A Review of Sub-Wavelength Wire Grid Polarizers and Their Development Trends

Abstract

There has been a significant rise in the fabrication of polarizing elements with the rapid advancement of polarization imaging technology, coinciding with a rise in research on such elements. This article provides a comprehensive review of sub-wavelength wire grid polarizers which can be applied in different operating wavelength ranges, specifically focusing on their design, as well as their related fabrication processes and metrology methods. First, structural parameters, designed and simulated via the finite-difference time-domain (FDTD) method or rigorous coupled wave analysis (RCWA), and their impact on wire grid performance are investigated based on the effective medium theory. Second, a comprehensive overview of domestic and international studies is provided, focusing on the developments in sub-wavelength wire grid polarizers with single-layer structures and bilayer structures at different operating wavelength bands—deep ultraviolet, visible, middle- and far-infrared, and terahertz wavelength bands. Research related to polarizers with multilayer structures, simulated and carried out via the use of specific software, is also presented. Finally, the progress regarding sub-wavelength wire grid polarizer research is summarized, and future prospects are forecasted, with emphasis on material selection, wire grid structure optimization, and innovation in manufacturing processes.

1. Introduction

Polarization, in addition to spectrum and light intensity, serves as another informative characteristic of objects [1]. Objects, or their various positions and states, emit or reflect light with distinct polarization states [2]. Polarization imaging technology integrates polarization and imaging techniques to detect and identify targets within intricate environmental settings by leveraging polarization discrepancies between targets and backgrounds [3]. A polarization device filters reflected or emitted light waves, and subsequent imaging by the device not only captures the target’s radiation intensity information but also its polarization degree and angle information [4,5]. In comparison to spectral imaging and thermal imaging [6], polarization imaging offers enhanced background noise suppression and significantly improves target detection and identification capabilities [7,8].

Polarizers are essential for light beam polarization separation, with various types being available, including reflective, dichroic, birefringent, and sub-wavelength wire grid polarizers (WGPs). Among these types, sub-wavelength WGPs have emerged as key components in optical systems due to their superior characteristics, such as their high extinction ratio and transmittance, wide operating wavelength range, wide angle acceptance, compact structure, ease of integration, and great versatility. Based on the spatial distribution of the subwavelength wires, subwavelength wire grids can be classified into space-invariant types (with constant phase and polarization spatial distribution, i.e., polarization homogeneity across the cross-section) and space-variant types [9] (with variable phase and polarization spatial distribution, i.e., polarization inhomogeneity across the cross-section). The notable characteristic of space-variant subwavelength wire grids is their ability to generate spatially non-uniform polarization fields through spatially nonhomogeneous grating structures, achieving polarization conversion such as linear to radial/azimuthal polarization [10] or circular to radial/azimuthal polarization [11]. Research on the application of sub-wavelength WGPs has focused on various operating wavelength ranges, encompassing visible WGPs [12,13,14] and mid/far-infrared WGPs [15,16], deep ultraviolet WGPs [17,18], and even terahertz WGPs [19,20,21]. This article, focusing specifically on space-invariant subwavelength wire grid polarizers, analyzes and summarizes the global research landscape on sub-wavelength WGPs, highlighting innovative aspects regarding design parameters, manufacturing techniques, and polarization performance at different wavelength ranges.

2. Wire Grid Polarizer Design Theory

WGPs have a sub-wavelength nanostructure which is significantly smaller than the operating wavelength and spatial arrangement. This microstructure is fabricated on various substrate materials, such as semiconductors [22,23], glasses [24,25], and polymer materials [26,27], and it adjusts the incident light’s electromagnetic field to achieve polarization separation. Any incident light decomposes into two mutually perpendicular polarized components. One component with polarization parallel to the wire grid axis is defined as the Transverse Electric wave (TE wave). The electric field vibration causes free electrons to undergo oscillation along the wire grid direction, resulting in current generation. Consequently, the wire grid behaves similarly to the properties of a metallic surface and effectively acts as a metal film. As a result, the TE wave is absorbed or reflected (see Figure 1). Conversely, another component with its polarization direction perpendicular to the wire grid axis is defined as the Transverse Magnetic wave (TM wave). In this case, the presence of air gaps between the wire grids restricts electron movement in this orientation. Thus, the wire grid exhibits the characteristics of a dielectric surface and effectively acts as a dielectric film, and the TM wave is transmitted directly (see Figure 1).

2.1. Basic Theory of Sub-Wavelength Wire Grid Polarizers

The polarization properties of sub-wavelength WGPs are based on the effective medium theory. When the wire grid period is significantly smaller than the incident light wavelength, these periodic nanostructures can be modeled as a homogeneous dielectric layer with an effective refractive index. The dielectric constant, permeability, and conductivity of the wire grid are dictated by structural parameters, including wire grid period, wire grid height, and duty cycle.

The Maxwell–Garnett model and the Bruggeman model can be used to determine the mathematical expression for the equivalent permittivity in wire grid structures. The Maxwell–Garnett model is suitable for wire girds with a lower duty cycle, according to Equations (1) and (2):

$${\epsilon }_{TE}={\epsilon }_d+f{\displaystyle \frac{{\epsilon }_m-{\epsilon }_d}{1+\left(1-f\right)\frac{{\epsilon }_m-{\epsilon }_d}{3{\epsilon }_d}}}$$

(1)

$${\epsilon }_{TM}\approx {\epsilon }_d$$

(2)

where ${\epsilon }_m$ is the relative permittivity of the metal, ${\epsilon }_d$ is the relative permittivity of the dielectric, and $f$ is the duty cycle.

