PROCESS FOR REDUCING DAMAGE SUSCEPTIBILITY IN OPTICAL-QUALITY CRYSTALS

Abstract: A process is disclosed for treating a crystal of MTiOXO4 (wherein M is selected from the group consisting of K, Rb, Tl and NH4 and mixtures thereof and X is selected from the group consisting of P and As and mixtures thereof) which has crystal structure deficiencies of M throughout the crystal, by immersing the crystal in molten salt consisting essentially of at least one low melting salt of one or more monovalent cations selected from the group consisting of Rb<+>, K<+>, Cs<+> and Tl<+> for a time sufficient to decrease the optical damage susceptibility throughout the crystal.

Description: PROCESS FOR REDUCING DAMAGE SUSCEPTIBILITY IN OPTICAL-QUALITY CRYSTALS FIELD OF THE INVENTION This invention relates to a-process for reducing the .damage susceptibility of optically useful crystals of KTiOP0 4 and certain of its analogs. EACKGRQUNP The need for optical-quality single crystals of materials exhibiting nonlinear optical properties is well established in the art. Potassium titanyl phosphate (i.e., KTP) is particularly useful in nonlinear optical devices, as described, for example, in U.S. Patent No. 3,949,323. For many optical applications (e.g., second harmonic and other frequency generation and electro-optic applications) optical- quality crystals having dimensions on the order of one millimeter or more are generally desired. Since KTiOP0 4 and its analogs are known to decompose upon melting, hydrothermal or flux methods have commonly been used to grow crystals of these compounds. U.S. Patent No. 4,231,838 discloses a method of crystal growth from a nonaqueous flux. U.S. Patent No. 3,949,323 discloses the preparation of crystals of compounds such as potassium titanyl phosphate by hydrothermal methods. Hydrothermal methods of crystal growth are considered particularly advantageous methods of crystal growth for certain applications since they often produce crystals of relatively low damage susceptibility. In part due to the popularity of the hydrothermal process, many improvements have been made in the basic process over the years. U.S. Patent No. 4,305,778 discloses a high temperature hydrothermal process for crystal growth of a group of materials including potassium titanyl phosphate which utilizes a mineralizer solution comprising at least in part a stable glass composition that minimizes the tendency of the seed crystals to dissolve in the aqueous mineralizer solution before nutrient can migrate to'the seed crystals. More recently, U.S. Patent 5,066,356 discloses an improved low temperature hydrothermal process using mineralizers relatively high in potassium which produces potassium titanyl phosphate crystals at relatively low pressures, as well as low temperatures. While the crystals produced by the known hydrothermal processes can have high optical quality, the rigors of certain optical applications, require high resistance to optical damage or lower susceptibility to optical damage than exhibited by most hydrothermally produced crystals. For example, in uses where the crystal is subjected to high electric fields or high power laser radiation, a typical hydrothermal crystal can develop "grey tracks" or even significantly darken throughout the crystal, severely limiting the continued use of the crystal. Such damage is discussed, for example, in V. V. Lemeshko et al., Ukr. Fiz. Zh. Russ. Ed 31, No. 11:1745-50 (1986); P. A. Morris et al., SPIE, The International Society for Optical Engineering, Vol. 1561, Inorganic Crystals for Optics, Electro-Optics, and Frequency Conversion, 104-110 (1991); and R. F. Belt et al., Proc. Soc. Photo-Opt. Instrum., Eng. 968, 100 (1988) . SUMMARY OF THE INVENTION A process is provided in accordance with this invention for treating a crystal of MTiOXθ 4 which has crystal structure deficiencies of M throughout the crystal, wherein M is selected from the group consisting of K, Rb, Tl and NH 4 and mixtures thereof and X is selected from the group consisting of P and As and mixtures thereof. The process comprises the step of immersing said crystal in molten salt consisting essentially of at least one low melting salt of one or more monovalent cations selected from the group consisting of Rb + , K + , Cs + and Tl + , for a time sufficient to decrease the optical damage susceptibility throughout the crystal. This process is considered particularly usef l for treating hydrothermally grown crystals . