This work demonstrates the design, fabrication, and characterization of the first piezoelectric micromachined ultrasonic transducers (PMUTs) based on bilayer X-cut lithium niobate (LiNbO₃). A comparison of PMUT materials based on different figures of merit (FoMs) is presented, highlighting LiNbO₃ as a promising and well-balanced alternative to more conventional materials. To leverage its superior material properties, PMUTs were designed based on bilayer X-cut LiNbO₃ to fully harnesses the in-plane stress associated with the bending of the structure, thereby enhancing transduction. The fabricated devices show high electromechanical coupling (k_t²) of 4.6 %, albeit significantly lower than the simulated value due to parasitic effects. Mechanical vibration characterization shows a high static displacement of 0.88 nm/V and excellent linear dynamic range. Based on this design, an 8 x 1 array is demonstrated showing excellent consistency among the elements, with a frequency spread of 0.006 MHz and a displacement sensitivity spread of 0.15 nm/V. Our devices show comparable performance to monocrystalline PZT-based PMUTs, and substantially outperform ScAlN-based PMUTs in terms of static displacement sensitivity by a factor of 5. These results underscore the strong potential of LiNbO₃ for high-performance PMUTs.

This work demonstrates a linear PMUT array based on a bilayer X-cut lithium niobate (LiNbO3) structure. Leveraging LiNbO3's high piezoelectric coefficients and the bilayer's ability to harness the sign-changing in-plain strain associated to plate bending, the device achieves high transmit sensitivity (0.48 kPa/V at 10 mm), high current receive sensitivity (0.56 nA/Pa), high voltage receive sensitivity ($2.26 \mu ~\mathrm{V} / \text{Pa}$), and a low noise-equivalent pressure ($0.18 \text{mPa} / \sqrt{\text{Hz}}$). The array operates at 1 MHz with a fractional bandwidth of $82.5 \%(-6 ~\text{dB})$ and has an active area of $0.173 ~\text{mm}^{2}$. Figures of merit (FoMs) that capture the sensitivity-bandwidth-area trade-offs are reported and benchmarked with other PMUT technologies. The results position bilayer LiNbO3 as a promising route to compact, high-performance ultrasonic transducers.

We present an analysis of the main mechanisms of dissipation of resonant multilayer double-clamped microbeams in the frequency range 200 to 500 kHz. The devices consist of 2μm thick silicon nitride (E ≈ 160 GPa) beams covered with a polymer IP-Dip (E ≈ 4 GPa) layer fabricated by two-photon polymerization. A laser-Doppler vibrometer was used to measure the resonant vibrations and energy dissipation of the devices in high vacuum (< 0.05 Pa) at room temperature. The experimental findings were compared with theoretical and finite element method (FEM) results. The quality factor, dominated by the intrinsic dissipation in the IP-Dip layer, has proven to have a strong dependence on polymer thickness. On this basis, a viscous model for intrinsic dissipation in a polymer layer was formulated.

The emerging high-resolution 3D printing technique called two-photon polymerization (2PP) enables to print devices bottom-up rapidly, contrary to the top-down lithography-based fabrication methods. In this work, various polymer microbeams are 3D printed and their resonant characteristics are analyzed to understand the origin of damping. The 2PP printed polymer resonators have shown less damping than other polymer devices reported earlier, with tensile-stressed clamped-clamped beams reaching a record quality factor of 1819. The resonant energy loss was dominant by bulk friction damping. These results pave the path towards using 3D printed microresonators as mass sensors with improved design and fabrication flexibility.

