Sapphire and Optical Crystals
These are single crystals bought, ground and polished like glass but chosen for what glass cannot do. Sapphire (single-crystal Al₂O₃) is 9 on the Mohs scale, transmits from 170 nm to 5.5 µm, holds strength past 1,800 °C and takes 400 MPa or more in bending; calcium and magnesium fluoride open the deep UV; zinc selenide, germanium and silicon are opaque to the eye but carry the 3–5 and 8–14 µm infrared bands for thermal imaging and CO₂ lasers; the alkali halides reach 25–40 µm for spectroscopy. The optical glasses N-SF11, N-LaK9 and their neighbours are listed here as the high-index, high-dispersion partners to N-BK7 in lens design.
Popular optical crystal grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Sapphire | 400 | 400 | 3.98 | Not recommended | Not recommended | Scratch-proof, hot and high-pressure windows |
| CaF2 | — | 37 | 3.18 | Fair | — | Deep-UV and excimer-laser optics, low dispersion |
| ZnSe | — | 50 | 5.27 | Fair | — | CO₂-laser lenses and 0.6–20 µm windows |
| Ge | — | 70 | 5.32 | Fair | — | 8–14 µm thermal-imaging lenses and windows |
| Si (optical grade) | — | 124 | 2.33 | Poor | — | Light, hard 3–5 µm optics and substrates |
| MgF2 | — | — | 3.18 | Poor | — | Vacuum-UV windows to 120 nm; hardest fluoride |
| N-SF11 | — | 45 | 3.2 | Poor | — | High-index flint for achromats and prisms |
Choose sapphire and optical crystals for
Scratch-proof and high-pressure windows, watch crystals and sensor covers; furnace, plasma and combustion viewports to 1,800 °C; deep-UV, excimer and high-power laser optics; thermal-imaging lenses and FLIR windows in the 3–5 and 8–14 µm bands; FTIR and spectroscopy windows into the far infrared; laser hosts, electro-optic modulators and nonlinear frequency converters; and wafers and substrates for LEDs, RF and epitaxy.
Look elsewhere when
The optic works in the visible only (N-BK7 or Fused Silica cost a fraction and polish faster), a large or complex shape is needed (crystals are grown as boules and every surface is ground), the window is thin and must survive drops rather than scratches (Gorilla Glass), the part is a structural or thermal-shock component rather than an optic (Zirconia Y-TZP or Silicon Nitride ceramics), or humidity is uncontrolled and the candidate is a water-soluble halide such as KBr or NaCl.
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Optical crystal by application
- Scratch-proof and pressure windows
- Sapphire discs 0.5–10 mm for watch crystals, barcode-scanner windows, sight glasses to 1,800 °C and pressure viewports; Al2O3 (Sapphire) wafers in c-plane for LED and RF epitaxy; MgO and Y3Al5O12 (YAG) where a hard, wide-band window is needed without birefringence.
- UV and laser optics
- CaF2 lenses, prisms and windows for 193–248 nm lithography and excimer beams; MgF2 and LiF for vacuum-UV to 120 nm and as protective coatings; CVD Diamond (Optical) for multi-kilowatt CO₂ and gyrotron windows.
- Thermal imaging
- Ge lenses and windows for the 8–14 µm long-wave band (opaque above about 100 °C, so keep the optic cool); Si (optical grade) for 3–5 µm mid-wave optics and lightweight mirrors; ZnSe and ZnS for broadband and rugged FLIR windows; IRG 24 as the chalcogenide glass alternative that can be molded.
- Spectroscopy and electro-optics
- KBr, NaCl, KCl and CsI windows and cells for FTIR to 25–50 µm, KRS-5 for humid environments, BaF2 for combined UV-IR coverage; LiNbO3 and LiTaO3 for modulators and SAW filters, KDP (KH2PO4) and YAG for nonlinear optics and lasers, GaAs and CdTe for infrared and detector substrates.
Working with optical crystal
- Crystal growth & forming
- Sapphire is grown by Kyropoulos, Czochralski or edge-defined film-fed methods into boules and ribbons; fluorides and halides by Bridgman or Czochralski; ZnSe and ZnS by chemical vapour deposition; germanium and silicon by Czochralski for optical grade. Blanks are cored and sliced from the boule with orientation set by X-ray — there is no molding or hot-forming, so every surface is ground.
