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Abstract
Directional energy weapons such as high-power microwaves and high-energy lasers pose a huge threat to optoelectronic detection systems. With that in mind, we designed an infrared optical window that has a nonlinear optical response to high-energy lasers and electromagnetic shielding to microwaves. By constructing a periodic metal circular hole array structure at the subwavelength scale, surface plasmons resonance is excited and its local field enhanced characteristics are utilized to form information transmission compatibility in the infrared band. At the same time, after laser etching off the subwavelength structure, the remaining metal forms a continuous conductive structure, forming an ultra-wideband shielding layer to achieve ultra-high and wide protection in the microwave band. Moreover, a layer of Ge2Sb2Te5 thin film was deposited between the transparent substrate and the metal film. Utilizing its nonlinear optical properties of high-temperature phase transition to reduce damage of directed energy weapons to the photoelectric detection system and equipment. Thus, when the photoelectric detection system or device is damaged or interfered by signals of different frequency bands or energies, the filtering window can achieve multi-mode shielding function.
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1. Introduction
Photoelectric detection technology and detection systems have achieved unprecedented development in the fields of target detection, early warning, fire control, and guidance. Therefore, electromagnetic radiation generated by the operation of various electronic devices and related infrastructure is becoming increasingly common. The bandwidth and intensity of radiation are widening, leading to a more complex electromagnetic environment [1–3]. Meanwhile, electromagnetic interference caused by disordered and high intensity electromagnetic radiation can interfere with the normal operation of the photoelectric detection system [4,5]. Especially the complex optical sensors, photoelectric conversion modules, and integrated circuits within the photoelectric detection system are particularly susceptible to electromagnetic interference [6]. An effective way to protect or reduce the damage such systems from electromagnetic radiation is to integrate electromagnetic functional materials into the optical window that the main channel for radiation to enter the system [7,6]. Therefore, the electromagnetic interference shielding material used for optical windows in optoelectronic systems must be able to effectively prevent microwave radiation and achieve high quality signal transmission and compatibility with high transparency [7–9]. It is worth noting that although research on transparent electromagnetic interference shielding materials is mostly focused on the visible light band, the demand for infrared optoelectronic systems in military, aerospace, and other fields highlight the importance of developing integrated protection for infrared transparent electromagnetic shielding windows [10–14].
With the development of micronanotechnology, subwavelength structured filters have been applied due to their unique field enhancement effect and functional integration [15–18]. Compared to the metal mesh structure [19,20], it solves the dual high decoupling problem of infrared transmittance and microwave shielding efficiency, namely high infrared transmittance and high electromagnetic shielding efficiency. Particularly, mesh shielding materials typically exhibit a decrease in shielding effectiveness with increasing frequency, resulting in significant high frequency characteristics; On the other hand, compared to the multilayer film structures [21–23], a single metal film structure also greatly improves the problem of film fall off. However, the subwavelength metal structure process is relatively complex, and once manufactured, it is difficult to modify and regulate its structure and performance, resulting in low adaptability to complex application scenarios. At the same time, with the rapid development of materials science and laser technology, the performance of coherent radiation sources in the infrared band has gradually improved, and corresponding infrared technology has also achieved rapid development [13]. Especially, laser interference can cause saturation, glare, and other interference on thermal imagers, causing serious impact on imaging quality [24,25]. Meanwhile, complex application scenarios and environments are increasingly driving its development towards multifunctionality, integration, and controllability.
In recent years, active device regulation based on functional materials [26–29] has been an effective modulation method. The unique characteristics of various functional materials, such as adjustable carrier concentration, adjustable atomic or molecular arrangement, and even adjustable surface stress, have become the solutions for researchers to actively regulate based on electrical, optical, thermal, mechanical and other means. Even more so, the surface plasmon surface is sensitive to the medium environment and geometric shape. This enables the application of actively regulated metasurfaces in switch modulators, sensor devices, high resolution color imaging, zoom lenses, and other fields [30–35], laying a solid foundation for the practical application of actively regulated metasurfaces. Therefore, by combining micro/nano structures with functional materials, the above problems can be effectively solved.
