Thin-film Resonators Research Articles

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators

Precise wavelength alignment of second-harmonic generation in thin-film lithium niobate resonators

6 GHz lamb wave acoustic filters based on A1-mode lithium niobate thin film resonators with checker-shaped electrodes

The first-order antisymmetric (A1) mode lamb wave resonator (LWR) based on Z-cut LiNbO3 thin films has attracted significant attention and is widely believed to be a candidate for next-generation reconfigurable filters with high frequency and large bandwidth (BW). However, it is challenging for traditional interdigitated electrodes (IDTs) based LWR filters to meet the requirement of a clean frequency spectrum response and enough out-of-band (OoB) rejection. To solve the problem, we propose LWRs with checker-shaped IDTs for the design of filters that meet the Wi-Fi 6E standard. By taking advantage of checker-shaped IDTs with unparalleled boundaries, the fabricated 6-GHz resonators successfully suppress higher-order A1 spurious modes, demonstrating a spurious-free impedance response and a high figure-of-merit (FOM) up to 104. Based on the demonstrated checker-shaped electrode design, the filter features a center frequency (f0) of more than 6 GHz, a 3 dB BW exceeding 620 MHz, and an excellent OoB rejection >25 dB, consistent with the acoustic-electric-electromagnetic (EM) multi-physics simulations. Furthermore, through the capacitance-inductance matching network technology, the filter’s voltage standing wave ratio (VSWR) is successfully reduced below 2, showing an excellent 50 Ω impedance matching. This study lays a foundation for ultra-high-frequency and ultra-wideband filters for the Wi-Fi 6/6E application.

Edge treatment for spurious mode suppression in thin-film lithium niobate resonators

Thin-film lithium niobate is an attractive material for RF acoustic devices because of its high electromechanical coupling. However, due to the large coupling and the high anisotropy, thin-film lithium niobate resonators are prone to accidental resonances called spurious modes. These modes compromise the frequency response of the resonators, limiting their use in filter and oscillator applications. In this work, we present a novel method of spurious mode suppression through a special edge treatment etch process. Two thin-film lithium niobate resonators were fabricated, one with smooth sidewalls and one with the edge treatment. It was found that the edge-treated resonators show a weaker spurious mode response. This is potentially a new way to mitigate spurious resonances, a major issue in lithium niobate Lamb wave devices.

Vibration Transfer in Spatially Separated Thin-Film Graphene Resonators

Vibration Transfer in Spatially Separated Thin-Film Graphene Resonators

Acoustically driven ferromagnetic resonance in YIG thin films

Acoustically driven ferromagnetic resonance (ADFMR) is a platform that enables efficient generation and detection of spin waves via magnetoelastic coupling with surface acoustic waves (SAWs). While previous studies successfully achieved ADFMR in ferromagnetic metals, there are only few reports on ADFMR in magnetic insulators such as yttrium iron garnet (Y3Fe5O12, YIG) despite more favorable spin wave properties, including low damping and long coherence length. The growth of high-quality YIG films for ADFMR devices is a major challenge due to poor lattice-matching and thermal degradation of the piezoelectric substrates during film crystallization. In this work, we demonstrate ADFMR of YIG thin films on LiNbO3 (LNO) substrates. We employed a SiOx buffer layer and rapid thermal annealing for crystallization of YIG films with minimal thermal degradation of LNO substrates. Optimized ADFMR device designs and time-gating measurements were used to enhance the ADFMR signal and overcome the intrinsically low magnetoelastic coupling of YIG. YIG films have a polycrystalline structure with an in-plane easy direction due to biaxial stresses induced during cooling after crystallization. The YIG device shows clear ADFMR patterns with maximum absorption for H ≈ 160 mT parallel to SAW propagation, which is consistent with our simulation results based on existing theoretical models. These results expand possibilities for developing efficient spin wave devices with magnetic insulators.

Characterization of parametrically amplified spin wave resonances in thin YIG films

We have experimentally shown that the perpendicular pumping geometry is optimal for parametric amplification of thermal magnons, which form Spin Wave Resonances (SWR) modes in in-plane magnetized Yttrium Iron Garnet (YIG) films. Above the parametric instability threshold, the parametric SWR spectrum exhibits a second order nonlinearity: the effect of modulational instability at Continuous Wave (CW) pumping. The parametric SWR modes possess a high quality factor, Qf = 2.0 × 106 that is two orders of magnitude more than the Q factor of the Ferromagnetic Resonance (FMR) in high quality YIG films.

