2. Engineering Periodic Domain Structure Crystalline defects play major role: it is easier to create uniform domain grating in the stoichiometric LN, LT that have minimal defects, than in congruent LN and LT. The kinetics of the domain structure in spatially nonuniform electric field produced by the electrode pattern depends on the number of factors: 1) shape of individual electrode, 2) electrode material, 3) variant of electrode structure (Fig.2), 4) geometry of the electrode pattern, 5) parameters of the dielectric layer, 6) poling waveform, 7) switching current limit, and 8) temperature. Moreover, the spatial uniformity of the switching characteristics, conductivity and thickness of the crystalline wafer is of crucial importance. The optimization of all technological factors can be done only the basis of the deep knowledge of the foundations of domain engineering in ferroelectrics.

Fig. 2 Variants of electrode structures for periodical domain patterning by electric field poling.
I – metal electrode pattern, II – metal electrode over insulator pattern, III – metal electrode pattern covered by insulator, IV – liquid electrode over insulator pattern, V – stamper electrode, VI – corona discharge method.

Table 2. Electrode structures for periodical domain patterning by electric poling

Spectralus’s efforts are directed towards identifying and optimizing those parameters contributing most significantly to repeatable, good-quality periodic domain pattern in LN and LT crystals.

An extensive discussion of domain structure development including investigations of the domain engineering aspects in LiNbO3 is contained in: Ferroelectrics, V.221, pp157-167 (1999) by V. Shur, E. Rumyantsev, R. Batchko, G. Miller, M. Fejer, R. Byer

The domain kinetics during periodical poling from the single domain state in a spatially inhomogeneous field can be divided into five main stages: 1) nucleation of new domains at the surface, 2) forward growth of nucleated domains in polar direction with subsequent coalescence, 3) broadening of the strip domains by sideways domain wall motion, 4) stabilization of the domain structure in external field, and 5) backswitching after removing of external field (Fig.3).

All stages have to be carefully optimized to produce a specified domain period and duty cycle with acceptable uniformity throughout the volume of the wafer.

Fig.3. The main stages of the domain evolution during periodical poling

After detail analysis of the published data and basing on our experience we have chosen for the poling of CLN to use the most simple and easier for realization design of the electrode structure, so-called “photoresist only” (Fig. 2 IV). In this case the photoresist pattern is deposited on one side. The liquid electrolyte in spatial sample holder was used for application of electric field.

The scheme of used experimental setup is shown on Figure 4. The setup allows us to apply arbitrary shape poling pulses from TREK 20/20 pulse source to the sample located inside the holder made from acrylic resin. Accuracy of the poling pulse: time resolution 10 ns (100 MHz sampling rate), voltage resolution 14 bit. The lowered voltage and switching current are monitored via the digital storage oscilloscope Tektronix TDS1002, triggered by the generator. Oscilloscope is connected to the computer, thus downloading the measured data via RS232 protocol. Optical polarizing microscope with video camera allows us to record simultaneously the video and instantaneous pictures with view area about 1 mm2.

Fig. 4. Scheme of the poling setup: 1 – sample in the holder, 2 - polarizer, 3 - analyzer, 4 - video camera, 5 - light source, 6 - acrylic resin box.

3” wafer with patterned photoresist is placed in the sample holder for poling with liquid electrolyte – saturated water solution of LiCl. Two holders different in the rubber pads shape: (1) circular and (2) rectangular with rounded corners were used during the poling process.

The periodical poling of the MgO doped LN can be achieved only at the elevated temperatures. As a result we use another variant of the electrode structure with the metal electrodes (Fig.2 I).

The whole poling process contains several operations.