Through engineering design and optimization of structural parameters, such as material, period, pore size, and especially film thickness, it is expected to achieve outstanding overall performance. In this article, the infrared optical window with nonlinear optical response and microwave electromagnetic shielding function is designed and fabricated. It constructs a periodic metal circular hole array structure at the subwavelength scale, excites surface plasmon resonance, and utilizes its local field enhancement characteristics to achieve the maximum transmittance over 0.7 within 3.7 to 4.8 µm band. At the same time, after laser etching off the microstructure, the remaining metal film forms a continuous conductive structure to form an ultra-wideband shielding layer, achieving electromagnetic shielding of over 40 dB in the microwave band range of 1-18 GHz. Furthermore, by depositing a layer of Ge2Sb2Te5 (GST) thin film between the transparent substrate and the metal thin film, utilizing the nonlinear optical properties of its high temperature phase transition, the energy of directional energy weapons is weakened, achieving protection for photoelectric detection systems and devices. In addition, our results also indicate that the thickness of GST thin film is a key parameter that has a significant impact on the bandwidth of the infrared communication window and the degree of transmittance change under different crystal states.
2. Structure and theory
The essence of electromagnetic shielding is to provide a passband for infrared wave and a stopband for microwave and high-energy. For devices with corresponding bands in the infrared band, it can be achieved through periodically arranged micron sized apertures. The microstructure used for transparent electromagnetic interference shielding can be designed with a cutoff frequency much higher than the cutoff frequency of the shielded wave. That is to say, which means significant attenuation when microwaves pass through, while ensuring effective transmission and even enhancement of infrared light.
As shown in Fig. 1, a subwavelength structure with periodic arrangement and continuity is designed. Considering the influence of the polarization direction of the incident light on the geometric parameters of the structure, a planar fully symmetric circular structure is adopted. In detail, the substrate material is sapphire; The light green part is the plated GST film. When simulated using CST simulation software, the refractive indices of amorphous GST (a-GST) and crystalline GST (c-GST) were obtained from literature [36]. The yellow part is a gold film, and the modified Drude model [37] is used to describe the relative dielectric constant of the metal material. Px and Py represent the unit period along the X and Y axes, respectively. The thickness of the metal film is h1, the thickness of the GST film is h2, and the thickness of the substrate is h3. Among them, a series of circular holes arranged periodically throughout the entire metal film layer are set on the metal film, with a radius of r. The incident light is perpendicular to the surface of the thin film in the Z direction, and the polarization direction of the electric field is in the X direction. Use CST simulation software to simulate the transmission process of light in periodic subwavelength metal holes.
Fig. 1. Unit structure diagram. Among of them, the structural period is P, the radius of the circular hole is r, the thickness of the metal film is h1, the thickness of the GST film is h2, and the thickness of the sapphire substrate is h3.
The enhanced optical transmission (EOT) effect of subwavelength hole arrays in metal films can be characterized by transmittance T [38]:
3. Simulation and discussions
Surface plasmons have excellent characteristics of local field enhancement, and can excite different resonance modes by etching periodic subwavelength structures of different sizes and structures on metal surfaces, thereby achieving the goal of selecting the frequency of a specific incident wavelength and enhancing its transmission effect.
As shown in Fig. 2(a), we ensure that the narrowest line width is 0.7 µm, as the process limit of the lithography equipment used in this experiment is 0.7 µm. By comparing structures with different periods and radii, it is found that as the period increases, the transmission peak undergoes a red-shifted phenomenon, and it basically shows linear offset. As shown in the literature, once the material used in the structure is determined ɛd and ɛm determines that the array period P is the main factor determining the position of the transmission peak [38], and this is also one of the manifestations of SPPs resonant mode excitation. It should be noted that when the value of period P is 3.5 µm, the distribution of the transmission spectrum is just suitable for information transmission in the infrared band. However, when the narrowest linewidth remains constant, as the period decreases, the overall transmittance also responds to a decrease. It is indicating that the duty cycle of the structure also plays a significant role in the amplitude of the transmittance. Therefore, when developing high performance transparent conductive materials, it is necessary to balance between optical transmittance and response range. As shown in Figs. 2(c-j), the transmittance of a single GST film at different thicknesses in both a-GST and c-GST states is compared. The results showed that as the thickness of the GST film increased, the overall transmittance decreased with increasing energy loss. But it can be clearly seen that when GST is in the crystalline state, there is a significant decrease in transmittance. From an energy perspective, the transition from an amorphous state to a crystalline state is the process in which the disorderly arrangement of atoms in the amorphous state enhances their thermal motion after heating and tends towards a more orderly atomic arrangement in the crystalline state [39]. That is to say, by controlling the temperature, the GST film can undergo mutations between amorphous and crystalline states, thereby forming a nonlinear response display to transmittance. By to characterized the extinction ratio of GST thin film transmittance in two states through the formula [40]:
Among of them, Tmax(a-GST) and Tmax(c-GST) are the transmittance at 4 µm for amorphous and crystalline conditions, respectively. The reason for choosing a wavelength of 4 µm is that, when designing the optical response of the filtering window, we chose the medium wave band in the communication band, which is 3-5 µm. When the central wavelength of the transmission peak is closer to 4 µm, it means that the proportion of high transmittance in the range of 3-5 µm is also larger, and the overall transmission effect is better. As shown in Fig. 2(b), when the wavelength λ at 4 µm, it is found that as the thickness of the GST film increased, more disordered atoms participated in the phase transition process, leading to η the larger. Based on the analysis of Figs. 2(b) and 2(c-j), as the thickness of GST increases, the overall transmittance and η mutual constraints. Therefore, in order to balance the overall transmittance and η the relationship between the GST film thickness, the GST film thickness we use is 40 nm.
