Resonator Surface Research Articles

AFM imaging and plasmonic detection of organic thin-films deposited on nanoantenna arrays

In this study, atomic force microscopy (AFM) imaging has been used to reveal the preferential deposition of organic thin-films on patterned nanoantenna array surfaces - identifying the localised formation of both monolayer and multilayer films of octadecanethiol (ODT) molecules, depending on the concentration of the solutions used. Reliable identification of this selective deposition process has been demonstrated for the first time, to our knowledge. Organic thin-films, in particular films of ODT molecules, were deposited on plasmonic resonator surfaces through a chemi-sorption process - using different solution concentrations and immersion times. The nanoantennas based on gold asymmetric-split ring resonator (A-SRR) geometries were fabricated on zinc selenide (ZnSe) substrates using electron-beam lithography and the lift-off technique. Use of the plasmonic resonant-coupling technique has enabled the detection of ODT molecules deposited from a dilute, micromolar (1 μM) solution concentration - with attomole sensitivity of deposited material per A-SRR – a value that is three orders of magnitude lower in concentration than previously reported. Additionally, on resonance, the amplitude of the molecular vibrational resonance peaks is typically an order of magnitude larger than that for the non-resonant coupling. Fourier-transform infrared (FTIR) spectroscopy shows molecule specific spectral responses – with magnitudes corresponding to the different film thicknesses deposited on the resonator surfaces. The experimental results are supported by numerical simulation.

The theory of acoustic sensors application in air quality control

Acoustic sensors based on piezoelectric resonators are widely used in the real-time monitoring of mass and in situ analysis of air composition, for example in environmental, health care and air quality control (AQC) applications, both indoor and outdoor. Recent advances in the QCM techniques include the possibility of measuring changes in resonance frequency, ∆f, and the dissipation, ∆D (so-called QCM-D sensors). Another field of application is measurements of different overtones, which can provide additional information about the system. The sensing layers in QCM devices are often polymer- or supported membrane-based, both of which are soft (viscoelastic) materials. Viscoelasticity of adsorbed layers must thus be taken into account for adequate interpretation of QCM data.This publication describes theoretical results, including analytical formulae for the resonance frequency shift and the dissipation of QCM-D, for two viscoelastic layers deposited onto resonator surface, as well as calculations for higher harmonics. In particular, the corrections in ∆f and ∆D due to the softness of the top layer, considered as a bulk, are written explicitly in terms of the layer's material parameters. The results can be useful for quantitative analysis of measured characteristics of QCM in AQC, environmental monitoring and biosensor applications.

Substrate Integrated Waveguide (SIW)-Based Wireless Temperature Sensor for Harsh Environments

This paper presents a new wireless sensor structure based on a substrate integrated circular waveguide (SICW) for the temperature test in harsh environments. The sensor substrate material is 99% alumina ceramic, and the SICW structure is composed of upper and lower metal plates and a series of metal cylindrical sidewall vias. A rectangular aperture antenna integrated on the surface of the SICW resonator is used for electromagnetic wave transmission between the sensor and the external antenna. The resonant frequency of the temperature sensor decreases when the temperature increases, because the relative permittivity of the alumina ceramic increases with temperature. The temperature sensor presented in this paper was tested four times at a range of 30–1200 °C, and a broad band coplanar waveguide (CPW)-fed antenna was used as an interrogation antenna during the test process. The resonant frequency changed from 2.371 to 2.141 GHz as the temperature varied from 30 to 1200 °C, leading to a sensitivity of 0.197 MHz/°C. The quality factor of the sensor changed from 3444.6 to 35.028 when the temperature varied from 30 to 1000 °C.

