Two-dimensional domain engineering in LiNbO_3 via a hybrid patterning technique

Two-dimensional domain engineering in LiNbO3

via a hybrid patterning technique

Michele Manzo,* Fredrik Laurell, Valdas Pasiskevicius, and Katia Gallo

Department of Applied Physics, Royal Institute of Technology (KTH), Stockholm, Sweden

*[email protected]

Abstract: We propose a novel electric field poling technique for the

fabrication of nonlinear photonic crystals in congruent LiNbO3 substrates,

based on a hybrid bi-dimensional mask, which combines periodic proton-

exchange and electrode patterns. With it we demonstrate rectangular bulk

lattices with a periodicity of 8 µm x 6.78 µm in 500 µm-thick substrates.

©2011 Optical Society of America

OCIS codes: (190.4400) Nonlinear optics, materials; (220.4000) Microstructure fabrication;

(160.2260) Ferroelectrics.

References and links

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#145259 - $15.00 USD Received 1 Apr 2011; revised 31 May 2011; accepted 3 Jun 2011; published 7 Jun 2011(C) 2011 OSA 1 July 2011 / Vol. 1, No. 3 / OPTICAL MATERIALS EXPRESS 365

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1. Introduction

The reversible polarization of ferroelectric materials is at the heart of their widespread use in

electronics [1] and photonics [2], for devices ranging from random access memories and high

density storage media [3], to nonlinear optical frequency converters. The field of nonlinear

optics has particularly benefited over the past years from the development of reliable

technologies to engineer ferroelectric gratings by electric field poling techniques [4],

providing effective means to implement the idea of Quasi-Phase-Matching (QPM), originally

proposed by Armstrong et al. in 1962 [5], in optical materials such as LiNbO3 [6], LiTaO3 [7]

and KTP [8].

In more recent years, the extension of electric field poling techniques to two-dimensional

lattices [9] has enabled the demonstration of purely nonlinear photonic crystals (NPC) [10]

and quasi-crystals [11]. Furthermore, the new degrees of freedom affordable through domain

engineering in 2D have led to variety of novel nonlinear optical devices, such as multiple-

beam frequency converters [12], tunable soliton switches [13] and Airy beam generators [14].

Most of such devices have been implemented in periodically poled LiNbO3 (PPLN) and

LiTaO3 (PPLT). PPLN is particularly appealing for its high nonlinear coefficients, the proven

scalability of its poling process to wafer sizes and the maturity of the waveguide technology

developed for congruent substrates, already exploited for integrated NPCs [15]. Yet several

challenges still remain to be faced in the fabrication of advanced NPC structures [16],

involving stringent control over domain sizes and complex two-dimensional (2D) topologies.

The conventional approach to fabricating NPCs consists in a direct generalization of the

standard 1D electric field poling (EFP) technique based on photoresist (insulator) patterning

[6], as illustrated in Fig. 1. The domain topology in the x-y plane of a z-cut CLN substrate is

defined by patterning in 1D (Fig. 1a) or 2D (Fig. 1b) a photoresist layer on one of the z-faces.

An electric field (Ez) exceeding the coercive value (Ec~21kV/mm) is then selectively applied

between electrical contacts made in the openings of the photoresist on one face of the crystal

and an uniform electrode on the other. The inhomogeneous (x-y) field distribution generated

by the patterned electrodes close to the CLN surface induces the polarization switching in the

areas where Ez(x,y) > Ec. In Figs. 1c and 1d, we illustrate the electrostatic distribution of

Ez(x,y) for the case of 1D and 2D electrodes, respectively, calculated for typical photoresist on

CLN.

In analogy to the 1D case, the main technological difficulties encountered for domain

engineering in 2D concern avoiding domain merging at short-periods. In standard EFP

configurations, part of the problem arises from the fringing fields at the edges of the

photoresist [17], apparent in the field plots of Figs. 1c and 1d. In order to overcome such

limitations, novel EFP techniques employing controlled domain back-switching [18] or

substrate chemical patterning [19,20] have recently been devised for short-period (<10μm)

poling of 0.5mm-thick CLN substrates.


