Hannes Pfeifer

Nanomechanical resonators with exceptionally low dissipation are advancing mechanics-based sensors and quantum technologies. The key for these advances is the engineering of localized phononic modes that are well-isolated from the environment, i.e., that exhibit a high mechanical quality factor, Qm. Membrane phononic crystals fabricated from strained thin films can realize high-Qm single or multiple localized phononic defect modes at MHz frequencies. These defect modes can be efficiently interfaced with out-of-plane light or coupled to a microwave quantum circuit, enabling readout and control of their motion. When membrane phononic crystals are fabricated from a crystalline film, they could offer built-in functionality. We demonstrate a membrane phononic crystal realized in a strained 90 nm thin film of aluminum nitride (AlN), which is a crystalline piezoelectric material. We engineer a high-Qm localized phononic defect mode at 1.8 MHz with a Qm×fm-product of 1.5×1013 Hz at room temperature. In future devices, the built-in piezoelectricity of AlN can be utilized for direct coupling to qubits or in situ tuning of mechanical mode frequencies, defect mode couplings, or acoustic bandgaps, which can be used as building blocks of tunable phononic circuits or low-noise sensors.

Nanomechanical resonators with high quality factors (Q _m) enable mechanics-based quantum technologies, in particular quantum sensing and quantum transduction. High-Q _m nanomechanical resonators in the kHz to MHz frequency range can be realized in tensile-strained thin films that allow the use of dissipation dilution techniques to drastically increase Q _m. In our work, we study the material properties of tensile-strained piezoelectric films made from aluminium nitride (AlN). We characterize crystalline AlN films with a thickness ranging from 45 nm to 295 nm, which are directly grown on Si(111) by metal–organic vapour-phase epitaxy. We report on the crystal quality and surface roughness, the piezoelectric response, and the residual and released stress of the AlN thin films. Importantly, we determine the intrinsic quality factor of the films at room temperature in high vacuum. We fabricate and characterize AlN nanomechanical resonators that exploit dissipation dilution to enhance the intrinsic quality factor by utilizing the tensile strain in the film. We find that AlN nanomechanical resonators below 200 nm thickness exhibit the highest Q m × f m -product, on the order of 10¹² Hz. We discuss possible strategies to optimize the material growth that should lead to devices that reach even higher Q m × f m -products. This will pave the way for future advancements of optoelectromechanical quantum devices made from tensile-strained piezoelectric AlN.

High‐quality factor ( Q _m ) mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for high‐ Q _m mechanical resonators made from amorphous materials and, recently, from crystalline materials such as InGaP, SiC, and Si. A strained crystalline film exhibiting substantial piezoelectricity expands the capability of high‐ Q _m nanomechanical resonators to directly utilize electronic degrees of freedom. In this work, nanomechanical resonators with Q _m up to 2.9 × 10 ⁷ made from tensile‐strained 290 nm‐thick AlN are realized. AlN is an epitaxially‐grown crystalline material offering strong piezoelectricity. Nanomechanical resonators that exploit dissipation dilution and strain engineering to reach a Q _m × f _m ‐product approaching 10 ¹³ Hz at room temperature are demonstrated. A novel resonator geometry is realized, triangline, whose shape follows the Al–N bonds and offers a central pad patterned with a photonic crystal. This allows to reach an optical reflectivity above 80% for efficient coupling to out‐of‐plane light. The presented results pave the way for quantum optoelectromechanical devices at room temperature based on tensile‐strained AlN.

Integrated micro and nanophotonic optomechanical experiments enable the manipulation of mechanical resonators on the single phonon level. Interfacing these structures requires elaborate techniques limited in tunability, flexibility, and scaling towards multi-mode systems. Here, we demonstrate a cavity optomechanical experiment using 3D-laser-written polymer membranes inside fiber Fabry-Perot cavities. Vacuum coupling strengths of ~ 30 kHz to the fundamental megahertz mechanical mode are reached. We observe optomechanical spring tuning of the mechanical resonator by tens of kHz exceeding its linewidth at cryogenic temperatures. The extreme flexibility of the laser writing process allows for a direct integration of the membrane into the microscopic cavity. The direct fiber coupling, its scaling capabilities to coupled resonator systems, and the potential implementation of dissipation dilution structures and integration of electrodes make it a promising platform for fiber-tip integrated accelerometers, optomechanically tunable multi-mode mechanical systems, or directly fiber-coupled systems for microwave to optics conversion.

Integrated micro- and nanophotonic optomechanical experiments enable the manipulation of mechanical resonators on the single phonon level. Interfacing these structures requires elaborate techniques limited in tunability, flexibility, and scaling towards multi-mode systems. Here, we demonstrate a cavity optomechanical experiment using 3D-laser-written polymer membranes inside fiber Fabry-Perot cavities. Vacuum coupling rates of g₀/2π ≈ 30 kHz to the fundamental megahertz mechanical mode are reached. We observe optomechanical spring tuning of the mechanical resonator frequency by tens of kilohertz exceeding its linewidth at cryogenic temperatures. The direct fiber coupling, its scaling capabilities to coupled resonator systems, and the potential implementation of dissipation dilution structures and integration of electrodes make it a promising platform for fiber-tip integrated accelerometers, optomechanically tunable multi-mode mechanical systems, and directly fiber-coupled systems for microwave to optics conversion.

Increasing the interaction between light and mechanical resonators is an ongoing endeavor in the field of cavity optomechanics. Optical microcavities allow for boosting the interaction strength through their strong spatial confinement of the optical field. In this work, we follow this approach by realizing a sub-wavelength-long, free-space optomechanical microcavity on-chip fabricated from an (Al,Ga)As heterostructure. A suspended GaAs photonic crystal mirror is acting as a highly reflective mechanical resonator, which together with a distributed Bragg (DBR) reflector forms an optomechanical microcavity. We demonstrate precise control over the microcavity resonance by change of the photonic crystal parameters. We find that the microcavity mode can strongly couple to the transmissive modes of the DBR. The interplay between the microcavity mode and a guided resonance of the photonic crystal modifies the cavity response and results in a stronger dynamical backaction on the mechanical resonator compared to conventional optomechanical dynamics.

We develop a model of a quantum field confined within a cavity with a movable wall where the position of the wall is quantized. We obtain a full description of the dynamics of both the quantum field and the confining wall depending on the initial state of the whole system. Both the reaction and back-reaction of the field on the wall, and the wall on the field, can be taken into account, as well as external driving forces on both the cavity and the wall. The model exactly reproduces the resonant cavity mode stimulation due to the periodic motion of the mirror (dynamical Casimir effect), as well as the standard radiation pressure effects on the quantized wall (optomechanics). The model also accounts for the interplay of the two scenarios. Finally, the time evolution of the radiation force shows the interplay between the static and dynamical Casimir effect.

Light is a powerful tool for controlling mechanical motion, as shown by numerous applications in the field of cavity optomechanics. Recently, small scale optomechanical circuits, connecting a few optical and mechanical modes, have been demonstrated in an ongoing push towards multi-mode on-chip optomechanical systems. An ambitious goal driving this trend is to produce topologically protected phonon transport. Once realized, this will unlock the full toolbox of optomechanics for investigations of topological phononics. Here, we report the realization of topological phonon transport in an optomechanical device. Our experiment is based on an innovative multiscale optomechanical crystal design and allows for site-resolved measurements in an array of more than 800 cavities. The sensitivity inherent in our optomechanical read-out allowed us to detect thermal fluctuations traveling along topological edge channels. This represents a major step forward in an ongoing effort to downscale mechanical topological systems.