Method of producing nonlinear optical crystal CdSiP
View Patent ↗CdSiP 2 crystals with sizes and optical quality suitable for use as nonlinear optical devices are disclosed, as well as NLO devices based thereupon. A method of growing the crystals by directional solidification from a stoichiometric melt is also disclosed. The disclosed NLO crystals have a higher nonlinear coefficient than prior art crystals that can be pumped by solid state lasers, and are particularly useful for frequency shifting 1.06 μm, 1.55 μm, and 2 μm lasers to wavelengths between 2 μm and 10 μm. Due to the high thermal conductivity and low losses of the claimed CdSiP 2 crystals, average output power can exceed 10 W without severe thermal lensing. A 6.45 μm laser source for use as a medical laser scalpel is also disclosed, in which a CdSiP 2 crystal is configured for non-critical phase matching, pumped by a 1064 nm Nd:YAG laser, and temperature-tuned to produce output at 6.45 μm.
1. A method for producing CdSiP 2 crystals comprising the steps of:
(a) vacuum sealing cadmium (Cd), silicon (Si), and phosphorous (P) in an ampoule in a molar ratio of approximately Cd:Si:P=1:1:2, not including any excess quantities added so as to account for a vapor phase above a melt, the Cd and Si being physically mixed at a first end of the ampoule and the P being located at a second end of the ampoule;
(b) heating the ampoule so that the Cd and the Si are maintained at a hot zone temperature at least near the melting point of CdSiP 2 while the P is maintained at a cold zone temperature sufficient to cause the P to volatilize and react with the Cd and the Si forming a CdSiP 2 compound; and
(c) cooling the CdSiP 2 compound so as to achieve directional solidification forming a CdSiP 2 crystal.
2. The method of claim 1 wherein the cold zone temperature is low enough such that the ampoule is not exploded.
3. The method of claim 1 wherein the cold zone temperature is low enough such that the internal vapor pressure does not exceed the burst strength of the ampoule.
4. The method of claim 1 wherein the cold zone temperature is approximately 485° C. and the hot zone temperature is approximately 1180° C.
5. The method of claim 1 wherein the step of cooling the CdSiP 2 compound so as to achieve directional solidification occurs at a cooling rate between approximately 0.02° C./hour and 1° C./hour.
6. The method of claim 1 wherein the directional solidification takes place at a crystal growth rate of between approximately 0.2 mm/hour and 2 mm/hour.
7. The method of claim 1 wherein the CdSiP 2 crystal formed has a diameter of at least 19 mm.
8. The method of claim 1 further comprising after step (c), the step of:
cooling the CdSiP 2 crystal to approximately room temperature.
9. The method of claim 8 wherein the step of cooling the CdSiP 2 crystal to approximately room temperature occurs at a cooling rate between approximately 5° C./hour and 100° C./hour.
10. The method of claim 1 further comprising between steps (b) and (c), the steps of:
cooling the CdSiP 2 compound to approximately room temperature; and
heating the CdSiP 2 compound adjacent to a seed crystal, using a furnace having a substantially uniform temperature gradient of between approximately 1° C./cm and 5° C./cm, to a melting temperature and adjusting the melting temperature so as to fully melt the CdSiP 2 compound and partially melt the seed crystal.
11. The method of claim 10 wherein the melting temperature is approximately 1130° C.
12. The method of claim 10 wherein the CdSiP 2 compound and the seed crystal are within a PBN-coated graphite boat during the steps where they are heated and cooled adjacent to one another.
13. The method of claim 12 wherein the PBN-coated graphite boat is vacuum-encapsulated in a fused silica ampoule during the steps where it is heated and cooled.