Diffractive optical elements (DOEs) are visual elements that diffract light. They can be made from any material and in many different shapes. Still, silicon for this application is so prevalent that it has been given the nickname “silicon optics.” This post will explore a few materials used in making DOEs and what they offer to engineers looking for a specific property.
Silicon is the most common material used in DOEs for various reasons. Silicon has a very low coefficient of thermal expansion, meaning that it does not expand or contract when heated or cooled. This makes silicon well-suited for applications where high precision is required. Additionally, silicon has a relatively low refractive index which helps to minimize optical aberrations caused by the DOE. The diffractive optics are also relatively easy to fabricate using standard microfabrication techniques.
Sapphire has a very high refractive index, which means it can be used for applications where more excellent light bending is required. Sapphire is also transparent in infrared frequencies up to about 20 micrometers wavelength, making it useful in communication systems involving near-infrared lasers. Because sapphire cannot withstand temperatures above 500 degrees Celsius (and typically needs to be under 200 degrees), this limits its use mainly to laboratory environments or other low-temperature conditions
Germanium has a very high refractive index, similar to a sapphire, and is transparent in the infrared spectrum. However, germanium can withstand much higher temperatures than sapphire making it more suitable for industrial applications. Additionally, germanium is a relatively rare material, making it costly compared to silicon.
- Gallium Arsenide
Gallium arsenide has a very high refractive index, which allows for the light-bending capabilities of diffractive optical elements. However, it also has a much higher coefficient of thermal expansion than silicon or sapphire, which can cause problems in use since gallium arsenide is more likely to crack when subjected to temperature changes because its atoms do not pack as tightly together.
Gallium arsenide can be used at temperatures up to 450 degrees Celsius, making it suitable for many industrial applications such as welding (which requires extremely high temperatures). Still, like germanium, gallium arsenide optics are expensive compared with those made from silicon.
There are a variety of materials that can be used to make diffractive optical elements, each with its advantages and disadvantages. Silicon is the most common material, thanks to its low thermal expansion and low refractive index. Still, other materials like sapphire, germanium, and gallium arsenide can also be used depending on the specific application requirements.