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Dichroic Mirrors

Definition: mirrors with significantly different reflection or transmission properties at two different wavelengths

Alternative terms: dual-wavelength mirrors, dual-band mirrors

More general term: dielectric mirrors

German: dichroische Spiegel, dichroitische Spiegel

Category: photonic devices

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A dichroic mirror is a mirror with significantly different reflection or transmission properties at two different wavelengths. This relates to one of two possible meanings of the term dichroism.

Dichroic mirrors are applied for different purposes. Some examples:

So-called harmonic separators (see Figure 1) are dichroic mirrors for separating the harmonic (frequency-doubled) beam from the pump beam, often outside the laser resonator. For that application, the two wavelengths for which the mirror needs to have very different reflectivities are so far apart from each other that it is easy to make such mirror designs.

harmonic separator
Figure 1: Use of a dichroic mirror as a harmonic separator. The frequency-doubled light is reflected to nearly 100%, whereas most of the pump light is transmitted, although a few percent of it may still be reflected to the output port.

Most dichroic mirrors are dielectric mirrors, but there are also crystalline mirrors where the multilayer structure consists of semiconductor materials. In both cases, the operation principle is that of a multilayer interference coating.

Short-pass and Long-pass Mirrors

In electronics, the terms low-pass and high-pass filter are common, where “low” and “high” refers to the frequency. In optics, where it is more common to refer to wavelengths, one uses the terms short-pass and long-pass mirror. Here, a short-pass mirror (or shortpass mirror) is one which has a high transmission at short wavelengths and high reflectance at longer wavelengths; it could also be called a high-pass filter (referring to optical frequencies).

It can be challenging to make mirrors such that the wavelength with high transmission and the wavelength with high reflectance are close together, as e.g. in Figure 2. They need more sophisticated designs and often also a higher precision of coating fabrication.

Fabrication of Dielectric Mirrors

Most dichroic mirrors are fabricated as dielectric mirrors, e.g. with electron beam deposition, ion beam sputtering (IBS) or ion-assisted deposition (IAD). Semiconductor-based dichroic mirrors are fabricated with epitaxial techniques such as MOCVD or MBE.

Depending on the case, the design of the required layer structure may be possible based on analytical considerations, possibly followed by a numerical optimization, or entirely on numerical optimization, e.g. with a Monte-Carlo method. In many cases, the design involves a compromise between the obtained optical properties, the required number of layers, and the required growth precision.

reflectance spectrum of a dichroic mirror
Figure 2: Reflectance spectrum of a dichroic mirror coating, designed with the software RP Coating for high transmittance (low reflectance) around 800–950 nm and high reflectance at 1064 nm.

For any dielectric mirror, the reflection spectrum depends on the angle of incidence and (for non-normal incidence) also on the polarization of the input light. Only to a limited extent, mirror designs can be made such that the desired dichroic properties are achieved over some range of input angles.

As a dichroic mirror has to be transparent for at least one wavelength of interest, the quality (e.g. transmission losses) of the substrate material and possible reflections from the back side need to be considered. An anti-reflection coating on the backside can help to reduce such a reflection, and a slight wedge form of the substrate can often eliminate the effects of residual reflection.

Suppliers

The RP Photonics Buyer's Guide contains 71 suppliers for dichroic mirrors. Among them:

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Bibliography

[1]T. Amotchkina et al.,“Broadband beamsplitter for high intensity laser applications in the infra-red spectral range”, Opt. Express 24 (15), 16752 (2016), doi:10.1364/OE.24.016752
[2]Design of a dichroic mirror with the RP Coating software

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See also: dichroism, mirrors, dielectric mirrors, dielectric coatings, Bragg mirrors, anti-reflection coatings, optical filters
and other articles in the category photonic devices

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