Enhanced cation ordering, electron-spin-phonon interactions and Fano resonance in half-metallic Sr2FeMoO6 thin films

We report the presence of electron-spin-phonon interactions in half-metallic ferromagnetic Sr2FeMoO6 (SFMO) double perovskite thin films using temperature-dependent Raman spectroscopy. A series of SFMO thin films have been prepared on LaAlO3 (001) single-crystal substrate using the Pulsed Laser Deposition technique. These depositions have been made in different gas-conditions such as in vacuum, and under nitrogen and oxygen gas pressures. At room temperature, Raman spectra manifest a Fano feature which indicates the presence of electron-phonon coupling in the films. The electron-phonon coupling strength further changes with a change in deposition conditions. Magnetization results show that the SFMO film grown in vacuum has the highest saturation magnetization which suggests better cation ordering as compared to the other films. For enhanced understanding, Raman spectra were recorded at varied temperatures and the data were analyzed by theoretical model fittings. A parameter quantifying temperature-dependent anharmonic nature of phonons has been derived using Balkanski model fits. This parameter shows a drastic deviation in the vicinity of Curie temperatures, manifesting a spin-phonon coupling in SFMO films. We further show that the spin-phonon coupling strengthens with improved Fe–Mo ordering. Any experimental observation of spin-phonon coupling has not been reported for SFMO systems till date. The magnetization data corroborate well with these observations made by Raman measurements. Our results of Raman spectroscopy, magnetization and resistivity collectively suggest that the SFMO films exhibit electron-spin-phonon interactions, which are influenced by the cation ordering. We also devised out the method of relating the anharmonic nature of Raman modes with the degree of Fe–Mo ordering and spin-phonon coupling in double-perovskite materials.

Modeling and radiation performance analysis of acoustically actuated magnetoelectric antennas

Bulk acoustic wave-mediated magnetoelectric (ME) antenna is the most promising acoustically actuated ultra-compact antenna scheme at ultrahigh frequency (0.3–3 GHz). However, the relevant research is still exploratory, and the antenna’s multi-field coupling modeling and performance analysis are imperfect. This work presents a cellular model based on finite element simulation to analyze the radiation characteristics of the ME antenna integrated on a thin-film bulk wave resonator (FBAR). Compared with the finite difference time domain (FDTD) method, the cellular model has the advantages of simplicity and high universality. The effects of mechanical and dielectric losses of piezoelectric (PE) material and mechanical and eddy current losses of magnetostrictive (MS) material on radiation efficiency are analyzed through the intensive research of unit cells of four different material combinations. Results show that the mechanical quality factors and piezomagnetic stress constant are the key parameters to determine the radiation efficiency. Besides conductivity and permeability, eddy current loss is also closely related to the thickness of the MS layer. This study provides a theoretical foundation for enhancing the radiation performance of ME FBAR antennas and identifying areas for further development.

Asymmetrical Lamb wave mode resonant infrared detector based on lithium niobate thin film

This work explores the infrared (IR) detection capabilities of 750 nm thick Z-cut lithium niobate (LN) thin film resonator at asymmetric modes of various orders: the first, third, fifth, and seventh order asymmetric (A1, A3, A5, and A7), with resonant frequencies of 2.35, 6.68, 11.09, and 15.49 GHz. Under the infrared radiation of 0.575 mW, the responses about admittance minima and frequency drift corresponding to these modes have been experimentally validated. Notably, the A1 mode exhibits the most significant admittance dip response with 2.62 dB, while the A7 mode demonstrates a maximum frequency drift response of 670 kHz. The optimal parameter for noise equivalent power reaches 15.41 pW/Hz1/2, and the optimal detectivity achieves 7.40 × 106 m Hz1/2/W. These findings indicate the immense potential of LN thin-film resonators for infrared sensing applications.

Photostrictive Actuators Based on Freestanding Ferroelectric Membranes.

Complex oxides offer a wide range of functional properties, and recent advances in the fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, photoactuation is demonstrated in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate (SrTiO3). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent holographic interferometry provide combined evidence for the photostrictive strain in BaTiO3 originating from a partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated bylight.

Tuning the plasmonic resonance in TiN refractory metal

Plasmonic coatings can absorb electromagnetic radiation from visible to far-infrared spectrum for the better performance of solar panels and energy saving smart windows. For these applications, it is important for these coatings to be as thin as possible and grown at lower temperatures on arbitrary substrates like glass, silicon, or flexible polymers. Here, we tune and investigate the plasmonic resonance of titanium nitride thin films in lower thicknesses regime varying from ~ 20 to 60 nm. High-quality crystalline thin films of route-mean-square roughness less than ~ 0.5 nm were grown on a glass substrate at temperature of ~ 200 °C with bias voltage of − 60 V using cathodic vacuum arc deposition. A local surface-enhanced-plasmonic-resonance was observed between 400 and 500 nm, which further shows a blueshift in plasmonic frequency in thicker films due to the increase in the carrier mobility. These results were combined with finite-difference-time-domain numerical analysis to understand the role of thicknesses and stoichiometry on the broadening of electromagnetic absorption.

