Surface domain engineering in congruent niobate single : A route to submicron A. C. Busacca, C. L. Sones, V. Apostolopoulos, R. W. Eason, and S. Mailisa) Optoelectronics Research Centre University of Southampton, Southampton SO17 1BJ, United Kingdom ͑͒

We describe a technique for surface domain engineering in congruent lithium niobate single crystals. The method is based on conventional electric-field poling, but involves an intentional overpoling step that inverts all the material apart from a thin surface region directly below the patterned photoresist. The surface poled structures show good domain uniformity, and the technique has so far been applied to produce domain periods as small as ϳ1 ␮m. The technique is fully compatible with nonlinear optical integrated devices based on waveguide structures. ͓͔

Domain engineering in ferroelectric crystals such as which when used in conjunction with a waveguide geometry, 10 LiNbO3 and LiTaO3 is an increasingly important and ever has produced a high conversion efficiency. more versatile technique for applications in areas as diverse This last result is significant in that, for waveguide ge- as harmonic generation and parametric processes,1 electro- ometries at least, it is not necessary to achieve domain inver- optic Bragg gratings,2 and piezoelectric microactuated sion to depths exceeding the guide depth itself. Many of the devices.3 Over the past decade or so, highly efficient quasi- earlier reports on domain inversion applied to typical com- phase-matched nonlinear interactions have been achieved via mercial material supplied as either 300 ␮mor500␮m thick precise periodic domain inversion in z-cut samples, wafers. It is clearly harder to maintain high aspect ratio, short using periods for example of the order of a few ␮m for period, high-quality domain patterning over these large and near-infrared to blue or near-UV harmonic generation.4,5 Re- ͑for waveguide geometries͒ unnecessarily large depths. In search on periodically poled lithium niobate, PPLN ͑and to a this letter, we discuss a method for achieving superficial, or lesser extent lithium tantalate͒, continues to generate consid- surface, domain inversion that has been used to achieve pe- erable interest from the fundamental viewpoint of materials riods of 1 ␮m, and that can be used, we believe, for achiev- research through to the fabrication of practical nonlinear op- ing the periods of ϳ0.3 ␮m required for waveguide imple- tical and electro-optical devices. PPLN with periods for stan- mentation of backward wave parametric generation and dard conversion wavelengths is now commercially available tunable Bragg grating structures. from several sources. The technique for surface domain inversion is based on Fabrication of periodically poled materials with arbi- conventional electric-field (E-field͒ poling at room tempera- trarily small values of period, particularly at submicron ture. The procedure is as follows: One of the z faces of the scales, remains an elusive goal however. The high coercive crystal is covered with a photolithographically patterned field, Ec , required for domain inversion in congruent photoresist layer with a thickness on the order of 1 ␮min ϳ Ϫ1 LiNbO3 (Ec 220 kV cm ), together with inherent non- order to achieve the appropriate E-field contrast which is uniformities and defects that are always present in commer- necessary for a spatially selective domain inversion. Both z cially available materials, restricts the applicability of the faces are then covered with conductive gel electrodes, and a standard electric-field poling technique to periods on the or- single high-voltage ͑HV͒ pulse is applied across the sample. der of Ͼ4–5 ␮m in samples of thicknesses ϳ500 ␮m. It is The value of the HV varies with the thickness of the sample not an easy task to routinely fabricate high-quality PPLN but the applied electric field must be on the order of 22 kV and, in many cases, the crystal must be polished down to mmϪ1. thicknesses on the order of 100–150 ␮m to achieve finer For normal E-field poling, the established practice is to periods than this.6,7 first calculate the charge, Q, corresponding to the patterned Two approaches to overcome this apparent limit in do- area intended for domain inversion. The formula used for ϭ ϫ ϫ main period have recently met with some success however. this calculation is Q 2 A Ps , where Q is the calculated The first technique, referred to as controlled spontaneous charge, A is the area corresponding to the developed part of backswitching, has been applied to bulk samples with a typi- the photolithographic pattern ͑the area where the