First Surface Mirrors

For a first surface mirror, the reflected light does not need to propagate through the mirror substrate. That has the following consequences for its performance, when comparing with a second surface mirror:

• The mirror substrate cannot reduce the reflectivity by absorption in certain wavelength regions. Therefore, front surface mirrors are preferable in situations where area wide low-loss reflection bands (e.g. in the infrared) are required.
• Wavefront distortions due to optical inhomogeneities of the substrate cannot occur. The substrate only needs to be sufficiently flat.
• For non-normal incidence of light, one avoids the transverse beam offset due to propagation in the glass substrate. In addition, in imaging applications one does not obtain ghost images due to the additional (weaker) front surface reflection of a second surface mirror.
• For normal incidence, strong chromatic dispersion based on interference of contributions from front and back surface reflections is avoided. (For details of such effects, see the article on Gires–Tournois interferometers.)

For many applications in optics, the mentioned advantages are vital; some examples:

• Laser mirrors (e.g. for CO₂ lasers) generally need to be first surface mirrors.
• Imaging applications generally need them for avoiding ghost images.
• Broadband mirrors for spectroscopy are also usually made as front surface mirrors.

Therefore, first surface mirrors are quite common in professional optics. However, compared with back surface mirrors, they have the important disadvantage of being substantially more sensitive. The front surface may be touched, and a metal coating on the front surface is substantially more sensitive than a bare glass surface. For example, fingerprints can easily cause oxidation of the metal, and it may not be possible to clean such mirrors while retaining the full optical quality. Also, moisture or aggressive gases may cause oxidation of the mirror coating.

In order to mitigate the sensitivity problem, and partly also for further improving the reflectivity, first surface mirrors are often made as protected or enhanced mirrors. This means that the metal layer is covered with a thin layer or multiple layers of a dielectric material such as amorphous SiO₂ (silica) or Si₃N₄ (silicon nitride), which protects the coating particularly against oxidation (tarnish) and to a lower extent against scratches.

Strictly speaking, the main reflection will no longer occur at the front surface, when there is a dielectric coating on the metal layer. That coating, however, is very thin, and therefore often not considered as providing an additional surface.

For enhancing the reflectivity, one often uses multilayer dielectric coatings; this leads to enhanced metal coatings e.g. for enhanced silver mirrors. One effectively combines the large reflection bandwidth of a metallic mirror with the higher reflectivity and damage threshold of a dielectric mirror. In some cases, the coating is also optimized concerning chromatic dispersion – mostly for use in ultrafast lasers.

A potential disadvantage of additional coatings arises from their deviating thermal expansion coefficients. This can cause problems particularly when mirrors need to be operated in wide temperature regions. That is the case for optical telescopes, for example.

Here are some examples for protected and enhanced metal-coated mirrors:

• Protected aluminum mirrors are relatively cheap mirrors for the visible and near-IR spectral region. They have a protective coating, e.g. made of silicon monoxide, but not particularly enhanced optical properties.
• There are UV-enhanced aluminum mirrors with particularly high reflectivity for ultraviolet light down to ≈250 nm. The enhancement of reflectivity, achieved with a dielectric multilayer coating, is typically obtained in a wide spectral range, i.e., also in the visible region. Besides the optical properties, the durability is also enhanced.
• Similarly, IR-enhanced gold mirrors are available, which have a high reflectivity in the near or even mid infrared, e.g. up to 20 μm.
• Ultrafast-enhanced silver mirrors have a very high reflectivity and low chromatic dispersion over the whole wavelength band of titanium–sapphire lasers (750–1000 nm). The coating also protects against the tarnishing of the silver, but it is still better to keep such mirrors in a dry environment.

It is also possible to apply laser line coatings, which can enhance the reflectivity at one or more specific laser lines.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.