Optical Material: Ceramics is the second article in this 5-part series of blog posts that aims to highlight the various optical materials used to design and manufacture optical components for the photonics industry.
The word ‘ceramic’ is derived from the Greek word keramos meaning pottery. Ceramics are most commonly associated with earthenware, porcelain and stoneware, which have been used for several thousand years and are still remain popular today. However, did you know that ceramics are a class of exceptionally versatile materials and have many more applications beyond pottery?
Today, ceramics are used in virtually every industry from construction (cements and tiles) to electronic components (insulators, semiconductors), and optics (optical windows, lasers, electro-optics).
Composition of Ceramics
Ceramics are inorganic, non-metallic compounds that are generally formed or densified under heat. They can be crystalline, glassy or possess a combination both crystalline and glassy phases. They are hard, brittle, and chemically inert, which makes them attractive candidates for high-temperature applications and operation under harsh environments. Although traditional approaches to ceramics typically used clay as the raw material, modern ceramics are made from oxides, carbides, nitrides, among other combinations of oxides and non-oxides.
As the name suggests, optical ceramics are a class of ceramics which are used in optical applications. The most common optical ceramics are Sapphire (Al2O3), Yttrium Aluminum Garnet (YAG, Y3Al5O12), Aluminum Oxynitride (ALON, (AlN)x • (Al2O3)1-x, 0.30 ≤ x ≤ 0.37), Magnesium Aluminum Spinel (Spinel, MgAl2O4), among others. Optical ceramics have excellent transmission across a wide spectral range, from UV to mid-IR. They can be doped with metal ions to engineer a range of optical phenomena such as fluorescence, phosphorescence, acousto-optic and electro-optic effects. Due to the versatility of these materials, they are used in a range of applications from defense and energy to electronics and sensor applications.
Applications of Optical Ceramics
The first demonstration of laser by Theodore Maiman in 1960 was carried out using an optical ceramic called Ruby, which is Al2O3 doped with Cr3+ ions. The chromium ions impart a red color to the otherwise colorless Al2O3 crystal and are the active material from which lasing occurs. Ruby lasers emit monochromatic radiation at 694.3nm. Although, a ruby crystal was used in the first demonstrated laser system, these lasers are not very popular and have limited applications in research due to their low efficiency.
Another class of solid-state lasers based on YAG ceramics have dominated the industry since they were first demonstrated at the Bell Laboratories in 1964. Among YAG lasers, Neodymium doped YAG or Nd:YAG lasers have found myriad applications due to their higher efficiency and possibility of generating high power through pulsing (Q-switching). Additionally, they can be used with non-linear materials to achieve frequency doubling and tripling. Nd:YAG lasers have fundamental emission at 1064nm, and a second harmonic at 532nm. Due to these advantages, they are used in applications including ophthalmology (cataract surgery), dentistry (surgery), manufacturing, military (range finders and laser weapons) and many others. Since the YAG ceramic acts as a suitable host for doping, other rare earths ions have been successfully doped to achieve laser action including, Erbium, Ytterbium and Thulium.
Optical ceramics are also routinely used as scintillators in medical imaging. Scintillators are materials that absorb high-energy radiation such as gamma rays and x-rays and emit visible light. The light emitted from these scintillators is typically coupled to a high-gain detector such as a Photomultiplier Tube (PMT) or an Avalanche Photodiode (APD). Ceramic scintillators were first developed for medical imaging in the 1980s. (Y,Gd)2O3:Eu and Gd3Ga5O12:Cr,Ce ceramics were among the early candidates that were demonstrated for Computed Tomography (CT) applications. Since then several ceramics have been used for scintillator applications including Cerium-doped Lutetium Ortho-silicate (LSO), Cerium-doped Gadolinum Ortho-silicate (GSO) and mixed Lutetium and Gadolinum Ortho-silicate (LGSO), among others. These scintillators are commonly used in Positron Emission Tomography CT (PET-CT) imagers to detect gamma rays that are emitted from radionuclide in the tissue. PET-CT scans are routinely used today to monitor lesions in cancer patients and have the benefit of providing functional imaging capability. Another class of transparent optical ceramic scintillators are Aluminum/Gallium Garnets (Gd,Lu)3(Al,Ga)5O12:Ce (GLuGAG), which are also great candidates for PET imaging. RMD offers a wide range of GLuGAG ceramic scintillators as cubes, cylinders and pixel bars, well suited for scintillator array applications.
Optical ceramics are excellent candidates for optical windows due to high transmission over a wide-spectral range, scratch resistance, high hardness and shock resistance properties. Sapphire or Al2O3 windows are well suited for pyrometry applications such as furnace windows, as they can withstand high temperatures up to 2000 oC without losing optical performance. Furthermore, sapphire windows are chemically inert, which makes them widely used in medical and pharmaceutical equipment where purity is of great importance. Transparent Optical Ceramics (TOCs) exhibit superior performance over their glass counterparts due to the above-mentioned attributes and therefore are great candidates for substrate materials.
Pigments and Photoluminescent Ceramics
Ceramics are also used in lighting applications such as phosphors in fluorescent lights and cathode ray tubes. For instance, Cerium-doped Yttrium-Aluminum Garnet or YAG:Ce is a common yellow phosphor used in white LEDs to impart a warm color palette. Examples of other phosphors include Calcium Tungstate for blue color, Manganese-activated Zircon for green color, and Europium-activated Yttrium Vanadate for red color. There are several other combinations of dopants and hosts to generate a wide range of colors palettes.
The combination of optical transparency and the ability to modulate optical properties using electric fields, is needed in electro-optic applications, and ceramics have dominated this field. Materials such Lithium Niobate (LiNbO3) and Lithium Tantalate (LiTaO3) are widely used in frequency doubling, non-linear optics, optical waveguides and optical switches, among a host of other acousto-optics and transducer applications.