conductive ␮ ␮ 8 ͒ cal thickness of 500 m, to generate periods of 4 m, and liquid or gel is in contact with the crystal surface and Ps is more recently 2.6 ␮m.9 The second technique, applied to the spontaneous polarization of lithium niobate ͑0.72 2 MgO:LiNbO3 which has the benefit of improved resistance ␮C/mm ). An additional external empirical factor (EF)is to photorefractive damage, utilized multiple short current also usually taken into account, to correct for variations in pulses, generating a period of 2.2 ␮m and depth of 1.5 ␮m, supplier dependent material , precise values of thickness across the sample, and specific electrical character- a͒l: istics of the poling supply itself. An EF value exceeding unity is often used to achieve the desired high-quality peri-

0003-6951/2002/81(26)/4946/3/$19.00 4946 © 2002 American Institute of Physics Downloaded 26 Jul 2008 to 18.51.1.222. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp FIG. 1. Schematic of the poling process as a function of the empirical factor ͑ ͒ ͑ Ͻ ͒ ͑ ͒ ϳ ͑ ͒ EF. a underpoling EF 1 , b normal poling (EF 1), and c overpol- FIG. 2. Single-pulse poling signatures for current and voltage. Note that no ӷ ing (EF 1) used for fabrication of surface domain structures. backswitching is observed in this process. odic domain patterning, resulting in a calculated Q value of graphic patterning for periods between 2.5 and 4.0 ␮m and ϫ ϫ ϫ 2 A Ps EF. The E-field is applied until the appropriate also exposure through a phase mask. For the latter tech- amount of charge according to the expression for Q is de- nique, final domain widths on the order of 0.5 ␮m and peri- tected. It is clear therefore, that the duration of the E-field ods of 1 ␮m have been obtained. We have examined these application is a function of both the area to be poled and the surface domains for the former case, and found that they EF value. extend to depths of between 6 and 11 ␮m which is entirely This EF thereby controls domain spreading within the compatible with waveguide depths and good overlap of crystal volume: Values of EFϽ1 lead to underpoling, guided modes. Ϫ whereby domains are inverted preferentially in areas where For the larger range of periods studied, the LiNbO3 z nucleation is easier, for example at the edges of the photore- face was spin coated with a 1.2 ␮m thick photoresist and UV sist patterns or areas of increased surface roughness. If exposed through a periodic amplitude mask. After photore- EFϳ1, normal poling occurs which can be of good quality sist development, gel electrodes were applied to both the with 50/50 mark-to-space ratio and large scale uniformity for unpatterned ϩz face and the patterned Ϫz surface. The long period poled structures. Finally, if EF is too large how- samples were poled using a computer-controlled supply that ever, then the inverted domains, once nucleated, spread lat- dynamically varied the applied field in order to maintain a erally, extending their volume more rapidly than required for constant current, and the poling process terminated when a ϭ ϫ ϫ ϫ an ideal 50/50 mark-to-space ratio grating. This case is re- predefined charge Q( 2 A Ps EF) had passed through ferred to as overpoling. The three regimes, according to the the crystal. A typical single-pulse poling curve is shown in domain spreading, are illustrated schematically in Fig. 1. Fig. 2, and illustrates the difference between this technique Concentrating our attention on Fig. 1͑c͒ which describes the and that reported in Refs. 8 and 9. The applied E field is on state of the sample after poling using large values of EF, the the order of 22.1 kV/mm which is the appropriate value for schematic shows that small regions of material beneath the domain reversal in lithium niobate. The high voltage is ap- photoresist can remain in their original poled state. If over- plied for a time duration that is proportional to the calculated poled, using values of EF exceeding the theoretical value of charge value, hence, it will depend on the area to be poled as ϳ2, then the sample appears almost uniformly poled when well as the EF value. No backswitching occurs in our over- observed between crossed polarizers. Once etched with poling process, and we feel this represents a fundamentally HF/HNO3 acids, however, careful investigation reveals that simpler technique for achieving controlled small period sur- some noninverted domain regions survive beneath the pho- face domain inversion. toresist patterned surface, and that these can extend a few A variety of surface poling results can be obtained that microns into the Ϫz crystal face. The technique which is depend