Over the past decade, superstructure-based surfaces based on micro-nano structures have shown significant advantages in light field regulation and have been proven to be a powerful platform for modulating the basic characteristics of electromagnetic waves . These planar optical components can localize electromagnetic energy at a sub-wavelength scale to enhance the interaction between light and matter, thereby regulating the amplitude, phase, polarization and other information of the light field in all aspects, providing new ideas for the integration and miniaturization of advanced photonic devices. However, most supersurface devices are static and cannot be changed once processed. Therefore, how to achieve dynamic light field regulation will become the key to promoting the practical application of superstructure surfaces.
At present, the design routes for dynamic superstructure surfaces roughly include three types: 1) Change the optical response of the structure through external excitation, such as electrical excitation, magnetic excitation, optical excitation, chemical reaction and thermal excitation, etc.; 2) Use special active materials, such as liquid crystals, phase change materials and functional crystals, etc.; 3) Apply external forces to deform the structure, such as microelectromechanical systems (MEMS), flexible stretching materials, etc. These regulatory schemes can enable the superstructure to exhibit a flexible dynamic response to the incident light field. Among them, the electrical tuning method is easier to integrate with mature optoelectronic devices and stands out among the numerous tuning mechanisms. Based on the different response mechanisms of some special materials to electric fields, the design of electrically tuned dynamic superstructure surfaces has developed many technical routes, such as the use of liquid crystal materials, transparent conductive oxides (TCOs), graphene, III-V- semiconductor materials, , transition metal dichalcogenide (TMD) materials, electro-optical crystals (EO-crystal), etc.
According to Mams Consulting, recently, researchers from the School of Electronics of Xi'an Jiaotong University published a review article titled "Research Progress in Electrical Tuning Superstructural Surface" in the journal Journal of Photonics, classifying and summarizing the dynamic superstructure surfaces of electric tuning in recent years, and dividing the current main tuning mechanisms into four design schemes: electronically controlled carrier excitation, liquid crystal regulation, MEMS drive and electro-optical crystals, and summarizing the physical mechanisms, tuning methods, research status and development trends of different schemes. It aims to systematically summarize this rapidly developing field and promote the research, development and application of nanophoton devices based on superstructure surfaces.
Design scheme for electro-modulation superstructure surface
In recent years, the research on light field regulation of dynamic superstructure surfaces has increased year by year (as shown in Figure 1). The dynamic regulation principle of superstructure surfaces can generally be divided into two categories: changing the refractive index of the material or changing the geometric structure of the superstructure surface.
Figure 1 Number of dynamic superfactory articles included in Web of Science in the past decade. Search keywords: adjustable/dynamic/reconstructible/active superstructure surface
Electronic control carrier regulation scheme
Electronic tuning superstructure surface
Electronic tuning superstructure surface
Article tuning by applying an external electric field to change the concentration of free carriers in the conductive material is one of the most common methods in electrical dimming. Indium Tin Oxide (Indium Tin Oxide, ITO) is one of the most widely used TCO materials at present, with a near-zero dielectric constant (Epsilon Near Zero, ENZ) point in the near-infrared band.
In 2016, HUANG Y W and others designed a gold grating-ITO layer-insulating layer-gold substrate structure. Using a 4V bias voltage, the carrier concentration of the ITO charge accumulation layer is increased by more than 3 times in the ENZ band; and using a 2.5V bias voltage, a 180° phase shift was generated, which achieved the switching between reflected light between ±1 order diffraction, and the modulation rate can reach 10MHz (as shown in Figure 2(a)). In 2020, SHIRMANESH G K and others designed a programmable super-surface multifunctional microcontroller using ITO and gold nanoantennas. Phase modulation of 270° is achieved near the wavelength of 1522nm, and beam deflection of 23.5° and focal length regulation of 1.5~3μm is achieved. In 2021, PARK J and others introduced an ITO layer between the gold grating and the aluminum substrate, designed 550 individually addressable resonators, , to form the superstructure surface, achieving phase regulation of 0~360°, the modulation rate theory can reach 5.4MHz, and three-dimensional depth scanning is realized within a distance of 4.7m (as shown in Figure 2 (b)).
