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. However, 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:
- In a diode-pumped laser, a dichroic short-pass mirror in the resonator, placed next to the laser crystal, may be used for injecting pump light.
- In a laser with intracavity frequency doubling, a dichroic end mirror may couple out the harmonic light while fully reflecting the pump wave.
- In case of external frequency doubling, a dichroic mirror may be used as harmonic separator (see below), i.e., as a kind of wavelength-dependent beam splitter.
- In laser microscopy (fluorescence microscopy), a dichroic mirror can be used for separating the fluorescence light (containing the image information) from the pump light.
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.
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.
There are also trichroic mirrors, having defined optical properties at three different wavelengths.
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.
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.
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