Optical properties of sapphire

Abstract

Sapphire (single crystal aluminum oxide) is a material commonly used in optical, electronic and chemical applications due to its material properties. Sapphire is usually used for optical applications due to its ability to transmit from the Ultra Violet (UV) wavelengths into the mid Infra-red (IR) wavelengths. The transmission characteristics of the material is determined by various factors, however the impurities content seems to play a significant role. These impurities can either come from the growth process or from the starting raw material (commonly called crackle). We studied the effect of impurities of the starting raw material with specific interest in hydrogen's effect on the optical properties (absorption, transmission) of sapphire crystals grown by different growth techniques. We have characterized these growth techniques into two categories: A)Large Thermal Gradient Method: (Czochralski (Cz), Edge Defined Film Fed Growth (EFG) or Stepanov) B.) Low Thermal Gradient Methods (Kyropoulos, Heat Exchange Method (HEM)) We used the following starting raw materials ("crackle"): a. Vernuil crystals produced by different manufacturers b. High purity aluminum oxide powder c. High Purity Densified Alumina (EMT HPDAR) produced by EMT, Inc thru their proprietary patented technology. Through Nuclear Magnetic Resonance (NMR) analytical techniques, it was found that the hydrogen concentration is very high in Vernuil crystals or in aluminum oxide powder. Consequently, sapphire crystals grown using Vernuil starting material or aluminum oxide powder also have a very high Hydrogen content. Utilizing the same NMR analytical techniques, EMT HPDA^R starting material showed very low Hydrogen concentration. Thus, sapphire crystals grown from EMT HPDA^R starting material has a very low Hydrogen content. It was found that optical properties in sapphire crystals grown using EMT HPDA^R starting material are more uniform and have higher transmission than in sapphire crystals grown using as starting material aluminum oxide powder or Vernuil crystals.