Figure 2 Electrically tuned superstructure surface of ITO material combined with metal structure
The main limitation of electro-optical regulation based on TCO materials is that changes in optical properties only occur near the charge accumulation layer, and the thickness is usually only 1~2nm, which is called the Debye length. Therefore, expanding the Debai length can further improve the optical response sensitivity of the ITO. In 2018, SHIRMANESH G K and others used dual-gated circuits to design an aluminum nanoantenna-insulating layer-ITO layer-insulating layer-metal substrate structure, and two independent voltage control channels were connected in series. Applying a 6.5V bias voltage, this structure can achieve 300° reflective phase modulation near wavelength 1550nm and 89% relative reflectivity modulation (as shown in Figure 3(a)).
Figure 3 Double-gated ITO superstructure surface and electrically tuned superstructure surfaces combined with ITO materials and dielectric materials
The inherent parasitic loss of metals and weaker light-matter interactions are another factor limiting this regulation method. Compared with the metal surface plasmon mode, high-refractive index dielectric materials can support stronger local electromagnetic mode, with more regulatory freedom and lower absorption losses. Therefore, combining ITO with full-media superstructure surfaces is another important route. In 2018, HOWES A et al. reported an electrically tuned full-die Huygens superstructure that consists of silicon nanoantennas top coated with ITO films and a fused silica substrate. The electromagnetic mode of the silicon nanoantenna occurs in the ENZ band of ITO. The local electric field in the ITO layer is changed through electrical tuning, achieving a transmittance regulation of 31%, and a beam deflection of about 26° is demonstrated (as shown in Figure 3(b)).
In addition to ITO, other TCO materials also have the potential for electrical tuning, such as Aluminum-doped Zinc Oxide (AZO), indium oxide (In
Electrically tuned superstructure surface
Graphene is composed of a single layer of carbon atoms arranged in a honeycomb lattice . Compared with TCO materials, graphene has higher carrier mobility and optical transparency, and has more flexible tunability, better robustness and environmental stability, making it an excellent material for photonics and optoelectronics.
In 2018, ZENG B et al. combined graphene with gold nanoantenna to realize a hybrid superstructure surface spatial light modulator . At 8 μm wavelength, a modulation depth of 90% to the reflected light intensity and a modulation rate up to 1 GHz are achieved by applying a gate bias of about 7V (as shown in Figure 4(a)). In addition, graphene has low absorption in the mid-infrared to terahertz band, and enhanced absorption can achieve effective regulation of light field intensity. In 2021, SUN Z and others designed a superstructure surface composed of a silver-opening ring resonator (SRR) and graphene. Applying a voltage of 60V can generate continuous phase changes of more than 330° in the mid-infrared band, achieving a beam deflection of 60° and an average efficiency of 22% (as shown in Figure 4(b)).
In addition to the plasmon mode on the metal surface, some studies have also explored the effect of combining graphene with other electromagnetic modes. YAO Y et al. achieved a modulation depth of almost 100% reflectivity in the 5~7μm band by combining graphene with the Fabry-Pérot (FP) mode in the dielectric layer by applying a voltage of 80V, and the modulation rate is as high as 20GHz (as shown in Figure 4(c)).
Figure 4 Graphene-based electrically tuned superstructure surface
Electrically tuned superstructure surface
TMDs is another widely-watched two-dimensional material .
In order to achieve the effect of electro-dimming, many studies have coupled the local surface plasmon resonance of metal nanoparticles and the exciton resonance of TMDs, and injected carriers using gate bias voltage, demonstrating excellent dynamic light field regulation capabilities. LEE B et al. and LIU W et al. used MoS
Over the past decade, superstructure-based surfaces based on micro-nano structures have shown significant advantages in light field regulation and have been proven to be a powerful platform for modulating the basic characteristics of electromagnetic waves . These planar optical components can localize electromagnetic energy at a sub-wavelength scale to enhance the interaction between light and matter, thereby regulating the amplitude, phase, polarization and other information of the light field in all aspects, providing new ideas for the integration and miniaturization of advanced photonic devices. However, most supersurface devices are static and cannot be changed once processed. Therefore, how to achieve dynamic light field regulation will become the key to promoting the practical application of superstructure surfaces.
