Scientific report on field force-driven gold nano-film high-sensitivity space electric field sensing based on microfiber interferometer

2021-11-24 03:54:37 By : Ms. Aileen Lee

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Scientific Reports Volume 5, Article Number: 15802 (2015) Cite this article

Traditional electric field sensing can be realized using electro-optical materials or liquid crystals, and has the limitations of easy breakdown, free assembly, and difficulty in low-frequency measurement. Here, we propose a new method for realizing the safe measurement of spatial dynamic electric field by using a microfiber interferometer integrated with a gold nanomembrane. The charge energy received by the antenna forms an intrinsic electric field with two microelectrodes, one of which is a 120nm gold film vibrating beam that is microprocessed by a femtosecond laser and integrated with microfibers. The change of the intrinsic electric field force caused by the space electric field will cause the vibration of the film beam. The electric field can be measured by demodulating the output signal of the micro-fiber interferometer. We proved that the detectable frequency range is from tens of Hz to tens of KHz, and the minimum electric field strength is ~200 V/m at 1 KHz. Our electric field measurement technology that combines optical fiber interference and gold nanostructures has the advantages of safety, high sensitivity, small size, multi-point multiplexing and remote detection.

Electric field sensing is very important for preventing electromagnetic interference 1, voltage balance 2, 3, 4, shielding near-field electromagnetic radiation 5 and other special applications (such as detecting charge 6, electrostatic precipitation 7 and millimeter wave to light wave signal converter 8). Although traditional electric field sensors, such as steered-electron9 (detectable range from several mV/m to tens of V/m), spherical electric field probe 10 (<12 kV/m), bistable microelectronic circuit 11 and THEMIS three-axis The electric field instrument 12 (mV/m to several V/m) can work accurately in some applications. Due to the unpredictable high-intensity electric field, they and their subsequent circuits are easily damaged and require active devices, which makes them not Suitable for remote detection. In addition, metal circuits and signal transmission cables are susceptible to electromagnetic interference.

In recent years, the photoelectric field detection 6, 13, 14, 15, 16, 17, 18, 19, 20, 21 has attracted more and more attention. They have excellent qualities such as remote and safe measurement, passive components, integrated structure, easy networking based on WDM technology, and extremely weak interference to the environment and electric field sources. The time-domain photoelectric field measurement is based on two materials 14, 15, 16, 17, 18, 19, 20, and 21, respectively. The first type is electro-optical (EO) materials 14, 15, 16, 17, 18, 19, which are mainly used for RF frequency electric field induction from MHz to GHz. The corresponding detectable range is higher than 2.5 V/m (or several mW/m2 of the smallest detectable electromagnetic energy flux density) for reference. 14 and 19 V/m to 23 kV/m reference. 15. However, the irregular frequency response caused by the piezoelectric effect or other effects of EO22, 23, 24, 25 materials is rarely reported in low frequency applications below tens of kilohertz15, 26. The second material is liquid crystal 16, 18, 20, 21, which is suitable for the measurement of low-frequency electric field. According to reports, the detectable range of electric field strength is more than tens of KV/m. 20 and 1 to 4.1 kV/mm reference. twenty one.

Considering that most of the power systems use low-frequency electric field sensing, it is particularly important to realize high-sensitivity and safe measurement of low-frequency electric fields. For the first time, we have proposed a new method of manufacturing sensors by integrating antennas and optical fibers. The detectable frequency range is from tens of hertz to tens of kilohertz. According to the half-wave electric field strength, the minimum detectable electric field strength is ~200 V/m at 1 KHz, and the maximum is about 5 kV/m. The sensitivity can be further improved by modifying the structure and parameters of the antenna (when the antenna length is ~27 mm, the limit of the minimum detectable electric field strength can be as low as ~0.015 V/m). The key component of the sensor is the microfiber interferometer integrated with the gold nanomembrane. The strong electric field formed by the coupling of the antenna and the space electric field realizes sensing through the tiny area in the sensor. The gold nano film can be used as the electrode of the micro area, and at the same time it deforms under the action of the ultra-weak electrostatic force generated by the strong electric field. The vibration beam formed by the gold nano film can be used as the reflector of the Fabry-Perot (FP) interferometer. Therefore, the change of the space electric field can be detected by demodulating the change of the cavity length of the interferometer under the action of electrostatic force.

