Paul G. Evans

Terahertz resonances embedded in crystalline heterostructures could close a spectral gap between conventional electronics and photonics while opening new windows on phenomena in non‐equilibrium lattice dynamics. We show that femtosecond optical screening of the depolarization field in epitaxial PbTiO ₃ /SrTiO ₃ superlattices launches a collective polar mode that oscillates near 1 THz and coherently spans the entire mini‐Brillouin zone. Wave‐vector‐resolved pump–probe X‐ray diffraction resolves a nearly dispersion‐less oscillation at 0.87 and 0.94 THz at the zone boundary and zone center, respectively, persisting for ∼2.5 ps, corresponding to a weakly damped resonance. Dynamical phase‐field simulations reveal the origin of the mode to mesoscopic rotation of closure‐domain textures during the photoexcited transition from an unscreened to a screened electrostatic state. Varying the PbTiO ₃ and SrTiO ₃ ratio tunes the mode frequency continuously from 0.9 to 1.4 THz, providing a quantitative design rule for frequency‐selectable THz oscillators in ferroelectric heterostructures. By coupling nanoscale polarization reconfiguration to long‐wavelength coherent dynamics, this work establishes depolarization‐field engineering to topology‐driven THz functionality and expanding the landscape of collective lattice dynamics.

Interfacial thermal and acoustic phenomena have an important role in quantum science and technology, including in spintronic and spincaloritronic materials and devices. Simultaneous measurements of the low-temperature thermal and acoustic properties of a metal/insulator heterostructure reveal distinct dynamics in the characteristic phonon frequency ranges of acoustic and thermal transport. The measurements probed a heterostructure consisting of a thin film of Pt on the ferrimagnetic insulator gadolinium iron garnet (Gd3Fe5O12, GdIG) grown epitaxially on a gadolinium gallium garnet substrate. Ultrafast structural dynamics within the Pt layer were tracked using time-resolved ultrafast x-ray diffraction and analyzed to probe interfacial acoustic and thermal properties. The rapid heating of the Pt layer by a 400 nm wavelength femtosecond-duration optical pulse produced transient structural changes that provided the stimulus for these measurements. Rapid heating produced a broadband acoustic pulse that was partially reflected by the Pt/GdIG interface. Temporal frequencies up to 740 GHz, corresponding to angular frequencies of several THz, were detected in a wavelet analysis of the acoustic oscillations of the strain in the Pt layer. The structural results were analyzed to determine (i) the acoustic damping coefficient and phonon mean free path in Pt at frequencies of hundreds of GHz and (ii) the Grüneisen anharmonicity parameter. The thermal conductance of the Pt/GdIG interface was tracked using the slower, tens-of-picosecond-scale, dynamics of the initial cooling of the heated Pt layer. Analysis using a model based on the Boltzmann transport equation shows that the phonon transmission is lower at the phonon frequencies relevant to thermal transport than for subterahertz regime acoustics.

Epitaxial growth of complex oxides on large-area wafers, such as sapphire and silicon, represents a key step toward scalable oxide device production. Solid phase epitaxy allows the synthesis of γ-Al2O3 on α-Al2O3 and provides a template with a matched lattice constant and appropriate cubic symmetry for subsequent heteroepitaxial growth of perovskite complex oxides. Nb-doped SrTiO3 thin films were deposited epitaxially on (111)-oriented γ-Al2O3 intermediate layers on (0001) c-axis-oriented sapphire α-Al2O3 crystals using pulsed laser deposition. The Nb:SrTiO3 thin films with a thickness of 53 nm, grown at 700 °C on γ-Al2O3, reached fully relaxed lattice parameters and were epitaxially oriented with respect to the substrate. Nb:SrTiO3 layers deposited using identical deposition conditions directly on α-Al2O3, without the γ-Al2O3 intermediate layer, were polycrystalline. The sheet conductivity of Nb:SrTiO3 grown on γ-Al2O3/α-Al2O3 is more than ten times higher than that of Nb:SrTiO3 grown directly on α-Al2O3 without the γ-Al2O3 layer. The results point to new directions for the integration of (111)-oriented pseudocubic perovskite complex oxides and the integration of epitaxial complex oxides over larger areas using α-Al2O3 single-crystal substrates.

The crystallization of complex oxide thin films on amorphous substrates presents a significant challenge because of the lack of long-range order in these substrates and the subsequent difficulty in controlling crystal growth. Nanocrystals with similar crystal structure have the potential to serve as nucleation sites for crystallization and can facilitate this integration. Isolated nanocrystals of strontium titanate (SrTiO3) can be produced on amorphous SiO2 surfaces through crystallization and ripening of initially amorphous layers of SrTiO3. The resulting SrTiO3 nanocrystals exhibit characteristic lateral radii ranging from tens to hundreds of nm and a consistent average height of 1–2 nm across this range. The area density and mean radii of the nanocrystals can be selected by adjusting the deposition and heating parameters, including the amount of deposited SrTiO3 and the heating duration. The heating-time dependence of the area density and mean radii of the nanocrystals is consistent with predictions based on Ostwald ripening kinetics. The selection of these parameters facilitates the use of SrTiO3 nanocrystals as nucleation sites to crystallize the subsequently deposited layer.

Surface acoustic wave devices are key components for processing radio frequency signals in wireless communication because these devices offer simultaneously high performance, compact size and low cost. The optimization of the device structure requires a quantitative understanding of energy conversion and loss mechanisms. Here we use stroboscopic full-field diffraction x-ray microscopy to reveal an unanticipated acoustic loss in a prototypical one-port resonator device. A non-uniform acoustic excitation in the active area was responsible for the substantial end and side leakages observed at the design frequency. Quantitative analysis of the strain amplitude using a wave decomposition method allowed the determination of several key device parameters. This high-resolution high-throughput spatiotemporal strain imaging technique is more generally applicable to the study of dynamic strain modulation in nanoscale acoustic, electronic, optical and quantum devices. The high sensitivity allows precise measurement of the strain modulation with picometer-scale amplitude.

R. Ramesh(*rr73@rice.edu) R. Ramesh(*rr73@rice.edu)