In particular, free-standing thin film bulk acoustic resonators (FBARs) have been widely adopted as the filter technology of choice for 5G bands. FBAR filters are composed of a thin film of a piezoelectric material sandwiched between electrodes in a capacitor geometry that is suspended over a cavity. The thickness of state-of-the-art FBAR filters needs to be reduced to meet the requirements posed by increasing telecom communication frequencies, since resonance frequencies are inversely proportional to thickness. However, scaling down current device geometries is challenging, both because of the complexity of manufacturing such ultrathin suspended heterostructures, but also since the piezoelectric performance [2,3] and breakdown voltage of polycrystalline ceramics diminishes. [4,5] Moreover, realizing uniform electrodes of nanometer-thickness with sufficiently high conductivity and low mass becomes increasingly difficult.Here, we investigate free-standing crystalline complex oxides as an alternative material platform that can mitigate some of the aforementioned drawbacks and enable resonant filters with improved performances. Single crystals are known to have larger dielectric breakdown voltages compared to their polycrystalline counterparts [6] while materials like BTO and PbZr x Ti 1−x O 3 (PZT) provide higher piezoelectric coefficients than commonly used AlN, and can thus handle higher voltages and power densities in the thin film form. Furthermore, single-crystal complex oxides in their ultrathin free-standing form are mechanically robust [7] withstanding large strains up to 8%, [8][9][10] are flexible enough to allow large curvatures [11] and have already been demonstrated as viable nanomechanical resonators. [12][13][14] Simultaneously, the electrodes, which also need to be scaled down, must be able to support high-GHz frequencies for 5G and 6G applications. In this regard, graphene is an ideal electrode material. Graphene conducts electricity down to the single atomic layer, [15] has ultra high mobilities, [16,17] is mechanically strong, [18,19] is able to withstand large strains [20] and has been demonstrated to support upto 300 GHz operating frequencies. [21] As a result of this, the use of graphene in various nano-electromechanical systems (NEMS) applications has been widely explored. [22][23][24][25][26][27][28][29] For Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of nextgeneration electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO 3 (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between...

Although 2D materials hold great potential for next-generation pressure sensors, recent studies revealed that gases permeate along the membrane-surface interface, necessitating additional sealing procedures. In this work, we demonstrate the use of free-standing complex oxides as self-sealing membranes that allow the reference cavity beneath to be sealed by a simple anneal. To test the hermeticity, we study the gas permeation time constants in nanomechanical resonators made from SrRuO 3 and SrTiO 3 membranes suspended over SiO 2 /Si cavities which show an improvement up to 4 orders of magnitude in the permeation time constant after annealing the devices. Similar devices fabricated on Si 3 N 4 /Si do not show such improvements, suggesting that the adhesion increase over SiO 2 is mediated by oxygen bonds that are formed at the SiO 2 /complex oxide interface during the self-sealing anneal. Picosecond ultrasonics measurements confirm the improvement in the adhesion by 70% after annealing.

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Conoscopic interferometry is a promising detection technique for ultrafast acoustics. By focusing a probe beam through a birefringent crystal before passing it through a polarizer, conoscopic interferences sculpt the spatial profile of the beam. The use of these patterns for acoustic wave detection revealed a higher detection sensitivity over existing techniques, such as reflectometry and beam distortion detection. However, the physical origin of the increased sensitivity is unknown. In this work, we present a model, describing the sensitivity behaviour of conoscopic interferometry with respect to the quarter-wave plate orientation and the diaphragm aperture, which is validated experimentally. Using the model, we optimize the detection sensitivity of conoscopic interferometry. We obtain a maximal sensitivity of detection when placing the diaphragm edge on the dark fringes of the conoscopic interference patterns. In the configurations studied in this work, conoscopic interferometry can be 8x more sensitive to acoustic waves than beam distortion detection.

During the process of measurement, quantum signals are amplified to human proportions. The question arises whether this amplification stage is responsible for the reduction of the state vector (also known as collapse of the wave function) during measurement. The cover image shows an artist’s impression of the experiment proposed by Tjerk H. Oosterkamp and co‐workers (article number http://doi.wiley.com/10.1002/pssb.202000567) to test this hypothesis. A single photon, indicated with sinusoids with a well‐determined amplitude, but unknown phase, is amplified within an interferometer. At some gain of the amplifiers the state might collapse unto a state with a well‐defined phase. The authors study how they can observe whether a collapse took place within the interferometer, or not.