Material Data Center | American Photonics — IR & Laser Substrate Properties

Side by Side

Eight Materials Compared

Key attributes at a glance. Refractive index quoted at 587.6 nm (n_d) for visible-range materials and at the primary IR working wavelength for IR materials.

Material	Useful Range (µm)	Index n	dn/dT (10⁻⁶/K)	Density (g/cm³)	Knoop (kgf/mm²)	CTE (10⁻⁶/K)	k (W/m·K)	Resistivity (Ω·cm)
ZnSe	0.6 – 16	2.403 @ 10.6 µm	+61	5.27	120	7.1	18	~10¹²
ZnS (FLIR)	0.4 – 12	2.192 @ 10.6 µm	+38.7	4.08	~230	7.6	16.7	~10¹²
ZnS MS / Cleartran®	0.37 – 14	2.192 @ 10.6 µm	+38.7	4.08	~160	7.6	27	~10¹²
Germanium	2 – 14	4.003 @ 10.6 µm	+396	5.33	780	6.1	59	5 – 40
GaAs	1 – 15	3.27 @ 10.6 µm	+149	5.32	750	5.7	52	>10⁷ (SI)
Silicon	1.2 – 7	3.422 @ 5 µm	+160	2.33	1150	2.55	148	10 – 1000
Sapphire	0.17 – 5.5	1.768 (n_d)	+13.1	3.97	2200	5.3	27	>10¹⁴
Fused Silica	0.18 – 2.5	1.458 (n_d)	+11.9	2.20	500	0.55	1.38	>10¹⁷
CaF₂	0.13 – 10	1.434 (n_d)	−10.6	3.18	158	18.85	9.7	Insulator
Chalcogenide	1 – 14	2.5–2.8 @ 10 µm*	+30 – 60*	~4.4 – 4.7*	~100 – 170*	~20*	~0.25*	High-R insulator

* Composition-dependent (Ge-As-Se / As-Se glass systems). Data compiled from published optical materials references (Edmund Optics, Coherent/II-VI, ISP Optics, Crystran); values typical at 293 K. Confirm per-lot specifications with American Photonics engineering.

Useful Transmission Ranges

0.1 µmUVVISSWIRMWIRLWIR · 20 µm

CaF₂

0.13–10

Sapphire

0.17–5.5

Fused Silica

0.18–2.5

ZnSe

0.6–16

ZnS MS

0.37–14

ZnS FLIR

0.4–12

GaAs

1–15

Chalcogenide

1–14

Silicon

1.2–7

Germanium

2–14

ZnSe

Zinc Selenide

The CO₂ laser workhorse — lowest absorption at 10.6 µm of any practical IR material.

Quote ZnSe Optics

Chemical-vapor-deposited zinc selenide is the standard transmissive material for high-power CO₂ laser systems and broadband IR imaging. Extremely low bulk absorption (<0.0005 cm⁻¹ at 10.6 µm for laser grade) minimizes thermal lensing, and its visible transmission (it's amber-transparent) makes system alignment with a visible pointer practical.

Optical
Useful range	0.6 – 16 µm (transmits to ~21 µm)
Refractive index	2.403 @ 10.6 µm · n_d 2.403
dn/dT	+61 × 10⁻⁶/K
Uncoated transmission	~71% (2 surfaces, Fresnel-limited)
Physical & Thermal
Density	5.27 g/cm³
Knoop hardness	120 kgf/mm² (soft — handle with care)
Thermal conductivity	18 W/m·K
Thermal expansion (CTE)	7.1 × 10⁻⁶/K
Electrical & Structure
Resistivity	~10¹² Ω·cm (semi-insulating)
Band gap	2.7 eV
Structure / growth	Polycrystalline cubic (zincblende), CVD

CO₂ Laser LensesDebris WindowsBroadband FLIRBeam Combiners

Typical uncoated transmission, 2 surfaces — representative curve

Engineering note: ZnSe is soft and moisture-tolerant but scratches easily — specify DLC or hard AR coatings for field-exposed surfaces. Preferred over GaAs where absorption must be absolute minimum.

ZnS

Zinc Sulfide — Three Grades

Harder and tougher than ZnSe — from rugged FLIR windows to water-clear multispectral apertures.