Fig. 1. Conventional electric field poling of z-cut CLN crystals with photoresist insulator

patterns. Insulating mask geometries for the: (a) 1D and (b) 2D case. Calculated in-plane (x-y)

distributions of the polar component (Ez) of the electrostatic field close to the patterned surface (z = 500nm) for: (c) 1D and (d) 2D patterns with a period Λ = 10 μm. Simulations done

with a commercial solver of the Poisson equation (Comsol Multiphysics@), for an external field

of 21 kV/mm applied to 0.5 mm-thick CLN (εLN = 34), with a 1.8 μm-thick photoresist layer (εpr = 3).

Additional constraints affecting the poling in 2D geometries (Fig. 1b) stem from the

crystal symmetry, which naturally favors hexagonal lattice topologies, making it significantly

more complicated to fabricate e.g., rectangular lattices with comparable periods in the two

orthogonal crystal directions (x-y). Specifically, due to the faster growth of CLN domains

along the y crystallographic axis with respect to the x-axis [6], it proves more challenging to

reduce the poling periods in the former than in the latter direction. The finest-pitch 2D bulk

domain structures in CLN to date have been demonstrated by Peng et al. [21]. With a

chemical patterning technique, they obtained rectangular domain arrays with periodicities of

6.6 µm and 13.6 µm along the x and y directions, respectively (implying a period along y

which is still twice the one along x).

Here we present a novel technique suitable for the fabrication of 2D bulk PPLN structures,

which relies on a hybrid 2D poling mask, obtained as the combination of 1D periodic

chemical patterning of the substrate (via proton-exchange) and 1D periodic electrodes

deposited on its surface (gel contacts through photoresist openings, as in Fig. 1a). With this

technique we successfully fabricated 2D ferroelectric rectangular lattices with periodicities of

8 x 6.78 µm2 (along x and y, respectively) in 0.5 mm-thick CLN, representing, to the best of

our knowledge, the densest 2D PPLN bulk structures achieved to date.

2. The hybrid mask

We performed our electric-field poling experiments on commercially available, 0.5 mm-thick

z-cut congruent LiNbO3 substrates (Castech Inc.). The poling masks consisted of rectangular

2D lattices, with periods of Λx = 8 μm and Λy = 6.78 μm along the x and y crystallographic

directions, as depicted in Fig. 2. The rectangular 2D mask patterns were a hybrid combination

of two orthogonal 1D gratings, made by periodic PE and periodic surface electrodes,

respectively. As illustrated in Fig. 2a, the PE grating lines were aligned with the y axis, while

the electrodes were parallel to x.


Λy Ps

Insulator

LiNbO3d

zy

x

Λx

’’

y

x

Elementary cell

LiNbO3

Insulatormask6.78µm

8µm

PE

a) c)

b)0

x (m)

Ez (a.u.)

010

10

55y (m

)

Λy Ps

Insulator

LiNbO3d

zy

x

Λx

’’

Λy Ps

Insulator

LiNbO3d

zy

x

zy

x

Λx

’’

y

x

Elementary cell

LiNbO3


8µm

PE

y

x

Elementary cell

LiNbO3


8µm

PE

a) c)

b)0

x (m)

Ez (a.u.)

010

10

55y (m

)

Fig. 2. EFP of CLN with a 2D hybrid mask. (a) Sketch of the mask geometry in 3D (blue stripes = PE regions, red stripes = photoresist). (b) top view of the mask, highlighting its

elementary cell. (c) calculated in-plane (x-y) distributions of the polar component (Ez) of the electrostatic field at a depth z = 2.3 μm beneath the patterned surface. Electrostatic simulations

under the same conditions as for Fig. 1 Eext = 21 kV/mm, CLN (εLN = 34) and insulator (εpr = 3)

thicknesses of 500 μm and 1.8 μm, respectively.

