Silicon is the most abundant element making up the Earth's crust, synthetic mono-crystalline silicon is the base for the modern electronics industry, and perhaps the most affordable infrared material. Silicon is commonly used as an optical window primarily in the 3-5μm band, and also could be used for 6-15μm transmission. The thickness is a very critical parameter of silicon optics. Silicon is often employed as a substrate for laser mirrors because of its high thermal conductivity and low density.

Silicon is commonly grown by CZ method and contains some oxygen which causes strong absorption band at 9μm  and limits its transmission applications in 8-12μm.
Optical Properties
  Bulk Absorption Coefficient                        <0.01/ cm @ 3μm
   Refractive Index                             3.4223 @ 5μm  3.4289 @ 4μm
 Temperature change of Refractive Index             160×10-6/K
Thermal Properties
       Thermal Conductivity at 25°C                            160 W/m/K
       Specific Heat                                                  703 J/Kg/K
       Thermal Expansion                                           2.6×10-6/K at 20? 
Mechanical Properties
        Young’s Modulus                                     131 GPa
       Rupture Modulus                                       125 MPa
       Shear Modulus                                          79.9 GPa
       Knoop Hardness                                        1150
       Poisson Ratio                                            0.266
       Density                        2.33 g/cm³ 
Typical Specification 
Dimensional Tolerance           +0.0/-0.1mm
Thickness Tolerance              ±0.2mm
Center Thickness Tol.             ±0.2mm
Surface Quality                     10/5
Centering                              3-5 arc minutes
Surface Accuracy                  1/10 λ @ 10.6μm
Parallelism                           < 1 arc minute 
Clear Aperture                        90%