on the value of EF used. Figure 3 shows a typical described here relies on overpoling the sample which scanning electron microscope ͑SEM͒ picture of a surface achieves the apparently undesirable effect of domain spread- poled sample, obtained with an EF value of 8. It is clearly ing and merging beneath the lithographically patterned pho- seen following the HF/HNO3 etching that the domains only toresist layer. It is also able to create large scale uniform fine exist in the near-surface region ͑shown here to a depth of ϳ3 period surface inverted domain structures. ␮m͒. Other EF values can and have been used but, to date, Using this technique, we have performed an initial para- we have not performed a full parametric study of depth or metric study of surface poling versus the value of EF and uniformity as a function of the EF value. The surface do- imposed photoresist period. It should be noted that this tech- main depth however is clearly an inverse function of the EF nique will not work with other electrode materials such as value. directly deposited metals, as charge accumulation is thereby In Fig. 4, we show the results of measured domain depth prohibited. We have used both conventional photolitho- as a function of the period of the imposed photolithographic FIG. 3. SEM picture of surface domains revealed by HF/HNO3 acid etch- ing. FIG. 5. SEM picture of 1 ␮m periodic surface domains written using a phase mask. pattern, for an EF value of 8. Although the variation of mea- ␮ sured domain depth ͑taken for between 30 and 100 periods͒ domain patterning down to periods on the order of 0.3 m is rather large, two clear points emerge. First, there is a mini- required for backward wave interactions at a wavelength of ␮ mum in the domain depth achieved, an obvious requirement 1.5 m should be readily achievable using exposure with for intended waveguide applications. Second, the mean depth near-UV laser irradiation. is seen to scale approximately linearly with the period. For In summary, therefore, we have presented a single-step applications that require submicron periodicity, this is, again, approach for achieving surface domain inversion to depths a useful observation as the overlap between the guided that are consistent with single-mode waveguides in LiNbO3 . modes and domain inverted regions is a prerequisite for ef- The overpoling technique is simple to implement, and ap- ␮ ficient nonlinear interactions. Figure 4 shows two fits: One pears to work down to periodicities of at least 1 m. Further ͑dashed line͒ includes the point ͑0,0͒ as a further implicit work is in progress to examine the optimum choice for the data point. The close agreement between these two gradients EF value used, and to fabricate first-order gratings in wave- ϳ ␮ further confirms the approximate linearity just stated. guide materials, with the required periodicities of 2 m. Finally, in Fig. 5, we show the details of a ϳ1 ␮m peri- The authors are pleased to acknowledge support from odicity surface grating, fabricated using exposure of the pho- the Engineering and Physical Sciences Research Council toresist via a phase mask. Following acid etching, sub-␮m ͑EPSRC͒ for research funding, under Grant No. GR/R47295, features are revealed that are on the order of 1 ␮m in depth. and thank Peter G. R. Smith from the ORC, University of We believe that such interferometric exposure ͑via phase Southampton, for discussions. mask or two beam interferometry͒ holds much promise, as 1 M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, Appl. Phys. Lett. 62, 435 ͑1993͒. 2 M. Yamada, M. Saitoh, and H. Ooki, Appl. Phys. Lett. 69,3659͑1996͒. 3 C. L. Sones, S. Mailis, V. Apostolopoulos, I. E. Barry, C. B. E. Gawith, P. G. R. Smith, and R. W. Eason, J. Micromech. Microeng. 12,53͑2002͒. 4 R. G. Batchko, V. Y. Shur, M. M. Fejer, and R. L. Byer, Appl. Phys. Lett. 75, 1673 ͑1999͒. 5 D. J. L. Birkin, E. U. Rafailov, G. S. Sokolovskii, W. Sibbett, G. W. Ross, P. G. R. Smith, and D. C. Hanna, Appl. Phys. Lett. 78, 3172 ͑2001͒. 6 K. Kintaka, M. Fujimura, T. Suhara, and H. Nishihara, Electron. Lett. 32, 2237 ͑1996͒. 7 M. Yamada and M. Saitoh, J. Appl. Phys. 84, 2199 ͑1998͒. 8 R. G. Batchko, V. Y. Shur, M. M. Fejer, and R. L. Byer, Appl. Phys. Lett. 75, 1673 ͑1999͒. 9 V. Y. Shur, E. L. Rumyantsev, E. V. Nikolaeva, E. I. Shishkin, R. G. Batchko, G. D. Miller, M. M. Fejer, and R. L. Byer, Ferroelectrics 236, 126 ͑2000͒. FIG. 4. Experimental measurements of domain depth vs domain period, as 10 T. Sugita, K. Mizuuchi, Y. Kitaoka, and K. Yamamoto, Jpn. J. Appl. Phys., determined by optical microscopy, using an EF value of 8. Part 1 40, 1751 ͑2001͒.