Study on shielding effect of several shielding cloths in terahertz band

With the rapid development of electronic technology, various weapon systems on the battlefield in the future are facing serious threats. Stealth technology has been considered as an effective means to improve the survivability and penetration capability of weapon systems. The development and application of shielding materials is one of the key factors in the development of stealth technology, and is highly valued by the major military countries in the world. A number of stealth material patents compatible with multi-band have appeared in foreign countries. At present, some units in China have also developed multi-band camouflage masks, which use visible light, infrared and radar compatible stealth design techniques to achieve multi-band stealth. Although stealth shielding materials have achieved great results in the microwave and infrared bands, little research has been done between the terahertz bands.

Terahertz radiation generally refers to far-infrared electromagnetic radiation at frequencies between (0.1 and 10) THz (wavelengths between 30 μm and 3 mm) with a wavelength between microwave and infrared. Its unique performance has had a profound impact on communications, radar, electronic countermeasures, electromagnetic weapons, security inspections, etc., and will also lead to new changes in military defense and security.

The application of terahertz spectroscopy to the study of shielding materials is of great significance in military, defense, aerospace and security fields. Therefore, the shielding properties of four shielding cloths in the terahertz band were studied experimentally, and the refractive index curves and shielding effectiveness curves of these shielding cloths were obtained. Experiments show that these materials have good shielding for terahertz waves. Performance can be used to some extent as a terahertz shielding material.

Electromagnetic shielding is mainly used to prevent the influence of high-frequency electromagnetic fields, thereby effectively controlling the electromagnetic wave propagation from one area to another. The basic principle is to use a low-resistance conductor material, which uses the electromagnetic wave to reflect on the surface of the shield conductor, the absorption inside the conductor and the loss of the transmission process to produce a shielding effect, which is usually expressed by the shielding effectiveness (SE). The value is measured in terms of the ratio of the field strength (E1 or H1) of the space protection zone in the absence of shielding to the field strength (E2 or H2) of the zone in the presence of shielding, in decibels (dB). The formula is defined as:

SE=20log(E1/E2) (1)

SE=20log(H1/H2) (2)

SE=10log(P1/P2) (3)

In the formula, E1 and E2 are the electric field strengths before and after shielding; H1 and H2 are the magnetic field strengths before and after shielding; P1 and P2 are the energy field strengths before and after shielding. There are many factors affecting the shielding effectiveness SE, mainly the frequency of the electromagnetic field and the electromagnetic parameters of the material.

Generally, the specific classification of shielding effectiveness is: 0~10dB has almost no shielding effect; 10~30dB has smaller shielding effect; 30~60dB medium shielding efficiency, can be used for general industrial or commercial electronic products; 60~90dB shielding effectiveness High, can be used for shielding of aerospace and military equipment; more than 90dB shielding material has the best shielding performance, suitable for demanding high precision, high sensitivity products. According to practical needs, for most electronic products, the shielding material is considered to be effective shielding at a frequency of 30~1000Hz with a SE of at least 35dB.

The experiment uses the terahertz time-domain spectral transmission system of the Physics Department of Capital Normal University (as shown in Figure 1), which is mainly composed of a femtosecond laser, a terahertz radiation generating device, a terahertz radiation detecting device and a time delay control system. This system uses a self-mode-lockable tunable titanium sapphire laser produced by Spectroscopic Physics, with a pulse center wavelength of 800 nm, a repetition rate of 82 MHz, a pulse width of 100 fs, and an output power of 1043 mW. The femtosecond laser pulse generated by the titanium sapphire is split into two beams by a splitting prism (CBS), one beam as the pump light and the other as the probe light. The pump light is modulated by a chopper with a frequency of 1.1 kHz, incident on the lens L1 through a time delay stage, and irradiated onto the surface of the <100>-InAS crystal by a lens focusing at an incident angle of 45°, thereby radiating a terahertz pulse. Two pairs of gold-plated off-axis mirrors collimate and focus, through a high-resistance silicon wafer, focused on the <110>-ZnTe crystal; another laser pulse—detection light passes through a series of mirrors, lens L2 The polarizer P and the terahertz pulse are simultaneously focused at the same position of the <110>-ZnTe crystal, at which time an electro-optic effect occurs in the ZnTe crystal. The polarization-modulated probe light is focused to the Wollaston prism (PBS) through a quarter-wave plate (QWP) and lens L3, and is divided into two components whose polarization directions are perpendicular to each other. Diode detection, demodulated by the lock-in amplifier, input to the computer using the control software written by Labview to obtain the terahertz pulse time domain information. In this paper, the effective spectral width detected by this device is (0.1~2.5) THz, the spectral resolution is 50GHz, and the signal-to-noise ratio is 600. In the experiment, we placed the sample at the focus of the high-intensity off-axis mirror PM2 and perpendicular to the direction of the incident light. In order to reduce the absorption of terahertz waves by air moisture in the experiment, nitrogen gas was flushed into the dotted line frame of the optical path, so that the humidity of the environment during detection was less than 5%, and the experimental temperature was 21 °C. In order to ensure the accuracy of the experiment, a reference signal is measured for each sample in the experiment.