Quote ZnS Optics

Attribute	CVD ZnS (FLIR Grade)	Multispectral ZnS	Cleartran® — Hellma Materials
Useful range	0.4 – 12 µm (best 8 – 12)	0.37 – 14 µm	0.35 – 14 µm
Appearance	Translucent yellow-orange	Water-clear	Water-clear, single-crystal-like
Process	CVD as-deposited	CVD + HIP (hot isostatic press)	CVD + HIP — Hellma Materials brand
Knoop hardness	~230 kgf/mm²	~160 kgf/mm²	~160 kgf/mm²
Thermal conductivity	16.7 W/m·K	~27 W/m·K	~27 W/m·K
Best for	Rugged LWIR windows & domes, rain/sand erosion	Shared VIS + IR apertures	Premium common-aperture systems: visible camera + IR detector + laser through one window

The HIP treatment removes zinc-hydride defects and normalizes the crystal structure, converting yellow FLIR-grade ZnS into a water-clear multispectral material transmitting from the visible through the LWIR. Cleartran® — produced by Hellma Materials — is the benchmark of that class, with single-crystal-like transmittance from 0.35 to 14 µm. Standard FLIR grade trades that visible clarity for higher hardness, making it the choice where erosion resistance matters more than a shared aperture.

Common Properties (all grades)
Refractive index	2.192 @ 10.6 µm · n_d 2.631
dn/dT	+38.7 × 10⁻⁶/K
Density	4.08 g/cm³
Thermal expansion (CTE)	7.6 × 10⁻⁶/K
Resistivity	~10¹² Ω·cm (semi-insulating)
Band gap	3.6 eV
Structure / growth	Polycrystalline cubic, CVD (± HIP)
Service limit	Avoid use above ~250 °C in normal atmosphere

FLIR Windows & DomesMultispectral AperturesChemically InertHarder than ZnSe

Multispectral / Cleartran® grade shown — FLIR grade cuts on near 0.4 µm with visible scatter

Engineering note: Choose FLIR grade when the window faces sand and rain; choose Multispectral/Cleartran® when one aperture must serve a visible camera, an IR detector, and a laser at once. Cleartran® is a registered trademark of its owner; American Photonics sources premium HIP-treated ZnS including Hellma Materials Cleartran®.

Ge

Germanium

The highest-index workhorse of thermal imaging — and the strategic material we reclaim domestically.

Quote Ge Optics

Germanium's very high refractive index (n ≈ 4.0) lets designers achieve high optical power with shallow, easy-to-manufacture curves — the reason it dominates LWIR thermal camera objectives. Its high Knoop hardness makes it rugged in the field, and DLC-coated Ge front elements survive sand, rain, and cleaning abuse. American Photonics operates a domestic Germanium reclaim program that recovers value from scrap and keeps this strategic material in the U.S. supply chain.

Optical
Useful range	2 – 14 µm (transmits to ~16 µm)
Refractive index	4.003 @ 10.6 µm
dn/dT	+396 × 10⁻⁶/K (highest of common IR materials)
Uncoated transmission	~47% (2 surfaces — AR coating essential)
Physical & Thermal
Density	5.33 g/cm³
Knoop hardness	780 kgf/mm²
Thermal conductivity	59 W/m·K
Thermal expansion (CTE)	6.1 × 10⁻⁶/K
Electrical & Structure
Resistivity (optical grade)	5 – 40 Ω·cm
Band gap	0.66 eV
Structure / growth	Mono- or polycrystalline, diamond cubic

LWIR ObjectivesFLIR WindowsSeeker DomesRugged IR Imaging

Typical uncoated transmission at 25 °C, 2 surfaces — representative curve

Engineering note: Ge absorption rises steeply with temperature (thermal runaway) — transmission degrades above ~70–100 °C. For hot environments or athermal designs, consider Chalcogenide or GaAs. Its extreme dn/dT demands athermalization in precision LWIR systems.

GaAs

Gallium Arsenide

The rugged alternative for high-power CO₂ delivery in harsh industrial environments.

Quote GaAs Optics

Semi-insulating gallium arsenide is harder and stronger than ZnSe, with better resistance to thermal shock and mechanical damage — the material of choice for CO₂ laser focusing optics in dusty, high-spatter cutting environments where a ZnSe lens would degrade. Its higher index buys thinner, stiffer lens elements at equal optical power.

