Metallic Reflectors

Nanoscale layers are gatekeepers of light. This is why reflectors are predominantly thin-film reflectors. Designing a mirror is a balancing act between reflectivity requirements, durability in specific environments, manufacturing technologies, and, of course, cost. In this post, we’ll explore the most common metallic mirrors and the physics that makes them reflect.

Reflectivity of mirror metals for unpolarized light with indicent angle at 45°

Universe's way of repeating a story

Light is an electromagnetic wave. When it hits metal, its oscillating electric field pushes and pulls free electrons. Because these electrons are free to move, at lower frequncies they wiggle at the exact same frequency as the incoming light. Moving charges create their own electromagnetic fields, generating a 'new' light wave—the reflection—that travels out of the metal.

This process is so efficient that the original light wave is extinguished almost instantly. This is why we can't see through a sheet of aluminum, but we can see our face in it. In metals like copper and gold, the 'knocking' process of electrons (interband transitions) also starts already with photon energies right in the visible spectrum; they absorb high-frequency blue light to move electrons between energy bands, leaving us with only the reflected warm yellow and red tones.

As we move toward the UV, this 'knocking' becomes dominant for all metals, and absorption begins to outpace reflection. Eventually, light hits the plasma frequency. At this point, the electrons have too much inertia to keep up with the field’s rapid oscillations. The metal stops acting like a mirror and becomes a transparent dielectric—the universe's way of turning a wall into a window. This is why X-rays can pass through metal sheets that stop visible light as the metal doesn't look like a solid wall of electrons anymore; it looks like a vast empty space populated by occasional nuclei.

With metallic mirrors a physical layer thickness of 70-100 nm is ideal for maximum reflection. Example is Silver.

Practical surface quality and scattering

If light hits optically smooth surface at 30°, light leaves at 30° degrees. Roughness is not a static label; it is a wavelength- and geometry-dependent interaction. The practical implication is that as wavelength increases, fixed surface features can become “electrically small” relative to the long wavelenght. This can cause a surface that appears matte in the visible to transition into a high-intensity specular reflector in the infrared.

To analyze how the surface flatness or roughness affects to reflectivity, one needs to measure the specular (mirror-like) and diffuse (scattered) components as a function of angle of incidence, repeating the process for varying levels of roughness.

Metallic reflectors

For metallic reflectors, extinction coefficient is high, which is why they reflect so well. However, this is why also get hot if they absorb even 1-2% of a high-power laser's energy. Dielectric mirrors in general have a higher laser induced damage threshold than metallic mirrors because they don't absorb as much energy into the material.

Gold (Au): Gold has the highest reflectivity in the infrared and great chemical stability, but very little scratch resistance because of its malleability, and it needs a primer layer.

Rhodium (Rh) & Chromium (Cr): These materials offer a high degree of hardness and abrasion resistance. Rhodium retains constant reflecting power without a protective layer, but since its material cost is astronomical, Chromium is often the preferred choice for applications such as automotive reflectors.

Silver (Ag): Conventional mirrors are silver rear-surface mirrors on transparent substrates. This is because exposed silver is not chemically or climatically resistant—the film remains permanently bright only on the side adhering to the glass. Front-surface silver mirrors require a barrier-adhesion layer and a specialized protective coating.

Aluminum (Al): The most common reflector material. Aluminum mirrors are generally superior to silver in terms of substrate adhesion and show no tarnish when exposed to the atmosphere. To improve mechanical and chemical stability, aluminum can be protected by a thin, optically transparent film, allowing it to be carefully cleaned.

In specialized cases, such as large astronomical reflecting telescopes, unprotected aluminum is used to achieve the highest quality. They use it primarily because protective dielectric layers can introduce polarization effects and slight phase shifts that mess up sensitive astronomical measurements. It's as much about "signal purity" as it is about raw % reflectance. However, because dust and dirt decrease reflectance and these mirrors cannot be cleaned without damage, they require periodic recoating—often once a year.

Polarization and angle of incidence

Incoming light can be randomly polarized, as seen in nature, or fully polarized, as with a laser. Because of this, the reflectance phase is a major factor in precision technology where wave interference is utilized.

When light hits a denser medium (for example, traveling from air to glass), the reflected wave undergoes a 180° phase shift—the wave essentially "flips" upside down. Conversely, the phase change is 0° when moving from glass to air. However, with metals, the phase shift is never a simple 0° or 180° because these materials absorb light and free electrons in metal layer respond more easily to to electric field that is parallel to surface. A mirror’s performance changes depending on the angle at which light hits it. There is a fundamental law in physics called the Kramers-Kronig relation, which implies that you cannot change the reflectivity of a mirror without also affecting the phase shift. They are mathematically locked together.

Reflectance Group Delay and group delay dispersion

Group Delay (GD) is a measure of the time delay experienced by the various frequency components of a light pulse as they reflect off a surface. While the reflectance phase tells you the shift of the wave's phase at a specific frequency, the Group Delay tells you how long the overall "envelope" or "packet" of light is held up by the reflection process.

In simpler terms: if the phase shift changes rapidly as the color (frequency) of the light changes, you will have a significant Group Delay.

Imagine a group of runners (different frequencies) hitting a wall. If the wall "holds onto" the blue runner longer than the red one, the group will be spread out when they continue their run. This spreading is what we call dispersion, or group delay dispersion (GDD).

Reflectance GD is critical in fields where the timing of light pulses is measured in femtoseconds, such as in ultrafast lasers and advanced fiber optics. If you design a mirror to have a very sharp "cutoff" (for example, reflecting blue but passing green perfectly), you will always create a massive spike in Group Delay right at that transition point. In the world of physics, you cannot have a "sharp" spectral change without a "messy" timing change.

In NGS, we can process planar substrates with maximum dimensions of 3x6 meters.

"The beauty you see in me is a reflection of you." — Rumi