characterization of charged domain walls in ferroelectric lithium niobate by near-field...
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Lab rotation for nanophysicsTRANSCRIPT
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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
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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
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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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0 50000 150000 0
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
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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