1. Introduction

The quasi-phase-matching (QPM) technique [1] offers the possibility to realize any second-order nonlinear interaction in a noncritical and efficient way with the capability to tailor its spatial and temporal properties. Although the QPM concept was proposed in 1962, it did not become of practical importance until the early 90’s with the introduction of periodically poled ferroelectric crystals [2]. The success of this technique critically depended on the development of ferroelectric domain engineering to realize QPM devices in materials as KTP [3], LiNbO₃ (LN) [2] and LiTaO₃ (LT) [4]. Thus, it is not surprising that a new branch of science and technology devoted to the creation of periodic ferroelectric domain structures rapidly evolved. The flexibility of tailoring nonlinear interactions by appropriately designing the QPM structure proved to be very appealing for many applications of nonlinear optics. Moreover, in recent years it became apparent that the QPM technology provides the possibility to realize unique nonlinear interactions that are impossible to obtain in ordinary birefringent phase matched nonlinear media. Examples of this are second-order nonlinear optical interactions involving counter-propagating photons as backward second harmonic generation, mirrorless optical parametric oscillators (MOPO) [5] employing distributed feedback of counter-propagating photons, and broadband counter-propagating optical parametric amplifiers which still wait for experimental demonstration. Due to the very large wavevector mismatch in all these interactions, the QPM nonlinear media should be structured on the scale of the optical wavelength. This in turn demands developing a reliable structuring technology for ferroelectric domains having the width of the order of a hundred nanometers and the height of the order of a millimeter along the polar direction, i.e. the aspect ratio of about 10⁴. Thus it is not surprising that the MOPO concept had to wait 41 years from its theoretical prediction [5] to its practical realization [6]. Consequently, the potential for applications for QPM devices with sub-µm periodicity has recently started to emerge. For instance, such structures have been proposed as all-optical switching components, which take advantage of increased efficiency in counter-propagating cascaded second-order interaction [7–9], as tunable photonic band gaps [10], and for applications as tunable slow-light structures [11]. Recently, structures with counter-propagating parametric interaction in an external cavity have been proposed as source of ultra bright single mode biphotons for quantum information processing [12].

However, fabrication of the structures with sub-µm domain engineering containing an optically usable domain depth and large enough grating area remains a challenge. The most popular nonlinear ferroelectric crystals, LN and LT, have a trigonal crystal structure that favors formation of domains of hexagonal or trigonal shapes [13–15], which make reduction of the lateral domain size rather challenging. Several attempts to fabricate sub-µm gratings in these materials have been reported [16–19], but none of those techniques has so far allowed for device implementation due to intrinsic limitations, either due to the small penetration depth of the domain grating, and/or due to the limited grating length.

On the other hand, KTP and its isomorphs contain a chiral crystal structure and have large anisotropy in the ferroelectric domain propagation velocities along the different crystal axes [20], which limits the domain broadening, making it easier to fabricate dense domain gratings. Moreover, they possess large nonlinearities similar to those found in LT, are suitable for QPM devices for near UV to mid-infrared generation, and present a damage-threshold exceeding that of LN and LT at room temperature. Indeed, we have recently successfully fabricated sub-µm domain gratings in bulk KTP crystals. The samples have been used to demonstrate MOPO [6], electro-optically switchable Bragg reflectors [21] and continuous-wave backward second harmonic generation [22].

In this paper we first review the fabrication process of bulk ferroelectric domain gratings with sub-micrometer periodicity in KTP. We demonstrate poling of gratings with periods ranging from 800 nm to 657 nm. We show that bulk Rb-doped KTP (RKTP) is a very attractive material for fabrication of fine pitch domain structures. Compared to undoped KTP, the RKTP presents superior properties in terms of reduced domain broadening and domain propagation along the c-direction. We demonstrate a periodic poling of a ferroelectric domain grating with a period of 690 nm in 1 mm-thick RKTP. This represents a domain aspect-ratio exceeding 4:10000.

2. Fabrication of sub-µm periodically poled KTP crystals

Since standard photolithographic techniques are not suitable for patterning lines much below 1 μm, an in-house built deep UV-laser lithography has been used. As a light source we employed frequency quadrupled CW single-mode Nd:YVO₄ laser delivering power up to 230 mW at 266 nm in a TEM₀₀ beam. The laser beam was launched through a microscope lens, followed by a pin-hole and a collimating lens. The collimated beam was then split into + 1 and −1 diffraction orders by a reflection diffraction grating. The two beams are then recombined at adjustable angles to form an interference fringe pattern, which can be then transferred to a photoresit layer deposited on the top of the ferroelectric polar surface.