The hybrid 2D mask was fabricated in two steps. First, we selectively proton-exchanged

the substrates through the openings of periodic ~100nm-thick Titanium stripes (patterned by

standard photolithography and reactive ion etching). The 1D Ti gratings had a periodicity Λx

= 8 μm and a duty cycle (stripe width over grating period) of 70%. A uniform thin layer of Ti

was additionally evaporated on the opposite (unpatterned) side of the crystals to prevent PE.

The samples were then exchanged for 24 hours at 200 °C in pure benzoic acid. This resulted

in PE surface gratings extending to a (measured) depth dPE~2.3 μm, with a duty cycle of 50%

(exceeding the 20% Ti-mask openings) due to the lateral diffusion of protons along x,

underneath the Ti stripes [22]. After PE, the Ti mask layers were removed by wet-etching,

leaving a surface chemical pattern in the crystals as illustrated in Fig. 2a (blue stripes = PE

regions).

The second patterning step consisted in depositing periodical electrodes on the substrate,

orthogonally to the chemical grating. This was done by patterning 1.8 µm-thick photoresist

(insulating) stripes with a period of Λy = 6.78 μm, a duty cycle of 50% (at the top) and a

trapezoidal (~80° wall slope) cross-section (red stripes in Fig. 2a). As in conventional poling

(Fig. 1), the openings of the photoresist were then filled with an electrolyte to achieve a

periodic electrical contact at the sample surface.

In the unitary cell of the final 2D hybrid pattern, four different areas can be identified,

corresponding to: bare CLN (grey), PE-CLN (blue), photoresist-covered CLN (red) and

photoresist-covered PE-CLN (violet) regions, respectively, as highlighted in Fig. 2b.

In Fig. 2c we also plot electrostatic calculations of the spatial distribution in the x-y plane

of the polar field (Ez) in the crystal. The latter results from the superposition of the internal

fields associated to the periodic proton-exchange [19] and of the external field applied via the

patterned electrodes. It is worth noticing how the field patterning due to PE mitigates the edge

effects of the external electrodes in comparison to the case of Fig. 1d, yielding a smoother 2D

field profile in the crystal. This significantly limits lateral domain broadening during the

poling, as previously demonstrated for the 1D case [19].

In order to evaluate also the effect of the substrate polarity on the poling, the hybrid

patterning of Fig. 2 was fabricated on multiple samples, either on the + z or on the –z face.


3. The poling experiments

Samples patterned with the 2D hybrid mask described above were poled with a standard EFP

technique, using gel electrodes to contact the crystals. We employed high voltage pulses of

the type of ref [6], with poling plateaux of durations Δτpol = 50 ÷ 100 ms, applied fields Eext

~22 kV/mm and voltage ramp-down times greater than 100 ms.

After the poling, the samples were etched in a solution of 40% hydrofluoric acid and water

for 60 min. The differential etching rates of domains of opposite polarity and of PE versus

non-PE regions [23], allowed the simultaneous visualisation of the chemical mask and of the

final domain distributions.

Substantially different results were obtained for patterning on –z and + z, as illustrated by

Fig. 3 and Fig. 5, respectively. In what follows, regardless of the original substrate polarity,

we will simply refer to the patterned face of the crystals as the „top‟ side and to the

unpatterned face as the „bottom‟ side.

When the masks were made on z, the double patterning due to periodic PE in one

direction and periodic contacts through the photoresist lines in the other resulted in 2D bulk

domain lattices with a rectangular topology, as shown in Fig. 3, a result which intuitively

agrees with the expected field distributions based on the simple electrostatic model discussed

in the previous section (Fig. 2c). Figures 3a and 3b provides more detailed views of the

structures observed after the etching on the top and bottom faces of the samples, respectively.

For a comparison, in Fig. 3 we have also sketched the original mask geometry, highlighting in

blue the (vertical) PE regions and in red the (horizontal) photoresist stripes.

20µm

y

x20µm

x

y

8µm PE

y

6.78µm

Resist

mask

Mask on -z a) b)

20µm

y

x20µm

x

y

8µm PE

yyy

6.78µm

Resist

mask

Mask on -z a) b)

Fig. 3. Results of EFP with a hybrid mask on -z. Top views of the patterns, revealed after the

poling by a wet-etch in an HF:H2O solution: (a) top (patterned) surface, originally -z and (b)

bottom (upatterned) surface, originally + z.