At present, the design routes for dynamic superstructure surfaces roughly include three types: 1) Change the optical response of the structure through external excitation, such as electrical excitation, magnetic excitation, optical excitation, chemical reaction and thermal excitation, etc.; 2) Use special active materials, such as liquid crystals, phase change materials and functional crystals, etc.; 3) Apply external forces to deform the structure, such as microelectromechanical systems (MEMS), flexible stretching materials, etc. These regulatory schemes can enable the superstructure to exhibit a flexible dynamic response to the incident light field. Among them, the electrical tuning method is easier to integrate with mature optoelectronic devices and stands out among the numerous tuning mechanisms. Based on the different response mechanisms of some special materials to electric fields, the design of electrically tuned dynamic superstructure surfaces has developed many technical routes, such as the use of liquid crystal materials, transparent conductive oxides (TCOs), graphene, III-V- semiconductor materials, , transition metal dichalcogenide (TMD) materials, electro-optical crystals (EO-crystal), etc.
According to Mams Consulting, recently, researchers from the School of Electronics of Xi'an Jiaotong University published a review article titled "Research Progress in Electrical Tuning Superstructural Surface" in the journal Journal of Photonics, classifying and summarizing the dynamic superstructure surfaces of electric tuning in recent years, and dividing the current main tuning mechanisms into four design schemes: electronically controlled carrier excitation, liquid crystal regulation, MEMS drive and electro-optical crystals, and summarizing the physical mechanisms, tuning methods, research status and development trends of different schemes. It aims to systematically summarize this rapidly developing field and promote the research, development and application of nanophoton devices based on superstructure surfaces.
Design scheme for electro-modulation superstructure surface
In recent years, the research on light field regulation of dynamic superstructure surfaces has increased year by year (as shown in Figure 1). The dynamic regulation principle of superstructure surfaces can generally be divided into two categories: changing the refractive index of the material or changing the geometric structure of the superstructure surface.
Figure 1 Number of dynamic superfactory articles included in Web of Science in the past decade. Search keywords: adjustable/dynamic/reconstructible/active superstructure surface
Electronic control carrier regulation scheme
Electronic tuning superstructure surface
Electronic tuning superstructure surface
Article tuning by applying an external electric field to change the concentration of free carriers in the conductive material is one of the most common methods in electrical dimming. Indium Tin Oxide (Indium Tin Oxide, ITO) is one of the most widely used TCO materials at present, with a near-zero dielectric constant (Epsilon Near Zero, ENZ) point in the near-infrared band.
In 2016, HUANG Y W and others designed a gold grating-ITO layer-insulating layer-gold substrate structure. Using a 4V bias voltage, the carrier concentration of the ITO charge accumulation layer is increased by more than 3 times in the ENZ band; and using a 2.5V bias voltage, a 180° phase shift was generated, which achieved the switching between reflected light between ±1 order diffraction, and the modulation rate can reach 10MHz (as shown in Figure 2(a)). In 2020, SHIRMANESH G K and others designed a programmable super-surface multifunctional microcontroller using ITO and gold nanoantennas. Phase modulation of 270° is achieved near the wavelength of 1522nm, and beam deflection of 23.5° and focal length regulation of 1.5~3μm is achieved. In 2021, PARK J and others introduced an ITO layer between the gold grating and the aluminum substrate, designed 550 individually addressable resonators, , to form the superstructure surface, achieving phase regulation of 0~360°, the modulation rate theory can reach 5.4MHz, and three-dimensional depth scanning is realized within a distance of 4.7m (as shown in Figure 2 (b)).
Figure 2 Electrically tuned superstructure surface of ITO material combined with metal structure
The main limitation of electro-optical regulation based on TCO materials is that changes in optical properties only occur near the charge accumulation layer, and the thickness is usually only 1~2nm, which is called the Debye length. Therefore, expanding the Debai length can further improve the optical response sensitivity of the ITO. In 2018, SHIRMANESH G K and others used dual-gated circuits to design an aluminum nanoantenna-insulating layer-ITO layer-insulating layer-metal substrate structure, and two independent voltage control channels were connected in series. Applying a 6.5V bias voltage, this structure can achieve 300° reflective phase modulation near wavelength 1550nm and 89% relative reflectivity modulation (as shown in Figure 3(a)).
