A polarizing filter transmits a single state of polarization of light while absorbing, reflecting, or deviating light with the orthogonal state of polarization. Applications include fluorescence polarization assays and imaging, second-harmonic-generation imaging and laser materials processing. Polarizers are characterized by the contrast ratio, or the ratio of the transmission through a pair of identical aligned polarizers to the transmission through the same pair of crossed polarizers. Contrast ratios typically vary from about 100:1 to as large as 100,000:1, but Semrock’s ion beam sputtering technology has enabled breakthrough improvements in performance of traditional thin-film plate polarizers. Semrock thin-film plate polarizers are guaranteed to achieve higher than 1,000,000:1 contrast, and only Semrock polarizers can achieve unique spectral performance of our patent-pending polarizing bandpass filters.
There are three common types of polarizers:
Thin-film Plate Polarizers
Semrock manufactures thin-film plate polarizers, which are based on interference within a dielectric optical thin-film coating on a thin glass substrate. These polarizers function as beamsplitters, diverting the unwanted polarization 90°. Thin-film plate polarizers have a number of unique advantages, including superior transmission, low scattering and very little wavefront distortion. They can be made with excellent environmental reliability, the highest laser damage threshold and very large aperture sizes. Thin-film plate polarizers function over a specific wavelength range because they are based on multiwave interference and are best suited to laser applications or systems with a limited signal band.
Birefringent Crystal Polarizers
In birefringent polarizers different polarizations of light rays incident on an interface at an oblique angle are deviated by different amounts. Light is incident at certain angles on the two interfaces formed by a gap between two birefringent crystals, such that one polarization is partially transmitted while the orthogonal polarization is totally internally reflected. For example, in a “Glan-laser” Calcite polarizer, there is an air gap and the angles are near or equivalent to Brewster’s angle for p-polarized light, which is therefore nearly completely transmitted, whereas at these angles s-polarized light is internally internally reflected. Birefringent crystal polarizers achieve high contrast ratios, high transmission and high optical damage thresholds. The main drawbacks include a limited aperture size due to the high cost of growing good optical quality crystals, and they are not well suited for precise laser deliver and imaging applications.
Glass Film Polarizers
Glass film polarizers work by selectively absorbing one orientation of linearly polarized light more strongly than the other. Glass film polarizers are similar to thin-film plate polarizers in that they work over a limited wavelength range, but have additional drawbacks to consider. The absorption characteristics of glass film polarizers lead to low transmission of the desired light and low laser damage threshold, making these polarizers unsuitable for laser applications.