Miniature Resonators Research Articles

Critical coupling in cavity-resonant integrated-grating filters (CRIGFs).

We experimentally demonstrate critical coupling in miniature grating-coupled resonators known as cavity-resonant integrated-grating filters (CRIGFs). Using previously proposed asymmetric grating coupler designs for non-linear CRIGFs, and introducing a dedicated variant of a coupled-modes theory model to estimate physical properties out of the measured reflection and transmission characteristics of these resonators, we demonstrate fine control over the in-and out-coupling rate to the resonator while keeping constant both the internal losses and the resonant wavelength. Furthermore, the critical coupling condition is also observed to coincide with the maximum enhancement of the second harmonic generation signal.

Miniature magneto-mechanical resonators for wireless tracking and sensing.

Sensor miniaturization enables applications such as minimally invasive medical procedures or patient monitoring by providing process feedback in situ. Ideally, miniature sensors should be wireless, inexpensive, and allow for remote detection over sufficient distance by an affordable detection system. We analyze the signal strength of wireless sensors theoretically and derive a simple design of high-signal resonant magneto-mechanical sensors featuring volumes below 1 cubic millimeter. As examples, we demonstrate real-time tracking of position and attitude of a flying bee, navigation of a biopsy needle, tracking of a free-flowing marker, and sensing of pressure and temperature, all in unshielded environments. The achieved sensor size, measurement accuracy, and workspace of ~25 centimeters show the potential for a low-cost wireless tracking and sensing platform for medical and nonmedical applications.

Fluid–structure model for disks vibrating at ultra-high frequency in a compressible viscous fluid

Radial mechanical modes of miniature disk-shaped resonators are promising candidates for probing the ultra-high-frequency rheological properties of liquids. However, the lack of an analytical fluid–structure model hinders the inference of liquid properties from the experimental measurement of such radial vibrations. Here, we develop analytical models for the case of a disk vibrating in a compressible viscous liquid. Closed-form expressions for the mechanical quality factor and resonant frequency shift upon immersion are obtained and compared with the results of numerical modeling for a few significant cases. At frequencies above 1 GHz, our model points out the significance of compressibility effects.

Pulsed Electron-Nuclear Double Resonance in the Fourier Regime.

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level molecular structural information. However, in molecules containing unpaired electron spins, NMR signals are difficult to measure directly. In such cases, data is obtained using the electron-nuclear double resonance (ENDOR) method, where nuclei are detected through their interaction with nearby unpaired electron spins. Unfortunately, electron spins spread the ENDOR signals, which challenges current acquisition techniques, often resulting in low spectral resolution that provides limited structural details. Here, we show that by using miniature microwave resonators to detect a small number of electron spins, integrated with miniature NMR coils, one can excite and detect a wide bandwidth of ENDOR data in a single pulse. This facilitates the measurement of ENDOR spectra with narrow lines spread over a large frequency range at much better spectral resolution than conventional approaches, which helps reveal details of the paramagnetic molecules' chemical structure that were not accessible before.

Surface Passivation of III-V GaAs Nanopillars by Low-Frequency Plasma Deposition of Silicon Nitride for Active Nanophotonic Devices.

Numerous efforts have been devoted to improve the electronic and optical properties of III-V compound materials via reduction of their nonradiative states, aiming at highly efficient III-V sub-micrometer active devices and circuits. Despite many advances, the poor reproducibility and short-term passivation effect of chemical treatments, such as sulfidation and nitridation, requires the use of protective encapsulation methods, not only to protect the surface, but also to provide electrical isolation for device manufacturing. There is still a controversial debate on which combination of chemical treatment and capping dielectric layer can best reproducibly protect the crystal surface of III-V materials while being compatible with readily available semiconductor-foundry plasma deposition methods. This work reports on a systematic experimental study on the role of sulfide ammonium chemical treatment followed by dielectric coating (either silicon oxide or nitride) in the passivation effect of GaAs/AlGaAs nanopillars. Our results conclusively show that, under ambient conditions, the best surface passivation is achieved using ammonium sulfide followed by encapsulation with a thin layer of silicon nitride by low-frequency plasma-enhanced chemical deposition. Here, the sulfurized GaAs surfaces, high level of hydrogen ions, and low-frequency (380 kHz) excitation plasma that enable intense bombardment of hydrogen, all seem to provide a combined active role in the passivation mechanism of the pillars by reducing the surface states. As a result, we observe up to a 29-fold increase of the photoluminescence (PL) integrated intensity for the best samples as compared to untreated nanopillars. X-ray photoelectron spectroscopy analysis confirms the best treatments show remarkable removal of gallium and arsenic native oxides. Time-resolved micro-PL measurements display nanosecond lifetimes resulting in a record-low surface recombination velocity of ∼1.1 × 104 cm s-1 for dry-etched GaAs nanopillars. We achieve robust, stable, and long-term passivated nanopillar surfaces, which creates expectations for remarkable high internal quantum efficiency (IQE > 0.5) in nanoscale light-emitting diodes. The enhanced performance paves the way to many other nanostructures and devices such as miniature resonators, lasers, photodetectors, and solar cells, opening remarkable prospects for GaAs active nanophotonic devices.