Fig. 2. (a) The transmission spectrum of single layer subwavelength structured metal films under different periods; (b) The extinction ratio distribution of the transmittance of GST thin films with different thicknesses in two states at a wavelength of 4 µm; (c-j) The transmission spectra of single layer GST thin films with different thicknesses and states.
When the thickness of the GST film is 40 nm, the geometric structure shown in Fig. 1 is used to optimize its parameters. As shown in Fig. 3(a), as the structural period increases, the pattern of transmission peaks is consistent with Fig. 2(a). The difference is that when the transmission peak position is at 4 µm, the corresponding period P is 2.5 µm. In the absence of GST film, as shown in Fig. 2(a), the period is greater than 3.5 µm. That is to say, when adding a layer of GST film, the transmission peak undergoes a red-shifted at the same period. It is easy to understand that due to the refractive index of GST thin films is higher compared to the substrate with air or sapphire. The increase in refractive index will lead to a red-shifted phenomenon in the transmission peak. Furthermore, when the period P of the Au-GST-Sub structure is 2.5 µm and the radius r of the circular hole is 0.9 µm, further optimization was carried out for the thickness of the GST film, as shown in Figs. 3(c-h). Obviously, two different transmission peak red-shifted occurred here. As the thickness of the GST film increases, the red-shifted of the transmission peak after phase transition becomes more pronounced. This is evident in the optical design of infrared antireflection protective film systems. The increase in film thickness is also known as an increase in optical path. In summary, when the refractive index of the first layer of film (gold film) and the third layer of film (substrate) increases or decreases, the impact on the transmittance is not significant, only the center wavelength shifts to the right and left. However, when the refractive index of the intermediate layer film (GST film) increases due to phase transition, not only does the central wavelength of the transmittance shift to the right, but the transmittance also slightly decreases due to Ohmic loss. On the other hand, the transmission peaks of GST films with the same thickness before and after phase transition also show a significant red-shifted. Due to changes in the dielectric constant of GST materials before and after phase transition, the degree of coupling with metal thin films with microstructures also changes. Furthermore, the resonance point also shifts. This has also been well validated in other studies [41]. At last, the electric field distribution on the surface of the metal structure under different states of GST thin films was monitored, and the results are shown in Fig. 4. When GST is in a crystalline state, the accumulated charges inside the circular hole are significantly reduced. Undoubtedly, the transmittance decreases accordingly [42].
Fig. 3. (a) The transmission spectrum of Au-GST-Sub structure at different periods; (b) The extinction ratio distribution of the transmittance of GST thin films with different thicknesses under two different states; (c-h) The transmission spectra of Au-GST-Sub structures under different thicknesses and states.
Fig. 4. Surface electric field distribution of metal thin films under amorphous and crystalline GST conditions.
Combined with Fig. 3(c) the distribution trend of η, the final selected parameters are: the period P of 2.5 µm, the circular hole radius r of 0.9 µm, the GST thickness h2 of 40 nm, the gold film thickness h1 of 100 nm, and the sapphire substrate thickness h3 of 0.45 mm.
4. Manufacture and experiment
Based on the above parameters, it is processed using mature stepper lithography technology and ion beam etching (IBE) technology. The main process design is shown in Fig. 5(a): (1) A layer of 40 nm thick GST film and 100 nm thick gold film were deposited on the surface of the sapphire substrate, respectively; (2) By spin coating photoresist on the surface of the gold film and performing step exposure processing; (3) Perform development processing; (4) Using IBE for circular hole etching; (5) Remove the photoresist and truing.
Fig. 5. (a) Fabricating process flowchart; (b-d) Partial enlarged view of the sample structure. Among of them, Figure (d) shows an oblique view.
