Metallic Mirror

Definition: The reflection of a mirror is caused by a metallic coating.

The metal mirror is a mirror obtained by using a thin metal coating, and the metal coating can be produced by evaporation technology or sputtering technology. As for the metal coating on the sink, it is usually glass or metal. Common metal coatings include aluminum, silver, or gold; copper, chromium, or other nickel-chromium alloys are less common.

The metal coating is usually coated with one or more thin dielectric materials, such as amorphous silicon dioxide (SiO2) or silicon nitride (Si3N4), to protect the metal coating from oxidation or scratches . These protective coatings are more wear resistant than uncoated and more sensitive than dielectric mirrors. Careful cleaning is required at the interface between metal and optics. And metal mirrors are sensitive to humidity and corrosive gases.

Reflectivity can also be enhanced with multiple protective coatings, which result in enhanced metal coatings. At this time, the metal dielectric coating not only has the large bandwidth of the metal mirror, but also has the high reflectivity and damage threshold of the dielectric mirror.

Broadband, Low Dispersion

A big advantage of metallic mirrors over dielectric mirrors is that their reflectivity is the same over a wide spectral range, independent of the angle of incidence and polarization state. Moreover, the metal reflector is simple to manufacture and relatively inexpensive. Therefore, ordinary mirrors are made of metal mirrors. Metal mirrors are sometimes required for ultrashort pulses with very wide bandwidths, because it is difficult to obtain sufficient reflection bandwidth from dielectric mirrors (although chirped dielectric mirrors already have large bandwidths). In addition, the chromatic dispersion of metal mirrors is also very weak, and the reflection phase shift has little wavelength dependence. Dielectric mirrors can operate at long infrared wavelengths, eg, up to 20 microns. In this wavelength region, it is difficult for the dielectric mirror to work, because the medium has strong absorption of the light of this wavelength.

limited reflectivity

One disadvantage of metal mirrors compared to dielectric mirrors is their large reflection losses. This is a property of its material, as metals inevitably absorb some of the incoming light (even if they are pure). The resulting reflectivity is therefore limited, for example, silver mirrors can reach 98%. Another consequence is a lower optical damage threshold: heat is generated due to absorbed light, and this heat is easily damaged when stored in very thin coatings. Regardless of whether it is expressed by average power or peak power, the damage threshold is relatively low. If the average power is high, there is significant thermal lensing, and thermal effects can cause beam distortion even below the damage threshold.

The use of multilayer dielectric coatings can reduce reflection losses. These coatings can also correspondingly increase the optical damage threshold. For example, an enhanced silver mirror has a damage threshold of a few J/cm2 for nanosecond pulses generated by a 1064nm YAG laser, compared to 0.5 J/cm2 for a simple silver mirror (less for aluminum). Dielectric mirrors can handle tens of J/cm2.

Partially transmitted mirror

Metallic coatings can be made very thin to obtain partial transparency. However, the loss is very large in this case, so the sum of the reflectivity and the refractive index is less than 1. In this case, a dielectric mirror is generally used, but when a large operating bandwidth is required and high power loss is allowed, it can be used Partially transmissive metal mirrors.

Common materials

Aluminum coatings are typically used in the visible and UV regions, and mirrors can be greater than 90% in the visible region and less than 90% in the UV region.

Silver coatings are suitable for wavelengths ranging from 500 nm to 20 microns. Due to the low loss of silver, its damage threshold is higher than that of aluminum coatings.

Gold mirrors are similar, but only for wavelengths of 600nm or longer. Reflectivity greater than 95% (sometimes around 99%) can be achieved. In the wavelength range of 700-2000 nm, an average reflectance of 97% can be obtained if a protected gold mirror is used. Unprotected gold mirrors are sometimes used, in order to avoid dispersion in the protective coating; since gold is not easily oxidized.

Gold-coated copper mirrors (sometimes prepared electrochemically) can be used in high-power infrared lasers, such as carbon dioxide lasers. Due to the high thermal conductivity of gold and copper, high thermal effects can be assumed.

First Surface Metal Mirror and Second Surface Metal Mirror

In the first surface metal mirror, the reflective coating is on the side of the incident light. Light passes through the coating, but does not reach the sink.

The reflective coating of the second surface metal mirror is on the other side of the sink, so the coating can be better protected. Light passes through the substrate both before and after reflection. Such mirrors are more common in home applications. Technically applied, Fresnel reflections on the first surface can be problematic (eg ghosting, power loss) and sometimes chromatic dispersion of the glass is also a problem.