Electron spin resonance microfluidics with subnanoliter liquid samples

Microfluidics is a well-established technique to synthesize, process, and analyze small amounts of materials for chemical, biological, medical, and environmental applications. Typically, it involves the use of reagents with a volume smaller than ~ 1 micro-l—ideally even nano- or picoliters. When the sample of interest contains paramagnetic species, it can in principle be quantified and analyzed by electron spin resonance (ESR) spectroscopy. However, conventional ESR is typically carried out with a sample volume of ~ 1 ml, thereby making it incompatible with most microfluidics applications. Here we show that by using a new class of miniature surface resonators combined with photolithography to prepare microfluidic patterns, ESR can be applied to measure small liquid samples, down to picoliter volumes, without considerably sacrificing concentration sensitivity. Our experiments, carried out with resonators whose mode volumes range from ~1 to 3.6 nL, showed that with a sample volume of ~0.25 nL good signals could be obtained from solutions with spin concentrations of less than 0.1 μM. The advantage of using microfluidics ESR is evident in our work, not only because it facilitates the use of a very small sample volume, but also because it makes it possible to apply huge ( ~ 1000 T/m) and fast ( ~ 1 μs) pulsed magnetic field gradients to the sample. This is a key capability to measuring unique properties such as nanoscale real-space diffusion and quantum spin diffusion. All our experiments are performed at room temperature, making our technique compatible with future microfluidics applications that might employ a complete system of compact resonators, microfluidic chips, miniature magnets, and a compact ESR-on-a-chip spectrometer. This could result in a completely new approach to processing and measuring paramagnetic liquid samples for use in a variety of chemical, biological, medical, and environmental applications.

Advanced surface resonators for electron spin resonance of single microcrystals.

Electron spin resonance (ESR) spectroscopy of paramagnetic species in single crystals is a powerful tool for characterizing the latter's magnetic interaction parameters in detail. Conventional ESR systems are optimized for millimeter-size samples and make use of cavities and resonators that accommodate tubes and capillaries in the range 1-5 mm. Unfortunately, in the case of many interesting materials such as enzymes and inorganic catalytic materials (e.g., zeolites), single crystals can only be obtained in micron-scale sizes (1-200 µm). To boost ESR sensitivity and to enable experiments on microcrystals, the ESR resonator needs to be adapted to the size and shape of these specific samples. Here, we present a unique family of miniature surface resonators, known as "ParPar" resonators, whose mode volume and shape are optimized for such micron-scale single crystals. This approach significantly improves upon the samples' filling factor and thus enables the measurement of much smaller crystals than was previously possible. We present here the design of such resonators with a typical mode dimension of 20-50 µm, as well as details about their fabrication and testing methods. The devices' resonant mode(s) are characterized by ESR microimaging and compared to the theoretical calculations. Moreover, experimental ESR spectra of single microcrystals with typical sizes of ∼25-50 µm are presented. The measured spin sensitivity for the 50-µm resonator at cryogenic temperatures of 50 K is found to be ∼1.8 × 106 spins/G √Hz for a Cu-doped single crystal sample that is representative of many biological samples of relevance.

Opto-mechanical oscillator in a nanoliter droplet.

