Encyclopedia … combined with a great Buyer's Guide!

Dichroic Mirrors

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

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

More general term: dielectric mirrors

German: dichroische Spiegel, dichroitische Spiegel

Category: photonic devicesphotonic devices


Cite the article using its DOI: https://doi.org/10.61835/1un

Get citation code: Endnote (RIS) BibTex plain textHTML

A dichroic mirror (or dual-band mirror, dual-wavelength mirror, dichroic reflector) is a mirror with significantly different reflection or transmission properties at two different wavelengths – actually meaning two wavelength regions of some often not so large width. The specifications often refer to frequently used laser lines, so that dichroic mirrors are often belonging to the category laser line optics.

There are also trichroic mirrors, having defined optical properties at three different wavelengths.

Some dichroic reflectors are used for broadband applications, e.g. for reflecting only ultraviolet light to some application but not so much infrared light which could lead to unwanted heating of the irradiated objects. Similar broadband devices are called hot mirrors or cold mirrors, depending on whether they reflect or suppress heat radiation.

The dichroic property relates to one of two possible meanings of the term dichroism.

Dichroic mirrors are applied for different purposes. Some examples:

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 transmittance 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 transmittance 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 (reflectance vs. wavelength) 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.

Alternative Approach: Using Polarization

In situations where the two relevant wavelengths of two light beams are rather close, it may be difficult to achieve e.g. high transmissivity for one and high reflectivity for the other. It may then be easier to work based on polarization, if non-normal beam incidence can be used.

More to Learn

Encyclopedia articles:


The RP Photonics Buyer's Guide contains 75 suppliers for dichroic mirrors. Among them:


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

(Suggest additional literature!)

Questions and Comments from Users


What is the difference between dichroic and dielectric?

The author's answer:

The term dichroic denotes the function – treating different wavelengths differently – while dielectric states what materials are used for a mirror.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Spam check:

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.


Connect and share this with your network:

Follow our specific LinkedIn pages for more insights and updates: