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Abstract
Miniature acoustic sensors with high sensitivity are highly desired for applications in medical photoacoustic imaging, acoustic communications and industrial nondestructive testing. However, conventional acoustic sensors based on piezoelectric, piezoresistive and capacitive detectors usually require a large element size on a millimeter to centimeter scale to achieve a high sensitivity, greatly limiting their spatial resolution and the application in space-confined sensing scenarios. Herein, by using single-crystal two-dimensional gold flakes (2DGFs) as the sensing diaphragm of an extrinsic Fabry-Perot interferometer on a fiber tip, we demonstrate a miniature optical acoustic sensor with high sensitivity. Benefiting from the ultrathin thickness (∼8 nm) and high reflectivity of the 2DGF, the fiber-tip acoustic sensor gives an acoustic pressure sensitivity of ∼300 mV/Pa in the frequency range from 100 Hz to 20 kHz. The noise-equivalent pressure of the fiber-tip acoustic sensor at the frequency of 13 kHz is as low as 62.8 µPa/Hz1/2, which is one or two orders of magnitude lower than that of reported optical acoustic sensors with the same size.
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1. Introduction
Acoustic sensors play an important role in applications such as acoustic navigation and communications [1,2], nondestructive structural testing [3], trace gas detection [4–6] and medical photoacoustic imaging [7,8]. The majority of modern acoustic sensors are based on piezoelectric [9,10], piezoresistive [11] and capacitive detectors [12], which convert the energy of acoustic waves into electric signals. However, for these acoustic sensors, a large element size on a millimeter to centimeter scale is usually required to achieve a high acoustic sensitivity, greatly limiting their spatial resolution and applications in space-confined sensing scenarios. Compared with these electric acoustic sensors, optical acoustic sensors (based on optical resonance structures including micro-resonators [13,14], fiber gratings [15–17], and Fabry-Perot interferometers [18–21]) offer potentials of high sensitivity, immunity to electromagnetic interference and safe operation in explosive or combustive atmosphere [22]. Among them, extrinsic Fabry-Perot interferometer (EFPI) based fiber-tip acoustic sensors have attracted much interest recently because of their unique advantages of compact size, high spatial resolution and simple configuration [23]. For an EFPI-based fiber-tip acoustic sensor, the key component is the flexible diaphragm which works both as a sensing element that can deflect under an external pressure and a reflecting mirror of the EFPI [24]. The comprehensive sensitivity of the fiber-tip acoustic sensors is determined not only by the deflection sensitivity (defined as the ratio of the deflection of the diaphragm to the external pressure) of the diaphragm but also by the optical sensitivity (defined as the ratio of the change in reflected light intensity to the deflection of the diaphragm) of the EFPI. Therefore, diaphragm with thin thickness, high reflectivity, excellent flexibility and mechanical strength is highly desired for a high-performance EFPI-based fiber-tip acoustic sensor. To date, a variety of materials have been employed as the sensing diaphragms, including polymers [25,26], silica [27,28], metals [29,30] and two-dimensional (2D) materials [31–35]. However, it is usually difficult for diaphragms based on polymers and silica to get a high deflection sensitivity [24] due to their large thickness (a few to tens of micrometers). The deflection sensitivity of 2D material-based diaphragms can be greatly improved for their ultrathin thicknesses (a few nanometers). But their low reflectivity (less than 8% [35], as well for diaphragms based on polymers and silica) greatly limits the optical sensitivity of the corresponding EFPI. In contrast, metal-based diaphragms can provide a high reflectivity in the near-infrared spectral region even with an ultrathin thickness (e.g., for a 10-nm thick gold film, its reflectivity is ∼60% around 1400-nm wavelength [36]). However, due to the difficulty in the fabrication of ultrathin freestanding metal films, the thickness of metal-based diaphragms used in fiber-tip acoustic sensors are usually larger than 100 nm, greatly limiting the deflection sensitivity and therefore the comprehensive sensitivity of the acoustic sensors.
Here, by using a single-crystal two-dimensional gold flake (2DGF) as the diaphragm, we demonstrate an EFPI-based fiber-tip acoustic sensor with a high sensitivity. The fiber-tip acoustic sensor constructed using a freestanding 2DGF with an ultrathin thickness of 8 nm shows a high acoustic pressure sensitivity of ∼300 mV/Pa in the acoustic frequency range from 100 Hz to 20 kHz. The noise-equivalent pressure (NEP) of the fiber-tip acoustic sensor at the frequency of 13 kHz is as low as 62.8 µPa/Hz1/2, which makes the miniature sensor attractive for the detection of weak acoustic pressure in space-confined scenarios with high sensitivity.