The Bruggeman model is applied to higher-duty-cycle wire grid structures, according to Equations (3) and (4):

$${\epsilon }_{TE}\left(\omega \right)=1-{\displaystyle \frac{{\omega }_p^2}{\omega \left(\omega +i\gamma \right)}}$$

(3)

$${\epsilon }_{TM}\approx {\epsilon }_d$$

(4)

Here, ${\omega }_p$ is the plasma frequency, and $\gamma$ is the damping coefficient.

When the wire grid period is significantly smaller than the wavelength of the incident light, higher-order diffraction in the diffraction field is effectively suppressed, leaving only zero-order diffraction. In accordance with the boundary conditions derived from Maxwell’s equations, the effective permittivity is determined by the relative volumes of the metal and medium. Consequently, the zero-order approximation of the equivalent refractive index for both the TE wave and TM wave can be expressed as

$${\epsilon }_{TE}\approx {\epsilon }_d+f\left({\epsilon }_m-{\epsilon }_d\right)$$

(5)

$$n_{TE}=\sqrt{{\epsilon }_{TE}}\approx \sqrt{{\epsilon }_d+f\left({\epsilon }_m-{\epsilon }_d\right)}$$

(6)

$${\epsilon }_{TM}^{-1}=f{\epsilon }_m^{-1}+\left(1-f\right){\epsilon }_d^{-1}$$

(7)

$$n_{TM}=\sqrt{{\epsilon }_{TM}}\approx {\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\sqrt{f\frac{1}{{\epsilon }_m}+\left(1-f\right)\frac{1}{{\epsilon }_d}}$}\right.}$$

(8)

Equations (6) and (8) provide the equivalent refractive indices for the TE wave and TM wave, respectively, with air as the medium.

$$n_{TE}={\left[f{n}_m^2+\left(1-f\right)n_0^2\right]}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$

(9)

$$n_{TM}={\left[f{n}_m^{-2}+\left(1-f\right)n_0^{-2}\right]}^{{\displaystyle \raisebox{1ex}{$-1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$

(10)

Here, $n_{TE}$ and $n_{TM}$ represent the equivalent refractive index of the TE wave and TM wave, respectively. $n_m$ is the refractive index of metal, $n_0$ represents the refractive index of air, and $f$ denotes the duty cycle of the metal wire grid.

The refractive index of metal is $n_m=n+ik$; $k$ represents its extinction coefficient and determines the extent of reflection and absorption of light. Substituting the metal’s refractive index into Equations (9) and (10) yields the following equations:

$$n_{TE}={\left[f{\left(n+ik\right)}^2+\left(1-f\right)n_0^2\right]}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$

(11)

$$n_{TM}={\left[f{\left(n+ik\right)}^{-2}+\left(1-f\right)n_0^{-2}\right]}^{{\displaystyle \raisebox{1ex}{$-1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$

(12)

In Equation (11), the metal’s refractive index dominates in the expression for the equivalent permittivity, resulting in a large imaginary part. The structure therefore behaves like a metal film, indicating significant absorption or reflection of the TE wave. Conversely, in Equation (12), the refractive index of air dominates, leading to an extremely small imaginary part. The structure thus acts like a dielectric film, allowing for the transmission of the TM wave.

2.2. Design and Simulation Analysis of Sub-Wavelength Wire Grid Polarizers

Vector diffraction theory is employed in the design and simulation of sub-wavelength WGPs. Design methods include modal, integral, differential, and rigorous coupled wave analysis (RCWA), as well as the finite-difference time-domain (FDTD) method. Among these, the most prevalent techniques are the finite-difference time-domain (FDTD) method [28,29,30] and the rigorous coupled wave analysis (RCWA) [31,32]. The FDTD method is applicable to any geometric structure, whereas RCWA is well suited to periodic structures. These two methods are complementary and, together, are pushing towards developments in the in-depth exploration of light–matter interactions.

The FDTD method is a numerical technique that directly solves Maxwell equations in the time domain to simulate the propagation of electromagnetic waves in different media. By discretizing Maxwell’s curl equation in the time domain and iteratively solving it, the method provides the temporal evolution of the electromagnetic field distribution across the spatial domain. Y.Y. Kong et al. [33] utilized the FDTD method to design and compare single-layer metal wire grid structures with bilayer structures which combined a metal wire grid with a silicon monoxide (SiO) dielectric film on a silicon (Si) substrate. They investigated wire grid structures made of aluminum (Al), gold (Au), silver (Ag), rhodium (Rh), and copper (Cu), as well as metal wire grids combined with a SiO dielectric film wire grid structure across identical time periods. The study revealed that aluminum (Al) is the most suitable material for wire grid polarizers. Specifically, the polarization performance was improved significantly when a 300 nm thick SiO film, with the exact same period as the metal wire grid, was introduced. The wire grid parameters were a period of 400 nm, a wire height of 100 nm, and a duty cycle of 0.5. At a wavelength of 4.0 μm, the TM wave transmittance increased to 94.8%, and the extinction ratio reached 28.3 dB. The introduction of the single-layer SiO thin film weakened the surface plasmons excited at the interface between aluminum metal and substrate. This structure improved TM wave transmittance, reduced TE wave transmittance, enhanced extinction ratio, and improved the polarization performance of the polarization element.

