A contact poling technique for domain engineering of ferroelectrics using a micro-structured silicon electrode is demonstrated on Rb:KTiOPO4. High quality QPM gratings were reproducibly fabricated. The silicon electrode is reusable and the technique potentially suitable when complex structures with sub-μm features are to be domain engineered, which otherwise is incompatible with conventional photolithography. A non-negligible domain broadening was seen and attributed to a low nucleation rate using this type of electrode. However, under the appropriate poling conditions, this could be exploited to obtain a QPM grating with a short pitch (2 μm), equal to half of the electrode period.
© 2015 Optical Society of America
The quasi-phase matching (QPM) technique allows realizing any second-order noncritical nonlinear optical interaction within the material transparency range in an efficient way, with the capability to tailor its spatial and temporal properties by appropriately designing the structure. First proposed in 1962 , the QPM approach has gained wide importance over the last two decades thanks to significant development in electric field poling  of ferroelectric oxide crystals, such as LiNbO3, LiTaO3 and KTiOPO4 (KTP). Although the electric field poling technique is the most mature way to implement QPM, it still suffers from some drawbacks. It usually involves the formation of a periodic electrode via photolithographic patterning on one of the polar faces of the ferroelectric crystal. The main disadvantage of the lithographic patterning is the need to process each wafer or sample individually, which is time consuming and expensive. The latter is especially true for KTP isomorphs, where the ionic conductivity variation across the wafer significantly affects the polarization-switching properties, and therefore, the wafer has to be cut into smaller samples, which must be treated separately in order to increase the poling yield . Thus, the need of individual sample-patterning introduces repeatability issues, which can significantly reduce the quality of the QPM grating. This can potentially be overcome by using a micro-structured contact electrode. Silicon is a mature, widely available material with well-established processes for micro- and nanostructure fabrication, offering the possibility to easily fabricate fine-pitch structures with very high precision . In fact, periodic poling with structured silicon as a contact electrode has been preliminarily tested on thin (0.15 mm) slabs of LiNbO3 , however the quality of the resulting domain structures was not reported in this case.
Among the ferroelectric oxides used for periodic poling, the crystals from the KTP family are considered to be the most suitable for fabrication of fine-pitch, one-dimensional QPM gratings because of the large anisotropy in ferroelectric domain-propagation velocity along the different crystalographic axes . Bulk Rb-doped KTP (RKTP) is a particularly attractive KTP isomorph with some advantageous properties relative to KTP for periodic poling . A low dopant concentration of 0.3% grants almost identical optical properties as those of ordinary KTP; however, it results in two orders of magnitude lower ionic conductivity, which is beneficial for periodic poling and fabrication of large aperture QPM structures [8, 9]. Moreover, a lower susceptibility to gray-tracking makes RKTP attractive for frequency conversion in the blue-green region .
In this work the feasibility of periodic poling of RKTP crystals using micro-structured silicon as a contact electrode has been investigated. We demonstrate that high quality bulk QPM gratings with a period of 9.01 μm can be fabricated by this method, achieving an effective nonlinearity as high as 10.8 pm/V. A larger domain-broadening was observed for samples poled with the silicon-contact electrodes than for those poled with standard photolithographically-patterned aluminum electrodes, which was attributed to a reduced nucleation rate under the silicon electrodes. Under the appropriate poling conditions, it could be exploited to obtain QPM gratings with a period equal to half of the electrode period. This is demonstrated by fabrication of a 2 μm grating pitch using a 4 μm grating silicon electrode.
2. Experiments and results
A 4-inch, p-doped, high conductive, 500 µm thick, silicon wafer with <100> orientation was used for fabrication of the contact electrodes. The high-conductivity (σ = 33.3 S/cm) was chosen to avoid power drop in the poling and get a necessary charge injection for polarization-reversal under the contact areas. The fabrication process of the silicon electrodes is illustrated in Fig. 1.
