IR Transmission of Sapphire – R/S vs HEM, Thickness & Surface Effect
In a joint project by KYBURZ SAPPHIRE and SAPPHCOM | SAPPHIRE COMPETENCE, new IR transmission measurements on R- and S-type samples were performed and compared to HEM sapphire. All measurements were carried out on highly polished samples. The discussion of surface effects here is based on theoretical considerations and literature — a systematic experimental study will follow in Part 3 of this series.
Background
Sapphire offers excellent transmission from the UV into the mid-IR. While in the UV range material quality and growth control dominate, beyond about 5.3–5.5 µm the multiphonon absorption of Al2O3 sets a hard limit. Our goal: precise measurements of R and S in 1–5 µm, comparison with HEM (2.5–25 µm), and a clear interpretation of surface and thickness effects.
Sample Preparation
Cylindrical raw material was sourced by SAPPHCOM and processed by KYBURZ SAPPHIRE to defined thicknesses (ground, lapped, polished) to minimize surface influences. R- and S-type samples were tested in multiple thickness levels (2–24 mm).
Results: R vs S (1–5 µm)
- 1–2 µm: both types with very high transmission (>85 %), practically identical.
- 2.5–3.5 µm: weak OH bands; S-type slightly ahead (≈ +0.1 %-points).
- 4–5 µm: expected decrease; S-type remains somewhat higher (≈ +0.15 %-points).
Influence of Thickness
Transmission follows Beer–Lambert:
T = (1 − R)2 · e−α·d.where R is the Fresnel reflection coefficient, α the absorption coefficient, and d the sample thickness.
Consequently, any increase in thickness yields an exponential decrease in transmission — even small α values become significant at larger d.- d≤ 5 mm: very high transmission; subtle R/S differences become measurable.
- d≈ 9–14 mm: noticeable reduction; material differences appear more clearly.
- d≈ 19–24 mm: strong reduction, thickness dominates over type differences.
Mechanical advantage of sapphire: Compared to other IR materials (e.g., quartz, ZnS, Ge), sapphire’s high hardness, bending strength, and toughness allow a significantly thinner design at equal load. Thinner parts directly translate into higher transmission (Beer–Lambert) — in many cases a 2 mm sapphire window is mechanically more robust and optically more transparent than a much thicker quartz window in the same spectral range.
HEM Comparison (2.5–25 µm)
The 5 mm HEM sample shows ~50 % transmission at 5 µm and drops to <1 % just above 6 µm. This behavior is intrinsic (lattice vibrations) — independent of growth method and orientation. R/S follow the same edge.
Practical note: For optics above 5.5 µm, 5 mm sapphire is essentially opaque.
Options: (i) significantly thinner parts, (ii) alternative LWIR materials (e.g., ZnS, ZnSe, Ge).
Surface Quality & Roughness
Beyond the bulk, surface quality governs effective transmission. Optically polished surfaces (Ra < 2 nm) minimize scattering and Fresnel losses. When roughness approaches the wavelength (Ra ≈ λ/10…λ), interference and scattering occur; local overcritical angles enhance internal reflection — measurable transmission losses result.
Note: The surface effects discussed here are based on theoretical considerations and literature.
The measurements shown were performed on highly polished samples only.
A systematic experimental investigation of surface roughness effects on transmission will be presented in the next blog post (Part 3).
Metasurfaces: functional surfaces without coatings
Laser-induced micro-/nanostructures (e.g., LIPSS) can act like intrinsic AR coatings and increase transmission in selected IR windows — a compelling option when conventional coatings are not feasible.
Applications
- Optical windows, lenses, domes for 1–5 µm (sensing, defense, process monitoring)
- IR analysis cells and windows in chemical/process environments
- High-pressure / high-temperature IR windows
- Protective windows for IR lasers
Summary
Combining selected raw material (SAPPHCOM) with precision machining (KYBURZ SAPPHIRE) ensures optimal IR transparency up to the physical limit near ~5.3–5.5 µm. R and S show very high transmission up to 5 µm, with S slightly superior. Thickness and surface are the key levers; thanks to its mechanical strength, sapphire can be designed thinner — delivering additional transmission benefits.
SAPPHCOM application support: material selection, orientation, thickness design and surface finish —
coordinated with European crystal growers and precision processors.
Additional steps (e.g., annealing, functional structuring) on request.
Let’s discuss your target wavelengths, tolerances, and budgets.
References
- Dobrovinskaya, Lytvynov, Pishchik: Properties of Sapphire, Springer (basics of n(λ), absorption, transmission range).
- Lye, Wang, Lam: Ultrashort-pulse laser processing of sapphire, Applied Surface Science (effect of rough exit surfaces, TIR & scattering).
- Wen et al.: Gold film-assisted structuring, Optics & Lasers in Engineering (nanostructures/metasurfaces for absorption/transmission).
- Born & Wolf: Principles of Optics (scattering/interference at rough interfaces).
- Beckmann & Spizzichino: Scattering from Rough Surfaces (classical theory for Ra ~ λ).