The PE regions can also be clearly recognised on the top face, as the darker areas in Fig.

3a. On the other hand, as discussed in [19], the actual ferroelectric domain patterns are best

identified by the images taken on the bottom face, where no PE layer is present. From Fig. 3b

it is apparent how the hybrid mask on –z results in regular 2D domain arrays, which, even on

the backside, follow well the periodicities of the hybrid mask created on the top. A

comparison between Figs. 3a and 3b illustrates also how the individual domain shapes evolve

from rectangles on the top surface to hexagons after propagation through the bulk, well

reflecting the symmetry of the CLN crystal. Additional sub-micrometric structures,

preferentially aligned along the crystallographic y axis, can also be distinguished within the

switched hexagons on the bottom face (Fig. 3b). We are currently further investigating their

nature. Due to their resemblance with structures reported elsewhere [24], we suspect these

features to be surface nanodomains, possibly originating from back-switching preferentially

occurring at the + y corners of the poled hexagons.

The dimension of the 2D poled array is ~4 mm x 1 mm. The hexagons on the bottom face

are ~4 µm x 4.6 µm (along x and y), corresponding to aspect ratios (domain width / depth) of

125 and 250, respectively. The inverted area related to each hexagonal domain is ~13.84 µm2.

Along y (where short pitch poling with a photoresist mask is normally extremely challenging,

because of the higher domain propagation speed) the inverted domains lie ~2 µm apart


without merging. To the best of our knowledge, this represents the shortest period achieved

along y in 2D bulk PPLN.

Second harmonic generation (SHG) measurements, made on these samples at higher order

QPM with a tuneable continuous-wave Ti:sapphire laser source, confirmed the microscopic

investigations. The 2D lattice results in multiple in-plane SHG resonances, as illustrated by

the SHG image of Fig. 4a, showing a picture of the blue output from the PPLN in the far field,

recorded at λp = 820.97 nm. The three blue spots in Fig. 4a correspond to SH beams emerging

at angles of ± 3.46° and 0°, generated by QPM via the reciprocal lattice vectors G1, ± 1 and G01

(collinear) of the 2D lattice, respectively. The spectral and angular positions of the SHG

resonances agree well with theoretical predictions based on Sellmeier equations for LiNbO3

[25]. In Fig. 4b we show also the calculated SHG tuning curve (magenta line) of an ideal

(4mm-long) grating for 5th order QPM via G01 and compare it with the corresponding

experimental data (blue dots), measured on the central lobe of Fig. 4a at temperature of

178°C.

Fundamental wavelength (nm)

0

0.2

0.4

0.6

0.8

1

SH

G n

orm

alis

ed p

ow

er (

a.u

.)

820.5 821 821.5

Exper FWHM = 2nm

Theor FWHM = 1.4nm

a)

b)

Fundamental wavelength (nm)

0

0.2

0.4

0.6

0.8

1

SH

G n

orm

alis

ed p

ow

er (

a.u

.)

820.5 821 821.5

Exper FWHM = 2nm

Theor FWHM = 1.4nm

a)

b)

Fig. 4. Optical characterization of the 2D PPLN sample by means of SHG. a) SH beams

emerging at ± 3.46° and 0° ; b) the ideal tuning curve (magenta line) calculated for the central SHG peak (5th order QPM with G01) in a 4mm-long grating and the measured ones: blue dots =

SHG tuning curve in the middle of the sample – black stars = SHG close to the patterned

surface

The experimental full-width at half maximum is Δλ = 2 nm, somewhat larger than the

theoretical one (Δλ = 1.4 nm), but this could also be attributed to the limited resolution we

could achieve in tuning the pump wavelength. The SHG measurements indicated a good

quality of the 2D PPLN pattern throughout the crystal thickness, with the only exception of a

shallow layer close to the patterned face, where we recorded a ~53% reduction of the peak

conversion efficiency (cf dark curve with black markers in Fig. 4b), presumably due to

scattering effects and surface perturbations of the domain pattern induced by the periodic PE,

similarly to what seen in 1D PE: PPLN [19].

