in which neutron diffuse scattering near the (110) B.Z. point is also maximum and the dielectric susceptibility exhibits the relaxor behavior. The maximum value of M appears to be due to the increasing number of polar nanodomains. In support of the proposed model, the observed value of ω L ≈ 0.7 THz is found to be similar to the estimate previously obtained by Girshberg and Yacoby. Alternatively, the TA phonon damping can be successfully fitted in the framework of an empirical Havriliak-Negami (HN) relaxation model that includes a strong resonancelike transient contribution.
Terahertz response of acoustically driven phonons
Physical Review B, 2010
The manipulation of transverse-optical TO-phonon polaritons and the associated terahertz THz light field by means of an ultrasound acoustic wave is proposed and illustrated by calculating the TO-phonon-mediated THz response of acoustically pumped CuCl and TlCl crystals. We show the high-contrast acoustically induced change in the THz reflectivity and multiple THz Bragg replicas, which are associated with the far-infrared active TO-phonon resonance driven by the ultrasonic wave. The effect, which stems from phonon anharmonicity and deals with the resonantly enhanced acousto-optical susceptibilities, refers to an operating acoustic intensity Iac~1–100 kW/cm2 and frequency vac ~0.1–1 GHz. Due to the anomalously small interaction length between the acoustic and optical fields, possible applications of the effect are in THz spectroscopy and THz acousto-optic devices.
Progress towards terahertz acoustic phonon generation in doping superlattices
2005
Progress is described in experiments to generate coherent terahertz acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation. Future experiments are proposed that would use the superlattice as a transducer in a terahertz cryogenic acoustic reflection microscope with sub-nanometer resolution.
Probing nanoscale ferroelectricity by ultraviolet Raman spectroscopy
2006
With the highly-polarizable SrTiO 3 in contact with the BaTiO 3 layers, the critical thickness is reduced to a single unit cell. Meanwhile, the mechanical boundary condition imposed by the SrTiO 3 substrate leads to strain in the BaTiO 3 layers and thus enhanced ferroelectricity. The interplay between the electrical and mechanical boundary conditions enables the tuning of T c by nearly 500 K. 7 This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online
MRS Proceedings, 2003
Using time resolved ultrafast spectroscopy, we have demonstrated that the far infrared (FIR) excitations in ferroelectric crystals may be modified through an arsenal of control techniques from the fields of guided waves, geometrical and Fourier optics, and optical pulse shaping. We show that LiNbO 3 and LiTaO 3 crystals of 10-250 µm thickness behave as slab waveguides for phonon-polaritons, which are admixtures of electromagnetic waves and lattice vibrations, when the polariton wavelength is on the order of or greater than the crystal thickness. Furthermore, we show that ferroelectric crystals are amenable to processing by ultrafast laser ablation, allowing for milling of user-defined patterns designed for guidance and control of phonon-polariton propagation. We have fabricated several functional structures including THz rectangular waveguides, resonators, splitters/couplers, interferometers, focusing reflectors, and diffractive elements. Electric field enhancement has been obtained with the reflective structures, through spatial shaping, of the optical excitation beam used for phonon-polariton generation, and through temporal pulse shaping to permit repetitive excitation of a phonon-polariton resonant cavity.
Physical Review Letters, 2008
The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric singledomain crystal of BaTiO 3 was determined by time-domain THz spectroscopy and Fourier-transform infrared reflectivity techniques. In addition to the three well-known polar lattice modes, the experiment shows an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the c-axis dielectric constant. A comparison of experimental results with ab initio based effective-Hamiltonian simulations allows us to elucidate its relation to the order-disorder model of Comes, Lambert, and Guinier [Solid State Commun. 6, 715 (1968)].