Widely-tunable, multi-band Raman laser based on dispersion-managed thin-film lithium niobate microring resonators

Stimulated Raman scattering is an attractive way to extend the operation spectral range of optical sources. However, the spectral extension range of a tunable Raman laser is limited by the Raman frequency shift and pump tuning bandwidth. This makes it challenging to realize chip-scale, widely tunable Raman lasers, as on-chip lasers only provide limited pump power and tuning bandwidth. Here, we tackle this by dispersion engineering of a thin-film lithium niobate microring resonator, where its high-quality factor ( ~ 2.5 million) ensures a sub-milli-watt (0.8 mW) threshold for Raman lasing while its strong normal dispersion with suppressed avoided mode crossing restrains the competing Kerr comb generation process. Combining the multi-wavelength Raman gain response of lithium niobate and cascaded Raman lasing, we demonstrate a widely tunable Raman laser covering 1592–1955 nm, showing a 335-nm spectral extension range from a 94-nm-tuning-bandwidth pump laser. Our demonstration paves the way to realize chip-scale, widely-tunable Raman lasers.

Development of Temperature Sensor Based on AlN/ScAlN SAW Resonators

Temperature monitoring in extreme environments presents new challenges for MEMS sensors. Since aluminum nitride (AlN)/scandium aluminum nitride (ScAlN)-based surface acoustic wave (SAW) devices have a high Q-value, good temperature drift characteristics, and the ability to be compatible with CMOS, they have become some of the preferred devices for wireless passive temperature measurement. This paper presents the development of AlN/ScAlN SAW-based temperature sensors. Three methods were used to characterize the temperature characteristics of a thin-film SAW resonator, including direct measurement by GSG probe station, and indirect measurement by oscillation circuit and antenna. The temperature characteristics of the three methods in the range of 30–100 °C were studied. The experimental results show that the sensitivities obtained with the three schemes were −28.9 ppm/K, −33.6 ppm/K, and −29.3 ppm/K. The temperature sensor using the direct measurement method had the best linearity, with a value of 0.0019%, and highest accuracy at ±0.70 °C. Although there were differences in performance, the characteristics of the three SAW temperature sensors make them suitable for sensing in various complex environments.

Full-Control and Switching of Optical Fano Resonance by Continuum State Engineering.

Fano resonance, known for its unique asymmetric line shape, has gained significant attention in photonics, particularly in sensing applications. However, it remains difficult to achieve controllable Fano parameters with a simple geometric structure. Here, a novel approach of using a thin-film optical Fano resonator with a porous layer to generate entire spectral shapes from quasi-Lorentzian to Lorentzian to Fano is proposed and experimentally demonstrated. The glancing angle deposition technique is utilized to create a polarization-dependent Fano resonator. By altering the linear polarization between s- and p-polarization, a switchable Fano device between quasi-Lorentz state and negative Fano state is demonstrated. This change in spectral shape is advantageous for detecting materials with a low-refractive index. A bio-particle sensing experiment is conducted that demonstrates an enhanced signal-to-noise ratio and prediction accuracy. Finally, the challenge of optimizing the film-based Fano resonator due to intricate interplay among numerous parameters, including layer thicknesses, porosity, and materials selection, is addressed. The inverse design tool is developed based on a multilayer perceptron model that allows fast computation for all ranges of Fano parameters. The method provides improved accuracy of the mean validation factor (MVF = 0.07, q-q') compared to theconventional exhaustive enumeration method (MVF = 0.37).

Identification and mitigation of conducting package losses for quantum superconducting devices

Low-loss superconducting rf devices are required when used for quantum computation. Here, we present a series of measurements and simulations showing that conducting losses in the packaging of our superconducting resonator devices affect the maximum achievable internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, different volumes of the stored electromagnetic energy were sampled, thus affecting Qi. When the backside of the sapphire substrate of the resonator device is adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi is found as the electromagnetic sampling volume is increased. This is a result of induced currents in large surface resistance regions and dissipation underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, we decrease the Ohmic losses and increase the maximum Qi for the larger size resonators.