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an optical absorption chart (i.e., a plot of absorption coefficient, α, versus the wavelength of incident radiation, λ) for crystals described in Example I. Figure 2 is an infrared spectrum (i.e., a plot of absorption coefficient, oc, versus wavenumber, 1/λ) for crystals described in Example I. Figure 3 is an optical absorption chart for crystals described in Example II. Figure 4 is an infrared spectrum for crystals described in Example II. Figure 5 is an optical absorption chart for crystals described in Comparative Example A. Figure 6 is an infrared spectrum for crystals described in Comparative Example A. Figure 7 is an optical absorption chart for crystals described in Comparative Example B. Figure 8 is an infrared spectrum for crystals described in Comparative Example B. All infrared spectra are for crystals cooled to less than 20°K. PSTAILEP PESCRIPTION This invention involves the discovery that crystals of MTiOXθ 4 (including flux grown and hydrothermally grown crystals where M is K, Rb, Tl and/or NH 4 and X is As and/or P) do not have perfect stoichiometry. Specifically, it has been found that such crystals of MT10X0 4 may have some vacant M and/or O sites in the crystal lattice or may have, particularly when hydrothermally grown, protons present in the crystal lattice. Crystals which have vacant M sites and crystals which have M sites occupied by protons are both considered deficient in M. While this invention is not bound by any theory or explanation of operation, it is believed to be those imperfections, including in particular crystal structure deficiencies of M (whether compensated for by 0 vacancies or proton presence) , that cause the susceptibility to optical damage, particularly when the crystal is hydrothermally grown. Optical damage in such crystals.can be electric field-induced and/or optical radiation-induced (e.g., laser induced). The process of this invention treats crystals of TiOXθ 4 which have crystal structure deficiencies of M throughout the crystal by immersing the crystals in molten salt consisting essentially of one or more salts of Rb + , K + , Cs + and/or Tl + which are low melting (i.e., melt at a temperature below the decomposition temperature of the treated crystal) . It is believed that the monovalent cation or cations from the molten low melting salt diffuse into the crystal of MTiOX0 4 , and substantially correct the crystal structure deficiencies of M, thereby reducing both electric field induced and optical radiation-induced optical damage susceptibility. It is preferred for simplicity and control of crystal composition that the monovalent cation be the same as the monovalent cation, M, in the crystal of MTiOX0 4 being treated. The contact or immersion time is determined in part by the size of the crystal. The treatment is particularly useful for treating crystals wherein each dimension is 0.8 millimeter or more, and typical treatment times for such crystals are at least about 2 hours. While crystals are typically contacted with the melt for several hours to a week, even longer contact times may allow for further treatment and will not generally harm the crystal. While flux grown crystals may be treated by this method, the treatment is considered particularly useful for treating hydrothermally grown crystals. All hydrothermally grown crystals of MTiOXθ 4 can benefit from treatment in accordance with the invention. Potassium titanyl phosphate, KTiOP0 4 is preferred. Typical hydrothermal crystals are grown at elevated temperature and pressure in a pressure vessel containing a means for nucleating crystal growth and a growth medium comprising a nutrient and an aqueous mineralizer solution. The preferred means for nucleating crystal growth are seed crystals which provide nucleation sites. In accordance with this invention seed crystals of MTiOX04, as defined above, can be utilized. The low melting salt containing a monovalent cation useful in the practice of this invention should melt at a temperature below the decomposition temperature of the MTiOXθ 4 . It is preferred that the low melting salt containing a monovalent cation melt at a temperature of less than about 600°C, more preferably less than about 500 C C, and most preferably less than about 475°C. Both inorganic and organic low melting salts containing a monovalent cation can be used in the practice of this invention, but inorganic low melting salts containing a monovalent cations (e.g., KNO3, KNO 2 ) are preferred for a combination of economics and availability. The melting point can be as low as 0°C, but for most practical contact times, the melting point of the low melting salt containing a monovalent