Optical
Useful range	1 – 15 µm (transmits to ~17 µm)
Refractive index	3.27 @ 10.6 µm
dn/dT	+149 × 10⁻⁶/K
Uncoated transmission	~55% (2 surfaces — AR coating essential)
Physical & Thermal
Density	5.32 g/cm³
Knoop hardness	750 kgf/mm²
Thermal conductivity	52 W/m·K
Thermal expansion (CTE)	5.7 × 10⁻⁶/K
Electrical & Structure
Resistivity (semi-insulating)	>10⁷ Ω·cm
Band gap	1.42 eV
Structure / growth	Single crystal, zincblende (LEC/VGF)

High-Power CO₂ LensesHarsh-Environment WindowsMWIR–LWIR Imaging

Typical uncoated transmission, 2 surfaces — representative curve

Engineering note: Choose GaAs over ZnSe when mechanical robustness and thermal-shock resistance outweigh the last fraction of a percent of absorption. Not visible-transparent — plan alignment accordingly.

Si

Silicon

Lightweight, hard, thermally excellent — the cost-effective choice for MWIR.

Quote Si Optics

Optical-grade silicon combines the lowest density of the IR semiconductors with the highest hardness and outstanding thermal conductivity — ideal where weight matters (airborne MWIR payloads) or where heat must be spread (laser mirrors on Si substrates). A strong lattice absorption band near 9 µm rules it out for CO₂ laser transmission, but for 3–5 µm imaging it is hard to beat on cost per performance.

Optical
Useful range	1.2 – 7 µm (strong absorption near 9 µm)
Refractive index	3.422 @ 5 µm
dn/dT	+160 × 10⁻⁶/K
Uncoated transmission	~54% (2 surfaces — AR coating essential)
Physical & Thermal
Density	2.33 g/cm³ (lightest IR semiconductor)
Knoop hardness	1150 kgf/mm²
Thermal conductivity	148 W/m·K
Thermal expansion (CTE)	2.55 × 10⁻⁶/K
Electrical & Structure
Resistivity (optical grade)	10 – 1000 Ω·cm
Band gap	1.12 eV
Structure / growth	Single crystal, diamond cubic (CZ/FZ)

MWIR Lenses & WindowsLaser Mirror SubstratesSpectroscopyTHz Optics

Typical uncoated transmission, 2 surfaces — note 9 µm lattice absorption band

Engineering note: Specify float-zone (FZ) material for demanding transmission applications — Czochralski silicon's oxygen content adds absorption near 9 µm. Excellent as a cooled-mirror substrate thanks to high k and low CTE.

Al₂O₃

Sapphire

Second only to diamond in hardness — the ultimate survivable window.

Quote Sapphire Optics

Single-crystal aluminum oxide transmits from the deep UV through the MWIR while shrugging off sand erosion, high-velocity rain, chemical attack, and extreme temperature. When the window is the last line of defense — aircraft sensor apertures, missile domes, sight glasses on pressure vessels — sapphire is the default answer.

Optical
Useful range	0.17 – 5.5 µm
Refractive index	n_d 1.768 (birefringent: n_o−n_e ≈ 0.008)
Abbe number	72.2
dn/dT	+13.1 × 10⁻⁶/K
Uncoated transmission	~86% (2 surfaces)
Physical & Thermal
Density	3.97 g/cm³
Knoop hardness	2200 kgf/mm² (Mohs 9)
Thermal conductivity	27 W/m·K
Thermal expansion (CTE)	5.3 × 10⁻⁶/K
Melting point	2040 °C
Electrical & Structure
Resistivity	>10¹⁴ Ω·cm
Structure / growth	Single crystal, hexagonal (Kyropoulos/HEM/EFG)

Sensor WindowsMissile DomesHigh-Pressure Sight GlassUV–MWIR Optics

Typical uncoated transmission, 2 surfaces — representative curve

Engineering note: Sapphire is birefringent — specify c-axis (0001) orientation for polarization-sensitive systems. Fabrication is slow and tooling-intensive; involve us early to keep tolerances economical.

SiO₂

Fused Silica

The precision standard for UV–NIR laser optics — near-zero thermal expansion.

Quote Fused Silica Optics

Synthetic amorphous silica offers the best combination of UV transmission, laser damage threshold, thermal stability, and figure retention of any common optical material. With a CTE twenty times lower than most glasses, fused silica optics hold their figure through thermal cycling — the reason interferometer references, laser windows, and 1 µm fiber-laser optics are made from it.