Droplets are very simple physical systems, whereby surface tension shapes liquids into ideal opto-mechanical devices. This has recently enabled low-viscosity liquid samples to serve as miniature acoustic resonators harnessing optical generation of bulk vibrations, capillaries, or surface waves. Uniquely, a simple room-temperature pendant droplet can be activated as a hypersound-laser emitter when illuminated by a free-space, low-power visible laser thanks to stimulated Brillouin scattering of optical and acoustic whispering-gallery modes. Here, we demonstrate continuous operation of a liquid polymer opto-mechanical resonator and characterize its quality factor and long-term frequency stability. Our results point to the feasibility of all-liquid micro-mechanical oscillators working in the 50-100MHz range. The stimulated generation of high-quality surface waves on nanoliter droplets gives momentum to new optical schemes for characterization of material viscous-elastic properties, laboratory investigation of atmospheric phenomena, and mass sensing for direct analysis of biological fluids based on ultrasound-hypersound coherent generation and detection.

Miniature hemispherical shell resonator with large-scale effective electrodes based on piezoelectric drive mechanism

Miniature resonators with three-dimensional curved surface are mostly driven by electrostatic capacitive. However, it is quite difficult to fabricate a curved surface electrostatic resonator with large-scale effective electrodes. This paper presents the first miniature hemispherical shell resonator with large-scale effective electrodes based on piezoelectric drive mechanism. The vibrating body and electrodes of the piezoelectric resonator are easily integrated without micro-scale or nano-scale narrow capacitive gap. Vibration experiment and finite element analysis both reveal that there exist seven significant vibration modes between 10 kHz and 100 kHz. Mode shape validation is also carried out by measuring the vibration velocity of upper perimeter and lateral perimeter with laser doppler vibrometer. Special vibration characteristics of each vibration mode are described in detail, based on which the resonator may be used for many specific applications. Compared with common electrostatic resonators, even smaller drive voltage applied to the piezoelectric resonator may produce larger vibration displacement at atmosphere. According to the experiment results, the resonator may provide a new way of realizing high performance three-dimensional miniature devices for communication and inertial navigation applications.

Investigating the Impact of Microwave Breakdown on the Responses of High-Power Microwave Metamaterials

We investigate the effect of microwave-induced breakdown on the frequency responses of a class of metamaterials composed of planar sub-wavelength periodic structures. When breakdown occurs in such a structure, its frequency response changes based on the nature of the plasma created within its unit cell. We examine how the frequency responses of such periodic structures change as a result of creation of microwave-induced discharges within their unit cells. To do this, we examine single-layer metasurfaces composed of miniature LC resonators arranged in a 2-D periodic lattice. These metasurfaces are engineered to be opaque at microwave frequencies when operated at low power levels but can be made transparent if a localized discharge is created within the LC resonators. By measuring their transmission and reflection coefficients under high-power excitation in different conditions, the impact of breakdown on the frequency responses of these devices is determined. Several prototypes of such structures are examined both theoretically and experimentally. It is demonstrated that when breakdown occurs in air and at atmospheric pressure levels, the responses of such periodic structures can be predicted with a reasonable degree of accuracy. Additionally, when the unit cell of the metasurface is composed of two different resonators, breakdown is always observed to occur in both resonators despite their different topologies and local field enhancement factors. In such structures, the discharge in one resonator appears to be mediated by the one in the other.

Cantilever resonator based on the electrostriction effect in Gd-doped ceria

Thin films of Ce0.8Gd0.2O1.9, which have recently been shown to demonstrate giant electrostriction, are proposed as the active material for miniature cantilever resonators and actuators. In the absence of strain, these films have an electrostriction coefficient within the range of 2–10kPa/(kV/cm)2, as compared with the somewhat larger values for the best commercial electrostrictors (e.g. PbMn1/3Nb2/3O3, 62kPa/(kV/cm)2. At the same time, Ce0.8Gd0.2O1.9 films can generate stress >70MPa which is competitive with materials currently in use and only limited by the strength of the film-substrate interface. In this report, we investigate two aspects of the practical application of Ce0.8Gd0.2O1.9 as a resonator: the fabrication conditions and the frequency dependence of the electrostrictive behavior. We show that the films can display electromechanical response with frequencies up to 6kHz. With respect to fabrication, we show that Ce0.8Gd0.2O1.9 films have a number of technological advantages when compared to the lead titanate based materials currently in use: (a) they can be deposited on a variety of metal contacts and substrates, including silicon; (b) they do not require high temperature processing; and (c) because Ce0.8Gd0.2O1.9 has cubic symmetry, it can in principle be used as a polycrystalline film with arbitrary texture and does not require poling. In addition, neither Ce nor Gd nor their oxides are toxic; the oxides have very low vapor pressure; and the cations, being highly charged do not diffuse into Si. Consequently, Ce0.8Gd0.2O1.9 films may be readily and advantageously integrated into existing semiconductor fabrication technologies.