The measurement system for detecting the electric field in the near space is shown in Figure 1. The output wavelength from the tunable laser AQ4321D is 1520-1620 nm, and the optical signal with a power of 0.1-4 mW passes through the coupler (50:50) to reach the sensor. The change of the spatial electric field is modulated into signal light by the sensor. And the signal light reaches the photodetector (PDA20C/M, PD) through the same coupler in the reverse direction. The output of the PD is input to subsequent signal processing equipment, such as frequency converters, amplifiers, oscilloscopes, etc., to record and demodulate changes in the spatial electric field.

Schematic diagram of electric field induction.

(a) Experimental device. (b) The simulated distribution of the electric field intensity of the sensing system. (c) The structure of the sensor and the internal electric field strength. (d) Photograph of the sensor.

The structure of the sensor is shown in Figure 1c. The sensor consists of two parts: The first part is a built-in plate capacitor composed of a 120nm gold nano-film vibrating beam and a metal probe end surface. The metal rods respectively connected to the gold nano film and the metal probe form a spatial electric field receiving antenna. Due to the electrostatic induction effect, the built-in electric field of the plate capacitor is equivalent to the amplified result of the electric field in the antenna space. The second part is the FP interferometer formed by the other side of the gold nano-film vibrating beam and the fiber end face. The change of the built-in electric field of the plate capacitor has the ability to drive the beam to vibrate, thereby realizing the modulation of the interference signal of the FP interferometer.

In order to explore the appropriate working wavelength, the reflectance spectrum of the FP interferometer was measured, as shown in Figure 2. We can see that the output voltage of the PD is from 3.9 V to 0.7 V and the wavelength is from 1536 nm to 1543 nm, and the slope is Kslope. Determine that the working wavelength is 1540.5 nm, and the corresponding output voltage amplitude is 2.0 V.

The reflectance spectrum of the sensor.

It is measured with a spectrometer (Si720).

It should be noted that in the absence of a magnetic field, only the near-field low-frequency electric field distribution is considered. In order to quantitatively describe the electric field sensing characteristics of the sensor, we simulated the intensity of the induced electric field with different structural parameters (see Figure 3). It can be seen that although the electric field intensity in the gap is large, the electric field exists in a large area, which is unexpected and inevitable (see Figure 3a). When we adjust the size of the probe end face and the center vibrating beam, the center electric field intensity of the gap remains almost unchanged. This can be explained by the fact that on a larger area scale, the amount of induced charge accumulated in the gap is only a small part of the total. In other words, even if the area of ​​the end face and the center vibrating beam changes, the equivalent capacitance inside the sensor remains almost unchanged. It can be concluded that the electric field strength of the gap Egap has nothing to do with the gap area Sgap, but is proportional to the magnitude of the external space electric field strength Eext. The magnitude of the electrostatic force is proportional to the product of Egap and Sgap. In addition, according to Figure 3c, Egap is almost inversely proportional to the gap length Lgap. We can derive the relationship between the peak optical signal change ΔIpeak and the peak external spatial electric field intensity Eextpeak (the direction parallel to the metal rod), as in equations (1) and (2) (see supplementary information for details):

The simulated distribution of the electric field strength within the sensor.

(a) Distribution within the entire sensor. (b) The distribution of the mid-plane between the vibrating beam and the probe surface. (c) The relationship between the electric field intensity at the center of the gap and the length of the gap.

Where εgap is the relative inductance of the gap (here 1), ε0 is the dielectric constant of free space, and L(structure, ε) is the equivalent antenna length, which depends on the structure of the antenna (except for the gap length) and insulating packaging materials and their The relative inductance ε and the gap εgap. The optimal L(structure, ε) is approximately the total length of the two antennas, D is the stiffness coefficient of the vibrating beam, and Lfp is the cavity length of the FP (here 37.4 μm) interferometer.

The low-frequency electric field experiment was carried out when the frequency range was 30 Hz to 27 KHz and the maximum electric field strength was 3600 Vpp/m. The experimental results are shown in Figure 4. For frequencies below 500 Hz, there is a drop in response (see Figure 5) and an equivalent phase difference (see Figure 4a). There is a response peak at 19 KHz. The minimum electric field strength that can be detected at 1 KHz is ~200 V/m, which is measured by reducing the electric field strength until the light signal cannot be extracted from the background noise. In our measurement system, the 10V output test signal generator is used to transmit the signal directly to the antenna pole through the wire to replace the previously used space electric field source. The dimensionless value of the voltage drive sensitivity shown in FIG. 5 is the normalized serial value of the optical signal change obtained when the same amplitude sweep voltage is applied to the sensor. The results show that the sensor has a flat response below 13KHz (the equivalent phase difference is small), and there is also a peak at 19KHz (the equivalent phase difference is large).