One of the main difficulties in poling of sub-μm periods arises from controlling the effect of the fringing transversal electric fields at the electrode edges [23]. The role of the fringing fields becomes increasingly important with shorter periods, due to the fact that the transversal fields give rise to the ferroelectric domain broadening. Two different approaches have proven useful to overcome this problem and are described below.

2.1 Poling using coercive field gratings

Flux-grown KTP presents deviations in stoichiometry dominated by potassium and oxygen deficiencies. Rosenman et al. [24] showed that the coercive field of KTP decreases when the potassium content increases. This fact can then be exploited to create a coercive field grating in the crystal by spatially selective increase in the stoichiomety in the crystal sub-surface region. First, the photoresist on the top of the crystal is patterned with the above-described UV-laser lithography. After development of the photoresist, an Al-film of the thickness of 50 nm is deposited on top of the pattern. A homogeneous Al-layer is deposited on the opposite face of the crystal, and the photoresist is then lifted-off. After that, the crystal is immersed for about 24 hours in a KNO₃ melt at 380 °C in order to in-diffuse additional concentration of K⁺ ions in the metal openings creating a grating of high and low coercive field regions. The difference in coercive field between the two regions is estimated to be 0.5 kV/mm. Afterwards, the metal layers are removed and the sample is poled by applying an homogeneous electric field across the sample. The difference in the coercive fields in the in-diffused and non in-diffused regions makes the use of periodic electrodes unnecessary.

Figure 1 shows an atomic force microscope (AFM) image of the topography of the etched periodically poled 1 mm-thick sample. The poling was accomplished by applying four 4.5 ms long square electric field pulses with an amplitude of 1.6 kV/mm. The total poled area was 7 × 4 mm². The period of the poled structure was 720 nm. Visual inspection by optical microscope showed that the poling was homogenous over an area 5 × 2 mm² and extended over the full sample thickness.

 

Fig. 1 AFM image showing the etched domain structure on the patterned face of a 1 mm-thick sample poled with Λ = 720 nm.

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2.2 Short electrical field pulse poling

In this technique the domain broadening is limited by using short electrical pulses to pole the crystal. First, a metal-insulator (photoresist) pattern is defined on the c^- face of the samples by the above described deep-UV lithography. Afterwards the crystals are poled by using electric-field pulses of length between 500 µs to 2 ms, and the amplitude of the pulse is kept in the high-field regime. By high-field regime here we mean an electric field strength substantially exceeding the coercive field of KTP (2 kV/mm). The high-field ensures rapid domain nucleation and propagation along the polar axis of the crystal [25] whereas the short length of the pulse prevents the domains from spreading beyond the electrodes. Essentially this technique exploits the inherent anisotropy of the ferroelectric domain growth rates along different crystal axes. Figure 2 shows the topography measured by an AFM of the chemically etched c surface of a KTP sample with a ferroelectric domain grating of 800 nm. The sample was poled by applying one 1.5-ms-long electrical pulse of 2.6 kV/mm. The periodic domain structure was uniform over a region of 5 mm and 1 mm in a- and b- directions respectively, and the extent of QPM grating in c-direction was 0.4 mm.Up to now, the shortest grating period that we have tried to pole with this technique was 657 nm. For this we have used a pulse length of 500 µs and an electric field magnitude of 3 kV/mm. The domain structure had a depth of 350 µm in c-direction. Although this technique is more straight-forward and requires less processing steps than the chemical patterning, it results in a limited penetration depth of the domain structures. Most probably the high ionic conductivity along the polar direction of KTP and the associated electric field screening slows down further propagation of the ferroelectric domains along this direction and the lateral domain growth rate becomes comparable with the domain growth along the polar axis. In such situation applying higher electric field amplitude would not help and would result in homogeneously inverted spontaneous polarization. A possible alternative could be the application of even shorter electric field pulses, however this could not be tested due to the limitation of the high-voltage amplifier.

 

Fig. 2 Topography of the etched domain structure on the patterned face of a KTP crystal with a domain period of 800 nm. The grating extends 0.4 mm in the c-direction.

