Enhancement and inhibition of coherent phonon emission of a Ni film in a BaTiO3/SrTiO3 cavity

Ultrafast magnetoacoustics in Galfenol nanostructures

Phonons and magnons are prospective information carriers to substitute the transfer of charge in nanoscale communication devices. Our ability to manipulate them at the nanoscale and with ultimate speed is examined by ultrafast acoustics and femtosecond optomagnetism, which use ultrashort laser pulses for generation and detection of the corresponding coherent excitations. Ultrafast magnetoacoustics merges these research directions and focuses on the interaction of optically generated coherent phonons and magnons. In this review, we present ultrafast magnetoacoustic experiments with nanostructures based on the alloy (Fe,Ga) known as Galfenol. We demonstrate how broad we can manipulate the magnetic response on an optical excitation by controlling the spectrum of generated coherent phonons and their interaction with magnons. Resonant phonon pumping of magnons, formation of magnon polarons, driving of a magnetization wave by a guided phonon wavepacket are demonstrated. The presented experimental results have great application potential in emerging areas of modern nanoelectronics.

Probing gigahertz coherent acoustic phonons in TiO2 mesoporous thin films

Ultrahigh-frequency acoustic-phonon resonators usually require atomically flat interfaces to avoid phonon scattering and dephasing, leading to expensive fabrication processes, such as molecular beam epitaxy. Mesoporous thin films are based on inexpensive wet chemical fabrication techniques that lead to relatively flat interfaces regardless the presence of nanopores. Here, we report mesoporous titanium dioxide-based acoustic resonators with resonances up to 90 GHz, and quality factors from 3 to 7. Numerical simulations show a good agreement with the picosecond ultrasonics experiments. We also numerically study the effect of changes in the speed of sound on the performance of the resonator. This change could be induced by liquid infiltration into the mesopores. Our findings constitute the first step towards the engineering of building blocks based on mesoporous thin films for reconfigurable optoacoustic sensors.

Evaluation of Optical and Acoustical Properties of Ba1-xSrxTiO3 Thin Film Material Library via Conjugation of Picosecond Laser Ultrasonics with X-ray Diffraction, Energy Dispersive Spectroscopy, Electron Probe Micro Analysis, Scanning Electron and Atomic Force Microscopies

Wide-range continuous spatial variation of the film composition in lateral compositionally graded epitaxial films requires the development of high throughput measurement techniques for their local and non-destructive characterization with the highest possible spatial resolution. Here we report on the first application of the picosecond laser ultrasonics (PLU) technique for the evaluation of acoustical and optical parameters of lateral compositionally graded film, the Ba_1−xSr_xTiO₃ (0 ≤ x ≤ 1) material library. The film was not dedicatedly prepared for its opto-acousto-optic evaluation by PLU, exhibiting significant lateral variations in thickness and surface roughness. Therefore, the achieved measurements of the sound velocity and of the optical refractive index, and characterization of the surface roughness confirm the robustness of the PLU technique for thin film evaluation. We hope that the first measurements of the acoustical and optical properties of epitaxial grown Ba_1−xSr_xTiO₃ (0 ≤ x ≤ 1) by PLU technique accomplished here provide the parameters required for more extended predictive design of the phononic, photonic and phoxonic mirrors and cavities with superior properties/functionalities for novel multifunctional nanodevices.

A Topological View on Optical and Phononic Fabry–Perot Microcavities through the Su–Schrieffer–Heeger Model

Advances in nanofabrication technologies have enabled the study of acoustic wave phenomena in the technologically relevant GHz–THz range. First steps towards applying concepts from topology in nanophononics were made with the proposal of a new topological acoustic resonator, based on the concept of band inversion. In topology, the Su–Schrieffer–Heeger (SSH) model is the paradigm that accounts for the topological properties of many one-dimensional structures. Both the classical Fabry–Perot resonator and the reported topological resonators are based on Distributed Bragg Reflectors (DBRs). A clear and detailed relation between the two systems, however, is still lacking. Here, we show how a parallelism between the standard DBR-based acoustic Fabry–Perot type cavity and the SSH model of polyacetylene can be established. We discuss the existence of surface modes in acoustic DBRs and interface modes in concatenated DBRs and show that these modes are equivalent to Fabry–Perot type cavity modes. Although it is not possible to assign topological invariants to both acoustic bands enclosing the considered minigap in the nanophononic Fabry–Perot case, the existence of the confined mode in a Fabry–Perot cavity can nevertheless be interpreted in terms of the symmetry inversion of the Bloch modes at the Brillouin zone edge.

Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction with a nanoscale probe beam for the quantitative retrieval of the time-dependent local deformation gradient tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformations close to the edge of a single-crystalline graphite membrane. Tracking the structural distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an acoustic membrane breathing mode with spatially modulated amplitude, governed by the optical near field structure at the membrane edge. Furthermore, an in-plane polarized acoustic shock wave is launched at the membrane edge, which triggers secondary acoustic shear waves with a pronounced spatio-temporal dependency. The experimental findings are compared to numerical acoustic wave simulations in the continuous medium limit, highlighting the importance of microscopic dissipation mechanisms and ballistic transport channels.

Spectral control of elastic dynamics in metallic nano-cavities

We show how the elastic response of metallic nano-cavities can be tailored by tuning the interplay with an underlying phononic superlattice. In particular, we exploit ultrafast optical excitation in order to address a resonance mode in a tungsten thin film, grown on top of a periodic MgO/ZrO₂ multilayer. Setting up a simple theoretical model, we can explain our findings by the coupling of the resonance in the tungsten to an evanescent surface mode of the superlattice. To demonstrate a second potential benefit of our findings besides characterization of elastic properties of multilayer samples, we show by micromagnetic simulation how a similar structure can be utilized for magneto-elastic excitation of exchange-dominated spin waves.

Picosecond optomechanical oscillations in metal-polymer microcavities

We experimentally study mechanical vibrations in planar Fabry-Perot microcavities made of metallic mirrors and a polymer spacer, using broadband pump-probe spectroscopy. These acoustic waves oscillate at a picosecond time-scale and result in spectral oscillations of the cavity transmission spectrum. We find that the oscillations are initiated at the metal mirrors and that their temporal dynamics match the elastic modes of the polymer layer, indicating that mechanical momentum is transferred within the structure. Such structures combine the strong optical absorption of metals with the elasticity and the processability of polymers, which open the road to a new class of optomechanical devices.

Towards GHz-THz cavity optomechanics in DBR-based semiconductor resonators

Resonators based on acoustic distributed Bragg reflectors (DBRs) were optimized to work in the GHz-THz regime, and grown by molecular beam epitaxy. We show that in structures made of GaAlAs alloys a simultaneous optimal confinement of light in the visible range and phonons in the tens of GHz range can be achieved. We report time resolved differential optical reflectivity experiments performed with fs-ps laser pulses. The experimental results are in excellent agreement with simulations based on standard transfer matrix methods. The resonant behavior of the photoelastic coefficient is discussed. The perfect optic-acoustic mode overlapping, added to a strongly enhanced coupling mechanism, implies that these DBR-based cavities could be the base of highly efficient optomechanical resonators.

Coherent control of GHz resonant modes by an integrated acoustic etalon

By carefully tuning the thickness of a compliant thin film placed within an acoustic cavity, we achieve coherent control of the cavity's acoustic resonances, analogous to the operation of an optical etalon. This technique is demonstrated using a supported membrane oscillator in which multiple high-frequency harmonic resonances are simultaneously optoexcited by an ultrafast laser. Theoretical and computational methods are used to analyze the selective strengthening or suppression of these resonances by constructive or destructive interference.