The electric field induction waveform includes the input voltage of the transformer (black), the intensity of the electric field around the sensor (red) and the light signal (blue).

(a) 50 Hz. (b) 500 Hz. (c) 2 kHz. (d) 10 kHz. (e) 19KHz. (f) 25 kHz.

Our sensors have the smallest detectable frequency characteristics of electric field strength, voltage drive sensitivity and electric field sensitivity.

The response of our sensor is flat in the mid-frequency range of 500 Hz~13 KHz, and the low-frequency response around 0 Hz is very weak. The obvious response peak appears at the high frequency of 19 KHz, which is understandable, because the natural resonance frequency of the vibrating beam may be ~38 KHz (this is also the maximum modulation frequency of the FP interferometer of our sensor). We can explain the decrease in low-frequency response near 0 Hz from the perspective of impedance. When the resistance of the sensor glass sleeve is assumed to be infinite, the sensor has a good amplitude-frequency response near 0 Hz. But in fact, the resistance of the glass sleeve is limited, even smaller than the capacitive reactance of the sensor. Considering the parallel connection of resistance and capacitance, the impedance of the sensor is smaller. Therefore, if we consider them along the electrodes of the electric field source in the partial voltage model, the voltage occupied by the strongly insulated external air is much higher than the voltage supplied to the sensor near 0 Hz. By using packaging materials with higher resistivity, the frequency response around 0 Hz can be flattened. We can also understand the response of the sensor from the perspective of the phase relationship. It is the parallel connection of resistance and capacitance that leads to the high-pass filtering effect of the sensor, that is, compared with Eext, Egap has a phase-advanced phase, and ΔI has a phase-retarded gold-film vibrating beam due to mechanical characteristics. These two opposite effects cause the light intensity change to have almost the same phase as the mid-frequency electric field intensity, but it has a phase lead at low frequencies and a phase delay at high frequencies (see Supplementary Figure S2). Regarding the vibrating beam as a spring vibrator, the light intensity change can be expressed as:

Where m and γ are the equivalent mass and damping coefficient of the vibrating beam, respectively. com(fre) is a function of compensation amplitude and frequency at the same time. When determining the source of the electric field, after determining m and γ, com(fre) can be calculated from Figure 5. In practical applications, since we are trying to measure a period rather than a DC signal, there is no need to use an unmodulated optical signal with significant noise as a zero electric field reference point. Instead, based on the square relationship between ΔI and Eext, we can calibrate the medium-intensity electric field and the corresponding optical signal change output value of the sensor to find the intensity of the target electric field.

According to the parameters of the sensor, the upper limit of the frequency response is ~20 KHz. Compared with the deformation speed of the beam, the charge and discharge rate is faster, which is implied by the existence of the peak response at high frequency, indicating that the upper limit of the frequency and the air resistance of the cavity can be further improved by reducing the mass of the center of the vibrating beam. ) Or design a gold film connection with a higher stiffness coefficient. In the frequency range of 1 KHz to 12 KHz, the minimum detectable electric field strength is ~200 V/m (internal gap length is 13 μm), which will be larger at lower frequencies, but smaller at higher frequencies. It can be improved by increasing the overall dimensions of the antenna rod, especially the length, or reducing the length of the gap inside the sensor. According to the light intensity noise of the interferometer in the sensor, the minimum detectable deflection of the vibrating beam can be estimated to be ~1 nm. If the length of the internal gap is reduced to 1 nm, while the size of the antenna remains unchanged, according to formula (1) we can calculate that the minimum detectable electric field strength can be as low as ~0.015 V/m (the total length of the antenna is about 27 mm , The equivalent length is about 13 mm). As for the theoretical sensitivity, our sensor is superior to other optical sensors that require waveguides with internal electrode gap lengths greater than 100 nm. The above analysis also shows that our sensor can be further reduced to a smaller size through nanotechnology. In terms of low-frequency electric field response, mixed metal impurity materials are used in the electro-optical material electric field sensor used in the production of optical waveguides to improve high-frequency sensitivity by sacrificing low-frequency electric field response. The insulation of the sensor. The liquid crystal electric field sensor cannot increase the low frequency sensitivity by increasing the liquid crystal resistance. In contrast, there is an air gap in our sensor, and with its high insulation resistivity, it has the ability to generate extremely low frequency responses. By using packaging materials with higher resistivity, the frequency response around 0 Hz can be made flatter. Moreover, it is easier to realize the packaging of the sensor, the antenna and the optical fiber. There is no doubt that our sensors are competitive due to their superior performance.