The RCWA is a numerical technique rooted in the frequency domain for precisely analyzing the diffraction behaviors of electromagnetic wave in periodic structures. The core principle involves performing Fourier expansion of the periodic permittivity profiles and solving Maxwell’s equations to rigorously determine the electromagnetic field distribution in both the incident medium and substrate regions. The accuracy of this method is governed by the number of terms retained in the Fourier expansion.

The permittivity in the wire grid region can be mathematically represented as

$$\epsilon \left(x\right)=\sum _h{\epsilon }_{h}exp\left(j{\displaystyle \frac{2\pi hx}{d}}\right)$$

(13)

where $d$ is the wire grid period, and ${\epsilon }_h$ is the $h$-th order Fourier component of the relative permittivity of the wire grid region.

X.W. Liu et al. [34] studied a method for enhancing the extinction ratio of sub-wavelength WGPs applied in the 1–2.5 μm near-infrared wavelength band. They demonstrated that the extinction ratio of a wire grid exhibited a positive correlation with the thickness of the metal wire grid layer (wire grid height) when simulating the extinction ratio using rigorous coupled wave analysis. It was observed that the extinction ratio was higher than 40 dB when the height was equal to or more than 300 nm. A metal–silver wire grid polarizer which had a wire period of 300 nm, a wire height of 450 nm, and an aspect ratio of 3:1 (the ratio of wire grid depth (height) and wire grid width) was successfully fabricated by subjecting a metal film to Laser Interference Lithography and Oblique Angle Deposition. This innovative method overcame the conventional limitation of metal layer thickness (<250 nm) in traditional etching or trimming processes. Scanning electron microscopy (SEM) analysis revealed that the wire grid height was 450 nm, and the wire grid width was 176 nm. The average transmittance of the TM wave was approximately 70%, and the average extinction ratio was as high as 40 dB within the 1 μm to 2.5 μm wavelength range.

3. Design of Sub-Wavelength Wire Grid

The sub-wavelength wire grid polarizer effectively segregates incident light’s polarization orientation through the manipulation of the electromagnetic field within the sub-wavelength structure and the optimization of materials. Its polarization performance is dependent upon factors such as wire grid period, wire grid height, duty cycle, wire grid materials, and the manufacturing process. The transmittance of the TM wave and the extinction ratio are two crucial parameters [15] for assessing sub-wavelength wire grid performance. Some studies [35,36] have also utilized the degree of polarization, instead of transmittance, as a parameter for evaluating wire grid performance [37]. The Degree of Polarization (DOP) can be defined by Equation (14) or Equation (15):

$$DOP={\displaystyle \frac{T_{TM}-T_{TE}}{T_{TM}+T_{TE}}}\times 100\%$$

(14)

$$DOP={\displaystyle \frac{R_{TE}-R_{TM}}{R_{TE}+R_{TM}}}\times 100\%$$

(15)

Here, $T_{TM}$ and $T_{TE}$ represent the transmittance of the TM wave and TE wave, and $R_{TM}$ and $R_{TE}$ represent the reflection of the TM wave and TE wave.

Equations (14) and (15) establish that perfectly polarized light achieves a degree of polarization of 1, while natural light (unpolarized light) has a degree of polarization of 0. The extinction ratio is defined as the ratio of transmittance between the TM wave and TE wave, and the decibel expressed (dB) is as Equation (16).

$$ER=10lg\left({\displaystyle \frac{T_{TM}}{T_{TE}}}\right)$$

(16)

The extinction ratio indicates the polarization separation effect of a polarizing element on incident light. The relationship between the transmittance of the TM wave and extinction ratio is inversely correlated, leading to a scenario where achieving maximum TM wave transmittance and maximum extinction ratio simultaneously is unfeasible. This presents a challenge in practical design and fabrication, so it becomes difficult to meet the dual requirements of high transmittance and high extinction ratio [38]. Therefore, rationally optimizing the parameters of the wire grid in design and simulation is essential to enhance TM wave transmittance and extinction ratio.

3.1. Selection of Substrate

The choice of substrate material is guided by considerations such as a broad working wavelength range, low refractive index, and high transmittance. In their study, G.H. Zhang et al. [39] emphasized that the permittivity and thickness of a substrate played a crucial role in determining the relative permittivity of the polarizing element. This, in turn, alters the surface plasmon resonance wavelength, consequently impacting the transmittance of the TM wave. Optional substrates for different operating wavelength bands are detailed in

Table 1. Substrates for different operating wavelength bands.

Operating Wavelength Bands	Optional Substrates
Ultraviolet	Fused Silica, MgF2, LiF
Visible	Fused Silica
Mid-IR	Sapphire, Ge, ZnS, Si, CaF2
Far-IR	CaF2
Terahertz	Fused Silica, HR-Si, PE

.

3.2. Selection of Wire Grid Materials

Metal is the predominant wire grid material utilized in the visible-to-terahertz wavelength due to its unparalleled dispersion characteristics compared to non-metallic materials. Typical metal grid materials include aluminum (Al), gold (Au), silver (Ag), copper (Cu), chromium (Cr), titanium (Ti), etc. According to the zeroth-order effective medium theory, the refractive index of TE-polarized light under normal incidence can be represented as follows:

$$n_{TE}\approx n_m\sqrt{f}$$

(17)

where n_m is the refractive index of the metal, and f is the duty cycle of the metal wire grid.