Substantially different results were obtained in the poling experiments performed, under

the same conditions, on samples patterned with the 2D hybrid mask on + z, as illustrated by

the images in Fig. 5 (next page). Etching of the bottom faces (z, Fig. 5b), revealed regular

1D PPLN domain structures, which followed the chemical but not the electrode patterns.

Optical SHG measurements confirmed that also in this case the structures seen on the bottom

face corresponded to the actual bulk domain distribution, extending through the sample

thickness. The results obtained for patterning on + z indicate that the sample polarity plays

also a major role in determining the balance between the actions of the PE and of the external

electrodes used in the poling.


y

x

20µm

x

y

8µm PE

y

6.78µm

Resist

mask

Mask on +z a) b)

20µm

y

x

20µm

x

y

8µm PE

yyy

6.78µm

Resist

mask

Mask on +z a) b)

20µm

Fig. 5. Results of EFP with a hybrid mask on + z. Top views of the patterns, revealed after the

poling by a wet-etch in an HF:H2O solution: (a) top (patterned) surface, originally + z and (b)

bottom (upatterned) surface, originally -z.

It is this balance/imbalance which ultimately determines whether the final bulk domain

distribution will result in a 2D or 1D pattern. In light of previous investigations on the effect

of PE on the coercive field of CLN [26], the poling selectivity associated to PE should be

stronger on + z than on -z and this would be consistent with our experimental observations.

Furthermore, our studies on 1D poling with photoresist masks of the same type (Fig. 1a) and

gel electrodes (as the ones used in the 2D experiments) gave better results for patterning on -z.

In light of this, it might see reasonable that for the case on Fig. 5 (mask on + z), the enhanced

action of the chemical patterning proves to be predominant over that of the photoresist,

yielding a complete switching of the non-PE regions in the bulk, regardless of whether

covered or not with insulator. A closer investigations of the domain patterns on the top

surfaces of our samples (Fig. 5a) reveals also the presence of fine structures between the PE

stripes (dark horizontal lines in Fig. 5a), which most likely are residual unswitched surface

nano-domains located under the resist.

4. Conclusions

We have presented a novel poling technique suitable for the fabrication of 2D NPCs in CLN,

combining two 1D masks, made by combination of periodic proton exchange and photoresist

patterning. With those we demonstrated rectangular domain lattices with periodicities of

8 µm x 6.78 µm in 0.5 mm-thick z-cut CLN substrates, which represent to the best of our

knowledge the densest 2D NPCs made in bulk CLN.

The proposed hybrid poling technology overcomes some of the constraints imposed by the

LN crystallographic structure in standard electric field poling and can in principle allow even

shorter-period bulk domain patterning, currently under investigation. Furthermore, the

experimental results provide a proof-of-principle for enhanced possibilities in tailoring the 3D

distributions of the electric field at the sample surfaces, by suitably weighting the

contributions arising from the chemical patterning inside the crystal with those created

externally by non conventional in-plane electrode geometries. The additional degrees of

freedom associated to the independent engineering of the internal and external poling fields

holds promise for enabling higher-resolution sophisticated 2D domain engineering suitable for

the implementation of a variety of novel nonlinear photonic crystals and quasi-crystals.

Further improvements of this technology would involve a 2D PE mask. Numerical

simulations of the field distributions suggest that this configuration might be the most

promising to attain even denser patterning in congruent lithium niobate by further weakening

2D later domain broadening.

Acknowledgments

This work was supported by the Swedish Scientific Research Council (Vetenskapsrådet,

VR 621-2008-3601) and the Linné centre for Advanced Optics and Photonics (ADOPT).

Katia Gallo gratefully acknowledges support from the EU and Vetenskapsrådet through Marie

Curie (PIEF-GA-2009-234798) and Rådforskare (622-2010-526) fellowships.


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Two-dimensional domain engineering in LiNbO_3 via a hybrid patterning technique

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