Magnetic field driven magnetic domains and ferromagnetic resonances in multilayer thin films Ta/FeGaB/Ta for microwave application

Magnetic field driven magnetic domains and ferromagnetic resonances in multilayer thin films Ta/FeGaB/Ta for microwave application

Radiative volume plasmon and phonon-polariton resonances in TiN-based plasmonic/polar-dielectric hyperbolic optical metamaterials

Ferrell and Berreman modes are absorption resonances in thin metal films and polar-dielectric media that arise from radiative bulk plasmon-polariton and phonon-polariton excitations. Compared to surface polaritons, Ferrell and Berreman modes occur due to volume charge oscillations across the medium and provide a unique pathway for light–matter interactions. Though the resonances are studied individually, stringent polarization and material requirements have prevented their observation in one host medium. Here, we show simultaneous excitation of Ferrell and Berreman absorption resonances in refractory epitaxial TiN/Al0.72Sc0.28N plasmonic metal/polar-dielectric hyperbolic metamaterials in the visible and far-infrared spectral ranges. The nanoscale periodicity of the superlattices enables the coupling of bulk plasmons (and longitudinal optical phonons) across different TiN (and Al0.72Sc0.28N) layers and allows polarization matching with free-space light that results in Ferrell (and Berreman) mode excitations. Ferrell and Berreman absorption resonances can be used for strong light confinement in radiative cooling, thermophotovoltaics, and other dual-band applications.

Thermally induced all-optical ferromagnetic resonance in thin YIG films

All-optical ferromagnetic resonance (AO-FMR) is a powerful tool for the local detection of micromagnetic parameters, such as magnetic anisotropy, Gilbert damping or spin stiffness. In this work we demonstrate that the AO-FMR method can be used in thin films of yttrium iron garnet (YIG) if a metallic capping layer (Au, Pt) is deposited on top of the film. Magnetization precession is triggered by heating of the metallic layer with femtosecond laser pulses. The heat pulse modifies the magneto-crystalline anisotropy of the YIG film and shifts the quasi-equilibrium orientation of the magnetization, which results in precessional magnetization dynamics. The laser-induced magnetization precession corresponds to a uniform (Kittel) magnon mode, with the precession frequency determined by the magnetic anisotropy of the material as well as the external magnetic field, and the damping time set by a Gilbert damping parameter. The AO-FMR method thus enables measuring local magnetic properties, with a resolution given by the laser spot size.

Fabrication of a New Air-Gap FBAR on an Organic Sacrificial Layer through an Innovative Design Algorithm

Realizing thin-film acoustic wave resonators presents many design and fabrication challenges. Actual material specifications always differ from nominal material properties employed in simulations, as they depend on the deposition technique and parameters used and on equipment type and status. Moreover, each deposition process introduces a degree of uncertainty regarding the thicknesses of the layers. All these factors have a substantial impact on the resonance frequency, which often differs from the designed value. This work details the design and fabrication of an aluminum nitride (AlN)-based thin-Film Bulk Acoustic wave Resonator (FBAR) showing one of the highest products of Q-factor and electromechanical coupling of 6895. The design process is based on an innovative, fast, and scalable design and fabrication approach that considers fabrication tolerances. The algorithm returns very fast results on the order of seconds, and successfully estimates the resonance of a designed stack at 2.55 GHz with a very low error of 0.005 GHz (about 0.2%). The FBAR layer stack is suspended on a polymeric membrane and an innovative rapid dissolving sacrificial layer made of Lift-Off Resist (LOR). This new fabrication protocol obtains resonators with an electromechanical coupling factor of 4.7% and a maximum quality factor of 1467, respectively.

A Non-Perturbative, Low-Noise Surface Coating for Sensitive Force-Gradient Detection of Electron Spin Resonance in Thin Films.

The sensitivity of magnetic resonance force microscopy (MRFM) is limited by surface noise. Coating a thin-film polymer sample with metal has been shown to decrease, by orders of magnitude, sample-related force noise and frequency noise in MRFM experiments. Using both MRFM and inductively detected measurements of electron-spin resonance, we show that thermally evaporating a 12 nm gold layer on a 40 nm nitroxide-doped polystyrene film inactivates the nitroxide spin labels to a depth of 20 nm, making single-spin measurements difficult or impossible. We introduce a "laminated sample" protocol in which the gold layer is first evaporated on a sacrificial polymer. The sample is deposited on the room-temperature gold layer, removed using solvent lift-off, and placed manually on a coplanar waveguide. Electron spin resonance (ESR) of such a laminated sample was detected via MRFM at cryogenic temperatures using a high-compliance cantilever with an integrated 100-nm-scale cobalt tip. A 20-fold increase of spin signal was observed relative to a thin-film sample prepared instead with an evaporated metal coating. The observed signal is still somewhat smaller than expected, and we discuss possible remaining sources of signal loss.