- Grinding & polishing
- Diamond grinding and lapping shape all of them; sapphire takes three to five times as long as glass and is polished with diamond, then colloidal silica, to λ/10 and 0.5 nm roughness. CaF2 and BaF2 cleave along (111) and crack under thermal shock — gentle feeds and cool coolant; ZnSe, KBr and the halides are soft (Knoop 100–150) and cut fast but scratch easily, and ZnSe dust is toxic, so machining is enclosed.
- Coatings & sealing
- High-index crystals reflect heavily uncoated — about 36 % per surface for Ge, 30 % for Si, 17 % for ZnSe — so anti-reflection coatings matched to the band are part of the specification, and diamond-like carbon hard coats protect Ge and ZnS on external windows. Sapphire is brazed to Kovar or niobium through an active-metal or metallized joint for hermetic viewports; halides are held with O-rings or indium seals and never wetted.
- Tolerances & design rules
- Diameters ±0.05 mm, thickness ±0.02 mm, flatness λ/10 and parallelism under 30 arc seconds are standard optical tolerances; state the crystal orientation (c-plane, random, or a specific axis) on the drawing because birefringence and cleavage depend on it. Design sapphire windows on 400 MPa flexural strength with a safety factor of 4 or more, support crystals on compliant gaskets, and keep germanium below 100 °C and halides below 40 % relative humidity.
Other glass series
Sapphire and Optical Crystals — frequently asked
What is optical crystal?
These are single crystals bought, ground and polished like glass but chosen for what glass cannot do. Sapphire (single-crystal Al₂O₃) is 9 on the Mohs scale, transmits from 170 nm to 5.5 µm, holds strength past 1,800 °C and takes 400 MPa or more in bending; calcium and magnesium fluoride open the deep UV; zinc selenide, germanium and silicon are opaque to the eye but carry the 3–5 and 8–14 µm infrared bands for thermal imaging and CO₂ lasers; the alkali halides reach 25–40 µm for spectroscopy. The optical glasses N-SF11, N-LaK9 and their neighbours are listed here as the high-index, high-dispersion partners to N-BK7 in lens design.
Which grade of optical crystal should I use?
Sapphire for windows, viewports, watch crystals, scanner and sensor covers, wafers and bearing jewels — anything that must not scratch or must run hot or under pressure; specify c-plane (0001) orientation when birefringence would disturb the optics. CaF2 for excimer-laser and deep-UV optics at 193–248 nm and for low-dispersion achromats in the visible and near-IR; MgF2 for vacuum-UV windows down to 120 nm and as the hardest of the fluorides. ZnSe for CO₂-laser (10.6 µm) lenses and windows and broadband 0.6–20 µm optics; ZnS where the same window must be harder and survive rain and sand. Ge for 8–14 µm thermal-imaging lenses and windows at room temperature, Si (optical grade) for lighter, cheaper 3–5 µm optics and mirror substrates. KBr, NaCl, KCl and CsI for FTIR windows to 25–50 µm (hygroscopic — keep them dry), KRS-5 where the window must also survive humidity. YAG (Y3Al5O12) as laser host and hard, wide-band window, LiNbO3 for electro-optic and SAW devices, KDP (KH2PO4) for frequency doubling, and CVD Diamond (Optical) when a window must pass kilowatts of laser power or microwave energy.
Which grade of optical crystal is easiest to machine?
IRG 24 (machinability excellent, 85/100), then KCl (60/100).
Sapphire or fused silica for a high-temperature viewport?
Fused Silica when the viewport sees thermal shock and temperatures to about 1,000 °C and nothing abrasive touches it — it is cheaper, easier to polish, transmits deeper into the UV and its 0.5 ppm/K expansion shrugs off a quench. Sapphire when the window must go hotter (usable to 1,800 °C, though transmission in the infrared falls with temperature), carry pressure in a small diameter (400 MPa or more in bending against about 50 MPa for silica), resist erosion by particles or molten splash, or stay unscratched; its 5–8 ppm/K expansion means it should be brazed or gasketed with care and not quenched.
Germanium, silicon or zinc selenide for an infrared lens?
Ge for the 8–14 µm long-wave band: refractive index 4.0 gives thin, powerful lenses with little chromatic aberration, but it absorbs increasingly above about 60–100 °C and is heavy (5.3 g/cm³). Si (optical grade) for 3–5 µm mid-wave optics: half the density, harder, cheaper, and transparent to 7 µm, but useless at 10 µm because of lattice absorption. ZnSe when one optic must cover both bands or handle CO₂-laser power at 10.6 µm: 0.6–20 µm transmission, low absorption, but soft and needing a hard coat outdoors. All three are opaque to the eye and need band-matched anti-reflection coatings.