2. Configuration and working principle
Figure 1(a) shows a schematic illustration of an EFPI-based fiber-tip acoustic sensor. It consists of a single-mode optical fiber, a section of hollow capillary (tens of micrometers in length), and two ultrathin gold films. The combination of the hollow capillary and the ultrathin gold films enables the formation of an EFPI which is critical for the acoustic sensing. The inner ultrathin gold film is attached on the end face of the single-mode optical fiber to work as the first reflective mirror of the EFPI, while the outer ultrathin gold film is suspended on the end of the hollow capillary to function not only as the second reflective mirror of the EFPI but also as a flexible diaphragm which can deform under an acoustic pressure. When an external acoustic pressure is applied to the outer ultrathin gold diaphragm, its deformation causes a change in the cavity length and subsequently a shift of the interference spectrum of the EFPI (Fig. 1(b)). To measure the change of cavity length, an intensity demodulation method is adopted due to its high speed and sensitivity among various demodulation technologies [37,38]. Therefore, the intensity of the acoustic pressure can be precisely detected by sending a continuous wave (CW) laser (which is tuned to the flank of a resonance dip, gray line in Fig. 1(b)) into the fiber-tip acoustic sensor, and measuring the change in the intensity of the reflected interrogation laser.
Fig. 1. Schematic diagram and working principle of EFPI-based fiber-tip acoustic sensor. (a) Schematic of an EFPI-based fiber-tip acoustic sensor with an ultrathin gold diaphragm. (b) Readout principle of the fiber-tip acoustic sensor. The acoustic waves induce deformation of the outer diaphragm and change the Fabry-Perot cavity length, resulting in a shift of the interference spectrum. The intensity of the acoustic pressure can be detected by sending a CW laser (tuned to the flank of a resonance dip, gray line) into the sensor, and measuring the change in the intensity of the reflected interrogation laser. (c) Calculated deflection sensitivity of 2DGFs with different thickness and prestress.
The deflection sensitivity of the gold diaphragm is crucial to the sensitivity of EFPI-based fiber-tip acoustic sensor. To investigate the deflection sensitivity of the gold diaphragm, it is approximated as a clamped circular diaphragm which is tensioned and suspended flatly on the hollow capillary. The relationship between the deflection of gold diaphragm ω and the pressure difference P may be expressed as [39]:
where E is the Young’s modulus of gold; r, t, υ and σ0 are the radius, thickness, Poisson’s ratio and prestress of the suspended ultrathin gold diaphragm, respectively. Then, the deflection sensitivity S can be obtained by [40]:In our configuration, the values of r, υ and E are 25 µm, 0.42 and 78 GPa, respectively. Figure 1(c) shows the deflection sensitivity of gold diaphragms with different thickness and prestress. The deflection sensitivity increases rapidly with the decreasing of the thickness of the gold diaphragm. Also, a higher sensitivity can be obtained with a smaller prestress. However, the reflectivity of gold diaphragm decreases quickly when its thickness is less than 10 nm, and a large prestress may be formed for a gold diaphragm with ultrathin thickness. Therefore, the thickness of the gold diaphragm is chosen to be around 10 nm to ensure both a high deflection sensitivity and reflectivity. Figure S1 in Supplement 1, section 1 further shows a comparison of the simulated reflectance spectra of EFPIs constructed using 2DGF and graphene with same thickness of 10 nm as the diaphragm. Benefitting from its high reflectivity, the output voltage amplitude of the EFPI based on the gold diaphragm is about 11 times higher than that of EFPI based on the graphene diaphragm, showing its advantage for acoustic sensing.