In the complex refractive index of metal materials $n_m=n+ik$, n characterizes the dispersion of light waves by the metal, while $k$ quantifies the absorption of incident light waves. A large $k$ value corresponds to a greater absolute value of the imaginary part of the permittivity, resulting in reduced TE wave transmittance, an increased extinction ratio, and enhanced polarization performance of metal wire grids. Common metallic materials and their complex refractive indices at 632.8 nm are presented in

Table 2. Refractive index and extinction coefficient of different metals at 632.8 nm.

Parameter	Al	Au	Ag	Cu	Cr	Ti
$n$	1.373	0.181	0.135	0.239	3.136	2.153
$k$	7.618	3.068	3.985	3.416	3.312	2.923

. As evidenced by Table 2, aluminum possesses the largest extinction coefficient, indicating that Al wire grid polarizers theoretically deliver optimal polarization performance. In a recent study by P.Y. Wang et al. [40], metal tungsten (W) was proposed as a novel metal material for investigation. The complex refractive index is $n=1.2+29.3i$. Using COMSOL software, the researchers simulated and compared the polarization properties of tungsten, aluminum, gold, and copper, with wire grid parameters set at a period of 200 nm, a height of 400 nm, and a width of 200 nm (with a duty cycle of 0.5). The findings indicated that, when tungsten and gold were utilized as materials for wire grids, their TM wave transmittance values were notably similar and higher than those of other materials, while tungsten demonstrated the highest extinction ratio.

In the deep ultraviolet (DUV) band, the performance of metal wire grid polarizers severely degrades due to reliance on free-electron intraband transitions, which are fundamentally limited by the plasma frequency constraint. Conversely, semiconductor wire grids rely on interband transitions, resulting in significant absorption and a high extinction ratio. Semiconductor wire grid materials are highly favored due to their broad bandgap, low electrical resistivity, and strong optical responsiveness to ultraviolet light. Titanium dioxide (TiO₂) [41,42], zirconium dioxide (ZrO₂) [43], and chromic oxide (C_r2O₃) [44] are commonly chosen as preferred semiconductor wire grid materials for deep ultraviolet applications.

Table 3. Refractive index and extinction coefficient of different metal oxides at 193 nm.

Parameter	Cr2O3	Ta2O5	TiO2
$n$	2.5405	1.7969	1.4902
$k$	1.4853	1.0974	1.1057

[44] presents three metal oxide materials suitable for the deep ultraviolet band, along with their respective complex refractive index values at 193 nm. These optical properties were measured using a spectroscopic ellipsometer (SCI, Filmtek 300). Furthermore, as indicated in Equation (16), Cr₂O₃ exhibited the highest extinction coefficient, indicating that the Cr₂O₃ metal wire grid polarizer theoretically offered optimal polarization performance.

3.3. Wire Grid Structural Parameters

The structure of the wire grid is shown in Figure 2. The structural parameters that determine the performance of the wire grid are the wire grid period P, wire grid height $h$, and duty cycle (w/p).

3.3.1. Wire Grid Period

According to rigorous coupled wave theory, diffraction periods of sub-wavelength elements can be defined as

$$P={\displaystyle \frac{\lambda \times m}{n+\sin \theta }}$$

(18)

where $P$ is the period of the wire grid, $\theta$ is the incident angle, $m$ is the diffraction order, $n$ is the refractive index of the substrate, and λ is the incident wavelength. The diffraction behavior of the wire grid is influenced by the relationship between the grid period and the operating wavelength.

Sub-wavelength refers to the grid period and is significantly smaller than the incident light wavelength. According to the zeroth-order effective medium theory, the maximum allowable diffraction period must be kept below the cutoff for m = 1.

$$P<{\displaystyle \frac{\lambda }{n+\sin \theta }}$$

(19)

Specifically, under normal incidence, the maximum period sustaining polarization selectivity can be calculated by Equation (19), given the operating wavelength and substrate material. In a study on an aluminum metal wire grid, Y.Y. Kong [45] highlighted that as the grid period increased, the transmittance of the TM wave decreased, while the transmittance of the TE wave increased, reducing the extinction ratio and consequently worsening polarization performance. Conversely, the grid period cannot be infinitely minimized. Inverse scaling between period and polarization performance also should be balanced against processing complexity, mechanical stability, repeatability of the fabrication process, and production costs, which must also be taken into account during the manufacturing stage.

3.3.2. Wire Grid Height

The transmittance of the TM wave serves as one crucial metric for assessing sub-wavelength wire grids. Equation (20) demonstrates that the effective refractive index of the TM wave is contingent upon the refractive index of the interstitial medium (air) and the duty cycle. Moreover, the effective refractive index of the TM wave is subject to variation in response to alterations in the duty cycle.

$$n_{TM}={\displaystyle \raisebox{1ex}{$n_0$}\!\left/ \!\raisebox{-1ex}{$\sqrt{1-f}$}\right.}$$

(20)

where $n_0$ is the refractive index of air and $f$ is the duty cycle of the wire grid.

A sub-wavelength wire grid acts as an equivalent dielectric film for the TM wave. When the optical thickness, which is product of the TM-equivalent refractive index of wire grid and its height (wire grid thickness), equals one-quarter of the incident wavelength, a TM wave can pass through the grid, resulting in decreased reflectivity and increased transmittance. Compared to single-layer dielectric anti-reflection coatings, one advantage of the sub-wavelength wire grid is that it allows one to achieve the desired equivalent refractive index by adjusting the duty cycle. Additionally, the height of the wire grid can be designed to match any specific value.