Figure 3 Double-gated ITO superstructure surface and electrically tuned superstructure surfaces combined with ITO materials and dielectric materials
The inherent parasitic loss of metals and weaker light-matter interactions are another factor limiting this regulation method. Compared with the metal surface plasmon mode, high-refractive index dielectric materials can support stronger local electromagnetic mode, with more regulatory freedom and lower absorption losses. Therefore, combining ITO with full-media superstructure surfaces is another important route. In 2018, HOWES A et al. reported an electrically tuned full-die Huygens superstructure that consists of silicon nanoantennas top coated with ITO films and a fused silica substrate. The electromagnetic mode of the silicon nanoantenna occurs in the ENZ band of ITO. The local electric field in the ITO layer is changed through electrical tuning, achieving a transmittance regulation of 31%, and a beam deflection of about 26° is demonstrated (as shown in Figure 3(b)).
In addition to ITO, other TCO materials also have the potential for electrical tuning, such as Aluminum-doped Zinc Oxide (AZO), indium oxide (In
Electrically tuned superstructure surface
Graphene is composed of a single layer of carbon atoms arranged in a honeycomb lattice . Compared with TCO materials, graphene has higher carrier mobility and optical transparency, and has more flexible tunability, better robustness and environmental stability, making it an excellent material for photonics and optoelectronics.
In 2018, ZENG B et al. combined graphene with gold nanoantenna to realize a hybrid superstructure surface spatial light modulator . At 8 μm wavelength, a modulation depth of 90% to the reflected light intensity and a modulation rate up to 1 GHz are achieved by applying a gate bias of about 7V (as shown in Figure 4(a)). In addition, graphene has low absorption in the mid-infrared to terahertz band, and enhanced absorption can achieve effective regulation of light field intensity. In 2021, SUN Z and others designed a superstructure surface composed of a silver-opening ring resonator (SRR) and graphene. Applying a voltage of 60V can generate continuous phase changes of more than 330° in the mid-infrared band, achieving a beam deflection of 60° and an average efficiency of 22% (as shown in Figure 4(b)).
In addition to the plasmon mode on the metal surface, some studies have also explored the effect of combining graphene with other electromagnetic modes. YAO Y et al. achieved a modulation depth of almost 100% reflectivity in the 5~7μm band by combining graphene with the Fabry-Pérot (FP) mode in the dielectric layer by applying a voltage of 80V, and the modulation rate is as high as 20GHz (as shown in Figure 4(c)).
Figure 4 Graphene-based electrically tuned superstructure surface
Electrically tuned superstructure surface
TMDs is another widely-watched two-dimensional material .
In order to achieve the effect of electro-dimming, many studies have coupled the local surface plasmon resonance of metal nanoparticles and the exciton resonance of TMDs, and injected carriers using gate bias voltage, demonstrating excellent dynamic light field regulation capabilities. LEE B et al. and LIU W et al. used MoS
Figure 5 Electrically tuned superstructure surfaces of TMDs combined with metal structures
Electrically tuned superstructure surfaces based on III-V semiconductors and multi-quantum wells
Group III-V semiconductor materials have an electrical tuning mechanism similar to those of TCO materials. By applying a gate bias voltage on a Group III-V semiconductor substrate, a charge accumulation region with adjustable thickness can be generated near the interface of the semiconductor layer, and its dielectric constant can change with the change of local carrier concentration, and the related regulation is mostly combined with the metal surface plasmon.
CHENHT and others integrate gold SRR on the GaAs semiconductor substrate. By changing the bias voltage, the carrier density of the GaAs substrate near the opening gap can be adjusted and its dielectric constant can be changed. In experiments, the relative transmittance modulation of 50% in the terahertz band under the 16V reverse gate bias voltage was demonstrated (as shown in Figure 6(a)). Subsequently, their team used similar structures to prepare a 4×4 pixel matrix. At a bias voltage of 0~14V, amplitude modulation in the 0.36THz band is achieved in the range of 35% to 50% (as shown in Figure 6(b)). In addition, PARK J and others made an aluminum grating on an epitaxially grown InAs substrate, and controlled the change of carriers in the InAs layer by electrically gating bias, which in turn led to a spectral shift of the radiation peak, which could change the absorption and heating characteristics of the superstructure surface, and achieved a 3.6% radiation efficiency change in the mid-infrared band.