3. Fabrication of EFPI-based fiber-tip acoustic sensor
For the proposed EFPI-based fiber-tip acoustic sensor, the key component is the ultrathin gold diaphragm (it requires a large size to obtain a high sensitivity) suspended on the end face of the hollow capillary, which is extremely difficult to be fabricated with conventional deposition approaches. Here, freestanding 2DGFs, which are fabricated by chemical etching [41] of substrate-supported gold flakes (see Methods for details), are exploited as the sensing diaphragm. They have a single-crystal structure and their thickness can be as thin as several nanometers while having a large lateral size of hundreds of micrometers. Figure 2(a) shows a reflective optical micrograph of a typical 2DGF with a thickness of 8 nm and a lateral size of ∼85 µm, which is semi-transparent in transmission (Fig. 2(a), inset) due to its ultrathin thickness. Figure 2(b) presents an atomic force microscopy image of a 2DGF, indicating that as-fabricated 2DGF has an ultrasmooth surface (RMS roughness ∼0.2 nm) and a constant thickness of 8.9 nm across the whole flake. The reflectivity of the 2DGFs, which is a key parameter for an EFPI acoustic sensor, is further characterized in the wavelength range from 900-1600 nm (Fig. 2(c)). The reflectivity at 1550 nm is around 63% and 73% for 2DGFs with thicknesses of 10 and 13 nm, respectively, which is about ten times higher than graphene with the same thickness [31]. Therefore, freestanding 2DGFs with single-crystal structure, high reflectivity and large size serve as an ideal metal diaphragm for the construction of EFPI-based fiber-tip acoustic sensors.
Fig. 2. Fabrication of EFPI-based fiber acoustic sensor. (a) Optical reflection image of an 8-nm-thick 2DGF. The inset shows the corresponding optical transmission image. Scale bar, 20 µm. (b) AFM image of a 2DGF with a thickness of 8.9 nm. (c) Measured reflectivity of 2DGFs with thicknesses of 10 and 13 nm, respectively. The gray horizontal line reveals the level of 60%. (d) Schematic diagram of the procedure for the fabrication of EFPI-based fiber-tip acoustic sensor. (e, f) Photos of a fabricated fiber-tip acoustic sensor. (g) Reflection spectrum of the fiber-tip acoustic sensor.
The procedure for the fabrication of an EFPI-based fiber-tip acoustic sensor with 2DGFs is schematically shown in Fig. 2(d). Firstly, on the tip of a single-mode optical fiber (SMF-28, Corning), a hollow capillary with the same outer diameter (50 µm in inner diameter) was fusion spliced and cleaved into a short section of tens of micrometers in length under an optical microscope. Secondly, a 2DGF was transferred onto the end face of the single-mode optical fiber using a polydimethylsiloxane-mediated transfer method (see Methods and Supplement 1, section 2 for details) to work as the first reflective mirror. Finally, another 2DGF was transferred to suspend on the end face of the hollow capillary to form an EFPI on the fiber tip (see Methods and Fig. S2(b) for details). Figure 2(e) shows a photograph of an as-fabricated fiber-tip acoustic sensor, in which 2DGFs with thicknesses of about 7 and 8 nm are used as the inner and outer gold films, respectively. Due to the ultrathin thickness of the suspended 2DGF, the inner 2DGF can be clearly seen through the suspended 2DGF (Fig. 2(f)). Figure 2(g) shows a reflection spectrum of the acoustic sensor, which gives a quality factor of about 135. The measured free spectra range (FSR) of the EFPI is 44.5 nm, corresponding to a calculated cavity length of about 27.9 µm, which is in good agreement with the measured length of the EFPI.
4. Response to static pressure
To characterize the performance of the fiber-tip acoustic sensor, its response to static pressures was first measured using the experimental setup shown in Fig. 3(a). The fiber-tip acoustic sensor was placed in an airtight chamber filled with nitrogen, whose pressure was controlled with a pressure controller and monitored by a micro differential pressure sensor. Then, light from a tunable CW laser (TSL-710, 1480-1640 nm, Santec) was launched into the fiber-tip acoustic sensor through a fiber-optical circulator, and the reflected light from the EFPI was measured by an optical power meter (synchronized with the tunable CW laser) after passing through the optical circulator. All the measurements were conducted under a constant room temperature to eliminate the possible influence of temperature change.
Fig. 3. Measurement of static pressure response. (a) Schematic of the setup used for the measurement of the static pressure response. (b) Evolution of the reflection spectrum of the fiber-tip acoustic sensor when the pressure difference is increased from 0 to 1000 Pa. (c) Dependence of the resonance wavelengths on the pressure differences for three fiber-tip acoustic sensors constructed using 8-nm-thick, 18-nm-thick and 45-nm-thick 2DGFs as the diaphragm.