Optical
Useful range	0.18 – 2.5 µm (IR grade to ~3.5 µm)
Refractive index	n_d 1.458
Abbe number	67.8
dn/dT	+11.9 × 10⁻⁶/K
Uncoated transmission	~93% (2 surfaces)
Physical & Thermal
Density	2.20 g/cm³
Knoop hardness	500 kgf/mm²
Thermal conductivity	1.38 W/m·K
Thermal expansion (CTE)	0.55 × 10⁻⁶/K (exceptionally low)
Softening point	~1600 °C
Electrical & Structure
Resistivity	>10¹⁷ Ω·cm
Structure	Amorphous SiO₂, synthetic (flame hydrolysis)

Fiber-Laser OpticsUV OpticsInterferometry FlatsHigh-LIDT Windows

Typical uncoated transmission, 2 surfaces — OH absorption limits IR edge

Engineering note: Specify low-OH (IR grade) material to push transmission toward 3.5 µm; standard UV grade absorbs in the 2.2–2.7 µm water bands. Highest laser damage threshold of the materials on this page.

CaF₂

Calcium Fluoride

Deep-UV to LWIR in one crystal — with famously low dispersion.

Quote CaF₂ Optics

Calcium fluoride's enormous transmission window (vacuum UV at 130 nm out to ~10 µm) and very high Abbe number make it indispensable in broadband and multispectral systems, excimer laser optics, and cooled MWIR imagers. Its negative dn/dT can be exploited to passively athermalize lens assemblies.

Optical
Useful range	0.13 – 10 µm
Refractive index	n_d 1.434
Abbe number	95.1 (very low dispersion)
dn/dT	−10.6 × 10⁻⁶/K (negative — athermalization tool)
Uncoated transmission	~94% (2 surfaces)
Physical & Thermal
Density	3.18 g/cm³
Knoop hardness	158 kgf/mm² (soft)
Thermal conductivity	9.7 W/m·K
Thermal expansion (CTE)	18.85 × 10⁻⁶/K (high — thermal shock sensitive)
Electrical & Structure
Electrical behavior	Insulator
Solubility	0.0017 g/100 g H₂O (slightly hygroscopic)
Structure / growth	Single crystal, cubic fluorite (Bridgman)

Excimer Laser OpticsCooled MWIR ImagingBroadband WindowsSpectroscopy

Typical uncoated transmission, 2 surfaces — representative curve

Engineering note: The high CTE makes CaF₂ vulnerable to thermal shock — avoid rapid temperature swings and clamp-induced stress in mount design. Pair its negative dn/dT against Ge or Si in athermal doublets.

ChG

Chalcogenide Glass

The moldable, athermal alternative to Germanium for volume LWIR production.

Quote Chalcogenide Optics

Chalcogenide glasses (Ge-As-Se and As-Se systems such as Ge₃₃As₁₂Se₅₅ and As₄₀Se₆₀) transmit across SWIR–LWIR with a dn/dT roughly an order of magnitude lower than Germanium — enabling passively athermalized thermal camera lenses. Because they are glasses, they can be precision-molded, cutting per-unit cost dramatically at volume and freeing designs from Germanium price volatility.

Optical (composition-dependent — typical)
Useful range	1 – 14 µm (some compositions 0.8 – 16 µm)
Refractive index	2.5 – 2.8 @ 10 µm
dn/dT	+30 – 60 × 10⁻⁶/K (~10× lower than Ge)
Uncoated transmission	~62 – 66% (2 surfaces)
Physical & Thermal (typical)
Density	4.4 – 4.7 g/cm³
Knoop hardness	100 – 170 kgf/mm²
Thermal conductivity	~0.25 W/m·K
Thermal expansion (CTE)	~20 × 10⁻⁶/K
Glass transition (T_g)	~185 °C
Structure
Electrical behavior	High-resistivity amorphous semiconductor/insulator
Structure / forming	Amorphous glass — grindable, polishable, precision-moldable

Athermal LWIR LensesMolded Volume OpticsGe AlternativeThermal Sights

Typical uncoated transmission, 2 surfaces — composition-dependent

Engineering note: No thermal-runaway behavior, unlike Ge — chalcogenide keeps transmitting in hot environments. Softer and lower-k than crystalline materials: design mounts and coatings accordingly. Ask us about diamond-turned prototypes that transition to molded production.

Substrate Data. Engineering Answers.

Eight Materials Compared

Useful Transmission Ranges

Zinc Selenide

Zinc Sulfide — Three Grades

Germanium

Gallium Arsenide

Silicon

Sapphire

Fused Silica

Calcium Fluoride

Chalcogenide Glass

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