Wire grid polarizers, as the name suggests, are polarizing elements that use arrays of fine metal wires to selectively transmit p-polarized light and reflect s-polarized light. Due to this function, they are used as linear polarizers and polarizing beam splitters. Wire grid polarizers can be fabricated with different metals like aluminum or gold on a range of substrates. This provides great design flexibility to produce polarizers that can operate in different spectral regimes, such as UV, visible, and IR. Additionally, they have high tolerance to heat and are suited for high-temperature applications. Due to these advantages they are a popular choice of polarizer in several applications from digital projectors to imaging and displays.
Since there are several choices of wire grid polarizers in the market, how do you go about buying right polarizer for your application? Here are some important parameters to consider when choosing a wire grid polarizer.
Spectral Characteristics and Substrates
One of the first parameters to consider when selecting wire grid polarizers is its spectral characteristics. Depending on the intended application, the operating spectral range needs to be determined. Some questions to ask here are:
- Is it intended for narrow-band or broadband operation?
- What are the desired spectral characteristics?
- Is it going to be used for UV, visible, NIR, SWIR, MIR, FIR, or terahertz applications?
For instance, glass substrates offer a spectral range of 420-700 nm, which is good for applications like digital projectors. In contrast, fused silica can be used over a much broader range from 250 nm – 4 microns. Naturally, fused silica substrates are 5-10 times more expensive than glass.
IR applications such as FTIR spectrometers and IR imagers require wire grid polarizers to operate from 2.5 – 30 microns. In such cases, Optometrics’ CaF2, ZnSe, BaF2, Ge and KRS-5 (Thallium Bromide) polarizers offer the best performance, although the transmission of these substrates varies across the IR range. KRS-5 has a flat transmission profile from 2-30 microns but is more expensive as compared to the other substrates. Optometrics offers holographic coatings on all these substrates with a high transmission of 80-90%, which is ideal for high-performance and low-loss applications.
Wire grid polarizers based on glass or fused silica substrates can operate over a wide temperature range, typically from -40oC to 200oC. They can be designed to have low thermal expansion and high stability at high temperatures. In contrast, thin film polarizers on plastic substrates can only operate in the -20oC to 80oC range. If high-temperature performance is an important design consideration for you, then wire grid polarizers are a good choice. Additionally, for applications involving high power lasers, wire grid polarizers offer high damage thresholds as compared to other types of polarizers.
Like most polarizers, a wire grid polarizer is characterized by its Extinction Ratio (ER) and Polarization Efficiency (PE). If, T1 is the transmission for p-polarized and T2 is the transmission when the polarizer is rotated by 90 deg (or crossed), then ER= T2/T1 and PE= (T1-T2)/(T1+T2). Extinction performance is typically expressed as the inverse of the ratio or (1/ER):1. E.g. If ER=0.0001, then extinction performance is 10000:1. Refer to our previous article on Understanding Polarization and Wire Grid Polarizers.
A high ER is desirable in order to achieve high performance. Manufacturers typically offer wire grid polarizers with an ER from 10:1 to 10000:1 over a wide spectral range, particularly in IR. If an extremely high ER is required, a combination of two or more polarizers can be used. For instance, Optometrics provides holographic wire grid polarizers with an ER of up to 300:1. Using two such polarizers in series can result in a ER of 90,000:1.
Coatings and Surface Quality
Many applications require anti-reflection (AR) coatings, particularly in displays and imaging. If your application demands a reflectance of under 1%, then you need a wire grid polarizer with an AR coating. Typically, the spectral response of the coating is designed to match the substrate transmission. However, AR coatings cannot be deposited on all substrates and they only offer the best performance at a particular wavelength.
For instance, the advantage of KRS-5 substrates is their high transmission over a wide spectral range. For such substrates, an AR coating is counterproductive as it interferes with the transmission range and narrows it down. Polarizers with a protective glass surface are more resistant to scratches, can be easily cleaned and therefore last longer. They are ideal in applications where the polarizer is subject to frequent cleaning or prone to damage.
Size, Shape, and Mounts
An important consideration is the size and shape of the polarizer. This depends on your application or product. Manufacturers mostly commonly offer square and round/circular polarizers. Here, circular polarizer refers to the physical shape of the polarizer and should not be confused with the circular state of polarization. Although more expensive, Optometrics can design a custom wire grid polarizer that meets your application needs.