To measure the static pressure response of the fiber-tip acoustic sensor constructed with an 8-nm-thick gold diaphragm (the same as the fiber-tip acoustic sensor shown in Fig. 2(e)), the pressure in the sealed chamber was first set to be the same as the pressure in the EFPI cavity at one atmosphere. With the increase of the pressure difference, a blueshift of the resonance dip in the reflection spectrum of the fiber-tip acoustic sensor can be observed (Fig. 3(b)), which is due to the decrease in the cavity length of the EFPI under an increasing ambient pressure. The good linear relationship between the wavelength of the resonance dip and the pressure difference for the fiber-tip acoustic sensor (Fig. 3(c), hollow red circles) gives a static pressure sensitivity of ∼15.2 nm/kPa, which is about 1.5 and 4 times of the sensitivities of fiber-tip acoustic sensors constructed with 18-nm-thick (Fig. 3(c), hollow orange circles) and 45-nm-thick (Fig. 3(c), hollow blue circles) gold diaphragms, respectively. This is understandable due to the easier deformation for diaphragms with a thin thickness under the same external pressure.
5. Response to acoustic pressure
The dynamic response of the fiber-tip acoustic sensor with an 8-nm-thick gold diaphragm to acoustic pressure was further characterized, using the experimental setup shown in Fig. 4(a). In an acoustic isolation box, a commercial loudspeaker, driven by a signal generator with a tunable frequency, was used to generate acoustic waves with specific frequencies and tunable acoustic pressure. The fiber-tip acoustic sensor and reference sensor (YSV5001, BYST) were placed symmetrically along the central axis of the loudspeaker for the detection of the acoustic pressure. The actual frequency and amplitude of the output acoustic waves was recorded and calibrated by the reference sensor. Then, light from a tunable laser (EXFO, tuned to the flank of a resonance dip of the tested fiber-tip acoustic sensor) was launched into the EFPI sensor as the interrogation laser after passing through a 50:50 beam splitter and a fiber-optical circulator. Finally, the reflected light from the fiber-tip acoustic sensor (together with the light from the other port of the 50:50 beam splitter) was detected by a balanced photodetector (PDB450C(-AC), Thorlabs) to generate electrical signal with high signal-to-noise ratio (SNR), which was subsequently received and analyzed by an oscilloscope (DPO3034, Tektronix). To avoid the influence of the temperature or pressure changes on the sensor, the acoustic response test was conducted at room temperature and a normal atmospheric pressure (a pressure balance hole can be drilled to avoid these problems in practical application [42]).
Fig. 4. Measurement of acoustic pressure response for the fiber-tip acoustic sensor with an 8-nm-thick gold diaphragm. (a) Schematic of the setup used for the measurement of the acoustic pressure response. (b, c) Time-domain response of the fiber-tip acoustic sensor (b) and the reference sensor (c) to acoustic waves with a frequency of 13 kHz. (d) Frequency-domain response of the fiber-tip acoustic sensor to acoustic pressure with different frequencies. (e, f) Time-domain response of the fiber-tip acoustic sensor to acoustic waves (13 kHz) with pressure increasing from 1.51 to 2.73 Pa (e), and corresponding acoustic pressure-dependent output voltage amplitude of the sensor (f). (g) Measured sensitivity (blue dots) and NEP (orange dots) of the fiber-tip acoustic sensor in the frequency range from 100 Hz to 20 kHz.
Figure 4(b) shows a typical time-domain output signal of the fiber-tip acoustic sensor under an acoustic pressure (2.73 Pa) with a frequency of 13 kHz (see Supplement 1 section 3 for its response to acoustic pressure with other frequencies), which responds in the same way with that of the reference sensor shown in Fig. 4(c). The positive and negative values of the output signal indicate the change in the cavity length of the EFPI caused by the deflection of the gold diaphragm under the applied acoustic pressure. By conducting fast Fourier transform (FFT) on the time-domain output signals, the frequency-domain response of the fiber-tip acoustic sensor to acoustic pressure with different frequencies can be obtained (Fig. 4(d)), showing a good SNR of more than 40 dB at each frequency. There are some fluctuations of SNR which is caused by the difference in the amplitude of the output pressure of the loudspeaker at different frequencies. The amplitude-frequency response of the fiber-tip acoustic sensor and corresponding pressure can be referred in Supplement 1 section 4 for more detail.
