Method of making large-volume CaF2 single crystals for optical elements with an optic axis parallel to the (100)-or (110)-crystal axis and CaF2 single crystal made thereby

US7837969B2 · kind B2 · utility

Assignee

• Hellma Materials GmbH & Co. KG · DE

Inventors

• Joerg Staeblein · Jena, DE
• Lutz Parthier · Kleinmachnow, DE

Key dates

• Filing date: Mar 25, 2009
• Grant date: Nov 23, 2010
• Expiry date: Mar 25, 2029

Classification

• Technology area (CPC Y): Emerging Cross-Sectional Technologies
• CPC primary: Y10T117/1024
• WIPO field: Surface technology, coating
• WIPO sector: Chemistry

Abstract

The method of making a single crystal, especially a CaF2 single crystal, includes tempering, in which the crystal is heated at <18 K/h to a temperature of 1000° C. to 1350° C. and held at this temperature for at least 65 hours with maximum temperature differences within the crystal of <0.2 K. Subsequently the crystal is cooled with a cooling rate of at maximum 0.5 K/h above a limiting temperature between 900° C. to 600° C. and then further below this limiting temperature at maximum 3 K/h. The obtained CaF2 crystals have refractive index uniformity <0.025×10−6 (RMS) in a (111)-, (100)- or (110)-direction and a stress birefringence of less than 2.5 nm/cm (PV) and/or a stress birefringence of less than 1 nm/cm (RMS) in the (100)- or (110)-direction. In the (111)-direction the stress birefringence is <0.5 nm/cm (PV) and/or the stress birefringence is <0.15 nm/cm (RMS).