Figure 6 Group III-V semiconductor/surface plasmon electrically tuning superstructure surface
is based on multiple quantum wells (MQWs) composed of semiconductor heterojunctions, such as InGaN/GaN, AlGaAs/GaAs and other structures, and has been widely studied in electrical dimming applications in visible to terahertz bands. Based on MQWs, WU P C and others developed an active medium superstructure surface with a distributed Bragg reflector. In the near-infrared band of 915~920nm, the modulation range of the system's refractive index real part can reach about 0.01~0.05, and a reflectivity modulation of up to 270% and a phase change of 0~70° can be obtained under a bias voltage of 7V (as shown in Figure 7(a)). LEE J et al. combined MQWs with plasmon resonance, and achieved an absorption change of more than 30% at a wavelength of 7μm by applying a 5V bias voltage. An ultra-fast response speed of 10ns was observed in the experiment. BENZ A et al. combined the transition homophobic light and strong coupling mechanism of the quantum well of the quantum well, and achieved the regulation of the center frequency shift by 8% line width at 2.5 THz (as shown in Figure 7(b)).
Figure 7 Electrically tuned superstructure surface
liquid crystal regulation scheme
liquid crystal As a mature and widely used optically active material, it has many advantages such as large refractive index adjustable range, high transmission efficiency, low power consumption, and strong integration, and occupies a very important position in tunable optical components.
BUCHNEV O and others use nematic liquid crystals and V-type gold nanoarrays to form a simple superstructure surface, and achieve 50% transmittance modulation near wavelength 1550nm by applying a voltage of 7V. Because the surface anchoring effect of liquid crystal will affect its dynamic response effect. They then cleverly designed a nematic liquid crystal and zigzag gold nanoarray to form a superstructure surface, reducing the surface anchoring of the liquid crystal. In experiments, they used a low-key control voltage of 1.5~2.7V to make the spectral offset reach 110nm, and can generate a phase change of π/4 at a voltage of 2V (as shown in Figure 8(a)). In addition to bringing changes in refractive index, the steering of molecules in liquid crystal can also change the polarization direction of incident light.XIE Z W and others designed nematic liquid crystal/aluminum grating superstructure surfaces, and controlled the conversion of transverse magnetic (TM) to transverse electrical (TE) modes through voltage changes of 0~4V, and achieved dynamic changes in the reflective structure color (as shown in Figure 8(b)).
Figure 8 Electrically tuned superstructure surface
electro-optical crystal regulation scheme
electro-optical crystal is the main material of commercial electro-optical modulators at present. It is a functional crystal with electro-optical effect. Combining electro-optical crystals with superstructure surfaces and using local electromagnetic modes in subwavelength structures to enhance the response sensitivity of the refractive index change of the light field has become an important route for electro-optical regulation in recent years. In 2021, WEISS A and others placed gold nanoparticles on an LN substrate, so that the surface plasmon mode of the gold nanoparticles interacts with the LN layer. By applying a 40V bias voltage, a modulation depth of 40% reflectivity is achieved at a wavelength of 1550 nm (as shown in Figure 9(a)). WEIGAND H et al. designed a silicon nanopillar array on thin film LN. The optical resonance introduced by the array can bring 80 times field enhancement; by applying a voltage less than 1V, broadband amplitude modulation is achieved 10Hz to 2.5MHz (as shown in Figure 9(b)). GAO B et al. processed the film LN into a two-dimensional dielectric grating, and obtained a quasi-continuous domain bound state (BIC) mode with ultra-high quality factors through the symmetry of the oblique incident grating system. Driven by ±150V, a phase change of about 47° was obtained (as shown in Figure 9(c)). KLOPFER E et al. combine silicon waveguides with thin film LN, and introduce Fano resonance with quality factors up to 30,000 in the length direction of a single waveguide through etching small defects. Simulation simulation shows that at a bias voltage of ±25V, a phase change of 0~360° can be achieved, and the reflectivity remains above 90% (as shown in Figure 9(d)).