First, the silicon substrate was wet-oxidized in a thermal furnace to obtain a 470 nm thick SiO2 layer. A photoresist grating with a period of 9.01 µm (with a duty-cycle of 30% for the photoresist openings) was then patterned by standard photolithography, and transferred to the SiO2 by anisotropic dry etching. Afterwards, isotropic silicon etching was performed in a magnetically enhanced reactive ion etching machine (Centura II, Applied Materials, Inc.) in two steps: first, 80 sccm flow of CF4 gas attacked the surface for 10 seconds, followed by a combination of 100 sccm SF6, 40 sccm CHF3, 100 sccm O2 and 100 sccm Ar for 30 seconds. In both steps the pressure was 30 mTorr, while ICP power was 900 W and the RF bias 400V. Then, the remaining layers of photoresist and SiO2 were removed by oxygen plasma and HF etching, respectively. Finally, the processed Si wafer was cut out into chips with dimensions 4 × 8 mm2. A scanning electron microscope (SEM) image of the cross-section of the electrode array is shown in Fig. 2. The ribs had a 2.65 µm wide unetched top, separated by 6.36 µm wide and 2.71 µm deep valleys.
Commercial, c-cut, flux-grown Rb-doped KTP (RKTP) was used for periodic poling. Due to conductivity reasons the RKTP samples were cut to the dimensions of 10 × 5 × 1 mm3 along the a-, b-, and c- crystallographic axes, respectively. The silicon rib-array chip was contacted to the electric poling circuit and pressed against the c- -face of the RKTP sample to get a tight, homogeneous contact over the whole crystal. The opposite polar face of the crystal was contacted to the poling circuit using a saturated solution of KCl.
Similar poling conditions to those used for poling of standard photolithographically patterned samples were used for the crystals with Si electrodes. It was previously demonstrated that poling with relatively short triangular electric field pulses mitigates the domain broadening problem and results in high overall QPM grating quality . Each RKTP sample was therefore poled by applying single, 5 ms long, symmetric triangular electric field pulse with a peak amplitude of 6.2 kV/mm in order to obtain fully formed QPM grating. The poling process was monitored by observing the electro-optic response  and by generating in situ SH with a continuous-wave (CW) Ti-Sapphire laser, in order to obtain information about the domain propagation and the uniformity of the poled structure . All crystals were periodically poled using the same Si electrode chip, which was cleaned in an ultrasonic bath of acetone between the poling events. After poling, each crystal was etched with a chemical selective etchant in order to reveal the domain structure on the polar faces . The microscope images of a representative crystal poled with 6.2 kV/mm electric field magnitude are displayed in Fig. 3, where Fig. 3(a) shows the domain structure on the polar face that was in contact with the Si-electrode, and Fig. 3(b) shows the corresponding domain structure on the opposite face.
Note that the resulting inverted domain duty-cycle is close to 50% throughout the crystal thickness, which represents the ideal case for an efficient first-order QPM frequency conversion.
In order to evaluate the optical performance, the PPRKTP crystals were placed in a SHG setup, pumped by a tunable, narrow linewidth, continuous wave (CW) Yb-doped fiber laser . The crystals were wrapped in an indium foil in order to improve the thermal contact, and placed in a temperature controlled oven, stabilized at 49 °C. The SH signal was homogeneous over the whole crystal aperture and a maximum power of 208 mW was generated for a pump power of 4.26 W at a fundamental wavelength of 1064 nm. The normalized conversion efficiency and effective nonlinearity were as high as ηeff = 1.22%W−1cm−1 and deff = 10.8 pm/V, respectively. The measured temperature acceptance bandwidth (FWHM) of 5.9 °C corresponds to the effective crystal length of 7.7 mm. This value is very close to the actual poled structure length of 8 mm, which further confirms the high quality of the obtained QPM grating.