Suspended drums made of 2D materials hold potential for sensing applications. However, the industrialization of these applications is hindered by significant device-to-device variations presumably caused by non-uniform stress distributions induced by the fabrication process. Here, we introduce a methodology to determine the stress distribution from their mechanical resonance frequencies and corresponding mode shapes as measured by a laser Doppler vibrometer (LDV). To avoid limitations posed by the optical resolution of the LDV, we leverage a manufacturing process to create ultra-large graphene drums with diameters of up to 1000 μm. We solve the inverse problem of a Föppl–von Kármán plate model by an iterative procedure to obtain the stress distribution within the drums from the experimental data. Our results show that the generally used uniform pre-tension assumption overestimates the pre-stress value, exceeding the averaged stress obtained by more than 47%. Moreover, it is found that the reconstructed stress distributions are bi-axial, which likely originates from the transfer process. The introduced methodology allows one to estimate the tension distribution in drum resonators from their mechanical response and thereby paves the way for linking the used fabrication processes to the resulting device performance.

The capacity to precisely pipette femtoliter volumes of liquid enables many applications, for example, to functionalize a nanoscale surface and manipulate fluids inside a single-cell. A pressure-controlled pipetting method is the most preferred, since it enables the widest range of working liquids. However, precisely controlling femtoliter volumes by pressure is challenging. In this work, a new concept is proposed that makes use of axisymmetrical phaseguides inside a microfluidic channel to pipette liquid in discrete steps of known volume. An analytical model for the design of the femtopipettes is developed and verified experimentally. Femtopipettes are fabricated using a multi-scale 3D printing strategy integrating a digital light processing printed part and a two-photon-polymerization printed part. Three different variants are designed and fabricated with pipetting resolutions of 10 picoliters, 180 femtoliters and 50 femtoliters. As a demonstration, controlled amounts of a water-glycerol mixture were first aspirated and then dispensed into a mineral oil droplet.

Resonant sensors hold great promise in measuring small masses, to enable future mass spectrometers, and small forces in applications like atomic and magnetic force microscopy. During the last decades, scaling down the size of resonators has led to huge enhancements in sensing resolution, but has also raised the question of what the ultimate limit is. Current knowledge suggests that this limit is reached when a resonator oscillates at the maximum amplitude for which its response is predominantly linear. We present experimental evidence that it is possible to obtain better resolutions by oscillation amplitudes beyond the onset of nonlinearities. An analytical model is developed that explains the observations and unravels the relation between ultimate sensing resolution and speed. In the high-speed limit, we find that the ultimate resolution of a resonator is improved when decreasing its damping. This conclusion contrasts with previous works, which proposed that lowering the damping does not affect or even harms the ultimate sensing resolution.

The use of nanoparticles has been growing in various industrial fields, and concerns about their effects on health and the environment have been increasing. Hence, characterization techniques for nanoparticles are essential. Here, we present a silicon dioxide microfabricated suspended microchannel resonator (SMR) to measure the mass and concentration of nanoparticles in a liquid as they flow. We measured the mass detection limits of the device using laser Doppler vibrometry. This limit reached a minimum of 377 ag that correspond to a 34 nm diameter gold nanoparticle or a 243 nm diameter polystyrene particle, when sampled every 30 ms. We compared the fundamental limits of the measured data with an ideal noiseless measurement of the SMR. Finally, we measured the buoyant mass of gold nanoparticles in real-time as they flowed through the SMR. [Figure not available: see fulltext.].

An RF oscillator has been demonstrated using a wideband SH₀ mode lithium niobate acoustic delay line (ADL). The design space of the ADL-based oscillators is theoretically investigated using the classical linear time-invariant (LTI) phase noise model. The analysis reveals that the key to low phase noise is low insertion loss (IL), large delay (τ_G), and high carrier frequency ( f_o). Two SH₀ ADL oscillators based on a single SH₀ ADL ( f_o = 157 MHz, IL = 3.2 dB, τ_G = 270ns) but with different loop amplifiers have been measured, showing low phase noise of -114 dBc/Hz and -127 dBc/Hz at 10-kHz offset with a carrier power level of -8 dBm and 0.5 dBm, respectively. These oscillators not only have surpassed other Lamb wave delay oscillators but also compete favorably with surface acoustic wave (SAW) delay line oscillators in performance. [2019-0223].