Piezoelectric MEMS resonant dew point meters

This work presents a new class of dew point meters using Thin-film Piezoelectric-on-Silicon (TPoS) resonators. The proposed approach utilizes microelectromechanical resonant balances coupled with a cooling element in which a standard chilled surface technique is used to deposit layers of moisture on the resonator surface. Reducing the temperature causes the resonance frequency of the resonator to increase due to its negative temperature coefficient of frequency (TCF). However, when the temperature of the cooled resonator reaches the dew point (DP) of the surrounding environment, a thin layer of frost starts to form on the resonator surface causing an abrupt negative shift in the resonant frequency due to mass-loading. Therefore, the resonance frequency will be at its maximum value when the dew or frost starts to form i.e., at DP. A custommade setup which includes a MEMS dew point sensor head, a dew point generator and a measurement/control/data collection interface is assembled to enable automated measurements. Using the proposed setup, dew points as low as -41 °C i.e., a dry air containing 145 ppmv H2O, are measured with a standard deviation of 0.1 °C. Long-term reliability and accuracy of the sensors presented here are investigated by conducting 35,000 cycles of uninterrupted measurements. The long-term stability, precision, and durability of the presented dew point meters make them suitable solutions for a wide variety of industrial applications.

Film bulk acoustic formaldehyde sensor with polyethyleneimine-modified single-wall carbon nanotubes as sensitive layer

Polyethyleneimine-modified single-wall carbon nanotubes were self-assembled as a sensitive coating for the micro-scale film bulk acoustic formaldehyde sensor. The film bulk acoustic resonator has a high working frequency (∼4.5 GHz) and can detect the ultra-small mass changes in the sensitive layer induced by the absorption of formaldehyde molecules. The proposed sensor demonstrated a reversible and linear response towards formaldehyde over the range of 50–400 ppb, with a limit of detection of 24 ppb. Both the response and recovery times were less than 1 min. The frequency response was improved with increasing coverage of carbon nanotubes on the resonator surface due to the larger surface area and adsorption space. The polyethyleneimine-modified single-wall carbon nanotubes provided an excellent selectivity towards formaldehyde over interfering organic vapors. The results show that the film bulk acoustic sensor is a promising candidate for the portable and convenient detection of indoor air pollution with high sensitivity.

Use of torsional resonators to monitor electroactive biofilms

Whereas the study of interfaces and thin films with the quartz crystal microbalance (QCM) is well established, biofilms have proven to be a difficult subject for the QCM. The main problem is that the shear wave emanating from the resonator surface does not usually reach to the top of the sample. This problem can be solved with torsional resonators. These have a resonance frequency in the range of tens of kHz, which is much below the frequency of the thickness-shear QCMs. The depth of penetration of the shear wave is correspondingly larger. Data acquisition and data analysis can proceed in analogy to the conventional thickness-shear QCM. Torsional resonators may also be operated as electrochemical QCMs (EQCMs), meaning that a DC electrical potential may be applied to the active electrode and that shifts of frequency and bandwidth may be acquired in parallel to the electrical current. Here we report on the formation of mixed-culture biofilms dominated by the microorganism Geobacter anodireducens. The viscoelastic analysis evidences an increase in rigidity as the films grows. Potential sweeps on electroactive biofilms reveal a softening under negative potentials, that is, under conditions, where the layer's metabolism was slowed down by insufficient oxidative activity of the substrate. For comparison, biofilms were monitored in parallel with a conventional thickness-shear QCM.

Optical absorption and oxygen passivation of surface states in III-nitride photonic devices

III-nitride surface states are expected to impact high surface-to-volume ratio devices, such as nano- and micro-wire light-emitting diodes, transistors, and photonic integrated circuits. In this work, reversible photoinduced oxygen desorption from III-nitride microdisk resonator surfaces is shown to increase optical attenuation of whispering gallery modes by 100 cm−1 at λ = 450 nm. Comparison of photoinduced oxygen desorption in unintentionally and n+-doped microdisks suggests that the spectral changes originate from the unpinning of the surface Fermi level, likely taking place at etched nonpolar III-nitride sidewalls. An oxygen-rich surface prepared by thermal annealing results in a broadband Q improvement to state-of-the-art values exceeding 1 × 104 at 2.6 eV. Such findings emphasize the importance of optically active surface states and their passivation for future nanoscale III-nitride optoelectronic and photonic devices.

