Silicon (Si) Optical Material

High-performance NIR/SWIR material for laser and infrared systems

Transmission
1.2–8 μm (opaque below 1.2 μm)
Refractive Index
~3.4 @ 5 μm
Hardness
7 Mohs
Density
2.33 g/cm³
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Quick Answer

Silicon (Si) is a widely used infrared optical material transmitting from 1.2 to 8 μm. It offers excellent thermal conductivity (130 W/m·K), low thermal expansion, and good mechanical strength. Commonly used for NIR imaging, SWIR optics, and as a substrate for IR coatings. It is opaque in the visible spectrum.

What Is Silicon (Si)?

Optical-grade Silicon is a crystalline semiconductor material widely used for infrared optics in the 1.2–8 μm range. It combines good optical transmission with exceptional thermal properties, making it ideal for applications requiring thermal stability.

Silicon's high thermal conductivity (130 W/m·K) and relatively low thermal expansion make it superior to Germanium in thermally demanding environments. It is widely used in NIR imaging, SWIR systems, and as beam splitters in FTIR instruments.

Optical & Physical Properties

Reference data — verify with PhotonEdge for project-specific specifications.

Transmission Range1.2–8 μm (opaque below 1.2 μm)
Refractive Index~3.4 @ 5 μm
Density2.33 g/cm³
Hardness7 Mohs
CTE2.6 × 10⁻⁶ /K
Thermal Conductivity130 W/(m·K)
dn/dT1.6 × 10⁻⁴ /K
Softening/Melting Point1414°C

Typical Applications

NIR Imaging Systems

SWIR cameras and sensors operating in 1.2–3 μm range

FTIR Beam Splitters

Silicon beam splitters for Fourier-transform infrared spectrometers

Laser Systems

Substrates and windows for NIR lasers (1.5–3 μm)

Thermal Imaging (MWIR)

MWIR optics (3–5 μm) with good thermal stability

Coating Considerations

AR 3-5 μm

Broadband AR for MWIR atmospheric window. R < 0.5% per surface.

Typical use: MWIR thermal imaging

AR 1.2-3 μm

AR for SWIR range. Essential due to ~30% uncoated reflection.

Typical use: SWIR imaging and sensing

Beam Splitter Coating

50/50 or custom ratio for FTIR applications.

Typical use: FTIR spectrometers

Engineering Considerations

Wavelength Range

Transmits 1.2–8 μm. Not suitable for visible or LWIR (>8 μm) applications.

Thermal Stability

Excellent thermal conductivity (130 W/m·K) dissipates heat quickly. Much better than Ge for high-power applications.

Refractive Index

High index (~3.4) requires AR coatings for transmission efficiency.

Weight Advantage

Density of 2.33 g/cm³ makes Si significantly lighter than Ge (5.32 g/cm³) — important for aerospace.

Material Comparison

vs Germanium

Si is lighter, better thermal conductivity, but narrower IR range (1.2–8 vs 2–14 μm)

vs ZnSe

Si is harder and more thermally stable, but ZnSe covers broader range including CO₂ laser

vs Sapphire

Si covers IR; Sapphire covers UV-VIS-NIR. Different application domains.

Frequently Asked Questions

What is Silicon used for in optics?

Silicon is used for infrared optics operating in the 1.2–8 μm range, including NIR imaging, SWIR sensors, FTIR beam splitters, and MWIR thermal systems.

Is Silicon transparent in visible light?

No. Silicon is opaque to visible light (below 1.2 μm). It appears metallic gray and reflective.

How does Silicon compare to Germanium for thermal imaging?

Silicon covers MWIR (3–5 μm) while Germanium covers LWIR (8–14 μm). For CO₂ laser at 10.6 μm, Germanium is required. For MWIR, Silicon offers better thermal stability and lighter weight.

Can Silicon withstand high-power lasers?

Yes. Silicon's high thermal conductivity (130 W/m·K) makes it excellent for high-power NIR laser applications. It dissipates heat much better than Germanium.

What coatings does Silicon need?

AR coatings are essential due to ~30% uncoated reflection. Use AR 3-5 μm for MWIR, or AR 1.2-3 μm for SWIR applications.

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Optical Material

BK7 Optical Glass

BK7 is the most widely used optical glass for visible and near-infrared applications. A borosilicate crown glass, BK7 offers excellent optical clarity, good homogeneity, and reasonable cost — making it the default choice for many general-purpose optical components.

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Key Optical Properties

Property Value
Transmission Range 350 nm – 2.0 μm
Refractive Index 1.5168 @ 587.6nm
Thermal Expansion 7.1 × 10⁻⁶ /K (20-300°C)
Density 2.51 g/cm³
Knoop Hardness 610 HK

Advantages

  • Excellent visible transmission (>92% per surface uncoated)
  • High material homogeneity and consistency
  • Good chemical resistance and stability
  • Cost-effective — widely available in stock sizes
  • Suitable for most visible and NIR applications

Limitations

  • Not suitable for UV below 350nm
  • Higher thermal expansion than fused silica
  • Lower LIDT than fused silica for high-power lasers
  • Hygroscopic under extreme humidity conditions
  • Limited IR transmission beyond 2μm

Typical Applications

Imaging lenses and objectives

Beam splitters and prisms

General-purpose windows

Spectrometer optics

Machine vision systems

Common Coating Options

Standard coating types available for this material. Custom coating designs available on request.

Broadband AR (VIS, VIS-NIR)
Anti-reflection single-layer MgF₂
Protected/enhanced aluminum
Dielectric HR and partial reflector
Beam splitter coatings

Products Using Silicon

Browse our catalog of precision optical components available in Silicon

Windows

Silicon Circular/Square Windows

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