Extinction ratio is another crucial parameter for assessing sub-wavelength wire grids. Equation (16) indicates that the extinction ratio can be enhanced by increasing the transmittance of the TM wave or decreasing the transmittance of the TE wave. The TM wave equivalent refractive index, Equation (19), reveals that, once the duty cycle is fixed, there is minimal variation in TM wave transmittance among various metal wire grid materials. Conversely, according to the TE wave equivalent refractive index, Equation (17), a higher refractive index of the metal leads to higher reflection and lower transmittance for the TE wave, resulting in a larger extinction ratio. For the remaining TE wave, G.G. Tang [46] was the first to introduce the concept of metals’ ‘skin depth’ into the design of sub-wavelength metallic wire grids. For the TE wave, the sub-wavelength metallic wire grids act as a metal layer. The ‘skin depth’ is defined as the depth at which the intensity of the TE wave passing through a metal layer decreases to 1/e of the original value. To maximize TE wave reflection or absorption, the sub-wavelength metal grid’s height should be greater than or a multiple of the ‘skin depth’. The skin depth is inversely related to the imaginary part of the metal’s refractive index. A higher imaginary part results in a smaller skin depth, leading to a shallower penetration of light into the metal and lower optical transmittance.

3.3.3. Duty Cycle

The duty cycle is defined as the wire grid width-to-wire grid period ratio. The equivalent permittivity can be tuned precisely, as indicated in Equations (5) and (7), by adjusting the duty cycle. This adjustment results in achieving an appropriate equivalent refractive index, enhancing TE wave reflection, improving TM wave transmission, and, ultimately, enhancing the extinction ratio of the polarizing element. In a polarization performance analysis of a Au metal wire grid, L.M. Liu [20] observed that, with a constant wire grid period and by using varying duty cycles of 0.3, 0.5, and 0.7, a higher duty cycle led to a larger degree of polarization. As the duty cycle increased, both TM wave and TE wave transmittance decreased, but TE wave transmittance decreased more significantly, resulting in a larger extinction ratio. The duty cycle is used to balance the extinction ratio with the transmission and absorption of the wire grid, with a common design choice being a duty cycle of 0.5.

4. Research Status of Wire Grid Structure

The wire grid structure can be classified into a single-layer structure, bilayer structure, or multilayer structure. The existing literature predominantly focuses on single-layer and bilayer structures. The bilayer structure [47] involves the addition of a dielectric anti-reflection film between the substrate and the wire grid in comparison to the single-layer structure. This bilayer structure significantly enhances the transmittance of the TM wave, decreases the transmittance of the TE wave, and, consequently, improves the extinction ratio of the polarizer. In a study conducted by J. Jeon [48] on enhancing the efficiency of infrared polarization imaging using a bilayer wire grid polarizer, comparisons were made among the single-layer wire grid structure A, bilayer structure B (dielectric anti-reflection coating + wire grid), and bilayer structure C (dielectric anti-reflection coating with identical periodicity + wire grid). Simulation results indicated that structure C exhibited the highest TM wave transmittance, the lowest TE wave transmittance, and the highest extinction ratio, as shown in Figure 3.

There are three distinct shapes—rectangular, trapezoidal, and circular—as shown in Figure 4. In a study on sub-wavelength metal wire grids for visible polarization imaging, P. Sun [49] analyzed the impact of these three shapes on polarization performance while maintaining the same wire grid height, wire grid period, and duty cycle. The findings revealed that the wire grid with circular shape demonstrated the highest transmittance, followed by the grid with a rectangular shape, while the one with a trapezoidal shape demonstrated the lowest transmittance. Conversely, the wire grid with a rectangular shape exhibited the highest extinction ratio, followed by the one with a trapezoidal shape and then the one with a circular shape. Due to its superior overall performance, the rectangular shape is commonly preferred in the design of most sub-wavelength metal wire grids.

4.1. Research on Single-Layer Structures

Thomas Siefke et al. [41] (2016) produced a titanium oxide wire grid polarizer on a fused silica substrate through Atomic Layer Deposition (ALD). The polarizer had a period of 104.5 nm, a wire width of 26 nm, and a wire height of 150 nm. TE wave and TM wave transmittance were measured using a lambda 950 spectrophotometer with a Glan–Taylor prism from 230 nm to 500 nm and a 193 nm laser system setup. The findings revealed that, at 244 nm, the extinction ratio peaked at 834, with a corresponding TM wave transmittance of 15%. Although the titanium oxide wire grid polarizer exhibited a transmittance of only 10% at 193 nm, which was lower than the 18.6% of chromium oxide and 44% of tungsten, the extinction ratio of 384 surpassed that of chromium oxide (138) by 2.8 times and that of tungsten (22) by 17.4 times. These results suggested that, in the deep ultraviolet spectrum, interband transitions in Wide-Bandgap (WBG) semiconductors can achieve higher extinction ratios compared to intraband transitions in metal wire grids. Consequently, the operating wavelength range of wire grid polarizers can be expanded to the shorter deep ultraviolet spectrum.