Near-field spectroscopy and tuning of sub-surface modes in plasmonic terahertz resonators

Highly confined modes in THz plasmonic resonators comprising two metallic elements can enhance light-matter interaction for efficient THz optoelectronic devices. We demonstrate that sub-surface modes in such double-metal resonators can be revealed with an aperture-type near-field probe and THz time-domain spectroscopy despite strong mode confinement in the dielectric spacer. The sub-surface modes couple a fraction of their energy to the resonator surface via surface waves, which we detected with the near-field probe. We investigated two resonator geometries: a λ/2 double-metal patch antenna with a 2 μm thick dielectric spacer, and a three-dimensional meta-atom resonator. THz time-domain spectroscopy analysis of the fields at the resonator surface displays spectral signatures of sub-surface modes. Investigations of strong light-matter coupling in resonators with sub-surface modes therefore can be assisted by the aperture-type THz near-field probes. Furthermore, near-field interaction of the probe with the resonator enables tuning of the resonance frequency for the spacer mode in the antenna geometry from 1.6 to 1.9 THz (~15%).

Designing of Universal WGM Biosensor from Micro Ring Resonator

The WGM biosensor using a nonlinear silica micro ring resonator coating by nano gold layer is designed and examined. The simulation results of Gaussian laser pulses propagating in the symmetry waveguide shows good behavior of the existence of whispering gallery mode outside the surface of the ring resonators coating by a very thin nano gold layer to trap biomolecules with the plasmonic interaction. The output shifted signals from the designed sensor have an accuracy to characterize types of the specific biomolecules as well.

A microwave cavity resonator sensor for water-in-oil measurements

Online monitoring of Water-Liquid Ratio (WLR) in multiphase flow is key in petroleum production, processing and transportation. The usual practice in the field is to manually collect offline samples for laboratory analysis, which delays data availability and prevents real time intervention and optimization. A highly accurate and robust sensing method is needed for online measurements in the lower end of WLR range (0%–5%), especially for fiscal metering and custody transfer of crude oil, as well as to ensure adequate flow assurance prevention and remedial solutions. This requires a highly sensitive sensing principle along with a highly precise measurement instrument, packaged together in a sufficiently robust manner for use in the field. In this paper, a new sensing principle is proposed, based on the open-ended microwave cavity resonator and near wall surface perturbation, for non-intrusive measurement of WLR. In the proposed concept, the electromagnetic fringe field of a cylindrical cavity resonator is used to probe the liquid near the pipe wall. Two of the cylindrical cavity resonance modes, TM010 and TM011 are energized for measurements and the shift in the resonance frequency is used to estimate liquid permittivity and the WLR. Electromagnetic simulations in the microwave frequency range of 4 GHz to 7 GHz are used for proof-of-concept and sensitivity studies. A sensor prototype is fabricated and its functionality demonstrated with flowing oil-water mixtures in the WLR range of 0–5%. The frequency range of the proposed sensors is 4.4–4.6 GHz and 6.1–6.6 GHz for modes TM010 and TM011, respectively. The TM011 mode shows much higher sensitivity (41.6 MHz/WLR) than the TM010 mode (3.8 MHz/WLR). The proposed sensor consists of a 20 mm high cylinder, with a diameter of 30 mm and Poly-Ether-Ether-Ketone (PEEK) filler. The non-intrusiveness of the sensor, along with the high sensitivity in the resonance shift, makes it attractive for practical applications.

Micro-Electromechanical Acoustic Resonator Coated with Polyethyleneimine Nanofibers for the Detection of Formaldehyde Vapor.

We demonstrate a promising strategy to combine the micro-electromechanical film bulk acoustic resonator and the nanostructured sensitive fibers for the detection of low-concentration formaldehyde vapor. The polyethyleneimine nanofibers were directly deposited on the resonator surface by a simple electrospinning method. The film bulk acoustic resonator working at 4.4 GHz acted as a sensitive mass loading platform and the three-dimensional structure of nanofibers provided a large specific surface area for vapor adsorption and diffusion. The ultra-small mass change induced by the absorption of formaldehyde molecules onto the amine groups in polyethyleneimine was detected by measuring the frequency downshift of the film bulk acoustic resonator. The proposed sensor exhibits a fast, reversible and linear response towards formaldehyde vapor with an excellent selectivity. The gas sensitivity and the detection limit were 1.216 kHz/ppb and 37 ppb, respectively. The study offers a great potential for developing sensitive, fast-response and portable sensors for the detection of indoor air pollutions.