By performing SEM microscopy on the sample, the results are shown in Figs. 5(b-d). Figures 5(b-c) show an enlarged view of the regional structure of the sample. Due to the lithography limit of the stepper process we used being 0.7 µm, this value was adopted for the narrowest line width of the structure in the preliminary design. At the same time, we also provided an oblique view of the metal surface in Fig. 5(d) to observe the flatness of its film layer. It was found that, due to the later photolithography and development processes, there was a slight error in the radius of the circular hole. At the same time, when ion beam etching is used, a certain degree of burrs appears on the surface of the metal film, especially at the edge of the circular hole. This has a significant impact on the transmittance of the sample. Moreover, the integrity of the GST film inside the circular hole is difficult to control.
Therefore, this also causes the overall low transmittance of the sample, as shown by the solid line in Fig. 6(a). It is worth noting that there is no significant nonlinear optical response in the transmittance under amorphous and crystalline conditions. This means that the GST film did not perform its intended function. In the operation of changing the GST state, the sample in the c-GST state is directly heated to 200 degrees centigrade through a heating device, thereby transforming into an a-GST state. According to the literature [42], as the temperature increases, there will be a slight decrease in thickness during the phase transition process of the GST film. Based on the analysis of GST thickness in Figs. 3(c-h), a decrease in thickness will result in a blue shift of its transmission peak. Meanwhile, combined with the loss of GST thin film in the circular hole caused by laser etching process, there is no significant shift in the transmission peak of GST thin film before and after the phase transition process. It is worth noting that this does not affect the shielding function of the sample against high energy lasers. But the difference is that, in order to improve the reduction of transmittance, we have optimized the process. In order to ensure that GST thin films are not lost during laser etching, we deposit metal thin films and GST thin films on both sides of the sapphire substrate, forming a sandwich structure of GST-Sub-Au. This will protect the GST film during the laser etching process and reduce the scattering loss caused by the unevenness of the GST film surface. The transmission spectrum distribution after the process improvement is shown by the dashed line in Fig. 6(a), which shows an increase of nearly 20%. Meanwhile, due to the fact that the sapphire substrate is located in the middle of the metal film and GST film, and its thickness is much greater than them, it to some extent reduces the coupling degree between the metal film structure and GST film. That is to say, the degree of red-shifted in the transmission spectrum of c-GST thin films will decrease with the decrease of the coupling degree. On the other hand, as shown in Fig. 6(b), the extinction ratio under the amorphous and crystalline states are shown. It is worth noting that after process optimization, the η has also increased by over 2 dB overall. Especially under the wavelength of 4 µm, the optimized η reached 7 dB. This enables the sample to provide good protection against high energy weapon attacks, ensuring the normal operation of internal components and systems in the light window.
Fig. 6. Comparison of performance parameters before and after process optimization. (a) The transmission distribution spectrum. The solid line represents before optimization, and the dashed line represents after optimization; (b) Comparison chart of extinction ratio before and after optimization; (c-d) The EMI SE distribution spectrum before and after optimization.
In the microwave band, according to the transmission line theory, the electromagnetic shielding effect of metal conductive films mainly depends on the sheet resistance [43]. The metal films with periodic subwavelength structures can be used as continuous conductive structures. Meanwhile, their thin layer resistance can be controlled by parameters such as metal film thickness and subwavelength structural period, especially for low frequency applications. In the testing platform, place the sample in a coaxial tester and connect it to the coaxial line through a waveguide coaxial converter. Finally, connect it to a vector network analyzer to collect S parameters to obtain its electromagnetic shielding effect as shown in Figs. 6(c) and 6(d). The results showed that the optimization of the process did not affect its electromagnetic shielding effect in the 1-18 GHz band. The reason is that the improvement of the process has not changed the metal structure that provides shielding effect. Similarly, changes in the state of GST thin films will not cause the significant changes of the EMI SE. And it is worth noting that due to the extremely small geometric size of the metal circular hole structure compared to microwaves, its EMI SE has reached over 40 dB. This indicates that it has extremely superior shielding performance when protecting photoelectric detection systems or devices in the microwave band.
