定义：
采用多层介质涂层得到的光学偏振片。

薄膜偏振片是光学偏振片的一种，利用了多层介质涂层的干涉效应。涂层通常放置在玻璃片上。（不需要采用双折射材料。）在某些入射角度时，可以得到随偏振变化的反射率。（通常是利用s偏振光被反射，而p偏振光则透射。）通常设计的薄膜偏振片工作时的入射角度为 45°，这时反射光束相比于入射光束角度变化为90°。 

存在不同种类的薄膜偏振片： 

1. 只包含一层镀膜的玻璃片的片状偏振器。通常设计入射角为布儒斯特角，这样可以避免背向的透射光的反射损耗。如果入射角为其它角度，例如45°（最常见），则需要在背面采用抗反射涂层，并且前面的设计层的结构也比较困难。

图1：工作在近布儒斯特角的片状偏振片，在顶层具有反射涂层，底部则没有涂层。 

1. 偏振立方体通常是对 45° 棱镜涂覆涂层，然后将另一个的 45° 棱镜黏在其上，得到立方体结构。入射和出射面通常都涂覆抗反射涂层。常用的MacNeille立方体设计的基本原理是在所有入射面以布儒斯特角入射消除p偏振的反射。因此引入布拉格反射镜很容易得到s偏振很高的反射率，并且不会引入对p偏振的反射。布儒斯特角条件需要衬底材料的折射率是合适的（给定涂层材料）。这种偏振片适合很大的波长范围，但是入射角度范围比较窄（几度）。 

在其它偏振立方体结构中，也可以不满足布儒斯特角条件，而利用干涉效应抑制p偏振的反射。这种干涉偏振片只工作在很窄波长范围内，但是可以很灵活的选取材料。 

偏振立方体中的粘接表面使纳米激光器脉冲情况下的光学损伤阈值减小到 1 J/cm2 的量级。而不需要粘接的光学接触的偏振立方体则可以承担更高的强度。 

  

图2：由薄膜涂层粘接两个 45° 棱镜得到的偏振立方体。 由于多层涂层之间的干涉效应与偏振有关，因此薄膜偏振片只能工作在有限的波长和角度范围。通过优化薄膜设计，工作波长能够到达几百纳米的范围。但是，这种宽带偏振片不能达到窄带偏振片（激光线偏振片）的性能。图3给出了一个工作带宽为50nm的偏振片的反射率曲线。 

图3：采用TiO2/SiO2的薄膜偏振立方体的反射率，工作在600-650nm范围内。 

为了优化薄膜偏振片的涂层，可以采用类似的数值模拟技术设计宽带分束器或二色性反射镜。 

薄膜偏振片的优势在于可以制作非常大尺寸，采用晶体（双折射）偏振片则非常困难。因此它可以工作于很高功率或能量的激光脉冲装置中。
 

Acronym: TFP

Definition: optical polarizers based on a multilayer dielectric coating

Thin-film polarizers are a kind of optical polarizers based on interference effects in a multilayer dielectric coating. That coating is usually placed on a glass plate. (A birefringent optical material as in various other types of polarizers is not needed.) A strongly polarization-dependent reflectivity is achieved for some range of incidence angles. (Basically always, s-polarized light is reflected and p-polarized light transmitted.) It is often convenient to design a thin-film polarizer such that it can be operated with an incidence angle of 45°, such that the reflected beam is obtained with an angular change of 90°.

There are different kinds of thin-film polarizers concerning optical fabrication technology and various properties which are of practical interest:

• Plate polarizers (Figure 1) consist only of a coated glass plate. The design is often made such that the angle of incidence is essential at Brewster's angle. In that way, reflection losses for the transmitted light on the back side are avoided easily. For other angles of incidence such as 45° (which is often convenient), an anti-reflection coating is required on the back side, and the layer structure design for the front side can also be more difficult.

Figure 1: A plate polarizer for operation near Brewster's angle, having a reflecting coating on the top side and no coating on the bottom side.

• There are also polarizing cubes (Figure 2), which are obtained by coating a 45° prism and cementing another 45° prism to it, so that a cube is obtained. The input and output faces usually obtain anti-reflection coatings. The basic idea of the common MacNeille cube design [2] is to eliminate the reflection for p polarization by having Brewster's angle at all internal interfaces. It is then easy to obtain high reflectivity for s polarization with a simple Bragg mirror design without introducing a significant reflection for p polarization. The Brewster angle condition requires that a substrate material with suitable refractive index (for given coating materials) is chosen. Such designs work well in a large range of wavelengths, but only in a narrow angular range (typically a few degrees).

In other polarizing cube designs than the MacNeille design, one does not fulfill the Brewster angle condition but uses interference effects to suppress reflection for p polarization. Such interference polarizers typically operate well only in narrow wavelength ranges but give more flexibility concerning the used materials.

The cemented interface of a polarizing cube leads to a lower optical damage threshold of the order of 1 J/cm² for nanosecond laser pulses. Optically contacted epoxy-free polarizing cubes, not requiring a cement, can stand several times higher fluences.

Figure 2: A polarizing cube based on a thin-film coating between two 45° prisms.

Since the interference effects in the multilayer coating are wavelength-dependent, a thin-film polarizer can work only in a limited wavelength range and angular range. However, operation in a range of tens or even a few hundred nanometers is possible by proper optimization of the thin film design. Such broadband polarizers, however, do not achieve the very high performance of narrowband polarizers (laser line polarizers), which are optimized for a narrow wavelength range. Figure 3 shows an example with a moderate operation bandwidth of 50 nm.

Figure 3: Reflectivities of a thin-film polarizer cube based on TiO₂/SiO₂, designed for operation at 600–650 nm with the software RP Coating.

For the optimization of the coatings for thin-film polarizers, similar numerical techniques can be used as for designing broadband beam splitters or dichroic mirrors, for example.

An advantage of thin-film polarizers is that they can be made with rather large dimensions, which is more difficult with crystalline (birefringent) polarizers. It is thus possible to operate such high-power or high-energy devices with laser pulses at very high peak power levels.

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