Film bulk acoustic formaldehyde sensor with layer-by-layer assembled carbon nanotubes/polyethyleneimine multilayers

Trace formaldehyde vapor was detected by a micron-scale AlN film bulk acoustic resonator based on mass-sensitive mechanism. The layer-by-layer carbon nanotubes/polyethyleneimine multilayers were assembled on the resonator surface as the sensitive coating. An almost linear decrease of the resonant frequency was observed as a function of the number of nanotubes/polyethyleneimine periods. The multilayers showed a random and porous structure and thus provided a large specific surface area for gas adsorption and diffusion. At the same time, the amine groups in polyethyleneimine had an strong affinity to formaldehyde with excellent selectivity. When exposed to gaseous formaldehyde, the attachment of gas molecules induced a small decrease in the resonant frequency, which made the sensor easily detect formaldehyde at ppb levels with 1 min response time. A linear relationship was observed between the formaldehyde concentrations and the frequency downshift of the resonator. The layer number had an obvious influence on the absorption/desorption behavior of formaldehyde. The gas sensitivity of FBAR sensors was 1.29–1.90 kHz ppb−1 with the limit of detection of 24–38 ppb.

A resonance system of an optoelectronic oscillator based on a transmission-type planar optical disk microcavity

It is noted that the best technical characteristics of optoelectronic microwave self-oscillators (OESO) are reached in schemes employing high-Q optical microresonators (OMR) working in the traveling wave modes (TWM). A possibility of using disk OMRs excited by fundamental whispering gallery modes (WGM) has been considered. Multielement coupling devices (CD) for such resonators have been investigated. They are constructed on the basis of planar optical waveguides (POWG) located over the disk resonator surface in a region bounded by the outer and inner caustics of the WGM. Models of this device have been proposed. The corresponding calculations have been provided.

Plasmonic Metal-Hybrid Hydrogen Sensor Based on Semiconductor Nanocrystal Micro Ring

ABSTRACTWe have proposed and demonstrated numerically an ultra-small (4×4μm2) hydrogen sensor based on micro ring resonator. With a palladium or platinum layer coated on the inner surface of the micro ring resonator, the device is highly sensitive to the low hydrogen concentration variation and the sensitivity is at least one magnitude order larger than the optical fiber-based hydrogen sensor. We have also investigated the tradeoff between the portion coverage of palladium/platinum layer and the sensitivity. The width of the hydrogen sensitive layer is also studied and the minimum feature width is determined to be the length of the ring waveguide evanescent wave. This ultra-small optical hydrogen sensor will be promising to realize highly compact sensor with integration capability for applications on hydrogen fuel economy.

ZnO piezoelectric film resonator modified with multi-walled carbon nanotubes/polyethyleneimine bilayer for the detection of trace formaldehyde

We fabricated a ZnO piezoelectric film resonator modified with multi-walled carbon nanotubes/polyethyleneimine bilayer as the sensitive layer for the detection of trace gaseous formaldehyde. The resonator consists of a ZnO piezoelectric stack and an SiO2/W Bragg reflector. The multi-walled carbon nanotubes were self-assembled on the resonator surface using the n-octadecyl mercaptan monolayer and further modified with polyethyleneimine. The formaldehyde molecules are absorbed on the multi-walled carbon nanotubes/polyethyleneimine bilayer based on the reversible nucleophilic addition reaction between formaldehyde molecules and the amine functional groups on polyethyleneimine. The high working frequency (~ 3.1 GHz) of the resonator provided enough mass sensitivity to probe the ultra-small mass change of the sensitive biolayer. The downshift of resonant frequency was linear with the increase of formaldehyde concentration. The experimental results show that our proposed sensor can yield rapid, sensitive, reversible and repeatable responses to formaldehyde in the concentration range of 50–400 ppb at room temperature. The piezoelectric film resonator is a promising and feasible sensor for the indoor pollution monitoring.