For square polarizers, typical sizes are 12.5 mm x 12.5 mm, 25 mm x 25 mm, and 50 x 50 mm. There is more flexibility in sizes with round polarizers. For instance, Optometrics’ holographic wire grid polarizers come in diameters of 25 mm, 29 mm, 35 mm, 38 mm and 50 mm.
Finally, it is important to consider the type of mount. If you are building a custom application and designing your own mounts, then you should choose the unmounted option. Typically, when setting up experiments on an optical table, mounts are required to hold the polarizer in position. In such cases, ring or square mounted options are available.
While designing a product, dimensional tolerances need to be considered. This helps minimize any optical misalignments and defects. Most manufacturers will mention dimensional tolerances for the thickness of the polarizer, and the diameter (round) or length (square). Typically, tolerances of +/- 0.2 mm – 0.5 mm are quoted for readily available polarizers. If your application needs lower tolerance and more precision, you will need to get custom parts.
Types of Polarizers
In general, the electric and magnetic fields in an electromagnetic wave oscillate in random planes perpendicular to the direction of propagation. A linear polarizer blocks all random oscillations and only permits one polarization state. Several applications are made possible through the manipulation of polarization of light. The most popular use of linear polarizers is in photography. Unwanted scattering and glare from flat or reflective surfaces are common problems faced by photographers. Linear polarizers offer a simple and straightforward solution to this problem. Light that is directly reflected from surfaces has a strong s-polarization component due to Fresnel reflection at the Brewster angle. A linear polarizer in the camera can be used to suppress the glare by rotating it such that s-polarization component is suppressed. This results in a much better color contrast in the images. Glare suppression is also used in sub-surface imaging particularly imaging under water surfaces.
Linear polarization can be achieved in a transmission mode or a reflection mode. In a transmission mode, all unwanted polarization states are absorbed, and only one is transmitted. These types of polarizers are called absorptive polarizers. Certain crystals, polaroid filters, and nano-particle based polarizers come under this category. Polarizer sheets made from stretched plastics are commonly found in inexpensive 3D viewing glasses.
In contrast, a reflective or beam-splitting polarizer creates two states of orthogonal polarizations, one that is transmitted and the other is reflected. Polarizers based on Fresnel reflections at the Brewster angle, thin-film and wire grid polarizers fall under this category. Reflective polarizers can be commonly achieved in thin film configurations, where dielectric multilayer films are coated on a range of transparent substrates. The dielectric stack can be designed to create wavelength dependent polarization due to interference effects. This offers greater flexibility in achieving polarizers with high damage thresholds to laser irradiation. Wire grid polarizers also offer the same advantages as thin film polarizers, as metal grids can be deposited over large area substrates.
In the case of wire grid polarizers, there are two types of polarizers based on the fabrication approach that is used to achieve the metal grid:
Ruled Wire Grid Polarizer
With ruled wire grid polarizers, a fine diamond ruler is used to carve fine lines on the substrate. Once the grid is generated, a thin metallic layer is evaporated on to the grid to create the polarizer. Although this method is relatively inexpensive, it can only be used on hard substrates and cannot achieve the grid spacing needed for short wavelength operation.
Optometrics, a Dynasil company, offers ruled wire grid polarizers on CaF2 (Calcium Fluoride) and ZnSe (Zinc Selenide) covering a wavelength region from 2.5 to 20 microns.
Holographic Wire Grid Polarizer
In contrast to the ruled fabrication approach, holographic wire grid polarizers are fabricated using interference lithography. Lasers are used to generate fine interference patterns that are incident on a substrate coated with a photoresist. After a certain duration of exposure, the film is developed to create the grid pattern. Finally, a metal layer is evaporated on to the grid. This method offers a finer grid spacing that is suitable for short wavelength operation. It can be deposited on softer substrates like KRS-5, which have excellent broadband transmission. Polarizers fabricated using this method have lesser defects and higher uniformity. Holographic wire grid polarizers have several advantages over their ruled counterparts.
Optometrics, a Dynasil company, offers holographic wire grid polarizers on CaF2 (Calcium Fluoride), ZnSe (Zinc Selenide), BaF2 (Barium Fluoride), KRS-5 and Ge (Germanium), covering a wavelength region from 2.5 to 30 microns.