Piezoelectric resonance sensors are widely used in chemical and biological sensing applications. They operate by detecting the resonant frequency shift of piezoelectric resonators caused by the deposition of analytes on their surfaces. To detect minute changes in analytes, resonators require a high quality factor (Q factor). Traditionally, methods to enhance the quality factor involve optimizing the vibration modes, structure, and materials of the resonator. However, these methods are often complex and costly. This paper proposes a novel approach to enhance the quality factor of piezoelectric sensors by utilizing Fano resonance, rather than optimizing the resonator’s structure or materials.

Fano resonance is a ubiquitous scattering wave phenomenon, originally discovered in atomic and solid-state physics. It occurs due to the interference between a discrete quantum state and a continuum of states, resulting in an asymmetric and steep spectral distribution. The narrow linewidth of Fano resonance makes it highly promising for applications in photonic devices. In this paper, by connecting an external shunt capacitor to the piezoelectric sensor, the sensor is brought into a Fano resonance state, significantly enhancing its quality factor.

Source of the Paper

This paper was co-authored by Mengting Wang, Jianqiu Huang, and Qing-An Huang, all affiliated with the Key Laboratory of MEMS of the Ministry of Education at Southeast University. The paper was published in 2024 in the journal Microsystems & Nanoengineering under the title “Putting Piezoelectric Sensors into Fano Resonances.”

Research Process and Results

1. Research Process

1.1 Theoretical Model and Experimental Design

The paper first proposes a theoretical model for enhancing the quality factor of piezoelectric sensors using Fano resonance. The asymmetry of Fano resonance can be described by the following equation:

[ \sigma = \frac{(\epsilon + q)^2}{\epsilon^2 + 1} \cdot h + \sigma_0 ]

where ( q ) is the shape parameter, ( \epsilon ) is the normalized energy, ( h ) is the gain parameter, and ( \sigma_0 ) is the offset. For piezoelectric resonators, the equivalent circuit model is the Butterworth-Van Dyke (BVD) model, which includes a static capacitance ( C_0 ) and a motional branch (( L_m ), ( C_m ), ( R_m )). By connecting an external shunt capacitor ( C_p ), the piezoelectric sensor can be brought into a Fano resonance state, thereby enhancing the quality factor.

1.2 Experimental Validation

To validate this theory, the authors designed and fabricated a one-port surface acoustic wave (SAW) resonator on a LiNbO3 substrate for humidity sensing. The resonator’s surface was coated with a composite of polymethyl methacrylate (PMMA) and graphene oxide (GO), which exhibits high sensitivity to humidity. By measuring the scattering parameter ( S_{11} ) at different shunt capacitances, the authors found that when ( C_p = 39 \, \text{pF} ), the quality factor increased from 929 to 7682, an improvement of approximately 8 times.

1.3 Simulation and Experimental Comparison

The authors used Advanced Design System (ADS) software to simulate the experiments, and the simulation results were in good agreement with the experimental data, further validating the theoretical model.

2. Main Results

2.1 Enhancement of Quality Factor

The experimental results show that by connecting an external shunt capacitor, the quality factor of the piezoelectric sensor was significantly improved. At the optimal shunt capacitance ( C_p = 39 \, \text{pF} ), the quality factor increased from 929 to 7682.

2.2 Humidity Sensing Performance

The authors also tested the sensor’s response at different humidity levels. The experiments showed that the sensor’s sensitivity was 0.358 kHz/%RH, and the sensitivity remained almost unchanged before and after connecting the shunt capacitor. However, the enhancement in quality factor significantly improved the overall performance of the sensor, with the figure of merit (FOM) increasing from 0.006 (%RH)^{-1} to 0.044 (%RH)^{-1}.

Conclusions and Significance

Through theoretical and experimental validation, this paper proposes a novel method to enhance the quality factor of piezoelectric sensors using Fano resonance. By connecting an external shunt capacitor, the piezoelectric sensor is brought into a Fano resonance state, significantly improving the quality factor. This method is not only applicable to SAW resonators but can also be extended to other types of piezoelectric resonators, such as quartz crystal microbalances (QCM) and film bulk acoustic wave resonators (FBAR).

This research provides new insights into the development of high-sensitivity chemical and biological sensors, with significant scientific and practical value.

Research Highlights

1. Novel Method: This paper is the first to propose the use of Fano resonance to enhance the quality factor of piezoelectric sensors, rather than optimizing the resonator’s structure or materials.
2. Significant Quality Factor Improvement: By connecting an external shunt capacitor, the quality factor was improved by approximately 8 times, from 929 to 7682.
3. Broad Applicability: This method is not only applicable to SAW resonators but can also be extended to other types of piezoelectric resonators, offering wide-ranging application prospects.

Other Valuable Information

The paper also provides a detailed description of the design and fabrication process of the SAW resonator, as well as the method for extracting circuit parameters. These details offer valuable references for researchers in related fields.