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3. Sub-µm periodically poled RKTP

Although sub-μm domain gratings in KTP have been successfully fabricated, the high-ionic conductivity of the material, the inhomogeneous stoichiometry over a single-crystal wafer as well as poor wafer-to-wafer consistency limit the yield of the periodic poling and the penetration-depth of the domain structures. A more promising candidate for sub-micrometer domain pattering is RKTP. This material has similar transmission and nonlinear properties as the flux-grown KTP, however it has orders of magnitude lower ionic conductivity. According to the reasoning above the reduced ionic conductivity should result in reduced ferroelectric domain broadening. Furthermore, this material shows substantially lower light-induced absorption compared to that of the flux-grown KTP [26]. Polarization switching in RKTP was reported by Jiang et al. [27], and Wang et al. [28] demonstrated periodic poling for frequency-doubling in the blue region. Recently, Zukauskas et al. showed consistent periodic poling in a 5 mm-thick crystals with a grating of 38.86 µm [29].

In this work we used commercially supplied c-cut, single-domain flux-grown RKTP. The crystals are grown by a top-seeded solution growth technique with a 1.4 mol % Rb in the flux melt, which results in 0.3% Rb⁺ replacing K⁺ in the as-grown crystal [30]. The low Rb-content in the material suggests that its linear and nonlinear optical properties are very similar to those of undoped flux-grown KTP. However, its ionic conductivity is two-orders of magnitude lower than that of KTP thanks to the larger Rb⁺ ionic radius [28]. The coercive field of RKTP is 3.8 kV/mm.

3.1 Periodic poling of RKTP

The superiority of RKTP in terms of domain-propagation along the polar direction and reduced lateral domain-broadening is illustrated in the following experiment. The c^- faces of 3mm-thick KTP and RKTP samples were photolithographically patterned with a 38.86 µm grating period that had a metal-isolator duty-cycle of 30-70%. In this case a standard UV lithography with predefined mask and following development, Al-deposition and lift-off techniques were used. The samples were poled using a single 8 ms-long square-shaped electrical-pulse of a magnitude of 2.9 kV/mm and 5 kV/mm for KTP and for RKTP, respectively. Figure 3 shows micrographs of the etched domain structures in (a) the patterned and (b) the non patterned faces of KTP, and (c) the patterned and (d) non patterned faces of RKTP.

 

Fig. 3 Micrographs showing the ferroelectric domain structure after chemical etching on the former patterned (a) and non patterned (b) faces of the KTP crystal, and former patterned (c) and non patterned (d) faces of the RKTP crystal.

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Note that the domain broadening under the insulated regions has occurred in KTP resulting in a domain duty-cycle (inverted to non-inverted) close to 49%. On the other hand, the domain grating in RKTP maintains the original duty-cycle of the photolithographic mask over the whole 3 mm thickness. Hence, no significant domain broadening occurred, which can be primarily attributed to the much lower ionic conductivity in RKTP compared to that of undoped KTP. Furthermore, it is worth noting that the homogeneity of the electrical properties of RKTP wafers is substantially higher than that of flux-grown KTP material, resulting in good reproducibility of the poling process over large areas and thus much higher yields of the process.

3.2 Sub-µm PPRKTP

Finally, we explored the potential of RKTP for sub-µm domain engineering. A 1 mm thick crystal was patterned on its c^- face by the deep-UV laser lithographic set-up with a grating period of 690 nm. The photoresist pattern was covered by a 100 nm thick Al-film. The sample was poled by applying a symmetric triangular pulse (maximum field 8 kV/mm, pulse width 2.5 ms). Defects in the photoresist grating prevented uniform electrical contact of the external circuit over the whole crystal surface resulting in a non-uniform grating. Nevertheless, successful periodic poling was achieved in the regions where we did have good electrical contact. Figure 4 shows scanning electron microscope (SEM) images of the etched domain pattern on (a) the patterned face and (b) the non patterned face. It is worth noting that the domain duty-cycle (inverted to non-inverted) on the patterned face is 49%, whereas it varies between 60 and 45% in the non patterned face suggesting that domain broadening is limited even for sub-µm periods in this material.

 

Fig. 4 SEM images of the former patterned (a) and non patterned (b) faces of the RKTP crystal poled with a period of 690 nm.

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4. Conclusions

We have demonstrated fabrication of bulk sub-micrometer ferroelectric domain gratings in KTP. Two techniques have been proven useful for minimizing the lateral ferroelectric domain broadening, namely, the creation of a coercive field grating by K⁺ enrichment and the use of short pulses for electric field poling. We have shown that RKTP is an excellent candidate material for high-aspect ratio domain poling and has superior properties in terms of reduced domain broadening and predominant domain propagation along the polar c-direction. A domain grating with a period of 690 nm has been produced in a 1 mm thick RKTP. This is, to the best of our knowledge the largest domain aspect ratio achieved in a bulk ferroelectric crystal.