1. Optical inspection of the wafer using polarized microscope to reveal the bulk macro-defects and residual domains (deviations from single domain state).
2. Creation of metal electrode pattern by lithography with controlled heating/cooling rates during baking of photoresist
3. Inspection of the domain structure in the wafer just before poling to avoid appearance of residual domains by polarizing or phase contrast microscopy without application of the external field.
4. Periodical poling at elevated temperature above 100OC in silicon oil with observation of domain kinetics in transmitted or reflected light with subsequent low cooling rate to avoid wafer cracking and to uncontrolled change of the tailored domain structure.

The quality of a periodically-poled (PP) structure is mainly determined by two factors: periodicity and duty cycle (DC). The periodicity of the PP structure strongly affects the phase-matching wavelength of a conversion device, while the DC of the PP structure affects conversion efficiency. Maximum conversion efficiency can be achieved for a perfect uniform DC of 0.5. Therefore, to estimate the uniformity of the DC, the optically microscope images of the etched (in pure hydrofluoric acid) periodically poled surface were carried out (Fig. 5). The widths of domain inverted region were measured on the Z+ and Z- surfaces in the fabricated PPLN element. Uniform PP structure with a 6.75 ?m period and 0.5?0.1 DC has been fabricated from the Z+ surface to more than 400 ?m depth over an area of 5 mm by 10 mm.

Fig. 5. Optical microscopic image of the etched PP structure for the 0.5 mm thick PPLN element with a 6.78 ?m domain inverted period on Z+ surface.

Fig. 6. Scanning probe microscopy observation of the periodical domain structure revealed by etching in 0.5 mm thick MgO:LN

3. Applications

An important application of periodically poled ferroelectrics is wavelength conversion of commercial diode and compact DPSS near-IR lasers to blue, green, and medium-IR spectral regions. Periodically poled LN and LT have clear advantages over the birefringence phase-matched KTP, LiB3O5 (LBO) and BaB2O4 (BBO) nonlinear crystals in terms of 3-5 times lager effective nonlinearity and a possibility to phase-match any second-order interaction within the transmission band of material. Due to high conversion efficiency, PPLN devices are well suited as a nonlinear component for SHG from compact, low power consumption (<10W) lasers, capable of producing 0.1-1W optical power. For the testing of our PPLN elements, the YAG:Nd DPSS laser, that produces 300mW cw at 1064?0.1nm in single-transverse mode, has been used.

Figure 6 present the dependence of the SHG (532nm) outputs on temperature. Excellent fitting of experiment and calculations demonstrates higher quality of fabricated periodically-poled grating, presented in Fig. 5.

A doubling of frequency of diode laser by high-efficient PPLN or PPMgOLN nonlinear-optical components is likely to be the winning technology for low-power blue-green laser market needs.

The diode laser used in present work was InGaAs VECSEL surface-emitting laser . Both their optical mode characteristics and wavelength were controlled by an extended compound optical cavity. These lasers produced ~0.15 W cw at 1064?0.2 nm in single mode operation.

The PPLN components have narrow acceptance bandwidths of about 2 nm/mm and therefore 5mm of length conversion elements are optimal for doubling of VECSEL diode lasers. On the other hands, conversion efficiency of periodically-poled elements is proportional to the fundamental optical power of laser. To increase the conversion efficiency, pulsed laser, modulated by an injection current, has been used. By using 2 A current pulse, 0.5 W optical power was generated in single-transverse mode.

Fig. 7. Temperature dependence of the green light maximum intensity (532 nm output power) for MgO PPLN period 6.95 microns. The dots are experimental data and the solid curve is theoretical curve for a 5 mm length.

Detail analysis of pulse regime has shown that a wavelength chirp of 5nm/?s rate takes place.
Therefore, for optimization of SHG process, pulse duration and length of PPLN element was restricted to 100 ns and 3mm. We have demonstrated 10mW average and 1W peak green power light generation from the intracavity-doubled VECSEL laser (Fig. 8).
No degradation of green light power was observed during 100 hours of operation.

Fig. 8. Pulsed 532nm output generated by the intracavity-doubled of VECSEL (1064nm) laser.