Figure 9 Electrically tuned superstructure surface
MEMS-driven regulation scheme
In the dynamic regulation route of the superstructure surface, in addition to changing the optical characteristics of the material itself, the structural units of the superstructure surface can also be reconfigured by changing the external stress. MEMS can accurately generate certain external forces on the target structure by applying electric field, magnetic field, thermal stimulation, etc. on the micro-nano scale, and quantitatively deform its geometric shape, thereby accurately and dynamically controlling its optical response. Many studies combine electrically tuned MEMS with superstructure structures to study dynamic superstructures from visible light to terahertz band.
MANJAPPA M and others designed a reconstructible MEMS supersurface composed of two SRRs. They can independently control the out-of-plane asymmetry of the superstructure surface resonator by two bias voltage channels, respectively, and excite Fano resonance. This change in anisotropy results in a hysteresis effect of the system, allowing logic operations to be performed through two independently controlled electrical inputs and an optical reading of a terahertz frequency, realizing the "AND" and "OR" gate logic in the terahertz band (as shown in Figure 10(a)). HOLSTEEN A L and others have proposed a multifunctional superstructure surface that can realize color control, dynamic beam control and light focus control in the visible range. They suspended the Mie-type resonator on the insulating substrate (Silicon on Insulator, SOI), and mechanically adjusted through MEMS, achieving a color change in the superstructure surface structure at different voltage regulation rates; phase regulation of 0~360° and beam deflection of 2°~12° at a voltage of 3.2V; at a wavelength of 600nm, a 2.2V voltage was applied to achieve a adjustment of the focal length from 26um to 5μm (as shown in Figure 10(b)). ZHANG X and others proposed a design solution for super-surface lidar based on Focal Plane Switch Array (FPSA). They designed a 128×128 grating nanoantenna array on a 1cm² SOI material, and used MEMS for separate site selection control of columns and rows, which can achieve a wide field of view effect of 70°×70° and a fine resolution of 0.6°×0.6°, and demonstrated the ranging and three-dimensional imaging effects of lidar through experiments (as shown in Figure 10(c)).
Figure 10 Superstructure surface functional device based on MEMS
With the rapid development of MEMS technology, active superstructure surface based on MEMS still has huge potential and is very promising to play an important role in advanced photonic devices such as zoom lenses, lidars, photonic chip , and photodetector .
Summary and Outlook
The continuous development of many dynamic superstructure surfaces is driving the transformation of the next generation of tunable optical components. According to the regulation mechanism, this paper divides the design route of the electrically tuned superstructure surface into four directions: electrically controlled carrier excitation, liquid crystal regulation, electro-optical crystal regulation and MEMS drive. They rely on their respective advantages to flourish in the past decade and have derived many different design solutions. With the help of the unique electromagnetic field local mode of the superstructure surface, each design scheme is constantly exploring towards 360° full-phase regulation and greater modulation depth. However, these design solutions are not simple alternative relationships, but are oriented towards different application environments and can complement each other according to their respective characteristics. For example, in the control band from visible light to terahertz, TCO, TMDs and liquid crystal regulation are mainly used in the control band from visible light to near infrared. Graphene regulation, Group III-V semiconductors and multi-quantum well regulation are mainly used in the mid-infrared to terahertz band. In terms of modulation rate, the regulation scheme based on the carrier excitation principle can basically achieve a modulation rate of GHz with the advantage of fast carrier migration speed, and can be used for many high-speed response applications. LCD regulation and MEMS regulation are limited by the hysteresis of their own response, and their modulation rates can only reach KHz and MHz respectively. However, with their own stability and mature process technology, they have successfully achieved many applications with low response speeds.
It is worth mentioning that electro-optical crystal regulation can cover the full band range of visible light to terahertz, and can have ultra-high modulation rates above GHz, making its mark among many modulation solutions. However, electro-optical crystals are not mature enough due to the processing technology and the thin film industry. The superstructure surface design based on electro-optical crystals started late, and there are still few related research at present. At the same time, due to the weak electro-optical effect at the micro-nano scale, it brings a contradiction between modulation depth and power consumption, which poses a higher challenge to the design of the superstructure surface. Nevertheless, as the dominant material of traditional electro-optical modulators, dynamic superstructure surface design based on electro-optical crystals still has great potential.