Ultra miniature resonators for electron spin resonance: Sensitivity analysis, design and construction methods, and potential applications

This paper describes a recently developed new family of miniature surface resonators, used for electron spin resonance spectroscopy and imaging. The first part of the paper provides a detailed description of the operational principles of the surface resonators. It also includes sensitivity analysis for a variety of configurations with inner dimensions ranging from 150 μm down to 2 μm, operating at the Ku, Q, and W frequency bands. Most of the data presented here is based on theoretical predictions; however, some of it is accompanied by experiential results for verification. The second part of the paper describes a new type of double-surface microresonator and its production method. This new configuration enables an efficient coupling of the microwave energy from millimetre-sized microstrip lines to micron structures even at relatively low frequencies. The resonator is analysed both theoretically and experimentally – exhibiting ultra-high spin sensitivity. The conclusion of the two parts of the paper is that micron-scale surface microresonators may achieve spin sensitivity of a few thousands of spins in one second of acquisition time for special samples, such as phosphorous-doped 28Si, at cryogenic temperatures. However, further miniaturization below 1–2 microns does not seem to be beneficial, sensitivity-wise. In addition to their high spin sensitivity, these resonators have a huge conversion factor, reaching in some cases to more than 500–1000 G of microwave magnetic field with input power of 1 W. Some possible applications of these unique capabilities are also described herein.

Replacement of Au and Ag with the Ni-P and Ni-B Alloys in Electronics

A new method of production of precise piezoelectric quartz resonators and filters, and monolithic piezoquartz filters with electrodes, made of electroless nickel-phosphorous alloy, for spacecraft, hydroacoustics and communication devices was developed. Electrical characteristics of quartz resonators with the deposited Ni-P coating are better than when gold and silver were used as electrode layers. A method of electroless deposition on polished quartz, glass and other nonmetallic polished materials was developed. Bulk and miniature resonators of piezoquartz and lithium niobate manufactured on the basis of the developed technology of electroless deposition of a nickel-phosphorus alloy, instead of gold and silver plating, were successfully used in industry. The technology was developed for the production of piezoceramic and ceramic devices by electroless deposition of electrode layers made of Ni-P or Cu.

Surface Adsorbate Fluctuations and Noise in Nanoelectromechanical Systems

Physisorption on solid surfaces is important in both fundamental studies and technology. Adsorbates can also be critical for the performance of miniature electromechanical resonators and sensors. Advances in resonant nanoelectromechanical systems (NEMS), particularly mass sensitivity attaining the single-molecule level, make it possible to probe surface physics in a new regime, where a small number of adatoms cause a detectable frequency shift in a high quality factor (Q) NEMS resonator, and adsorbate fluctuations result in resonance frequency noise. Here we report measurements and analysis of the kinetics and fluctuations of physisorbed xenon (Xe) atoms on a high-Q NEMS resonator vibrating at 190.5 MHz. The measured adsorption spectrum and frequency noise, combined with analytic modeling of surface diffusion and adsorption-desorption processes, suggest that diffusion dominates the observed excess noise. This study also reveals new power laws of frequency noise induced by diffusion, which could be important in other low-dimensional nanoscale systems.

Miniature density–viscosity measurement cell utilizing electrodynamic-acoustic resonator sensors

Miniaturized physical sensors for precise density and viscosity measurement are required as supplement or replacement for complex and expensive laboratory instruments. Utilizing miniature mechanical resonators as transducers, one can greatly reduce the necessary liquid volume, measurement time, and complexity of the experimental setup. In this contribution, we describe the development of a miniature, modular measurement setup for a large viscosity range. We devised and fabricated an inexpensive flow-through cell using milling technology of multiple PCB layers and Parafilm or Teflon sealing for easy assembly and disassembly. The mechanical resonator is designed as a suspended plate using electrodynamic Lorentz force excitation and movement induction detection. Quality factor and frequency shift of the resonator are evaluated and related to the density–viscosity product of different test liquids at multiple resonant modes of vibration at low operating frequencies for different temperatures and over a wide range of different viscosities.

