Reflecting Optical Components: An Overview
Almost as important as transmitting optical components are those that reflect a significant portion of incident light. In most cases, the main requirement is for specular reflectance to be as high as possible. However, certain specialized applications demand not only high reflectance but also extremely low absorption.
For mirrors used in optical instruments, simple metallic layers often suffice, and these will be discussed first. In cases requiring higher reflectance than simple metallic layers provide, additional dielectric layers can enhance reflectance. Multilayer all-dielectric reflectors, designed for maximum reflectance with minimal absorption while transmitting unreflected energy, are reserved for later tutorials.
1. Metallic Layers
The performance of commonly used metals for reflective coatings is shown in Figure 5.1.
Silver
Silver was historically the principal reflecting material for precision optics until the mid-1930s. It was typically deposited chemically in a liquid process, which remains in use for second-surface architectural mirrors. In these applications, a thin tin layer bonds the silver to the glass, which is then protected by copper and paint layers.
For front-surface applications, silver tarnishes over time, forming silver sulfide. Despite this, its high initial reflectance and ease of evaporation make it a useful choice for temporary coatings or short-term use, such as in interferometer plates for flatness testing. We will explore protected silver coatings in the next section.
Aluminum
John Strong introduced evaporated aluminum coatings for astronomical mirrors in the 1930s. Aluminum adheres well to most materials, including plastics, and provides excellent reflectance across the ultraviolet, visible, and infrared regions. While its reflectance gradually decreases over time, a protective oxide layer forms quickly after deposition, slowing further degradation.
Primary mirrors of large telescopes are recoated annually, using multiple sources to ensure uniformity for these large, heavy mirrors. Aluminum remains the standard for telescope mirrors, though some newer instruments are adopting protected silver coatings for improved performance.
Gold
Gold is the preferred material for infrared reflecting coatings due to its high reflectance in this region. However, its utility in the visible spectrum is limited as its reflectance drops sharply below 700 nm. Gold films are soft and require stabilization with an underlayer, often chromium or Nichrome (a nickel-chromium alloy), to improve adhesion.
Rhodium and Platinum
These metals are less reflective than silver or aluminum but are highly resistant to corrosion. Rhodium is commonly used for dental mirrors, which must endure sterilization and harsh environments, and for automobile rear-view mirrors exposed to weather and cleaning.
2. Protection of Metal Films
Metal films, while effective reflectors, are soft and easily scratched. They are also prone to atmospheric corrosion. Protective dielectric layers can improve durability and resistance.
Dielectric Layers on Metals
Adding a dielectric layer changes the reflectance characteristics. The admittance diagram (Figure 5.2) qualitatively illustrates the behavior of the system as a dielectric layer is added. Initially, reflectance decreases until the locus of the assembly’s admittance crosses the real axis. This minimum reflectance occurs when the dielectric layer thickness is less than a quarter-wave.
The phase thickness of the dielectric layer can be calculated using:
\[
\delta_f = \arctan \left( \frac{2\beta\eta_f}{\eta_f^2 – \alpha^2 – \beta^2} \right) + m\pi, \quad m = 0, 1, 2, \ldots
\]
Here, \(\delta_f\) is the dielectric layer thickness, and \(\alpha – i\beta\) represents the metal’s optical admittance.
Performance of Protected Coatings
For aluminum \((n – ik = 0.82 – i5.99)\) at 546 nm, silica (\(n = 1.45\)) and cerium oxide (\(n = 2.3\)) are common dielectric materials. Reflectance values for these combinations, calculated using equations in this section, are listed in Table 5.1. High-index layers reduce the reflectance minimum significantly but require precise monitoring of layer thickness to avoid performance drops.
3. Enhanced Reflectance Systems
Adding dielectric layers to metal coatings enhances reflectance in specific wavelength ranges.
Dielectric Pairing
Two quarter-wave dielectric layers, with indices \(n_1\) (outer layer) and \(n_2\) (inner layer), improve reflectance if:
\[
n_1 n_2 > 1
\]
For aluminum at 550 nm, applying silicon oxide (\(n_1 = 1.45\)) and titanium oxide (\(n_2 = 2.40\)) increases reflectance from 91.6% to 96.4%. Additional pairs of quarter-wave layers can raise reflectance further, achieving 99% or higher for multiple pairs. However, enhancement is limited to specific spectral regions.
4. Reflective Coatings for the Ultraviolet
Producing reflective coatings for ultraviolet applications is more challenging than for visible or infrared regions.
Aluminum Coatings
Aluminum is the best material for ultraviolet reflectance, particularly in the range up to 100 nm. The coating process involves rapid evaporation at high rates (40 nm/s or more) onto cold substrates, with pressures below \(10^{-6}\) torr. The use of ultra-pure aluminum improves performance, as shown by Hass and Tousey.
Protective Layers for UV Reflectance
Magnesium fluoride and lithium fluoride are effective for protecting aluminum films. These layers prevent oxidation and stabilize reflectance over time. They must be deposited immediately after the aluminum layer to minimize exposure to oxygen. Figures 5.5 and 5.6 illustrate the improvement in reflectance with these protective layers.
5. Conclusion
Metallic layers are crucial for reflective optical components. While aluminum remains the standard, advancements in protective coatings and enhanced reflectance systems allow for improved performance in specialized applications across the visible, infrared, and ultraviolet regions. Techniques like dielectric layering and the use of protective materials ensure durability and optimized reflectance for diverse optical requirements.