Kosuke Asano et al. [44] (2014) utilized metal oxides, such as Cr₂O₃, as wire grid materials to enhance both stability and optical performance. A 90 nm periodic Cr₂O₃ wire grid was produced on a quartz substrate using the double-patterning technique in conjunction with KrF lithography and dry etching. A flow chat of the process is illustrated in Figure 5. Scanning electron microscopy (SEM) analysis revealed wire widths of 37.5 nm (with a duty cycle of 0.417) and 34 nm (with a duty cycle of 0.378), a wire height of 120 nm, and an aspect ratio of approximately 3:1. TM wave and TE wave transmittance were measured using a self-constructed test system, as shown in Figure 6. At a wavelength of 193 nm, the TM transmittance for the high duty ratio sample was 18.6%, the TE transmittance was 0.134%, and the extinction ratio was 21.4 dB. For the low-duty-ratio sample, the TM transmittance was 31.1%, with an extinction ratio of 18.7 dB. By introducing material innovation through the use of metal oxides and process innovation via double-patterning technology, the issue of metal wire grid polarizers being prone to oxidation in deep ultraviolet was resolved. Consequently, the stability and reliability of wire grid polarizers were significantly enhanced.

J.L. Qi [50] (2024) proposed a graphene wire grid polarizer with a flat polarization extinction ratio curve across an ultra-wide wavelength band ranging from 1 μm to 10 μm. By theoretically modeling and optimizing the number of graphene layers and Fermi level to enhance carrier concentration, a polarizer fabricated on a quartz substrate achieved remarkable performance. With 100 graphene layers and a Fermi level of 3 eV, it transmittance of TM-polarized light was more than 80%, while the transmittance of TE-polarized light was less than 1%, and the polarization extinction ratio exceeded 20 dB, reaching up to 84.5 dB within the 1 μm to 10 μm range. Furthermore, the polarization extinction ratio curve remained exceptionally flat within this entire wavelength band, as shown in Figure 7. Although traditional metal wire grid polarizers demonstrate stable performance, their insufficient carrier concentration limits their applicability to broadband wavelengths. Graphene, as a semimetal material, enables the realization of a novel plasmonic resonance effects. Its exceptionally high and flat polarization extinction ratio properties filled a technical gap in this spectral wavelength band. This advancement offered a simple and feasible implementation path for developing ultra-broadband, high-performance graphene polarizers. Future research should focus on experimentally validating strategies of carrier concentration modulation in thick-layer graphene.

C.Y. Wang [51] (2023) proposed a novel design with a high aspect ratio, the ratio of wire height and wire width. Dry etching was conducted on a Si substrate using the ICP-GSE200 system manufactured by NMC Company. Compared to wet etching, dry etching resulted in greater etching depth and better grid rectangularity. Following the etching process, a grid structure with a period of 550 nm, wire width of 76.5 nm, and wire height of 1850 nm was achieved, corresponding to a high aspect ratio of 24:1, as shown in Figure 8. Subsequently, the grid was positioned at an inclination angle of 10.8°, and a single silver metal layer was deposited on the grid surface. The structure and manufacturing process are illustrated in Figure 9. The TM wave transmittance and TE wave transmittance were measured by applying a Fourier-transform infrared spectrometer (Nicolet iS 50R), and the extinction ratio (ER) was subsequently calculated. The experimental findings demonstrated that the polarizer exhibited an average transmittance of 56.57% for the TM wave, as well as an average extinction ratio of 32.33 dB and a peak of 40 dB within the 3–15 μm operating wavelength band, compatible with the 3–5 μm and 7–12 μm atmospheric windows. Within a radius of 15 mm in the central area, the deviation in TM wave transmittance was less than 3%, and the extinction ratio deviation was less than 2 dB. This breakthrough surpassed the size constraints of traditional infrared grid polarizers. Through design optimization and process improvements, a wire grid structure with a high aspect ratio was fabricated, enabling the manufacture of large-area, broadband, high-extinction ratio infrared polarizers. This advance not only reduced the quantity and weight of optical components in broadband remote sensing systems but also enhanced the performance of broadband remote sensing imaging systems.

Y.Q. Chen et al. [19] (2018) fabricated a metal aluminum wire grid polarizer for terahertz band application on a Teflon substrate with a thickness of 3 mm via the use of a femtosecond laser micromachining system. The finite-difference time-domain (FDTD) method was applied to simulate and analyze the influence of duty cycle and wire grid height on polarization performance. The simulation results indicated that, under the same period condition, a smaller duty cycle (wire width/wire period) resulted in lower loss and higher transmittance for TM-polarized light. However, constrained by the micromachining accuracy of the femtosecond laser system, they selected a grid structure with a period of 30 μm and a width of 20 μm grid (i.e., duty cycle of 0.667). At the same THz frequency of 7 THz, the electric field intensity distributions caused by surface plasmon resonance coupling within the grid slits with different wire heights of 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm were compared. It was observed that greater wire height caused more interference fringes due to the strong electric fields. In order to reduce the number of fringes, they chose a wire height of 10 μm. The time domain signals of bare the Teflon substrate and wire grid polarizer were measured by using a self-built terahertz time domain spectroscopy (THz-TDS) system. These two time domain signals were Fourier-transformed, and then the TM wave transmittance and TE wave transmittance were calculated and obtained at different frequencies. Based on the extinction ratio calculation formula, Equation (16), and the formula of polarization calculation Equation (14), the extinction ratio was between 40 dB and 45 dB, and the degree of polarization was 1, achieving perfect linear polarization.

