fabrication of submicrometer quasi-phase-matched devices in ktp and rktp [invited]

Fabrication of submicrometer quasi-phase-

matched devices in KTP and RKTP [Invited]

Andrius Zukauskas, Gustav Strömqvist, Valdas Pasiskevicius, Fredrik Laurell,

Michael Fokine, and Carlota Canalias*

Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 10691, Stockholm, Sweden

*[email protected]

Abstract: We review the techniques used for fabrication of bulk sub-

micrometer ferroelectric domain gratings in KTiOPO4 (KTP) and

demonstrate that bulk Rb-doped KTiOPO4 (RKTP) is an excellent candidate

for implementation of dense domain gratings. Compared to KTP, RKTP

presents predominant domain propagation along the polar c-direction,

substantially reduced lateral domain broadening, and higher poling yield. As

a result we obtain homogeneous sub-µm periodic poling of RKTP with a

period of 690 nm in 1 mm thick samples.

©2011 Optical Society of America

OCIS codes: (160.2260) Ferroelectrics; (190.4410) Nonlinear optics, parametric processes.

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#154805 - $15.00 USD Received 19 Sep 2011; revised 18 Oct 2011; accepted 18 Oct 2011; published 21 Oct 2011(C) 2011 OSA 1 November 2011 / Vol. 1, No. 7 / OPTICAL MATERIALS EXPRESS 1319

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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.

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.


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.

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.

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.

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.

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.

Acknowledgments

This work has been possible thanks to the generous support of the Linneus Centre ADOPT,

the Swedish Research Council (VR), the Swedish Foundation for Strategic Research and the

Göran Gustafsson Foundation.


fabrication of submicrometer quasi-phase-matched devices in ktp and rktp [invited]

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