To evaluate the acoustic pressure sensitivity of the fiber-tip acoustic sensor, its response to acoustic waves (13 kHz) with different pressure were measured. Figure 4(e) shows the output voltage amplitude of the fiber-tip acoustic sensor as a function of the applied acoustic pressure, which increases linearly with the increasing acoustic pressure from 1.51 to 2.73 Pa (Fig. 4(f)), giving an acoustic pressure sensitivity of ∼315 mV/Pa. In addition to the dynamic pressure sensitivity, frequency response is another important parameter for an acoustic sensor. Figure 4(g) shows the measured acoustic pressure sensitivities and NEP of the fiber-acoustic sensor at frequencies from 100 Hz to 20 kHz. The relatively uniform pressure sensitivity over this frequency range is due to the much higher resonant frequency of the 2DGF (∼1.18 MHz, see Supplement 1 section 5 for more details), which makes the EFPI-based sensor suitable for acoustic pressure sensing. The NEP of the fiber-tip acoustic sensor at acoustic frequencies is basically at the level of a few hundreds of µPa/Hz1/2, and at the frequency of 13 kHz, the NEP is as low as 62.8 µPa/Hz1/2 (see Supplement 1 section 6 for NEP measurement method). This value is one or two orders of magnitude lower than that of other reported acoustic sensors of the same size [24,43,44], and almost five orders of magnitude lower than that of the commercial reference sensor of the same size (see Supplement 1 section 7 for a summary of the experimental results in comparison with reported acoustic sensors).
The excellent performance of the fiber-tip acoustic sensor can be attributed to the ultrathin thickness and relatively low Young's modulus (∼78 GPa) of the ultrathin gold diaphragm which enable a higher deformation sensitivity of the sensor, as well as the high reflectivity of the ultrathin gold diaphragm which enables a higher optical sensitivity of the sensor.
6. Conclusion
In summary, we have demonstrated a miniature EFPI-based fiber-tip acoustic sensor by using an 8-nm-thick single-crystal 2DGF as the sensing diaphragm. Taking advantage of the ultrathin thickness and high reflectivity of the 2DGF diaphragm, the fiber-tip acoustic sensor shows an acoustic pressure sensitivity of ∼300 mV/Pa in the frequency range from 1 to 20 kHz, and a NEP as low as 62.8 µPa/Hz1/2. The performance of fiber-tip acoustic sensors enabled by 2DGFs can be further improved by reducing the thickness of 2DGFs, enlarging the diameter of 2DGF or reducing the prestress of the suspended 2DGFs introduced during the transferring process. Meanwhile, the structure of the planar Fabry-Perot cavity can be replaced by some novel structures [45–47] combined with 2DGF to break the limitations of functional materials on mechanical response and further enlarge frequency spectrum response range of the sensor. The advantages, such as high sensitivity, miniature size and easy fabrication, make it an attractive device for acoustic wave detection in applications ranging from high-resolution medical imaging to ultra-wide band acoustic signal monitoring and industrial nondestructive evaluation.
7. Methods
7.1 Fabrication of single-crystal 2DGFs
Single-crystal 2DGFs were fabricated using a two-step approach. In the first step, single-crystal gold flakes were grown on a substrate (e.g., glass slide) using a modified wet-chemical method [48,49]. Briefly, 10-mL ethylene glycol (99.8%, Signa-Aldrich) and 270 µL 0.1-M chloroauric acid (98%, Sigma-Aldrich) aqueous solution were mixed in a 20-mL glass vial as a growth solution. Then, a cleaned glass slide was put into the growth solution at a slightly tilted angle. Finally, the growth solution was heated in an oven to 95°C to start the growth of gold flakes. After about six hours, single-crystal gold flakes with lateral sizes from 40 to 120 µm and thickness from 15 to 40 nm can be obtained on the glass slide. In the second step, an atomic-level-precision chemical etching approach was used to thin as-fabricated gold flakes. Typically, the glass slide with thick gold flakes grown on it was immersed into a cysteamine solution to start the chemical etching. The gold atoms on the surface of the gold flakes can be etched monolayer by monolayer, while the lateral sizes of the gold flakes remain almost unchanged. With this approach, gold flakes with an initial thickness of ∼30 nm can be readily thinned down to several nanometers to obtain 2DGFs by controlling the etching time.