The ultra-thin GST film has extremely fast heating ability under laser irradiation. Therefore, the phase transition process can be completed in a shorter response time. As shown in Fig. 7(a), we have built a laser irradiation testing platform. Among them, the filter filters the stray light generated by the laser, and the remaining highly monochrome laser passes through the focusing lens and is irradiated on the surface of the sample. It is worth noting that the presence of a focusing lens does not have a significant impact on testing. Finally, record on the other end of the sample using a power meter. As shown in Fig. 7(b), after laser irradiation of the sample, we were able to observe very obvious pigmented circular spots on its irradiation point. This is the same result as described in Ref. 46. The reflectivity of the GST film after phase transition is higher, therefore it will present a darker color compared to before phase transition. And this is also one of the characteristics to prove that the GST film undergoes phase transition after laser irradiation. As shown in Figs. 7(c-d), the transmission power spectrum of the samples after laser irradiation with different powers are presented. When the laser power is 3 W, after absorption and reflection of the sample, the transmission power decreases to about 0.7 mW; When the laser power is 10 W, after absorption and reflection of the sample, the transmission power decreases to about 2.5 W; That is to say, the energy attenuation of the sample towards the laser reaches over 70%. It is worth noting that the power meter time sampling point we used in the test was set to 0.1 s (equipment limit value), so the response time during the before and after the phase transition process cannot be accurately obtained from the graph. The damage level and corresponding power density diagram of sensitive electronic devices [44,45] are shown in Fig. 7(e). From the figure, it can be seen that when the aperture of the laser is 3 mm, the power density of the 10 W laser irradiated on the surface of the sample is 140 W/cm2, reaching the destroy level. After passing the sample, the power density decreased to around 25 W/cm2, which means it has reached the damage level. Although it still has an impact on the photoelectric detection system, it has greatly reduced the energy of the laser.
Fig. 7. (a) Laser testing platform; (b) The sample image after laser irradiation; Among them, the red mark represents the spots generated after irradiation. (c-d) The transmission power spectrum of lasers with powers of 3 W and 10 W during the irradiation process are used; (e) Damage level and corresponding power density of sensitive electronic devices.
Furthermore, we have built a microwave pulse testing platform. After amplifying the output power of the signal source through a power amplifier, it is connected to the coaxial tester. Finally, observe the changes in its signal directly through an oscilloscope. Among them, a circulator and a matching load are connected to one end of the power amplifier to ensure the safety of its platform.
Herein, we use a signal source with a pulse width of 1 µs, a repetition rate of 50 µs, and a frequency of 12 GHz in Fig 8. As shown in Table 1, by adjusting the output power of the signal source, it is possible to compare the changes in output power with and without samples. It should be noted here that we did not increase the load when there were samples in the coaxial tester. On the contrary, with a load of 40 dB, we did not place the sample in the coaxial tester. When the output power of the signal source increases from −30 dBm to −1 dBm, the situation with samples and a 40 dB load is basically the same. That is to say, the sample has a 40 dB EMI SE under pulse irradiation. More importantly, its EMI SE does not vary with the power of the pulse. This also indicates that the metal film aperture structure has excellent electromagnetic shielding stability in the microwave band.
In summary, the Table 2 compares the performance of metal mesh structure, multi-layer film system structure, and phase change material thin film structure in three aspects: high transmittance in the infrared band, ultra-wideband EMI SE in the radar band, and nonlinear response against laser. From the comparison results in the table, it can be seen that the combination of metal micro/nano structures and phase change material film structures has unique advantages in protecting the optical channel of the photoelectric detection window and the nonlinear response of high-energy lasers. The details are presented below:
- (1) Firstly, by optimizing the geometric dimensions of metal micro/nano materials, surface plasmon resonance modes can be excited in the infrared band to enhance their transmission ability. The decoupling of high transmittance in the infrared band and ultra-wideband EMI SE in the radar band for the optical channel shielding of the photoelectric detection system was ultimately achieved.
- (2) On the other hand, by optimizing the structure and thickness parameters of the phase change material GST film, the EMI SE is not dependent on the phase change state. Thus, achieving decoupling between high transmittance in the infrared band, ultra-wideband EMI SE in the radar band, and nonlinear response to high-energy lasers.
- (3) Finally, we conducted high-energy laser irradiation experiments and strong field irradiation experiments on the samples. The experimental results have verified that the device has high transmittance in the infrared band, high EMI SE in the radar band, and adaptive nonlinear response to laser. Importantly, the experimental results well demonstrate that the device is in an independent state in all three performance aspects, achieving decoupling in all aspects. Moreover, this experiment also provides solidly support for our theoretical research.
Table 1. EMI SE under the different signal source outputs
Table 2. Performance comparison of different jobs
5. Conclusions
This article designs and manufactures a high-power density infrared nonlinear filtering window with EM shielding based on a subwavelength periodic circular hole structure. The maximum transmittance in the 3-5 µm band can reach over 0.7 through the local field enhancement effect of the surface plasmons resonance mode inside the hole. In addition, using the remaining continuous metal film after etching, an electromagnetic shielding effect of over 40 dB was achieved in the microwave band. Finally, based on the phase transition characteristics of GST materials, the autonomous shielding response function was achieved when subjected to high-energy laser irradiation.
Funding
Foundation of Hunan Talents (2020RC3028); National Natural Science Foundation of China (62293491).
Disclosures
The authors declare that they have no conflicts of interest.
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.
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