With the deepening of Industry 4.0, many technical applications will have greater demand for easy integration of tunable optical components. These include a variety of wearable devices, autonomous driving, robotics, augmented and virtual reality, communications, sensing, imaging and display technologies, and more. The combination of electro-modulation superstructure surface technology and semiconductor integrated circuit technology has the hope to play an important role in the transformation and upgrading of these technologies. Many novel regulatory methods are constantly being born, such as programmable superstructure surfaces designed through deep learning, and wireless remote control of superstructure surfaces through brain waves , etc. However, tunable superstructure surface technologies currently have different advantages and disadvantages. Although many excellent applications have been achieved, there is no perfect tuning method that can meet these more advanced industrial application needs in various indicators. The research on tunable metasurfaces has only been developed for more than a decade, and there is still huge research potential worth exploring. For example, promote the upgrading of micro-nano processing technology, study new active materials and new regulatory methods, or combine multiple regulatory solutions to complement their advantages to build a composite tunable superstructure surface. Although it is quite challenging to realize industrial-grade applications of dynamic superstructure surfaces, it is still foreseeable that the research on tunable superstructure surfaces in the future will develop rapidly with the coordinated efforts of cross-field and multidisciplinary issues, and can become an important force in promoting the application of the new optoelectronic devices .
XIE Z W and others designed nematic liquid crystal/aluminum grating superstructure surfaces, and controlled the conversion of transverse magnetic (TM) to transverse electrical (TE) modes through voltage changes of 0~4V, and achieved dynamic changes in the reflective structure color (as shown in Figure 8(b)).Figure 8 Electrically tuned superstructure surface
electro-optical crystal regulation scheme
electro-optical crystal is the main material of commercial electro-optical modulators at present. It is a functional crystal with electro-optical effect. Combining electro-optical crystals with superstructure surfaces and using local electromagnetic modes in subwavelength structures to enhance the response sensitivity of the refractive index change of the light field has become an important route for electro-optical regulation in recent years. In 2021, WEISS A and others placed gold nanoparticles on an LN substrate, so that the surface plasmon mode of the gold nanoparticles interacts with the LN layer. By applying a 40V bias voltage, a modulation depth of 40% reflectivity is achieved at a wavelength of 1550 nm (as shown in Figure 9(a)). WEIGAND H et al. designed a silicon nanopillar array on thin film LN. The optical resonance introduced by the array can bring 80 times field enhancement; by applying a voltage less than 1V, broadband amplitude modulation is achieved 10Hz to 2.5MHz (as shown in Figure 9(b)). GAO B et al. processed the film LN into a two-dimensional dielectric grating, and obtained a quasi-continuous domain bound state (BIC) mode with ultra-high quality factors through the symmetry of the oblique incident grating system. Driven by ±150V, a phase change of about 47° was obtained (as shown in Figure 9(c)). KLOPFER E et al. combine silicon waveguides with thin film LN, and introduce Fano resonance with quality factors up to 30,000 in the length direction of a single waveguide through etching small defects. Simulation simulation shows that at a bias voltage of ±25V, a phase change of 0~360° can be achieved, and the reflectivity remains above 90% (as shown in Figure 9(d)).
Figure 9 Electrically tuned superstructure surface
MEMS-driven regulation scheme
In the dynamic regulation route of the superstructure surface, in addition to changing the optical characteristics of the material itself, the structural units of the superstructure surface can also be reconfigured by changing the external stress. MEMS can accurately generate certain external forces on the target structure by applying electric field, magnetic field, thermal stimulation, etc. on the micro-nano scale, and quantitatively deform its geometric shape, thereby accurately and dynamically controlling its optical response. Many studies combine electrically tuned MEMS with superstructure structures to study dynamic superstructures from visible light to terahertz band.