7.2 Transferring of 2DGFs
2DGFs can be readily transferred onto fiber-tips via a PDMS-mediated approach. As shown in Fig. S2(a), a 2DGF on a glass slide was first pick up by a cylindrical PDMS stamp (∼40 µm in diameter and ∼50 µm in length) with the aid of a droplet of aqueous ammonia solution (5% in concentration) to weaken the Van der Waals and electrostatic force [50–52] between the 2DGF and the substrate. Then, the PDMS stamp with the 2DGF on it was dried by nitrogen to remove the residual water, and subsequently aligned to make a contact with the target substrate (e.g., fiber tips). Finally, the PDMS stamp was moved away carefully and the 2DGF was left on the target substrate. Specifically, for the transferring of 2DGFs to suspend on the end face of hollow capillaries, a ring-shaped PDMS stamp (50 µm in inner diameter, 200 µm in outer diameter and 6 µm in length) was used instead to reduce the attractive force between the 2DGF and the stamp (as shown in Fig. S2(b)).
Funding
National Natural Science Foundation of China (12004333, 62075195, 62305293, 92150302, 92250305); Fundamental Research Funds for the Central Universities (2022QZJH28, 2023QZJH13).
Acknowledgment
The authors thank Professors Yanhua Liu, Changchong Chen and Ying Sun in Micro and Nano fabricating Center at Zhejiang University for their help in AFM imaging and template PDMS preparing experiment.
Disclosures
The authors declare no competing financial interests.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. Y. Chen, H. Liu, M. Reilly, et al., “Enhanced acoustic sensing through wave compression and pressure amplification in anisotropic metamaterials,” Nat. Commun. 5(1), 5247 (2014). [CrossRef]
2. C. Lang, J. Fang, H. Shao, et al., “High-sensitivity acoustic sensors from nanofibre webs,” Nat. Commun. 7(1), 11108 (2016). [CrossRef]
3. R. Di Sante, “Fibre optic sensors for structural health monitoring of aircraft composite structures: Recent advances and applications,” Sensors 15(8), 18666–18713 (2015). [CrossRef]
4. X. Zhao, H. Qi, Z. Wang, et al., “Cantilever enhanced fiber-optic photoacoustic microprobe for diffusion detection of sulfur dioxide,” Sens. Actuators, B 405, 135340 (2024). [CrossRef]
5. M. Guo, K. Chen, C. Li, et al., “High-sensitivity silicon cantilever-enhanced photoacoustic spectroscopy analyzer with low gas consumption,” Anal. Chem. 94(2), 1151–1157 (2022). [CrossRef]
6. X. Zhao, C. Li, H. Qi, et al., “Integrated near-infrared fiber-optic photoacoustic sensing demodulator for ultra-high sensitivity gas detection,” Photoacoustics 33, 100560 (2023). [CrossRef]
7. B. Fischer, “Optical microphone hears ultrasound,” Nat. Photonics 10(6), 356–358 (2016). [CrossRef]
8. Y. Hazan, A. Levi, M. Nagli, et al., “Silicon-photonics acoustic detector for optoacoustic micro-tomography,” Nat. Commun. 13(1), 1488 (2022). [CrossRef]
9. M. Toda and M. L. Thompson, “Contact-type vibration sensors using curved clamped PVDF film,” IEEE Sens. J. 6(5), 1170–1177 (2006). [CrossRef]
10. J. Xu, M. J. Dapino, D. Gallego-Perez, et al., “Microphone based on polyvinylidene fluoride (PVDF) micro-pillars and patterned electrodes,” Sens. Actuators, A 153(1), 24–32 (2009). [CrossRef]
11. M. Papila, R. T. Haftka, T. Nishida, et al., “Piezoresistive microphone design pareto optimization: Tradeoff between sensitivity and noise floor,” J. Microelectromech. Syst. 15(6), 1632–1643 (2006). [CrossRef]
12. A. Torkkeli, O. Rusanen, J. Saarilahti, et al., “Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate,” Sens. Actuators, A 85(1-3), 116–123 (2000). [CrossRef]
