While industries such as optical image recording, data storage, and displays await the coming of the blue diode laser, many companies are already offering access to the green, blue, and UV spectrums through frequency-converting nonlinear crystals.
Two of the most widely used nonlinear materials employed for frequency conversion of low-power Nd:YAG and other mid-infrared-class lasers are lithium niobate (LN or LiNbO3) and lithium tantalate (LT or LiTaO3). "Lithium niobate is grown by the tons per year," said Lawrence Myers, project leader of Lightwave Electronics Corp. (Mountain View, CA) and cochair of SPIE's 1999 Photonics West conference on Laser Material Crystal Growth and Nonlinear Materials and Devices (Proceedings of SPIE Vol. 3610). "There's a lot of maturity with this material and it has good efficiency."
Figure 1. Researchers at NIRIM have developed the double-crucible Czochralski, which feeds powered LN and LT into an outer crucible to boost the lithium content towards stoichiometry. The melting process uses radio-frequency heating and an automatic diameter control system. An off-congruent lithium-rich melt (Li2O) of 58 to 60 moll-percent is placed in the inner crucible. Stoichiometric powder is supplied to the outer melt based on the weight increase of the crystal as determined by a load cell.
Figure 2. Stoichiometric lithium niobate rests in the double-crucible melt cylinders the growth process. In collaboration with The Pennsylvania State University's V. Gopalan, the NIRIM group discovered that both coercive fields and internal fields of lithium niobate and lithium tantalate are strongly dependent on stoichiometry.
So far, LN and LT have been predominately used for frequency conversion into the infrared, but research is under way to extend their utility into the visible and ultraviolet.
Nonlinear crystals require matching the phases of the interacting waves for efficient frequency conversion. Conventional crystals utilize birefringence to overcome phase velocity dispersion. More recently, techniques for quasi-phasematching (QPM) have led to the commercial introduction of periodically poled ferroelectric materials. In QPM, manufacturers take a raw crystal such as LN and switch the orientation of ferroelectric domains of the material 180 deg., forming sections with different periods. In a standard Nd:YAG application, the original 1064-nm beam is converted to its second harmonic, or 532-nm green light. Periodically poled crystals can also be designed with periods to achieve other frequency-mixing processes, such as sum-and-difference frequency generation, and multiple grating sections can be incorporated to achieve cascaded processes in a single monolithic crystal.
Although LN is easier to produce, its sister material LT has a shorter wavelength UV absorption edge, meaning that it can convert light to a shorter wavelength with less absorption of UV light, reducing the tendency toward crystal failure. Typical UV edges for LN are around 350 nm, while LT continues down to 280 nm. However, new manufacturing methods that produce stoichiometric LN and LT rather than congruent composition materials are reducing the UV wavelength edge while improving crystal quality.
Nature's not perfect
Myers, like others in the field of nonlinear optics, is closely watching developments in stoichiometric LN and LT manufacturing. Despite the utility and flexibility of commercially available congruent LN and LT, the performance levels of both crystals fall short of their potential.
According to Myers, stoichiometric crystals offer better performance in several areas when compared with congruent crystals naturally pulled from the melt. "One deficiency is that standard congruent material is grown without the perfect stoichiometric composition," he said. "The lithium content is something like 48 percent, whereas the ideal chemical formula for stoichiometric balance in the crystal would be 50 percent lithium. The fact that the crystals are missing some lithium means that the crystal is rich in defects, and defect sites can cause absorption and incorporation of impurities."
Stoichiometric LN crystals are not easy to produce because the ideal crystal must have just the right chemical composition and near-perfect uniformity. Congruent crystals are uniform, meaning that the index of diffraction stays the same through the crystal, but their chemical composition is off -- a crucial consideration for periodically poling the material and maintaining phase matching throughout the crystal. "So uniformity is the key, but at the price of a defective structure," Myers said.