Figure 1, terahertz spectroscopy experimental system

The four special electromagnetic wave shielding materials in the sample were purchased from Beijing Jingmian Huaxing Special Shielding Material Technology Co., Ltd., and the relevant units have proved that the patented shielding cloth produced by the company has the shielding effectiveness of B or above in the microwave, and has the characteristics of waterproofing and sun protection. It can be used as a shielded tent, screen door, shielded zipper, etc. in the military. It can also be used as a mobile phone shielding cover, microwave protective cover and maternity clothes to reduce or prevent damage from microwave and infrared radiation. The shielding material is based on cotton fiber fabric, and is physicochemically treated to infiltrate metal elements into its structure. It has a shielding effect on the basis of maintaining the original fabric characteristics. The thickness of the shaded color shielding cloth is 0.38 mm, and the thickness of the brick red and gray shielding cloth is 0.28 mm and 0.12 mm, respectively.

The terahertz time-domain spectroscopy system can obtain time-domain data of the terahertz wave incident and transmitted electric field, and then obtain the corresponding frequency domain data by fast Fourier transform, using the formula:

SE=10log(P1/P2)

The shielding effectiveness SE can be obtained. In the formula, P1 and P2 are the energy of the terahertz incident electric field and the energy of the terahertz transmission electric field, respectively.

Figure 2 shows the shielding performance curves of (0.2~2.5) THz for gray shielding cloth, brick red shielding cloth, deep color shielding cloth and light camouflage shielding cloth.

(a) Shielding effectiveness curve of gray shielding cloth

(b) Shielding effectiveness curve of brick red shielding cloth

(c) Shielding effectiveness curve of light colored shielding cloth

(d) Deep absorption spectrum of colored shielding cloth

Figure 2. Shielding effectiveness curve of four shielding cloths

By observing the shielding effectiveness curves of these samples, we can see that in the range of (0.20~2.20) THz, the shielding effectiveness of the three kinds of shielding fabrics of gray and shade can be more than 30, with medium or even medium and above. The shielding effect can be used for shielding of general industrial or commercial electronic equipment. Figure 3 shows the shielding effectiveness of the four shielding cloths. It can be seen from the numerical value that the shielding effectiveness of the four shielding cloths in the low frequency band is relatively large. Among them, the shielding effectiveness of the brick red shielding cloth is relatively poor, but the shielding performance between the (0.28~0.47) THz is also more than 30dB, which has a medium shielding effect; the shielding performance of the gray shielding cloth is relatively best, and the minimum is also 40dB. Above, and at (0.33~1.15) THz shielding performance exceeds 60dB, the highest is 72.5dB, shielding effect is good, can be used for shielding of aerospace and military equipment. This shows that if such a shielding cloth is applied to the terahertz band, it will have a better shielding effect, which will bring great difficulties to the corresponding terahertz radiation detection and detection.

Figure 3. Comparison of shielding effectiveness curves of four shielding cloths

Experiments show that the four special shielding cloths produced by Beijing Jingmian Huaxing Special Shielding Material Technology Co., Ltd. have better shielding effectiveness in the terahertz band. The shielding effectiveness of the four kinds of shielding cloths is above 30dB, which can be used for shielding the microwave and terahertz waves of electronic equipment to prevent information leakage or interference. Among them, the shielding effect of gray and brick red shielding cloth is better. The shielding performance of (0.33~1.15) THz exceeds 60dB, up to 72.5dB, and because these shielding materials have the characteristics of waterproofing and sunscreen, these substances It can be considered as a shielding material for the terahertz band, and it can detect the detection of terahertz radiation in military, aerospace and communication information. It may also be used to shield the packaging raw materials against terahertz explosives, firearms and other arms items to ensure Protection of important items in military warfare. For example, it is envisaged to apply terahertz "confrontation" techniques for sensitive detection to detect targets or detect their internal topography. If the surface of the target enables the terahertz wave to be absorbed or scattered, the probability of being discovered by the other party can be greatly reduced, thereby achieving the invisible purpose. Of course, the development of terahertz shielding materials still needs to solve many problems, such as improving the power of terahertz radiation, improving the sensitivity and signal-to-noise ratio of detecting terahertz radiation, and further analyzing and processing image information. With the development of technology, the application prospect of terahertz radiation technology will be very broad and will play an important role in the military field.

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