cation should preferable be at least about 100°C, and, more preferably at least about 300°C. It is preferred for simplicity and control of crystal composition that the monovalent cation be the same as the monovalent cation ,M, in the hydrothermal crystal of MTiOX0 4 being treated. To avoid thermal shock to the crystal of MTiOX0 4 it is preferred that the crystal be preheated to around the temperature of the molten salt prior to contact therewith. Susceptibility of crystals to optical damage from exposure to an electric field is believed to generally correlate with susceptibility to optical damage from exposure to optical laser radiation. The crystals treated in accordance with this invention have utility for frequency generation (e.g., SHG) and for electro- optic applications. Practice of the invention will become further apparent from the following non-limiting examples. Example I Two samples (about 2 x 6 x 0.9 mm) were cut from the high conductivity side, as determined by measurement of the dielectric constant in accordance with P. A. Morris et al., SPIE, The International Society for Optical Engineering, Vol. 1561, Inorganic Crystals for Optics, Electro-Optics, and Frequency Conversion, 104- 110 (1991) , of a single z-cut wafer of a crystal of KTiOPθ 4 , grown by the high temperature hydrothermal process described in U.S. Patent 4,305,778, and polished. One of the samples was placed in a KNO3 bath in a quartz crucible at 425°C for 93 hours to allow for K ion exchange. The ion exchanged sample and the second sample were then damaged by being subjected to an electric field of 300 V/cm in dry air at 200°C for 20 minutes. The optical absorption spectra (i.e., absorption coefficient, oc, versus wavelength, λ) of the damaged ion exchanged sample (1) , and the damaged "as- grown" sample (2), were obtained and are shown in Figure 1, along with an "as-grown" or untreated sample, which was not subjected to the electric field (3) for reference. The data of Figure 1 clearly show the reduction in damage susceptibility for the ion exchanged KTiOP0 4 . Figure 2 shows the infrared spectrum (i.e., absorption coefficient, CX, versus wavenumber, 1/λ) of "as-grown" (3), ion exchanged (4), and ion exchanged/damaged KTiOPθ 4 (1) , indicating that the number and distribution of protons in the ion exchanged KTiOPθ 4 are distinct from those of the "as-grown" or untreated KTiOPθ 4 and that the damage process leaves the number and distribution of protons in the ion exchanged KTiOPθ 4 substantially unaltered. Example II Two samples (about 3 x 8 x 0.9 mm) were cut from the low conductivity side, as determined by measurement of the dielectric constant in accordance with P. A. Morris et al., SPIE, The International Society for Optical Engineering, Vol. 1561, Inorganic Crystals for Optics, Electro-Optics, and Frequency Conversion, 104- 110 (1991) , of a single z-cut wafer of a crystal of KTiOP0 4 , grown by the high temperature hydrothermal process described in U.S. Patent 4,305,778, and polished. One of the samples was then ion exchanged in accordance with the procedure of Example I. The ion exchanged sample and the second sample were then damaged as described in Example I. The optical absorption spectra of the damaged ion exchanged sample (1) and the damaged "as-grown" sample (2) were obtained and are shown in Figure 3, along with an "as-grown" or untreated sample, which was not subjected to the electric field (3) for reference. The data of Figure 3 clearly show the reduction in damage susceptibility for the ion exchanged KTiOPθ 4 . Figure 4 shows the infrared spectrum of "as-grown" (3), ion exchanged (4), and ion exchanged/damaged KTiOPθ 4 (1) , indicating that the number and distribution of protons in the ion exchanged KTiOPθ 4 are distinct from those of the "as-grown" or untreated KTiOP0 4 and that the damage process leaves the number and distribution of protons in the ion exchanged KTiOPθ substantially unaltered. Comparative Example A A z-cut sample (about 2 3 x 1 mm) was cut from the high conductivity side, as determined by measurement of the dielectric constant in accordance with P. A. Morris et al., SPIE, The International Society for Optical Engineering, Vol. 1561, Inorganic Crystals for Optics, Electro-Optics, and Frequency Conversion, 104- 110 (1991) , of a single z-cut wafer of a crystal of KTiOPθ 4f grown by the high temperature hydrothermal process described in U.S. Patent 4,305,778, and polished. The sample was placed in a molten salt bath containing 87 mole % of KNO 3 and 13 mole % of Ba(N0 3 ) 2 bath in a quartz crucible at 425°C for 93 