13. S.-W. Huang, S.-L. Chen, T. Ling, et al., “Low-noise wideband ultrasound detection using polymer microring resonators,” Appl. Phys. Lett. 92(19), 193509 (2008). [CrossRef]
14. A. Maxwell, S. W. Huang, T. Ling, et al., “Polymer microring resonators for high-frequency ultrasound detection and imaging,” IEEE J. Sel. Top. Quantum Electron. 14(1), 191–197 (2008). [CrossRef]
15. R. Gao, M. Zhang, and Z.-m. Qi, “Miniature all-fibre microflown directional acoustic sensor based on crossed self-heated micro-Co2+-doped optical fibre bragg gratings,” Appl. Phys. Lett. 113(13), 134102 (2018). [CrossRef]
16. W. Ni, P. Lu, X. Fu, et al., “Highly sensitive optical fiber curvature and acoustic sensor based on thin core ultralong period fiber grating,” IEEE Photonics J. 9(2), 1–9 (2017). [CrossRef]
17. H. Yin, Z. Shao, F. Chen, et al., “Highly sensitive ultrasonic sensor based on polymer bragg grating and its application for 3D imaging of seismic physical model,” J. Lightwave Technol. 40(15), 5294–5299 (2022). [CrossRef]
18. P. Fan, W. Yan, P. Lu, et al., “High sensitivity fiber-optic michelson interferometric low-frequency acoustic sensor based on a gold diaphragm,” Opt. Express 28(17), 25238–25249 (2020). [CrossRef]
19. L. Liu, P. Lu, H. Liao, et al., “Fiber-optic michelson interferometric acoustic sensor based on a PP/PET diaphragm,” IEEE Sens. J. 16(9), 3054–3058 (2016). [CrossRef]
20. X. Wang, L. Jin, J. Li, et al., “Microfiber interferometric acoustic transducers,” Opt. Express 22(7), 8126–8135 (2014). [CrossRef]
21. F. Yang, W. Jin, H. L. Ho, et al., “Enhancement of acoustic sensitivity of hollow-core photonic bandgap fibers,” Opt. Express 21(13), 15514–15521 (2013). [CrossRef]
22. C. Zhu, H. Zheng, L. Ma, et al., “Advances in fiber-optic extrinsic Fabry-Perot interferometric physical and mechanical sensors: A review,” IEEE Sens. J. 23(7), 6406–6426 (2023). [CrossRef]
23. C. Ma, D. Peng, X. Bai, et al., “A review of optical fiber sensing technology based on thin film and Fabry-Perot cavity,” Coatings 13(7), 1277 (2023). [CrossRef]
24. Q. Huang, S. Deng, M. Li, et al., “Fabry–perot acoustic sensor based on a thin gold diaphragm,” Opt. Eng. 59(06), 1 (2020). [CrossRef]
25. L. Cheng, C. Wang, Y. Huang, et al., “Silk fibroin diaphragm-based fiber-tip Fabry-Perot pressure sensor,” Opt. Express 24(17), 19600–19606 (2016). [CrossRef]
26. S. Wang, P. Lu, L. Liu, et al., “An infrasound sensor based on extrinsic fiber-optic Fabry-Perot interferometer structure,” IEEE Photonics Technol. Lett. 28(11), 1264–1267 (2016). [CrossRef]
27. O. Kilic, M. J. F. Digonnet, G. S. Kino, et al., “Miniature photonic-crystal hydrophone optimized for ocean acoustics,” J. Acoust. Soc. Am. 129(4), 1837–1850 (2011). [CrossRef]
28. W. Wang, N. Wu, Y. Tian, et al., “Miniature all-silica optical fiber pressure sensor with an ultrathin uniform diaphragm,” Opt. Express 18(9), 9006–9014 (2010). [CrossRef]
29. F. Guo, T. Fink, M. Han, et al., “High-sensitivity, high-frequency extrinsic Fabry-Perot interferometric fiber-tip sensor based on a thin silver diaphragm,” Opt. Lett. 37(9), 1505–1507 (2012). [CrossRef]
30. F. Xu, J. Shi, K. Gong, et al., “Fiber-optic acoustic pressure sensor based on large-area nanolayer silver diaghragm,” Opt. Lett. 39(10), 2838–2840 (2014). [CrossRef]
31. J. Ma, W. Jin, H. L. Ho, et al., “High-sensitivity fiber-tip pressure sensor with graphene diaphragm,” Opt. Lett. 37(13), 2493–2495 (2012). [CrossRef]
32. Y. Wu, C. Yu, F. Wu, et al., “A highly sensitive fiber-optic microphone based on graphene oxide membrane,” J. Lightwave Technol. 35(19), 4344–4349 (2017). [CrossRef]