These defects lead to an increase in both linear and nonlinear absorption such as green-induced infrared absorption (GRIIRA), which means the crystal absorbs more of the IR pump light when green light is also present. While more than 2 W of green light have been generated in a PPLN crystal without material failure, the GRIIRA can be a problem for IR pump levels at or above 5 W when 1 W of green light is present.
However, Myers said stoichiometric crystals exhibit lower linear and nonlinear absorption, a lower coercive field (allowing for shorter poling periods) and, in some cases, lower susceptibility to photorefraction.
Myers points to two groups on opposite sides of the Pacific Ocean-one in California and the other in Japan -- who are making progress in pursuing new manufacturing processes for stoichiometric LN and LT crystals.
Stanford Univ. group: oxidation vs. vapor transport equilibration (VTE)
Researchers at Stanford University's Center for Nonlinear Optical Materials (Stanford, CA), with equipment and material assistance from Crystal Technology Inc. (Palo Alto, CA), have looked at several approaches to improving the performance of congruent LN and LT crystals, including increasing the oxidation of the crystals and lithium indiffusion1.
Oxidation experiments were conducted separately with ambient air and O2 at atmospheric pressures at 600 deg. C. Results indicated an increase in green absorption from 0.01 cm-1 with congruent material, and a drop down to 0.0008 for oxygen annealed. IR absorption at 1064 nm, the UV edge absorption band, and photorefraction stayed relatively constant for all three samples.
The second method holds greater promise for reaching shorter wavelengths. Called the vapor transport equilibration (VTE) process, a wafer of congruent LN is placed in a platinum crucible over a mixture of Li3NbO4 and LiNbO3 and heated to temperatures between 1050 deg. C and 1100 deg. C for 100 hours.
VTE-processed LN crystals showed a significant drop in the UV absorption edge from 316.2 nm to 305.5 nm for crystals heated to 1050 deg. C, down to 301.4 nm for VTE crystals treated at 1100 deg. C. Green absorption also dropped from the 0.01 level of the congruent material to 0.006 for the highest temperature VTE procedure. Again, IR absorption remained relatively constant; however, the procedure enhanced the crystals photorefraction.
The VTE process also exhibited a welcome side effect by lowering the coercive field as more lithium was diffused into the crystals. The Stanford group said, "The lowest value reached was 7.2 kV/mm, which is almost three times less than the coercive field of congruent LiNbO3 (21.5 kV/mm). Such a low value could allow the periodic poling of thicker samples."
Similar experiments on LT showed even further reduction of linear and nonlinear absorption in the green and IR as well as less susceptibility to photorefraction. However, researchers point out that these advantages have to be weighed against the difficulty in producing the crystals. The group believes that an improved VTE process could further reduce the UV absorption edge to shorter wavelengths, which, when combined with an efficient oxygen-annealing process, could greatly reduce linear absorption of the IR pump or output green light and GRIIRA.
Figure 3. NIRIM has grown single crystals of stoichiometric lithium tantalate measuring 45 mm in diameter and 50 to 85 mm in length. The crystal showed a ferroelectric domain switching field of 1.7 kV/mm at lithium levels of 0.4999 compared to congruent crystals with lithium levels of 0.485. Internal field also dropped from 4 to 5 kV/mm for congruent crystals to 0.1 kV/mm for stoichiometric crystals.
NIRIM: the double-crucible method
Controlling the melt and lithium levels during the manufacturing processes are also at the heart of an effort by the National Institute for Research in Inorganic Materials (NIRIM; Tsukuba, Ibaraki, Japan). Led by Kenji Kitamura, NIRIM's 13th research group has developed a double-crucible Czochralski method (Figure 1) to imbue LN (Figure 2) and LT crystals (Figure 3) with higher percentages of lithium.
The group has grown single crystals of LT measuring 45 mm in diameter and 50 to 85 mm in length through a double-crucible device connected to an automatic powder supply system. The melting process uses radio frequency heating and automatic diameter control system. An off-congruent lithium-rich melt (Li2O) of 58 to 60 moll-percent is placed in the inner crucible. Stoichiometric powder is supplied to the outer melt based on the weight increase of the crystal as determined by a load cell.