The detection of formaldehyde using microelectromechanical acoustic resonator with multiwalled carbon nanotubes-polyethyleneimine composite coating

A micro-scale gas sensor based on mass-sensitive film bulk acoustic resonator is demonstrated for the detection of trace formaldehyde at room temperature. The composites mixed with multiwalled carbon nanotubes and polyethyleneimine (MWNTs-PEI) were coated on the resonator surface as the sensitive layer to specifically absorb formaldehyde molecules using a facile spray process. The influence of spraying processes on the formaldehyde sensing properties were investigated. Different response behaviors were determined by both the chemical absorption between formaldehyde molecules and the amine functional groups on PEI and the increase of absorption surface came from the nanostructure. The combination of high frequency of the film bulk acoustic resonator (~4.3 GHz) and the specific absorbability of MWNTs-PEI composites provided a high sensitivity in the detections of trace formaldehyde. The obtained ultra-low limit of detection was as low as 60 ppb with linear response, quick response/recovery time, good reproducibility and selectivity. The proposed sensor shows potential as a portable and convenient gas-sensing system for monitoring the low-level concentration of indoor air pollution.

Nonlinear aeroacoustic characterization of Helmholtz resonators with a local-linear neuro-fuzzy network model

The nonlinear acoustic behavior of Helmholtz resonators is characterized by a data-based reduced-order model, which is obtained by a combination of high-resolution CFD simulation and system identification. It is shown that even in the nonlinear regime, a linear model is capable of describing the reflection behavior at a particular amplitude with quantitative accuracy. This observation motivates to choose a local-linear model structure for this study, which consists of a network of parallel linear submodels. A so-called fuzzy-neuron layer distributes the input signal over the linear submodels, depending on the root mean square of the particle velocity at the resonator surface. The resulting model structure is referred to as an local-linear neuro-fuzzy network. System identification techniques are used to estimate the free parameters of this model from training data. The training data are generated by CFD simulations of the resonator, with persistent acoustic excitation over a wide range of frequencies and sound pressure levels. The estimated nonlinear, reduced-order models show good agreement with CFD and experimental data over a wide range of amplitudes for several test cases.

Modeling the Effect of Imperfections in Glassblown Micro-Wineglass Fused Quartz Resonators

In this paper, we developed an analytical model, supported by experimental results, on the effect of imperfections in glassblown micro-wineglass fused quartz resonators. The analytical model predicting the frequency mismatch due to imperfections was derived based on a combination of the Rayleigh's energy method and the generalized collocation method. The analytically predicted frequency of the n = 2 wineglass mode shape was within 10% of the finite element modeling results and within 20% of the experimental results for thin shells, showing the fidelity of the predictive model. The postprocessing methods for improvement of the resonator surface quality were also studied. We concluded that the thermal reflow of fused quartz achieves the best result, followed in effectiveness by the RCA-1 surface treatment. All the analytical models developed in this paper are to guide the manufacturing methods to reduce the frequency and damping mismatch, and to increase the mechanical quality factor of the device.

Fabrication of poly(N-isopropylacrylamide) nanoparticles using a simple spray-coating method and applications for a QCM-based HCl gas sensor coating

We propose a new simplified fabrication process for poly(N-isopropylacrylamide) (PNIPAM) nanoparticles for a quartz crystal microbalance (QCM)-based HCl gas sensor coating. PNIPAM solution, synthesized via atom transfer radical polymerization (ATRP) below a lower critical solution temperature (LCST), is simply sprayed onto a heated quartz resonator surface at 50°C; nanoparticles with a mean diameter of 100nm were successfully formed with this method. The PNIPAM nanoparticles produced exhibited a large HCl gas sensor response, and the magnitude of the response increased linearly with PNIPAM loading on the resonator. However, the value of the recovery was unexpectedly less than 90%. The hydrogen-bonding structure of the nanoparticles produced was characterized by water vapor adsorption measurement and FT-IR analysis in a humid atmosphere.

Ring Resonator-Based Optical Hydrogen Sensor

We have proposed and demonstrated numerically a ultra-small and highly sensitive hydrogen sensor, based on micro ring resonator with a footprint size as small as 4×4 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . With a palladium or platinum hydrogen sensitive layer coated on the inner surface of the micro ring resonator, the device is highly sensitive to the low hydrogen concentration variation and the sensitivity is at least one magnitude order larger than the optical fiber-based hydrogen sensor. We have also investigated the tradeoff between the portion coverage of palladium/platinum layer and the sensitivity. The width of the hydrogen sensitive layer is also studied and the minimum feature width is determined to be the length of the ring waveguide evanescent wave. This ultra-small optical hydrogen sensor will be promising to realize highly compact sensor with integration capability for applications on hydrogen fuel economy.