The most striking difference between samples poled with Si electrodes and those poled with standard photolithographic patterning is the significant domain broadening in the Si electrode case. In order to investigate this further, we compared samples in the early stages of polarization-reversal, i.e. the domain nucleation phase, which were poled with either Si electrodes or with photolithographically-patterned electrodes. Figure 4(a) shows the former c--face of the RKTP sample poled with Si electrode at low-field electric field regime with a peak amplitude of 6 kV/mm. The poling resulted in domains formed at the electrode edges, where the normal component of the electric field is the highest. As can be clearly seen, the sideway expansion of the nucleated domains is similar under and outside of the electrode area. The duty-cycle of the newly formed domain tips together with the remaining unswitched area under the electrode is already ~50%, while the Si-rib electrode duty-cycle was 30%. The domain structure corresponding to a similar stage of polarization reversal in a sample poled with an aluminum-photoresist grating electrode is shown in Fig. 4(b). Here the duty-cycle for the aluminum mask was 32%. In this case, the lateral growth of the domains occurs mainly under the electrode area, resulting in a duty cycle of 37%.
This difference may be explained by the different properties of the periodic conductor-insulator gratings. It is well known that the electrode material strongly influences the domain nucleation rate during poling . In photolithographically patterned samples, the aluminum acts as an electrode, providing high nucleation site density, while the photoresist prevents charge injection and, consequently, the nucleation and growth of the domains outside the electrode area are limited. By comparing poling with the Si-electrode and the normal aluminum mask, Fig. 4(a) vs. 4(b) it is clear that the domain inverted area is smaller under the Si-electrode compared with Al-electrode, which is attributed to the lower domain nucleation rate in the Si-rib poling technique than standard photolithography technique. In addition, the air gaps between the silicon ribs provide less insulation than photoresist, allowing faster expansion of the domain tips outside the electrode area.
The aforementioned properties of Si electrodes may be advantageous for achieving reduced periodicities. With an appropriately designed electrode duty-cycle and a carefully chosen electric field magnitude it should be possible to form inverted domains with a width equal to a quarter of the electrode period. In order to test this concept, we fabricated an electrode with a period of 4 µm, with a duty-cycle of 28% using the same fabrication process as described above. The cross-section of this electrode array is shown in Fig. 5.
With this Si chip as an electrode, we poled a RKTP crystal by applying a single 5 ms long symmetric triangular electric field pulse with an amplitude of 6.0 kV/mm. The resulting domain structure on the contacted face of the crystal is shown in Fig. 6(a), and the corresponding domain structure on the opposite face is shown in Fig. 6(b). It can be clearly seen that the resulting structure consists of a domain grating with an average periodicity of 2 μm. The width of the inverted domains on the contacted face varies between 300 nm and 900 nm, indicating that this technique may be used for obtaining QPM structures even with sub-μm periods. Note, though, that some domain merging can be observed on both polar faces, suggesting that more carefully optimized poling conditions would be required in order to obtain higher quality QPM structures with divide-by-two periodicity.
In this work a new method for fabrication of periodically poled RKTP samples using an array of Si electrodes is presented. This poling technique has several advantages over traditional poling. The mature silicon fabrication technology guarantees high accuracy and reproducibility in the electrode fabrication. In addition, when a more complicated electrode with small features (sub-μm) and different geometry is required, which cannot be reproduced in conventional photolithography, a contact electrode can be formed in Si using electron-beam lithography and then used for poling. Furthermore, as the Si-electrode is reusable it will speed up the poling process and thereby reduce the costs.
We demonstrate that consistent high quality periodic poling using a silicon contact electrode with a period of 9.01 μm can be achieved in RKTP. The obtained QPM devices were evaluated in a CW SHG setup and present an effective nonlinear coefficient of 10.8 pm/V, close to the optimum value. Furthermore, it was observed that under proper poling conditions, the reduced domain nucleation rate under the silicon ribs and increased domain expansion rate outside of the ribs result in a regular domain-grating with a divide-by-two periodicity relative to the contact electrode period. Taking advantage of this phenomenon, we were able to fabricate domain gratings with an average period of 2 μm, by using a silicon contact electrode with a period of 4 μm. To conclude we believe that contact electrode poling is a convenient, cheap and flexible method, potentially suitable for obtaining QPM gratings with sub-μm periodicity and/or complex structures .