4.2. Research on Bilayer Structures

H. Kim et al. [52] (2024) produced metal wire grid polarizers on a Si substrate, also producing InAs/GaSb type-II superlattice (T2SL) photodetectors (PDs) utilizing nanoimprint lithography with the same fabrication processes. The wire grid consisted of a Su-8 dielectric anti-reflection film with a thickness of 450 nm and a Au metal wire grid with a period of 400 nm, a wire width of 200 nm, and a wire height of 150 nm. The structure of the anti-reflection film and metal wire grid is shown in Figure 10. Due to the different thermal conductivities of the Si substrate and the InAs/GaSb T2SL PDs, distinct femtosecond laser (FSL) polishing parameters were selected for post-processing of the metal wire grid polarizers. The polishing reduced surface roughness, thereby improving the surface quality of the wire grid and enhancing the extinction ration (ER), as demonstrated by the test data presented in Figure 11. After femtosecond laser polishing of a metal wire grid polarizer fabricated on a silicon substrate, structure C exhibited the best performance. The TM wave transmittance increased to a maximum of 99%, while the TE wave transmittance decreased to a minimum of 0.13%, achieving a maximum extinction ratio (ER) of 658 dB. For the metal wire grid polarizer integrated with the InAs/GaSb II-type superlattice detector, as shown in Figure 12, the extinction ratio (ER) increased to 1044 dB after femtosecond laser polishing, 2.6 times higher than with the pre-polishing ER, as shown in Figure 13. This study demonstrated that the innovative process of combining nanoimprint lithography and femtosecond laser polishing post-treatment broke through the performance bottleneck of mid-infrared polarizers. This offers a significant technical pathway for applications in high-performance infrared imaging systems.

A. Chicharo et al. [53] (2021) utilized nanoimprint lithography (NIL) to fabricate bilayer wire grid polarizers on Si substrates and a cyclic olefin polymer (COC) film. They used COMSOL Multiphysics to simulate and compare the impact of various parameters, such as wire period (p), wire height (d), and gold layer thickness (t), on the polarizer performance, while maintaining a constant duty cycle of 0.5. Wire grid polarizers with three distinct structures as shown in

Table 4. List of materials and structural parameters of bilayer wire grid polarizer.

Structure	Substrate	Period (μm)	Height (μm)	Au Thickness (μm)
P1	COC	2	3	0.1
P2	COC	3	3	0.1
P3	Si	2	2	0.1

were fabricated through a process involving direct laser writing (DLW), inductively coupled plasma reactive ion etching (ICP-RIE), nanoimprint lithography (NIL), and gold film sputtering, as shown in Figure 14. Testing results obtained by using a terahertz time-domain spectrometer (THz TDS) in the frequency range of 0.1–25 THz and a Fourier-transform infrared spectrometer (FTIR) in the range of 0.9–20 TH indicated that the wire grid polarizer fabricated on the COC substrate exhibited higher TM wave transmittance and a higher extinction ratio than one fabricated on a silicon substrate within the 0.1–25 THz range. Specifically, the TM-polarized light transmittance of the COC polarizer was more than twice that of the silicon substrate polarizer. Notably, at 4.2 THz, the extinction ratio of the P1 polarizer achieved the highest extinction ratio of 65.4 dB. The characteristics of the COC substrate wire grid polarizer exhibited high TM wave transmittance, a broad operating bandwidth, and a high extinction ratio, marking its significant potential for applications in the terahertz band.

X.H. Fu et al. [54] (2021) fabricated a metal aluminum wire grid polarizer with a period of 300 nm, a height of 100 nm, and a duty cycle of 0.5 on a Si substrate via holographic lithography. A multilayer dielectric anti-reflection film was coated between the substrate and the wire grid to improve TM wave transmittance. Si and SiO were selected as high-refractive index and low-refractive index materials, respectively, for the anti-reflection film. An innovative way to reduce the influence of radiation temperature on a photoresist grid via intermittent coating of an aluminum film was studied, thus improving the rectangularity of a metal aluminum wire grid. The TM wave transmittance and TE wave transmittance in the 3 μm~5 μm wave band were measured using a Fourier infrared spectrometer with a polarizer inside, and then, the extinction ratio was calculated. The results showed that the average TM wave transmittance was 89.1%, and the average extinction ratio was 21.9 dB.

P. Sun et al. [49] (2011) designed and prepared an aluminum-clad wire grid for visible-light spectra. A sub-wavelength quartz wire grid with a period of 250 nm, a duty cycle of 0.34, and a height of 230 nm was fabricated via nanoimprint and ion beam etching techniques. The aluminum film was deposited twice on the quartz wire grid, as shown in Figure 15a, and the quartz wire grid was placed at an angle of 54 degrees, calculated as shown in Figure 15b. The wire grid structure was simulated and calculated according to the wire grid structures obtained by SEM, as shown in Figure 16b. The actual aluminum film coated on the top of quartz grating exhibited a circular structure, rather than an ideal rectangular structure; see Figure 16a. This deviation was due to the inverse relationship between the shield area and the aluminum deposition area. During the coating process, with the increase in the thickness of the aluminum film coated on the top of the wire grid, the larger the shielded part was, the smaller the aluminum deposition area. The intensities of the TM and TE waves were measured via a self-built test system; then, the TM wave transmittance, TE wave transmittance, and extinction ratio were calculated. The results are shown in

Table 5. Transmittance and extinction ratio values of TM and TE waves corresponding to different wavelengths.

Wavelength (nm)	413.1	457.9	476.5	488	501.7	514.5	532	632.8
TE Transmittance (%)	68.31	58.11	56.37	55.36	51.03	66.67	60.51	60.24
TE Transmittance (%)	12.46	8.04	6.18	6.16	5.36	8.78	7.53	2.23
extinction ratio	5.48	7.23	8.61	8.99	9.52	7.59	8.03	29.97

.

