An unusually broad window
Most optical materials are transparent in only part of the spectrum. Glass passes visible light but blocks much of the ultraviolet and the far infrared. Diamond stands out because a high-quality crystal transmits across a very wide range: from the deep ultraviolet, through the visible, and far into the infrared, out to wavelengths well beyond those that many infrared materials can handle. That breadth is what makes optical transparency one of diamond’s most valuable properties alongside its hardness and heat conduction.
Uses such as diamond optics and diamond windows build on this range; here the focus is on what sets it and what narrows it.
The short-wavelength edge
Transparency ends where photons carry enough energy to lift electrons across the band gap. Diamond’s gap is large, corresponding to ultraviolet light with a wavelength a little above 200 nanometers. Visible light, which has much less energy, cannot excite electrons across it, so it passes straight through a pure crystal. Below roughly that wavelength, absorption becomes strong and the crystal turns opaque.
This is why diamond is useful for ultraviolet optics where many materials fail, and why its wide gap is also what makes it interesting as a wide bandgap semiconductor.
Into the infrared
At the long-wavelength end, the limit comes from the lattice itself rather than from electrons. Diamond’s atoms vibrate, and photons of certain infrared wavelengths can be absorbed by creating those vibrations. In a perfect diamond crystal, the simple one-vibration absorption is forbidden by symmetry, which is a large part of why diamond is so transparent in the infrared. However, absorption by combinations of two vibrations is allowed, which produces a weak absorption band in the mid-infrared, typically described as lying in the region of several micrometers. Beyond that band, diamond becomes transparent again and stays so out to very long wavelengths.
For most uses, the absorption band is modest, especially in thin pieces, so diamond can be used for laser windows across both visible and infrared wavelengths, including high-power systems where its thermal conductivity helps remove heat.
What narrows the window in real material
Ideal transparency belongs to ideal crystals. In practice, several things interfere:
- Nitrogen. The most common impurity absorbs in the ultraviolet and blue, giving a yellow tint, and adds extra absorption features in the infrared. This is why optical grades are usually specified as very low in nitrogen impurities.
- Boron. Adds absorption in the red and infrared and gives a blue color.
- Other defects. Vacancies, dislocations and strain can create color centers, and they add scatter and unwanted birefringence, which matters in precision optics.
- Graphitic or non-diamond carbon. Especially in films, small amounts of non-diamond carbon in the material absorb strongly and make the material gray or black.
- Grain boundaries. In polycrystalline diamond, boundaries between grains can scatter light, so thick polycrystalline plates may look hazy unless they are grown and polished carefully.
- Surface quality. Rough or damaged surfaces scatter light, and diamond is hard to polish, so good surfaces take real effort.
Refractive index and reflection
Diamond has a high refractive index, which is part of its sparkle in gem use. For optical windows, the high index means a noticeable fraction of light reflects at each surface unless an anti-reflection coating or surface texture is applied. Not all light lost at a window is absorbed; some is simply reflected, and the fix is different.
Natural versus synthetic
Natural stones vary widely in impurity content. Synthetic diamond grown by chemical vapor deposition can be made with very low nitrogen and controlled defects, which is why optical-grade material is generally synthetic. Both high-pressure growth and vapor growth can produce clear material, but they differ in impurity profile; the two routes are often framed as HPHT versus CVD.
A cautious summary
Diamond is transparent over one of the widest ranges of any solid, from the ultraviolet edge near the band gap to the far infrared, with a weak mid-infrared absorption band in between. Real pieces are less perfect, and the quoted range depends on grade, thickness, surface finish and whether the material is a single crystal or polycrystalline. Specific transmission data should come from measurements on the actual grade being considered.