In addition, according to the internal electric field strength, the electrostatic force generated on the vibrating beam ranges from 10-11 to 10-9 N (see Supplementary Equation S(2)). According to the actual measurement of the optical signal amplitude, the calculated maximum deformation (center deflection) can reach several hundred nanometers. However, the large electric field will cause large deformation of the vibrating beam, and the Egap will be disturbed due to the change of Lgap, which will cause the optical signal to be distorted. Due to the square field effect, the vibrating beam will be affected by the bias force, which will affect the operating wavelength of the sensor (see Supplementary Figure S3) and change the dynamic range of the sensor accordingly. In practical applications, tightening the mechanical structure of the sensor, especially the FP cavity and gap, helps to eliminate the original light intensity and electric field intensity response fluctuations. The influence of temperature and vibration on the electric field measurement should also be considered. It is recommended to fix our sensor on a stable platform and introduce temperature compensation structures and algorithms to overcome structural deformation caused by temperature changes. The power consumption includes the very small amount of energy extracted from the electric field source and the laser source power (0.1~4 mW), which depends on the initial charging energy (<0.1 μJ) of the equivalent capacitance of the sensor, and the initial kinetic energy is gold film (<10-22f 2 J/s2, f is the electric field frequency), air resistance power consumption (<1 nW) and sensor resistance heat (resistance from the encapsulation glass, <10-11 W). Our sensor has the characteristics of miniaturization and small cross-effect with electric field source or environment. If three sensors are assembled in different directions, it is suitable for directional space electric field sensing and even three-dimensional sensing.

In summary, we have proved the feasibility of a new method for safe measurement of spatial dynamic electric fields by using a microfiber interferometer integrated with a gold nanomembrane. Although the internal electric field intensity of the equivalent drive belongs to the same class as the existing optical sensing mechanism, by reducing the gap length to the 1 nanometer scale, the external-to-internal electric field gain factor of our sensor may be larger than any other optical sensor. Principle limitation (the EO14,15 method requires the gap of the optical waveguide to be greater than 300 nm, while the LC20 method requires 30 μm). This means that our limit minimum detectable intensity (15 mV/m) is better than other optical sensors14,15,20,21. And the resistivity of our sensor (air dielectric) may be much larger than any other sensor (EO material or LC dielectric), which illustrates the competitiveness of our sensor in low-frequency electric field detection, such as photoelectric pulse detection19,27, Monitoring and direction measurement of electric field in power grid.

The electrical signal from the signal generator of tens of Hz to KHz is input to the power amplifier, and then used as the input of the transformer (there are several different transformers for different frequency ranges). Measure the actual transformation ratio and phase characteristics in advance to calculate the high voltage output of the transformer. High voltage is used to generate the required medium to high intensity electric field. The electric field intensity around the sensor is calculated based on the simulation of the electric field distribution.

The end of the stainless steel capillary tube (inner diameter 250 μm) is covered with a whole piece of gold film. It is made of femtosecond laser and high-precision processing platform. The center wavelength of a femtosecond laser is about 790 nm. The pulse repetition frequency is 1 KHz. And the focal spot size is ~3 μm. Adjust the energy of the laser pulse so that it is just enough to vaporize gold (~1 μJ) to ensure smooth beam edges.

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This work was funded by the National Natural Science Foundation of China. 61405020, 61475029, 61377066, Chongqing Outstanding Youth Science Foundation CSTC2014JCYJJQ40002. Thanks Huang Wei, Deng Ming, Cao Zhengzhou, Tang Xiaosheng and Zhang Zhigang for their helpful discussions.

Key Laboratory of Optoelectronic Technology and System of Ministry of Education, Chongqing University, Chongqing 400044

Zhu Tao, Zhou Liming, Liu Min, Zhang Jingdong, Shi Leilei

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TZ proposed and coordinated this work. LZ jointly developed this idea, established an experimental device and conducted experiments. TZ, LZ, ML, JZ and LS analyzed the theory. We conducted data analysis and drafted the manuscript together.

The author declares that there are no competing economic interests.

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Zhu, T., Zhou, L., Liu, M. etc. Scientific Report on Highly Sensitive Space Electric Field Sensing Based on Field Force Driven Gold Nano-Film Microfiber Interferometer 5, 15802 (2015).


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