What is a Wire Grid Polarizer?
To understand the function of wire grid polarizers, one must first understand polarization of light. Light is most commonly characterized by its frequency (or wavelength) and intensity. In the visible spectrum, the wavelength is related to the color of light, whereas intensity represents its brightness. These are two typical properties for common applications such as lighting in your home or car.
However, there are other applications that exploit another important characteristic of light called polarization. When we invoke classical electrodynamics, light is an electromagnetic wave comprised of an electric and a magnetic field. These fields oscillate in space and time depending on the wavelength and frequency, respectively. The electric field (E) is perpendicular to the magnetic field (B), and both oscillate perpendicular to the direction of propagation. When the electric field oscillates randomly, we call that light unpolarized (and this is what we mostly observe around us).
What is Light Polarization?
For light to be polarized, electric (or magnetic) field oscillation needs to be confined to a single plane or follow a helical trajectory along with direction of propagation. Basically, the plane of oscillation needs to be well-defined or non-random for the light to be termed as polarized. When the oscillation is confined to a single plane, the light is linear polarized. Here, the plane can either be aligned with, or have a tilt with respect to, the vertical plane. When the oscillation follows a helical pattern, the light is circularly polarized. Depending on the direction of the helical trajectory, we can either have right or left circularly polarized light. When the helix traces an ellipse instead of a circle, the light is elliptically polarized. In all these cases, we see that the electric field oscillation follows a certain well-defined trajectory and is not random.
How to Create Polarized Light
Since most light around us is unpolarized, optical elements called polarizers are used to achieve the desired polarization state. Polarizers are designed to either restrict the electric field oscillation of unpolarized light to create polarized light or transform one polarization state to another. For instance, a linear polarizer transforms unpolarized into linearly polarized light. Another commonly used polarizing element is the quarter wave plate, which converts linearly polarized light into circularly polarized light.
What is Bifringence?
Additionally, there are numerous other special optical elements that utilize a property called birefringence to create polarized light. Let us assume that unpolarized light can be represented by two orthogonal polarization states called s (TE) and p (TM). Both s-polarization and p-polarization refer to the E-field component perpendicular and parallel to the plane of incidence, respectively. Birefringent materials have different refractive indices for the two orthogonal polarization states, which creates a phase difference between the two. In such cases, one component can travel slower than the other or even experience spatial separation. Crystals such as calcite and quartz are naturally birefringent. Special prisms like the Wollaston, Glan-Thompson, or the Nicol prisms are designed to be birefringent to achieve polarization.
Fresnel reflections can also be used to create polarized light. Depending on the refractive indices of the reflecting surface and ambient medium, there exists a special angle called the Brewster angle, at which the reflected light is s-polarized. Hence, polarized sunglasses have lenses that only pass p-polarization, which effectively cuts out the glare. This is commonly used in photography lenses as well.
How Do Wire Grid Polarizers Work?
Wire grid polarizers are another class of polarizers that use fine metal wires to restrict E-field oscillation for s-polarized light. Fine metal wires typically 100 nm to several microns apart are lithographically deposited on substrates depending on the operating wavelength range. For the s-component, the metal grid works like a typical metal surface as electrons are excited along the wire length. As a result, the s-polarization is almost completely reflected. In case of the p-component, electrons can only be excited along with wire width, which is in the sub-micron range. Hence, most of the p-polarized light is transmitted. A wire grid polarizer is typically characterized by the extinction ratio (ER) and polarization efficiency (PE). Let us say transmission is T1 when the wire grid polarizer is oriented to maximize p-polarized light transmission. If T2 is the transmission when the polarizer is rotated by 90 degrees (or crossed), then ER = T2/T1 and PE = (T1-T2)/(T1+T2). The extinction performance is typically expressed as the inverse of the ratio, (1/ER):1. E.g. if ER = 0.0001, then extinction performance is 10000:1.
Common Uses of Wire Grid Polarizers
Wire grid polarizers offer several advantages over conventional polarizers. For instance, they are suited for applications that require a high ER. Wire grid polarizers also are inherently broadband and can be designed to operate across UV-Visible-IR spectrums by choosing appropriate substrates. Finally, wire grid polarizers have greater thermal stability and can operate under high temperature or high flux conditions. These features make them widely used in high quality projection systems, military applications, medical imaging, simulators and even digital cinema. The choice of grid material, wire spacing, and substrates in designing wire grid polarizers achieves a wide range of utility.
A noteworthy application of wire grid polarizers is in Liquid Crystal On Silicon (LCOS) type digital projectors. LCOS-type projectors are becoming increasingly popular and offer several advantages over traditional LCD counterparts. For instance, they provide smooth film-like images in contrast to the pixelation seen in LCD projectors. The projection quality of LCOS projectors is heavily dependent on the quality of the polarizers. Inside a projector, bright lamps are in close proximity to optical components, and subject to high flux and temperature. Additionally, LCOS projectors demand polarizers with high ER. Based on these requirements, wire grid polarizers are ideal for polarizers, analyzers and polarizing beam splitters in the LCOS-type projectors.
Holographic Wire Grid Polarizers
Several materials that are suited for IR applications cannot be converted to typical ruled polarizers. In such cases, holographic methods are used to achieve a wire grid polarizer. In this method, a photoresist-coated substrate is exposed to an interference pattern generated by a laser. Due to the intensity profile of the interference fringes, a sinusoidal surface feature is generated once the resist is developed. This surface feature can be now coated with metals like aluminum to obtain a wire grid like structure.
Optometrics offers holographic wire grid polarizers on several IR-compatible substrates like CaF2, ZnSe, BaF2, Ge and KRS-5 (Thallium Bromide) over an operating wavelength range of 2.5-30 microns. Typical ERs for a single polarizer are 300:1. If applications require larger ER, a combination of 2 polarizers can provide ERs up to 90,000:1.