Characterization of Charged Domain Walls in Ferroelectric

Lithium Niobate by Near-Field Second-Harmonic

Generation Microscopy

Introduction

Lithium niobate (LiNbO3 or LNB, Fig.1) crystals are characterized, between others, by

ferroelectric polarization, non-linear optical polarizability and negative uniaxial birefringence,

characteristics that make this bulk material an excellent candidate for domain wall observations.

In this paper we experiment on congruent LNO, doped with 5% Magnesium, using the z-cut

crystals propriety of having the ferroelectric polarization always perpendicular to the surface.

A domain wall (Fig.2), also described as topological soliton, is a phenomenon generated by the

change of polarization in a localized symmetric area. When the walls are tilted they are charged

and they are known as charged domain walls (CDW). The positive ions are a bit more shifted

against the negative ions, and that gives remnant electric polarization. The domain walls also

produce the strongest second-harmonic light (SHG), which can give high resolution

measurements due to the fact that it is a non-linear process. Although, when the z-cut crystals are

illuminated vertically, the signal is rather weak.

Fig.1 Structure of

LiNbO3 crystal

Fig. 2 Charged Domain Walls a) as seen through a microscope, b) given by SHG c) given by TPPL

a) b) c)

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The aim of the experiments was to measure and characterize the size and appearance of the

domain walls using the data offered by second-harmonic generation (SHG) and two-photon

photoluminescence (TPPL).

Experimental setup

The measurements were carried out with the aid of a short pulse (fs) laser giving 100fs pulses at

790nm wavelength, focused on nanoparticles, with an average power of 170mW and a repetition

rate of 75MHz. The scheme showed in Fig.3 represents the arrangement of the system with the

beam coming from the fs laser arriving at the oil objective and unto the sample where the

particles are scanned through focus using the piezo-scanner. The detection is realized in

backscattering geometry, the beam being redirected through the splitter S1 towards the filter F1

which blocks the infrared light above 750nm wavelength by 8 orders of magnitude. Passing

through splitter S2, the waves between 420 and 640nm are caught by the less sensitive

spectrometer (luminescence light) with an integration time of 25ms, while the rest of the waves

are filtered again by filter F2 transmitting only within the range of 390 400nm wavelength (second harmonic generation

light). The SHG waves are

focused and detected by the

highly sensitive avalanche

photo diode (ADP).

For observation we used a 0.5

mm thick substrate of LNB

doped with magnesium at a

concentration of 5%. Second-

harmonic generation (SHG)

(Fig.2b) and two-photon-

photoluminescence (TPPL)

(Fig.2c) techniques were

applied for comparison reasons.

Experiments

One interesting characteristic of ferroelectric domains is, as previously mentioned, its

polarization. We wanted to determine if we can get any notable dependency of the signal given

by the domain walls on the direction and/or the rotation of the electric polarization of the

incident light. Having the electric field of the exciting beam linearly polarized and parallel to the

surface, we inspected the horizontally and vertically polarized components given by the SHG

signal. Fig.4 and Fig.5 show the 90 rotation around the optical axis realized with the /2 plate, in 10 steps, where 0 means we are parallel to the surface.

There seem to be no apparent dependence on polarization on any of the domain walls. For both,

the horizontal and the vertical polarization measurements the intensity seems to remain constant.

Fig. 3 Experimental setup

a) b)

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Fig.4 Horizontally polarized light given by the SHG signal of the CDW.

Fig.5 Vertically polarized light given by the SHG signal of the CDW.

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Investigating further, we took single line scans of two parallel walls of a domain. Going from

10m to 100m focal depth in steps of 10m we observed the variations in SHG signal. Based

on previous experiments (ref) little changes in the wall conductivity are to be expected at

different depths, but what really stands out is the apparent shrinkage of the distance between the

two walls of the domain. It seems that the domain walls in 5% Mg doped LNO z-cut crystals are

not completely parallel to the ferroelectric polarization, instead they are tilted by an angle so

the distance dependency is given by eq.1 below.

where d0 is the distance between the walls (m) measured with SHG, z is the focal depth (m)

and =0.08 as seen in Fig.6

This relation leads to a decrease in distance

between the two parallel walls dependent on the

focal depth. Fig.7 is a graphic representation of

this distance as a function of depth measured by

SHG and adjusted according to the equation

above. The total error was estimated to be around 0.3%. As it can be seen in the graph, some of

the values are more off than expected, but it could be due to the poor quality of the domain walls,

as shown in the magnification of the measurement at 50um depth.

Eq.1

Fig.7 Graphic representation of the distance between walls as a function of depth, where

parameter A represents d0, parameter B represents a = 2*tan(), and X is the depth. The

magnification shows the distance between the two walls at 50um as measured with SHG

The chosen scan line of the two parallel walls for the measurements can be seen in Fig.8 for both

SHG and TPPL.

Silver decorated CDWs

Decorating the domain walls with silver particles, we aimed to observe the near fields generated

by them. We plunged a sample containing domain walls in a solution of HAuCl4 and water and

we illuminated it with a Hg lamp for a several minutes. The silver decorated domain walls

obtained were inspected by dark field microscopy (fig.9).

With an untilted beam, we tried reaching the

surface of our sample, as close as 1m (Fig.10).

The decorated LNO sample was attached upside-

down on a cover glass at only few m distance,

space filled with a thick sheet of air, and then put

over the oil objective. During the measurements,

the mode-locking broke down, due to the

reflection of the beam directly into the objective,

but whenever the mode-locking was working we

were able to acquire stronger SHG signal from

the area around our sample than on the actual

sample. Comparing to the literature, this effect

was not unusual.

Fig.8 Scan line chosen to measure a) SHG and b) TPPL for the distance between two parallel walls

a) b)

0 54000 48000 53000

Counts/0.025s Counts/0.025s

Fig.9 Dark field image of Ag decorated domain

walls

Shifting parallel the laser beam that enters the objective, our aim was to gather more SHG signal

as field components parallel to ferroelectric polarization appear. This is due to the fact that the

incident beam is not perpendicular to the ferroelectric polarization when the incident angle is

tilted.

Counts/0.025s Counts/0.025s

Counts/0.025s

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34000 5300 3500 0 Counts/0.025s

Fig.10 5%Mg-LNO Ag decorated domain walls observed with a) SHG and b) TPPL at different distances from the surface

Fig.11 5%Mg-LNO Ag decorated domain walls observed with a) SHG and b) TPPL for 0mm, 1mm and 2mm shift

25um 10um 5um 1um

25um 10um 5um 1um

0mm 1mm 2mm

0mm 1mm

2mm

We shifted the beam in steps of 0.5mm with 1mm shift corresponding to 20 and 2mm to 45

(Fig.11), however we could not observe an increase in SHG signal as expected. We can see in

Table 1 that the power of our beam is lost as we shift further. This was probably due to the

deterioration of the focus as the beam passes the objective in a different direction from the

optical axis. We also observed that closer to the surface, at 2mm shift, the domain walls appear

as dotted chains rather than compact walls. This could happen because of the silver particles, but

the quality of the scan is not the best either.

Table 1. The tendency of the laser beams power to decrease as the beam is shifted

Displacement

(mm)

0 0,5 1 1,5 2

Number of

photons

127 796 136 610 32 362 7 490 1 897

Conclusions