3D imaging of subsurface structures at the nanoscale is a longstanding challenge in microscopy. Ultrasound enables subsurface imaging, but nanometer depth resolution requires high-frequency sound waves, achieved by heating with pulsed lasers. Combined with Atomic Force Microscopy (AFM), which offers atomic-scale surface resolution a promising candidate for full 3D nanoscale imaging emerges.

This thesis focuses on the development of such a combined ultrasound-AFM system.
Novel optical detection techniques are introduced for both the ultrasound and the AFM, enabling simultaneous operation.

Acoustic waves up to 100 GHz are generated and detected, used to characterize the thickness and adhesion of thin films. We demonstrate that the conical AFM tip acts as an acoustic lens, focusing the wave into the tip apex. When in contact with a sample, changes in the reflection signal confirm acoustic transmission—opening the door to true 3D nanoscale imaging.

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 behavior 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 18 dB more sensitive to acoustic waves than beam distortion detection.

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 SrRuO3 and SrTiO3 membranes suspended over SiO2/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 Si3N4/Si do not show such improvements, suggesting that the adhesion increase over SiO2 is mediated by oxygen bonds that are formed at the SiO2/complex oxide interface during the self-sealing anneal. Picosecond ultrasonics measurements confirm the improvement in the adhesion by 70% after annealing.

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation 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, BaTiO₃ (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 the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.