Miniature flow-through resonator cell for density and viscosity sensing

Miniaturized physical sensors for precise density and viscosity analysis are required as supplement or replacement for complex and expensive laboratory instruments. Utilizing miniature mechanical resonators as transducers, one can greatly reduce the necessary liquid volume, measurement time, and complexity of the experimental setup. In this contribution, we describe the development of a miniature, modular measurement setup for a large viscosity range. We devised and fabricated an inexpensive flow-through cell using milling technology of multiple PCB layers and Parafilm sealing for easy assembly and disassembly. The mechanical resonator is designed as a suspended plate using Lorentz force excitation and movement induction detection. The Qfactor of the resonator is evaluated and related to the density-viscosity product of different test liquids at multiple resonant modes of vibration at low operating frequencies for over a wide range of different viscosities.

Realization of Fabry-Perot resonators for hard X rays using micro- and nanotechnology

An X-ray Fabry-Perot resonator has been proposed and investigated for more than three decades. The difficulty in realizing such a resonator is mainly due to the strict requirement on coherence for cavity resonance, which is governed by the energy resolution of the incident X-ray photons and the size of the resonator. With the aid of modern micro-and nanotechnology for silicon, such as X-ray lithography, miniature resonators, approximately a few micrometers or smaller, can be prepared. Together with X-ray photons of ultrahigh-energy resolution from synchrotron sources, the required conditions on coherence for cavity resonance are fulfilled. Several X-ray cavities prepared by this means are used in X-ray back-diffraction experiments, where back diffraction acts as mirror reflection for X rays. Resonance interference fringes are observed unambiguously. Attempts to use multilayered materials for cavity fabrication are discussed in relation to X-ray synchrotron experiment. Possible future applications of this X-ray resonator are also mentioned.

Ratio for polycrystalline silicon used in disk-shaped microresonators

Integrated circuit technology has been used to fabricate miniature disk resonators of polycrystalline silicon that operate at frequencies above 100 MHz. The ratios of low-order resonant frequencies in these resonators can be used to determine the value of Poisson's ratio and to confirm assumptions regarding homogeneity and isotropy.

Detection of proteins and intact microorganisms using microfabricated flexural plate silicon resonator arrays

We are developing biosensor arrays that are based on microfabricated silicon flexural plate wave (FPW) resonators coated with molecular recognition chemistry. The resonators within the micro-chemical analysis array (μCANARY) are micro-electromechanical (MEM) sensors that have been miniaturized to allow many independently addressable sensors to be integrated within a single silicon chip. The target analyte of an individual sensor within the chip is selectively detected by depositing molecular recognition component (or “coating”) onto the sensor surface, and monitoring changes in the frequency and phase of the resonance as the coating interacts with the analyte. The ultimate goal of this project is integration of hundreds of miniature resonators within a single chip for detection of biological species. As proof of concept demonstration, we describe here the detection of proteins and intact microorganisms using 2-element and 8-element μCANARY sensor chips and address electronics. Preliminary results of sensitivity, selectivity, and surface regeneration methods of the sensor are presented. Detection of proteins and microorganisms with the μCANARY sensor were confirmed by optical measurements.

High-temperature superconducting dual-spiral resonators and their application in filter miniaturization

Dual-spiral resonators, each of which consists of two spirals connected in series, exhibit much smaller sizes than existing miniature microstrip resonators, and their geometrical layouts enable them to be easily integrated into microwave circuits. Dual-spiral resonators have very high quality factors if they are fabricated with high-temperature superconducting (HTS) thin films. HTS dual-spiral resonators are very suitable for developing miniature filters in MIC and MMIC cellular communication systems. A full-wave electromagnetic simulator is used to calculate the resonant properties of dual-spiral resonators, and numerical results are reported. As an application example of HTS dual-spiral resonators, a three-stage bandpass filter with central frequency 633 MHz is designed and fabricated with YBa2Cu3O7- thin films deposited on a 20 mm × 8 mm × 0.5 mm LaAlO3 substrate. Experiments show that the dual-spiral resonator structure presented in this paper is very effective for filter miniaturization.