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Si

Related link:  Silicon Windows and Lenses   

 

Silicon Characteristics & Transmission Curve

 

A semiconductor material that is commonly used in infrared optical systems operating in the 3 to 5µm spectral band. The refractive index is near 3.4 throughout the range. Silicon is also useful as a transmitter in the 20µm to 300µm range.

 

Silicon is used as a mirror substrate for lasers because of its thermal conductivity, light weight, and hardness. It is also used for windows and lenses in the 1.2µm to 6.7µm range. Due to the strong absorption at 9µm, Silicon is not suitable for use with CO2 lasers as a transmitting optic but is widely used for CO2 mirrors.

 

Silicon has one of the lowest densities of the common infrared materials making it ideal for systems with weight constraints. The density of Silicon is only half that of Germanium, Gallium Arsenide and Zinc Selenide. Silicon is harder than Germanium and not as brittle. Silicon is the lowest material cost option of all the infrared materials.

 

 Property

 Specification

 Transmission Range

 1.2µm to 7.0µm & from 25µm out to beyond 300µm

 Density

 2.329 g/cm3

 Thermal Expansion

 2.55 x 10-6 /°C @ 25°C

 Surface Finish

Typical specifications for surface quality in the infrared are a 40-20 scratch-dig in the 1.2 to 3µm spectral region & 60-40 scratch-dig for the 3-7µm area. Diamond Turned surface finishes of 120 Å rms or better are typical.

 Surface Figure

In the infrared, typical required surface figure ranges from 1/2 wave to 2 waves @ 0.6328µm & are usually specified depending on the system performance requirements.

 AR Coating Options

The most common anti-reflectance coating for Silicon is BBAR for 3 to 5µm. Many other specialized wavelength bands are possible within the 1.2 to 7.0µm range.

 Typical Applications

 Thermal imaging, FLIR.

 Products 

 Manufactured

 Lenses, Aspheric Lenses, Binary (Diffractive) Lenses, Windows, Optical Beamsplitters,

 Optical Filters, Wedges.

 

Wavelength (µm)

Index of Refraction (n)

1.5

3.484

 2.0

3.456

3.0

3.436

4.0

3.429

5.0

3.426

6.0

3.424

7.0

3.423

 

Silicon Transmission Curve