Photonics West ‘99: Periodically Poled Materials Broaden Beyond PPLN

Low-temperature techniques allow researchers to effectively pole large-area potassium titanium phosphate (KTP); poling rubidium titanium arsenide (RTA) requires in situ monitoring.

By: Kristin Lewotsky

For the last several years, periodically-poled lithium niobate (PPLN) has been the hot technology in photonics, allowing researchers and manufacturers to perform frequency conversion without the angle-tuning constraints imposed by nonlinear crystals. Lithium niobate is not the optimal material for poling, merely the easiest to fabricate. (Click here to view sidebar; see PPLN—Challenges of Commercialization). At short wavelengths, photorefractivity compromises the performance of lithium niobate, and the material is susceptible to optical damage. Meanwhile, potassium titanium phosphate (KTP) offers a higher surface damage threshold and is generally more robust than lithium niobate.

Until recently, however, conductivity properties of KTP presented significant barriers to fabrication. Now David Eger and collaborators at Soreq NRC have developed a low temperature electric field poling (LTEP) technique for KTP (paper #3610-06, Photonics West, January 25-30, San Jose, CA), that yields samples with 3.8- to 10-Å periods on 400 to 900 mm³ area, 0.5- to 1.0-mm thick flux-grown KTP plates. In the same session, Richard Stolzenberger and colleagues discussed the poling potential for RTA (paper #3610-07, Photonics West, January 25-30, San Jose, CA).

Low-temperature poling
Although KTP is more resistant to optical damage than lithium niobate, most commercially-available KTP crystals have relatively large ionic conductivity that complicates the poling process. Conventional poling methods can only be applied to insulating KTP crystals, or on those in which conductivity has been chemically modified to increase resistivity.

The Soreq NRC technique reduces the effects of conductivity, allowing engineers to pole large-area KTP crystals. "The essence of our method is to reduce the poling temperature and determine poling conditions so that each wafer, regardless of its conductivity can be poled properly," said Soreq's David Eger. The method, which is patent pending, permits the poling of wafers 30 x 30 cm² and 1mm thick. Eger anticipates that the technique can be extended to wafers as thick as 2 mm.

Fabrication
The wafers used for this work were z-cut, flux-grown KTP. After coating the C⁺ surface of the sample with photoresist and patterning it lithographically with strips, Eger and collaborators coated the patterned surface with a 1000-Å-thick titanium layer.

For poling, the KTP wafers were mounted in a vacuum chamber on a temperature controlled stage. A DC power supply generated the high voltage necessary for poling; the current was integrated by an electronic unit to get the poling charge. The transition temperature T_tr that defined the maximum temperature for poling varied considerably from wafer to wafer, ranging from -70°C to -120°C . The values of the applied electric fields used in this work ranged from 50 KV/cm to 150 KV/cm.

After removing the electrodes from the wafer surface by etching, the researchers diced the wafers into 10-mm-long bars perpendicular to the domain boundaries, in some cases applying antireflection coatings. According to Eger, the fabrication process is deterministic and suitable for high-yield production.

Results
The class of samples designed for green light generation required 9-µm periods. In experiments with diode-pumped solidstate (DPSS) laser sources, the PPKTP samples demonstrated efficiencies as high as 75%.

The production of PPKTP for blue light generation is more difficult than for longer wavelength. According to Eger, strong fringe fields formed at the metal stripe edges tend to wipe out the microdomain structure. Propagation of the narrow domains through the wafer becomes difficult and in some cases the domains are blocked and do not reach the lower surface. Blue light generation requires samples with 4 µm periods, corresponding to an inverted domain width of about 2 µm; these feature sizes pushed the performance of the group's lithography equipment.

By optimizing the LTEP parameters, the group was able to fabricate PPKTP that demonstrated highly uniform domain structure throughout the sample depth. Normalized doubling efficiencies in these samples were 3-6%W^-1cm^-1 for multimode Ti:S laser and 1.5-3%W^-1cm^-1 measured with single longitudinal mode diode lasers.

Periodically poled RTA
In addition to KTP and lithium niobate, researchers are experimenting with poling materials like RTA and lithium tantalate (LTA). Using an optical parametric oscillator (OPO), Richard Stolzenberger and colleagues at Crystal Associates Inc. have developed a system for poling RTA that provides in situ optical monitoring to determine when domain reversal has been achieved. The group produced poled RTA samples with 41.8-µm periods, suitable for near infrared OPOs.

In experiments, the group obtained up to 2 mJ of total output (signal plus idler) from a PPRTA-based OPO pumped by a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 10 Hz. For a 1064-nm pump wavelength, the system generated a signal beam at 1911.4 nm and an idler beam at 2400 nm. The measured slope efficiency for the sample was approximately 17% (see Figure 1).

Periodically-poled materials show great potential for harmonic generation at a variety of wavelengths. KTP is nearing commercial availability, and as work continues on RTA and LTA, those materials may also become viable options frequency doubling.

References
1. Eger, Oron, et. al., "Frequency doubling in periodically-poled flux-grown KTP of short period length," Proc. SPIE #3610, Photonics West '99, San Jose, CA (1999).

2. R. Stolzenberger and M. Scripsick, "Recent advancements in the periodic poling and characterization of RTA and its isomorphs," Proc. SPIE #3610, Photonics West '99, San Jose, CA (1999).