J.T Yang et al. [55] (2021) developed and fabricated a sub-wavelength bilayer metal wire grid polarizer for visible-light spectra on a flexible polycarbonate (PC) substrate. The wire grid fabricated on the PC substrate had a period of 278 nm, a width of 139 nm, and a height of 110 nm, ensured through a nanoimprinting process. Subsequently, a 70 nm thick single-layer aluminum metal film was deposited by magnetron sputtering. The fabrication process is shown in Figure 17. TM wave and TE wave transmittance were measured using a self-constructed test system, as shown in Figure 18. The experimental findings indicated that the TM wave transmittance was as high as 48%, and the extinction ratio was 100,000 within the 350 nm to 800 nm range. The fabrication processes, which only involved nanoimprinting and magnetron sputtering, were simpler than those including photoresist, coating, lift-off, and etching; thus, this process showed obvious advantages in low-cost and large-scale polarizer manufacturing.

4.3. Research on Multilayer Structures

Using simulation software, S.H. Liu et al. [56] (2023) conducted a comparative analysis of the extinction ratios of a single-layer Au wire grid (with a grid height of 400 nm), a single-layer Al wire grid (with a grid height of 400 nm), and a multilayer Au-Pt-Ti wire grid (with a grid height of 440 nm), maintaining the same period of 800 nm and with a duty cycle of 0.5. The findings indicated that, within the 3 μm~5 μm wavelength range, the extinction ratio value of the single-layer Al wire grid was optimal, while that of the multilayer Au-Pt-Ti wire grid was close to that of the single-layer Au wire grid. Taking into account various factors, such as the fabrication process, stability, and reliability, Au, Pt, and Ti were ultimately chosen as the preferred materials for the final grid, fabricated with a grid period of 800 nm, a grid height of 440 nm, and a duty cycle of 0.5. See the structure shown in Figure 19. The novel aspect of the fabrication process was that a single SiO₂ dielectric film was initially deposited on a Si substrate to enhance TM wave transmittance. Then, Ti, Pt, and Au metal layers were sequentially deposited. Ti served as an adhesion layer between the Pt and the SiO₂ film, enhancing the adhesion force between the metal and dielectric layers, thereby ensuring a more robust interface connection between the metal and dielectric layers. Pt acted as a bonding layer between Ti and Au. This multilayer structure enhanced the adhesion force between the metal and the substrate compared to a single-layer metal grid, thereby improving the reliability and stability of the metal wire grid. Simulations indicated that the extinction ratio exceeded 28.68 dB at 3.89 μm and 32.77 dB at 4.57 μm when a 3.2 μm metal isolation band was introduced between adjacent pixels.

Y.Y. Kong et al. [57] (2020) investigated the polarization characteristics of a metal–dielectric–metal (MDM) structure in an infrared wire grid polarizer. They conducted simulations using the finite-difference time-domain (FDTD) method to analyze the impact of magnetic polarization polaritons on TM wave transmittance and extinction ratio in an MDM structure, as shown in Figure 20a. The researchers elucidated resonance phenomena by integrating an equivalent LC circuit model into their analysis. In comparison to various wire grid configurations, such as single-metal Al wire grids, dielectric metal–metal wire grids, and metal–metal–dielectric wire grids, the metal–dielectric–metal wire grid structure exhibited superior polarization performance. It achieved an extinction ratio (ER) exceeding 40 dB across the 1–5 μm wavelength range, as shown in Figure 20b. The optimized structural parameters included a wire grating period of 500 nm, a duty cycle of 0.5, and a layer thickness of 100 nm. At a wavelength of 4 μm, this configuration realized an ER exceeding 42 dB and a TM wave transmittance of 89%, outperforming traditional single-layer metal gratings.

5. Conclusions

The rapid development of polarization imaging technology has led to a growing need for wire grid polarizers. This article provides a comprehensive review of wire grid polarizers, focusing on their characteristics, construction materials, structures, fabrication methods, and measurement approaches. In terms of structural design evolution, wire grids have progressed from being single-layer and bilayer structures to multilayer structures, enhancing the overall transmittance and polarization capabilities of wire grids. Additionally, by selecting appropriate substrate materials, such as flexible polymers like PET, viable alternatives to the conventional glasses used for wire grid substrates can be found. The materials used for fabricating wire grids typically consist of metals, metal oxides, and wide-bandgap semiconductors. Metal oxides, in particular, are preferred as wire grid materials due to their ability to resolve the issue of oxidation, commonly observed in metal wire grids when they are exposed in deep ultraviolet high-energy environments. This choice enhances the reliability and stability of polarization elements. Various techniques are employed in the preparation of wire grids, such as electron beam lithography, nanoimprinting, and holographic lithography. Among these techniques, nanoimprinting stands out for its capacity to overcome the challenges associated with high costs, low efficiency, and complex processes inherent in electron beam lithography, primarily due to its applicability in large-area fabrication. Furthermore, the application of femtosecond laser post-treatment can effectively resolve the surface roughness of metal wire grids, thereby enhancing the optical performance of polarizers. In terms of wavelength band applications, extending from visible and mid-infrared to deep ultraviolet and terahertz bands presents a promising direction. Future research on wire grid polarizers will advance towards larger-scale dimensions, high uniformity, high extinction ratios, and high transmittance values. This review paper serves as a valuable reference for researchers in this field and has practical significance for further exploration of the advancements in wire grid polarizer technology.