The group found LT crystals showed a 70 percent increase in conversion efficiency and a domain (coercive) switching field that is thirteen times lower than congruent crystals, giving this crystal an edge for periodically poled devices.2
These crystals, exhibiting higher Curie temperatures than their congruent counterparts (692 deg. C compared with 600 deg. C for congruent crystals, while it is 695 deg. C for a stoichiometirc ceramic sample), evidenced a ferroelectric domain switching field of 1.7 kV/mm at lithium levels of 0.4999 compared to congruent crystals with lithium levels of 0.485. Internal field also dropped from 4 to 5 kV/mm for congruent crystals to 0.1 kV/mm for stoichiometric crystals. In collaboration with V. Gopalan of The Pennsylvania State Univ., they have first revealed that both coercive fields and internal fields are strongly dependent on stoichiometry.
The group discovered that stoichiometric crystals also showed a difference in domain shapes. Conventional congruent LT exhibits triangular shapes, while the isolated domain forms a hexagonal shape in stoichiometric crystals, providing smoother domain walls.
Kitamura said that because of the many advantages for optical devices provided by stoichiometric LN and LT created through the double-crucible Czochralski method, NIRIM is developing procedures that will result in commercial production in Japan and the United States.
Applications hunger for improved crystals
Low-cost laser systems that can produce green and UV light have already found markets in many applications, including metrology, printed circuit board processing, and machining. As the performance of these devices improves, these application areas will only expand as the crystals, and therefore the laser systems, become more efficient and robust.
Up-conversion to UV wavelengths from commercially available Nd:YAG and other solid state lasers holds special promise for companies such as Lightwave in metrology and large-scale lithography of printed circuit boards.
"We don't grow crystals," Myers said. "We buy the substrate crystals and we do the poling, but we really just want to put the crystals to use with our lasers and serve these applications. We'd be happy to buy the crystals already poled, but because there is not a big market out there, we pole them ourselves."
The recent advances in stoichiometric crystals excite Lightwave and other companies that incorporate these optics into their systems to deliver compact, relatively low-cost solid state sources for various applications. Meyers said, "The shorter the wavelength the higher the resolution. If people are using it for material processing, they want to be able to focus more tightly, requiring a shorter wavelength. If they're using it for metrology with a single-frequency laser, they want interference fringes that are more closely spaced," all of which could be improved by the use of mass producible stoichiometric LN and LT crystals.
Myers said obtaining blank crystals is relatively easy, although most companies still conduct their own periodic-poling processes. Although Lightwave Electronics sees itself as a laser system designer and manufacturer, he said the company poles its own crystals because of a lack of some types of commercially available periodically-poled crystals. Until high-power, tunable semiconductor lasers can stretch into the shorter wavelengths of the visible and UV spectrums, companies such as Lightwave will have to look to researchers to improve nonlinear optics in order to meet a growing number of marketable applications.
National Institute for Research in Inorganic Materials (NIRIM)
The Science and Technology Agency
1-1 Namiki , Tsukuba , Ibaraki , 305-0044 JAPAN
Lightwave Electronics Corp.
1161 San Antonio Road
Mountain View, CA 94043
Phone: (1) 650/962-0755
Fax: (1) 650/962-1661
Crystal Technology, Inc.
1040 E Meadow Circle
Palo Alto, CA 94303
Phone: (1) 650/856-7911
Fax: (1) 650/354-0173
Center for Nonlinear Optical Materials
Edward L. Ginzton Laboratory
Stanford, CA 94305-4085
Robert L. Byer , Director
Marty Fejer, Co-Director
Phone: (1) 650/723-0228
R. Winn Hardin
R. Winn Hardin is a science and technology writer based in Jacksonville, FL.