hours to allow for K and Ba ion exchange. This Comparative Sample A was then damaged by subjection to an electrical field as described in Example I. The optical absorption spectra of the damaged Comparative Sample A (1) and the damaged "as-grown" sample (2) were obtained and are shown in Figure 5, along with an "as-grown" or untreated sample, which was not subjected to the electric field (3) for reference. The data of Figure 5 clearly show that there was not only no reduction in damage susceptibility for the ion exchanged sample (i.e., Comparative Sample A) but the damage susceptibility actually increased when Ba was added to the KNO 3 melt. Figure 6 shows the infrared spectrum of "as-grown" (3), Comparative Sample A (4), and damaged Comparative Sample A (1) indicating that the crystal composition/structure of the Comparative Sample A is distinct from that of the "as-grown" or untreated KTiOPθ 4 , but not in a way which reduces the susceptibility to damage. Comparative Example B A z-cut sample (about 3 x 3 x 1 mm) was cut from the low conductivity side, as determined by measurement of the dielectric constant in accordance with P. A. Morris et al., SPIE, The International Society for Optical Engineering, Vol. 1561, Inorganic Crystals for Optics, Electro-Optics, and Frequency Conversion, 104- 110 (1991) , of a single z-cut wafer of a crystal of KTiOP0 4 , grown by the high temperature hydrothermal process described in U.S. Patent 4,305,778, and polished. The sample was ion exchanged as described in Comparative Example A. This Comparative Sample B was then damaged by subjection to an electrical field as described in Example I. The optical absorption spectra of the damaged Comparative Sample B (1) and the damaged "as-grown" sample (2) were obtained and are shown in Figure 1 , along with an "as-grown" or untreated sample, which was not subjected to the electric field (3) for reference. The data of Figure 7 clearly show that there was not only no reduction in damage susceptibility for the ion exchanged sample (i.e.. Comparative Sample B) but the damage susceptibility actually increased when Ba was added to the KNO 3 melt. Figure 8 shows the infrared spectrum of "as-grown" (3), Comparative Sample B (4), and Comparative Sample B/damaged (1) , indicating that the crystal composition/structure of the Comparative Sample B is distinct from that of the "as-grown" or untreated KTiOP0 4 , but riot in a way which reduces the susceptibility to damage. The examples serve to illustrate particular embodiments of the invention. Other embodiments will become apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is understood that modifications and variations may be practiced without departing from the spirit and scope of the novel concepts of this invention. It is further understood that the invention is not confined to the particular formulations and examples herein illustrated, but it embraces such modified forms thereof as come within the scope of the following claims.

Claims: QLHMS. 1. A process for treating a crystal of MT10X0 4 which has crystal structural deficiencies of M throughout the crystal, wherein M is selected from the group consisting of K, Rb, Tl, NH 4 and mixtures thereof and X is selected from the group consisting of P, As and mixtures thereof, comprising the step of: immersing the crystal in molten salt consisting essentially of at least one low melting salt of one or more monovalent cations selected from the group consisting of Rb + , K + , Cs + and Tl + , for a time sufficient to decrease the optical damage susceptibility throughout the crystal. 2. The process of Claim 1 wherein the crystal is hydrothermally grown. 3. The process of Claim 1 or Claim 2 wherein each dimension of the treated crystal is 0.8 millimeter or more. 4. The process of Claim 1 or Claim 2 wherein the monovalent cation of the molten salt is the same as M in the crystal treated. 5. The process of Claim 1 or Claim 2 wherein the crystal treated is a crystal of KTiOXθ 4 . 6. The process of Claim 5 wherein the molten salt consists essentially of KNO 3 or KNO 2 . 7. The process of Claim 1 or Claim 2 wherein the low melting salt melts at a temperature below about 600°C. 8. The process of Claim 1 or Claim 2 wherein the crystal being treated has M sites occupied by protons. 9. The process of Claim 1 or Claim 2 wherein the crystal being treated has vacant M sites.

Inventors: MORRIS PATRICIA A, ROELOFS MARK G, MORRIS, PATRICIA, A., ROELOFS, MARK, G.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1993
Main classification: C30B29
IPC codes: C30B29, C30B33, G02F1