MANJAPPA M and others designed a reconstructible MEMS supersurface composed of two SRRs. They can independently control the out-of-plane asymmetry of the superstructure surface resonator by two bias voltage channels, respectively, and excite Fano resonance. This change in anisotropy results in a hysteresis effect of the system, allowing logic operations to be performed through two independently controlled electrical inputs and an optical reading of a terahertz frequency, realizing the "AND" and "OR" gate logic in the terahertz band (as shown in Figure 10(a)). HOLSTEEN A L and others have proposed a multifunctional superstructure surface that can realize color control, dynamic beam control and light focus control in the visible range. They suspended the Mie-type resonator on the insulating substrate (Silicon on Insulator, SOI), and mechanically adjusted through MEMS, achieving a color change in the superstructure surface structure at different voltage regulation rates; phase regulation of 0~360° and beam deflection of 2°~12° at a voltage of 3.2V; at a wavelength of 600nm, a 2.2V voltage was applied to achieve a adjustment of the focal length from 26um to 5μm (as shown in Figure 10(b)). ZHANG X and others proposed a design solution for super-surface lidar based on Focal Plane Switch Array (FPSA). They designed a 128×128 grating nanoantenna array on a 1cm² SOI material, and used MEMS for separate site selection control of columns and rows, which can achieve a wide field of view effect of 70°×70° and a fine resolution of 0.6°×0.6°, and demonstrated the ranging and three-dimensional imaging effects of lidar through experiments (as shown in Figure 10(c)).
Figure 10 Superstructure surface functional device based on MEMS
With the rapid development of MEMS technology, active superstructure surface based on MEMS still has huge potential and is very promising to play an important role in advanced photonic devices such as zoom lenses, lidars, photonic chip , and photodetector .
Summary and Outlook
The continuous development of many dynamic superstructure surfaces is driving the transformation of the next generation of tunable optical components. According to the regulation mechanism, this paper divides the design route of the electrically tuned superstructure surface into four directions: electrically controlled carrier excitation, liquid crystal regulation, electro-optical crystal regulation and MEMS drive. They rely on their respective advantages to flourish in the past decade and have derived many different design solutions. With the help of the unique electromagnetic field local mode of the superstructure surface, each design scheme is constantly exploring towards 360° full-phase regulation and greater modulation depth. However, these design solutions are not simple alternative relationships, but are oriented towards different application environments and can complement each other according to their respective characteristics. For example, in the control band from visible light to terahertz, TCO, TMDs and liquid crystal regulation are mainly used in the control band from visible light to near infrared. Graphene regulation, Group III-V semiconductors and multi-quantum well regulation are mainly used in the mid-infrared to terahertz band. In terms of modulation rate, the regulation scheme based on the carrier excitation principle can basically achieve a modulation rate of GHz with the advantage of fast carrier migration speed, and can be used for many high-speed response applications. LCD regulation and MEMS regulation are limited by the hysteresis of their own response, and their modulation rates can only reach KHz and MHz respectively. However, with their own stability and mature process technology, they have successfully achieved many applications with low response speeds.
It is worth mentioning that electro-optical crystal regulation can cover the full band range of visible light to terahertz, and can have ultra-high modulation rates above GHz, making its mark among many modulation solutions. However, electro-optical crystals are not mature enough due to the processing technology and the thin film industry. The superstructure surface design based on electro-optical crystals started late, and there are still few related research at present. At the same time, due to the weak electro-optical effect at the micro-nano scale, it brings a contradiction between modulation depth and power consumption, which poses a higher challenge to the design of the superstructure surface. Nevertheless, as the dominant material of traditional electro-optical modulators, dynamic superstructure surface design based on electro-optical crystals still has great potential.
With the deepening of Industry 4.0, many technical applications will have greater demand for easy integration of tunable optical components. These include a variety of wearable devices, autonomous driving, robotics, augmented and virtual reality, communications, sensing, imaging and display technologies, and more. The combination of electro-modulation superstructure surface technology and semiconductor integrated circuit technology has the hope to play an important role in the transformation and upgrading of these technologies. Many novel regulatory methods are constantly being born, such as programmable superstructure surfaces designed through deep learning, and wireless remote control of superstructure surfaces through brain waves , etc. However, tunable superstructure surface technologies currently have different advantages and disadvantages. Although many excellent applications have been achieved, there is no perfect tuning method that can meet these more advanced industrial application needs in various indicators. The research on tunable metasurfaces has only been developed for more than a decade, and there is still huge research potential worth exploring. For example, promote the upgrading of micro-nano processing technology, study new active materials and new regulatory methods, or combine multiple regulatory solutions to complement their advantages to build a composite tunable superstructure surface. Although it is quite challenging to realize industrial-grade applications of dynamic superstructure surfaces, it is still foreseeable that the research on tunable superstructure surfaces in the future will develop rapidly with the coordinated efforts of cross-field and multidisciplinary issues, and can become an important force in promoting the application of the new optoelectronic devices .
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