What is the upper part of the beam splitter called

The upper layer of a beam splitter is typically a thin optical coating that controls the proportion of light reflected and transmitted, and it is crucial for defining the device's splitting behavior.Function of the Upper LayerThe upper layer of a beam splitter, often called the first surface, is where the incident light initially interacts. This layer can be a metallic coating (e.g., aluminum or silver), a dielectric thin-film coating, or a partially reflective surface on a glass or plastic substrate . Its primary function is to partially reflect and partially transmit light, determining the splitting ratio, which can be 50/50, 30/70, or other ratios depending on the application .Types of CoatingsMetallic Coatings: Thin layers of metals like aluminum or silver are deposited on the upper surface. They reflect a significant portion of light while absorbing a small amount, providing a broad wavelength response but slightly higher energy loss .Dielectric Coatings: These consist of multiple alternating layers of materials with high and low refractive indices. The thickness of each layer is optimized to exploit interference effects, allowing precise control of reflection and transmission at specific wavelengths .Antireflection Coatings: Applied to the back surface or sometimes the upper layer to minimize unwanted reflections and ghost images, typically reducing reflectivity to around 0.5% at 45-degree incidence .Impact on Polarization and WavelengthThe upper layer can influence polarization-dependent splitting. Dielectric coatings may reflect and transmit differently for p- and s-polarized light, which is important in applications requiring polarization control . Dichroic coatings on the upper layer can also selectively reflect or transmit light based on wavelength, enabling shortpass, longpass, or multiband splitting .Structural ConsiderationsIn cube beam splitters, the upper layer is applied to the hypotenuse face of one prism before the prisms are cemented together, protecting the coating from environmental damage and ensuring precise 90-degree beam separation . In plate beam splitters, the coating is applied directly to the first surface of a flat glass plate, with the back surface often antireflection-coated to reduce ghosting . In summary, the upper layer of a beam splitter is a carefully engineered optical coating that defines the device's reflection, transmission, polarization, and wavelength characteristics, making it a critical component in optical systems .