33. F. Yu, Q. Liu, X. Gan, et al., “Ultrasensitive pressure detection of few-layer MoS2,” Adv. Mater. 29(4), 1603266 (2017). [CrossRef]
34. L. Hou, Y. Li, L. Sun, et al., “High-fidelity optical fiber microphone based on graphene oxide and Au nanocoating,” Nanophotonics 12(19), 3707–3719 (2023). [CrossRef]
35. W. Ni, P. Lu, X. Fu, et al., “Ultrathin graphene diaphragm-based extrinsic Fabry-Perot interferometer for ultra-wideband fiber optic acoustic sensing,” Opt. Express 26(16), 20758–20767 (2018). [CrossRef]
36. R. A. Maniyara, D. Rodrigo, R. Yu, et al., “Tunable plasmons in ultrathin metal films,” Nat. Photonics 13(5), 328–333 (2019). [CrossRef]
37. K. Chen, Z. H. Yu, Q. X. Yu, et al., “Fast demodulated white-light interferometry-based fiber-optic Fabry-Perot cantilever microphone,” Opt. Lett. 43(14), 3417–3420 (2018). [CrossRef]
38. C. Li, H. Qi, X. Han, et al., “Ultrahigh-speed phase demodulation of a Fabry-Perot sensor based on fiber array parallel spectral detection,” Opt. Lett. 49(3), 714–717 (2024). [CrossRef]
39. C. Li, X. Gao, T. Guo, et al., “Analyzing the applicability of miniature ultra-high sensitivity Fabry-Perot acoustic sensor using a nanothick graphene diaphragm,” Meas. Sci. Technol. 26(8), 085101 (2015). [CrossRef]
40. M. G. Allen, M. Mehregany, R. T. Howe, et al., “Microfabricated structures for the in situ measurement of residual stress, young’s modulus, and ultimate strain of thin films,” Appl. Phys. Lett. 51(4), 241–243 (1987). [CrossRef]
41. C. Pan, Y. Tong, H. Qian, et al., “Large area single crystal gold of single nanometer thickness for nanophotonics,” Nat. Commun. 15, 2840–2849 (2024). [CrossRef]
42. Z. Han, W. Bingkun, S. Mingguang, et al., “Optical fiber Fabry-Perot acoustic sensor based on large PDMS diaphragm,” in Proc.SPIE (2019), p. 110531W.
43. C. S. Monteiro, M. Raposo, P. A. Ribeiro, et al., “Acoustic optical fiber sensor based on graphene oxide membrane,” Sensors 21(7), 2336 (2021). [CrossRef]
44. J. A. Guggenheim, J. Li, T. J. Allen, et al., “Ultrasensitive plano-concave optical microresonators for ultrasound sensing,” Nat. Photonics 11(11), 714–719 (2017). [CrossRef]
45. X. Lu, Y. Wu, Y. Gong, et al., “A miniature fiber-optic microphone based on an annular corrugated MEMS diaphragm,” J. Lightwave Technol. 36(22), 5224–5229 (2018). [CrossRef]
46. M. Yao, Y. Zhang, X. Ouyang, et al., “Ultracompact optical fiber acoustic sensors based on a fiber-top spirally-suspended optomechanical microresonator,” Opt. Lett. 45(13), 3516–3519 (2020). [CrossRef]
47. H. Wei, Z. Wu, K. Sun, et al., “Two-photon 3D printed spring-based Fabry-Perot cavity resonator for acoustic wave detection and imaging,” Photonics Res. 11(5), 780–786 (2023). [CrossRef]
48. E. Krauss, R. Kullock, X. Wu, et al., “Controlled growth of high-aspect-ratio single-crystalline gold platelets,” Cryst. Growth Des. 18(3), 1297–1302 (2018). [CrossRef]
49. L. Liu, A. V. Krasavin, J. Zheng, et al., “Atomically smooth single-crystalline platform for low-loss plasmonic nanocavities,” Nano Lett. 22(4), 1786–1794 (2022). [CrossRef]
50. B. Zhao, Z. Wan, Y. Liu, et al., “High-order superlattices by rolling up van der waals heterostructures,” Nature 591(7850), 385–390 (2021). [CrossRef]
51. J. Cai, H. Chen, Y. Ke, et al., “A capillary-force-assisted transfer for monolayer transition-metal-dichalcogenide crystals with high utilization,” ACS Nano 16(9), 15016–15025 (2022). [CrossRef]
52. X. Ma, Q. Liu, D. Xu, et al., “Capillary-force-assisted clean-stamp transfer of two-dimensional materials,” Nano Lett. 17(11), 6961–6967 (2017). [CrossRef]
