Infrared Wire Grid Polarizers
Wire grid polarizers are commonly used to polarize radiation from an unpolarized molecular laser, attenuate radiation from a polarized laser or, using two in series, to both polarize and attenuate a laser beam. A second polarizer can be inserted in the reflected beam for applications requiring a polarizing beam splitter. Wire grid polarizers are also used in reflectance accessories for dispersive and FT-IR spectrophotometers. Applications include the investigation of metal surfaces and crystal structures at grazing incidence, where polarization of the incident radiation is required.
Wire grid polarizers transmit radiation when the “E” vector is perpendicular to the wire (E ^). Radiation with the “E” vector parallel to the wire (E II) is reflected. Due to surface reflections, the reflected beam contains both polarizations.
The extinction ratio of a polarizer is a measure of its ability to attenuate a plane polarized beam. Two principle transmissions are necessary to calculate an extinction ratio, T1 and T2. Assuming a perfectly plane polarized beam, T1 is defined as the maximum transmission for which the polarizer can be oriented. Minimum transmission (T2) is the transmission through the polarizer when it is rotated 90 degrees from T1. The extinction ratio is given as E = T2/T1 and expressed as a decimal or percentage. The inverse of E, expressed as a ratio (R = 100: 1), is used in our specifications. Wire grid polarizers can also be characterized by the degree of polarization, defined as P = (T1 – T2)/(T1 + T2).
Extinction ratios greater than 40,000:1 can be achieved by the use of two wire grid polarizers in series with their grids parallel (the overall extinction ratio is the product of the extinction ratio of the individual polarizers)
Ruled Wire Grid Polarizers
Produced on one of our diffraction grating ruling engines, precisely spaced grooves are ruled directly into CaF2 or ZnSe substrates. The ruling process forms sharp, well defined peaks which are then deposited with aluminum, forming an array of parallel “wires”. This technique produces polarizers which are useful in relatively high power laser applications.
Holographic Wire Grid Polarizers
Working with experts in the field, we have developed a special holographic technique to produce infrared polarizers with sub-micron grid spacing. Produced in one of our holographic laboratories, coherent laser beams are projected onto a photo-resist coated substrate to produce a precise pattern of equally spaced grooves with nanometer precision. Once processed, the resist has a sinusoidal surface relief profile which is then vacuum aluminized at an oblique angle to create the array of parallel conductors. The fabrication of holographic wire grid polarizers permits the use of a wide variety of infrared materials that do not lend themselves to the ruling process. They are available with a spacing of 2700 grooves/mm for optimum short-wavelength efficiency.
Calcium Fluoride and Barium Fluoride have low refractive index (high Tx) values and do not require anti-reflective (AR) coatings. Zinc Selenide has a high refractive index and transmission at specific wavelength regions which can be enhanced by an AR coating on the rear surface only. Zinc Selenide is usually optimized for transmission at specific laser lines, typically from 9 to 11 microns. KRS-5 is not normally AR coated, because this would limit its broad transmission range, which is its primary advantage. Germanium is AR/AR coated to maximize transmission @10.6μ.
The surface of a wire grid polarizer, like a diffraction grating, is extremely delicate. Nothing should ever be allowed to touch the surface of the polarizer. Handling, when necessary, should be by the edge only and with protected fingers. Always wash your hands after handling a polarizer. Careful removal of dust by gentle air flow is the only cleaning procedure recommended.