Resonant sensors determine a sensed parameter by measuring the resonance frequency of a resonator. For fast continuous sensing, it is desirable to operate resonant sensors in a closed-loop configuration, where a feedback loop ensures that the resonator is always actuated near its resonance frequency, so that the precision is maximized even in the presence of drifts or fluctuations of the resonance frequency. However, in a closed-loop configuration, the precision is not only determined by the resonator itself, but also by the feedback loop, even if the feedback circuit is noiseless. Therefore, to characterize the intrinsic precision of resonant sensors, the open-loop configuration is often employed. To link these measurements to the actual closed-loop performance of the resonator, it is desirable to have a relation that determines the closed-loop precision of the resonator from open-loop characterisation data. In this work, we present a methodology to estimate the closed-loop resonant sensor precision by relying only on an open-loop characterization of the resonator. The procedure is beneficial for fast performance estimation and benchmarking of resonant sensors, because it does not require actual closed-loop sensor operation, thus being independent on the particular implementation of the feedback loop. We validate the methodology experimentally by determining the closed-loop precision of a mechanical resonator from an open-loop measurement and comparing this to an actual closed-loop measurement.

In this work, thin-film lithium niobate (LiNbO₃) acoustic delay line (ADL) based oscillators are experimentally investigated for the first time for the application of single-mode oscillators and frequency comb generation. The design space for the ADL-based oscillator is first analyzed, illustrating that the key to low phase noise lies in high center frequency (fo), large delay (τ G), and low insertion loss (IL) of the delay. Therefore, two self-sustained oscillators employing low noise amplifiers (LNA) and a low IL, long delay (fo=157MHz, IL =2.9dB, τG= 200-440ns) SH₀ mode ADLs are designed for a case study. The two SH₀ ADL oscillators show measured phase noise of -109 dBc/Hz and -127 dBc/Hz at 10-kHz offset while consuming 16 mA and 48 mA supply currents, respectively. Although the carrier power of the proposed oscillator is lower than published state-of-the-art ADL oscillators, competitive phase noise performance is still attained thanks to the low IL. Finally, frequency comb generation is also demonstrated with the same delay line and a commercial RF feedback amplifier, showing a comb spacing of 3.4 MHz that matches the open-loop characterization.

This paper demonstrates low-loss acoustic delay lines (ADLs) based on shear-horizontal waves in thin-film LiNbO ₃ for the first time. Due to its high electromechanical coupling, the shear-horizontal mode is suited for producing devices with large bandwidths. Here, we show that shear-horizontal waves in LiNbO ₃ thin films are also excellent for implementing low-loss ADLs based on unidirectional transducers. The high acoustic reflections and large transducer unidirectionality induced by the mechanical loading of the electrodes on a LiNbO ₃ thin film provide a great tradeoff between delay line insertion loss and bandwidth. The directionality for two different types of unidirectional transducers has been characterized. Delay lines with variations in the key design parameters have been designed, fabricated, and measured. One of our fabricated devices has shown a group delay of 75 ns with an IL below 2 dB over a 3-dB bandwidth of 16 MHz centered at 160 MHz (fractional bandwidth = 10%). The measured insertion loss for other devices with longer delays and different numbers of transducer cells are analyzed, and the loss contributing factors and their possible mitigation are discussed.

This paper presents the design and experimental results of the first chip-scale radio frequency MEMS gyrator based on hybridizing Lorentz-force and piezoelectric transduction. The MEMS gyrator has a non-reciprocal phase response of 180° and can be used as the building blocks for synthesizing complex non-reciprocal networks. The equivalent circuit and measured performance of a fabricated MEMS gyrator are presented, both showing the anticipated 180° phase difference. The demonstration marks the first time that non-reciprocity is attained at radio frequencies with an entirely passive chip-scale mechanical device. Various challenges in achieving strong coupling and low insertion loss for the designed devices will be discussed.