This work was partly supported by the Swedish Research Council (VR) through its Linnæus Center of Excellence ADOPT. The authors also thank the Göran Gustafsson Foundation and the Carl Trygger Foundation for financial support.
References and links
1. J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, “Interactions between light waves in a nonlinear dielectric,” Phys. Rev. 127(6), 1918–1939 (1962). [CrossRef]
2. M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, “First-order quasi-phase matched LiNbO3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation,” Appl. Phys. Lett. 62(5), 435–436 (1993). [CrossRef]
3. H. Karlsson and F. Laurell, “Electric field poling of flux grown KTiOPO4,” Appl. Phys. Lett. 71(24), 3474–3476 (1997). [CrossRef]
4. J. D. Plummer, M. D. Deal, and P. B. Griffin, Silicon VLSI Technology (Prentice Hall, 2000).
5. K. Kintaka, M. Fujimura, T. Suhara, and H. Nishihara, “Fabrication of ferroelectric-domain-inverted grating in LiNbO3 by applying voltage using etched-Si stamper electrode,” Electon. Lett. 34(9), 880–881 (1998). [CrossRef]
6. C. Canalias, J. Hirohashi, V. Pasiskevicius, and F. Laurell, “Polarization-switching characteristics of flux-grown KTiOPO4 and RbTiOPO4 at room temperature,” J. Appl. Phys. 97(12), 124105 (2005). [CrossRef]
7. Q. Jiang, P. A. Thomas, K. B. Hutton, and R. C. C. Ward, “Rb-doped potassium titanyl phosphate for periodic ferroelectric domain inversion,” J. Appl. Phys. 92(5), 2717–2723 (2002). [CrossRef]
8. S. Wang, V. Pasiskevicius, and F. Laurell, “High-efficiency frequency converters with periodically-poled Rb-doped KTiOPO4,” Opt. Mater. 30(4), 594–599 (2007). [CrossRef]
9. A. Zukauskas, N. Thilmann, V. Pasiskevicius, F. Laurell, and C. Canalias, “5 mm thick periodically poled Rb-doped KTP for high energy optical parametric frequency conversion,” Opt. Mater. Express 1, 201–206 (2011).
10. A. Zukauskas, V. Pasiskevicius, and C. Canalias, “Second-harmonic generation in periodically poled bulk Rb-doped KTiOPO₄ below 400 nm at high peak-intensities,” Opt. Express 21(2), 1395–1403 (2013). [CrossRef] [PubMed]
11. C. Canalias, V. Pasiskevicius, F. Laurell, S. Grilli, P. Ferraro, and P. De Natale, “In situ visualization of domain kinetics in flux grown KTiOPO4 by digital holography,” J. Appl. Phys. 102(6), 064105 (2007). [CrossRef]
12. H. Karlsson, F. Laurell, and L. K. Cheng, “Periodic poling of RbTiOPO4 for quasi-phase matched blue light generation,” Appl. Phys. Lett. 74(11), 1519–1521 (1999). [CrossRef]
13. F. Laurell, M. G. Roelofs, W. Bindloss, H. Hsiung, A. Suna, and J. D. Bierlein, “Detection of ferroelectric domain reversal in KTiOPO4 waveguides,” J. Appl. Phys. 71(10), 4664 (1992). [CrossRef]
14. P. Zeil, V. Pasiskevicius, and F. Laurell, “Efficient spectral control and tuning of a high-power narrow-linewidth Yb-doped fiber laser using a transversely chirped volume Bragg grating,” Opt. Express 21(4), 4027–4035 (2013). [CrossRef] [PubMed]
15. G. D. Miller, “Periodically poled lithium niobate modeling, fabrication and nonlinear optical performance,” Ph.D. Dissertation, Stanford University (1998).
16. J. R. Kurz, A. M. Schober, D. S. Hum, A. J. Saltzman, and M. M. Fejer, “Nonlinear physical optics with transversely patterned quasi-phase-matching gratings,” IEEE J. Sel. Top. Quantum Electron. 8(3), 660–664 (2002). [CrossRef]