Fused silica and kodial optics
When we consider transparent materials, often the first one we think of is glass! It is one of the most commonly used materials in people’s homes for windows, as well as within optical use-cases such as telescopes, microscopes, cameras and glasses and for packaging drinks and condiments.
Glass in and outside of Torr
Glass isn’t just one material; it has a base of silicon dioxide, but can have many different additives such as oxides of sodium, calcium or boron. These additives provide different properties to the glass, all of which can be useful depending on how the glass is being used. For our manufacturing purposes at Torr Scientific, we use an alkali borosilicate glass called Kodial. This is due to its low thermal expansion and low outgassing properties!
Pure silicon dioxide glass is also used here at Torr, which is also known as our fused silica. This is available in different grades, which are formed based on purity and manufacturing methodologies! The trait of transparency is based on how the glass is manufactured, which changes the internal structure and physical properties of the material!
Amorphous structure
The first one is due to glass being an amorphous solid. This means that it has the properties of a solid, as it has a fixed shape and is mechanically rigid. However, unlike a typical crystalline solid, which has a structured neat organisation of atoms, the arrangement of atoms in in an amorphous solid is scattered and random.
Amorphous solids are formed when materials are heated to a high temperature and then cooled very quickly. When the raw materials are heated their atoms become free moving and flow around the structure. Then as they are cooled so quickly the atoms become stuck before they can move back into a crystal structure. The scattered atomic formation is part of the reason for transparency. This is because there are fewer internal structures than that of a polycrystalline material. Polycrystalline structures have numerous crystals of different refractive indices that causes internal reflections. Therefore, as amorphous structures do not have time to form crystals, they do not have these structural differences! This is true for both borosilicate glass (Kodial) and fused silica as well as all their different grades!
Absorption
The other reason that glass is transparent is to do with the limited absorption of energy. As the photons in light contain energy, it is possible for them to transfer their energy to other particles or sub-atomic structures, which in this case includes electrons. This would mean that the energy is absorbed by the structure and does not pass through, therefore cannot be received by the human eye and is not transparent. However, to be absorbed by the material, the energy from the photons needs to be able to raise the energy level of the electron to the next layer of the atomic shell. The energy needed to raise the level is known as the band gap.
Luckily, visible light does not have enough energy to raise the energy level within this chemical composition! Therefore, the energy does not get transferred and is free to pass through the material. This means that light is not absorbed by the glass! You may notice that fused silica has an absorption band, differing based on the grade, generally between 2700 – 3300 nm. This is where the band gap is achieved. The placement of the absorption band depends on the presence of hydroxyls within the material and other metallic impurities within the fused silica! This limited absorption is also the reason that crystalline structures such as sapphire and our CVD diamond optics are also visually transparent. They have large band gaps caused by their structure, where all of the electrons are utilised within chemical bonds and not free to interact with the photons!
What types of glass do we work with at Torr?
If you require a viewport with transparent glass, we have various grades of optics that we can offer! We have standard grades of Kodial and fused silica as well as IR and UV grades to tailor our viewports for your experiments! For our glass cells we can also offer Pyrex as a borosilicate alternative. To learn more you can reach out to our sales team at sales@torrscientific.co.uk
