# Formation ofMgO:LiNbO3 Domain-Inverted Gratings by Voltage Application under UV Light Irradiation at Room Temperature

Authors: Masatoshi Fujimura; Toshiaki Suhara
Journal: Advances in OptoElectronics
 (2008)

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## Abstract

MgO:LiNbO3  is an attractive nonlinear-optic crystal for quasiphasematched (QPM) nonlinear-optic devices. This paper reports a new formation method of domain-inverted gratings for QPM in  MgO:LiNbO3. Domain inversion of  MgO:LiNbO3 by voltage application under UV light was characterized, and reduction of the voltage required for inversion was demonstrated. Results of voltage application under periodic UV light suggested that suppression of excess lateral expansion of the domain inverted regions on −Z surface was crucial for domain-inverted grating formation. Voltage application to a crystal with a photoconductive cladding layer under periodic UV light was proposed. The cladding layer suppressed the expansion, and the domain-inverted gratings with period of 18 μm and area of 25×5
mm2 were obtained. The formation method does not require the photolithography process and allows the formation by voltage application at room temperature, and therefore, is quite simple and productive.


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## Body

## 1. Introduction

There
has been a great
interest in quasiphasematched (QPM)
nonlinear-optic
devices
for applications in variety of fields including optical memory, optical
communication, display, and quantum optics, because they can provide efficient
wavelength conversion through the largest nonlinear-optic tensor element at any
wavelength in the transparency range [1]. Ferroelectric-domain-inverted grating
in nonlinear optic crystals is the mostly used structure to accomplish QPM. Formation
technique of the grating by voltage application through periodic electrodes has
been established for LiNbO3 crystal [2–4], and a lot of QPM devices
have been demonstrated in LiNbO3 [1]. However, applications of the
devices may be limited due to the photorefractive damage caused by intense
short wavelength optical waves. The devices are often kept at high temperature during
the experiments to avoid the damage.MgO-dopedLiNbO3 has large nonlinear-optic coefficients comparable to those of
the nondoped LiNbO3 and higher resistance to the photorefractive
damage [5], and therefore, it is an attractive nonlinear-optic crystal for QPM
devices. Domain-inversion characteristics of MgO:LiNbO3 are quite
different from those of the nondoped LiNbO3 [6–8]. Formation of
domain-inverted gratings in MgO:LiNbO3 has been studied extensively,
and several methods have been reported [9–13]. These methods require
photolithography process for fabrication of periodic electrodes and also
special treatments for voltage application, such as corona discharge, crystal
heating, insulation layer cladding, and/or vacuum environment. Simpler and more
productive methods are preferable for practical application.We
recently found reduction of voltage required for MgO:LiNbO3 domain
inversion by ultraviolet (UV) light irradiation and showed possibility of grating
formation at room temperature by a simple method without photolithography
process [14]. The reduction was confirmed also by another research group [15].In
this paper, we report the characteristics of the domain inversion under UV in
detail, and demonstrate formation of domain-inverted gratings in MgO:LiNbO3.


## 2. Reduction of Inversion Voltage by UV Irradiation

We characterized domain inversion of Z-cut
MgO(5mol%):LiNbO3 by voltage application under UV light irradiation
using a setup shown in
Figure 1. On the +Z face of a MgO:LiNbO3 crystal of 0.5 mm thickness a
NESA glass plate was settled, and
LiCl aqueous solution filled the gap between the plate and the crystal. The
solution served as a liquid electrode. The −Z face was
grounded through a liquid electrode and a series capacitor for monitoring the
transferred charge. The area of the electrodes was about 4 mm2. An
ultra-high-pressure mercury lamp was used to irradiate the crystal through the NESA.
The intensity of the light was ~0.1 W/cm2 on the NESA. Voltage of a
single cycle symmetric triangular waveform was applied to the crystal at room
temperature. The amplitude was ~3 kV and the period was 10 seconds. Recording
the transferred charge during the voltage application, we obtained a hysteresis
loop as shown in Figure 2(a), which implies domain inversion. Voltage
application was stopped at point A in
Figure 2, and the crystal was etched in
HF:HNO3 mixture to visualize the domain structure. Inversion of
domain was confirmed. For application of the identical voltage without the UV irradiation,
the charge transfer was much smaller, as shown in
Figure 2(b), and no domain
inversion took place. The results indicated that UV light irradiation reduces
the voltage required for domain inversion.Figure 1
Setup for characterization of domain inversion
under UV irradiation.Figure 2
Hysteresis loop of MgO:LiNbO3 crystal at room temperature (a) under UV irradiation and (b) without UV
irradiation.By
application of a voltage of−2 kV for 4 seconds to the −Z surface with
irradiating the UV light of ~0.1 W/cm2 intensity from −Z side, domain-inverted
region was obtained only on −Z  surface. By
application of a voltage of +2 kV for 4 seconds to +Z surface with irradiating
from +Z side, inverted region was not obtained on +Z surface but on −Z surface. This result
suggests that domain inversion nucleates only on  −Z  face under the
UV irradiation. It is quite different from the other methods, where inversion
nucleates on +Z face.The
light from the high-pressure mercury lamp consists of many emission lines, and
they include the emissions at 289, 313, 334, and 365 nm wavelengths. Each
wavelength component was extracted by a wavelength filter, and used for the
domain inversion experiments to clarify the wavelength components effective to
the reduction of the inversion voltage. The inversion took place for UV irradiation
at 289 or 313 nm wavelength, which is in the absorption wavelength range of MgO:LiNbO3 crystal. The mechanism of the UV-assisted domain inversion is not clear yet. It
may be related to local field due to migration of free electrons caused by UV
light absorption.


## 3. Domain Inversion by Voltage Application under Periodic UV Light

We considered that voltage application
under UV light with spatially periodic intensity modulation would result in
formation of MgO:LiNbO3 domain-inverted grating. We made an apparatus for the voltage application
at room temperature under periodic UV light. On the −Z surface of a
MgO:LiNbO3, a Cr photomask with grating pattern is settled. A UV light from an ultra-high-pressure mercury lamp goes
through the photomask, and periodic intensity distribution of UV light is
formed in the crystal. LiCl aqueous solution fills the
gaps on both surfaces of the crystal. A negative high voltage is applied to the −Z  surface at room
temperature through the uniform liquid electrode. Domain inversion takes place in
the UV irradiated region, while no inversion takes place in the nonirradiated
region. Therefore domain-inverted gratings are formed.Voltages
were applied under UV light of 0.1 W/cm2 from −Z  side, and the
resultant domain structures were examined after etching. The crystal thickness
was 0.5 mm, and the period of the Cr pattern was 15.4 μm. The applied voltages ranged
from −0.2 to −4 kV, and the
durations ranged from 10 to 90 seconds. Typical waveforms of the applied
voltage and the accumulated inversion charge are shown in
Figure 4 for
electrode area of 50 mm2. The charge increased almost
proportionally.  Typical result is shown
in Figure 5. In a part (central part of
Figures 5(a) and
5(b)),
grating was obtained on  −Z  face, but inverted region was
not obtained on +Z face yet. In the surrounding part, inverted regions had
expanded laterally and merged one another on −Z face. Only in
such part, inverted regions were obtained on +Z. Nucleation and evolution
of domain-inverted regions on the −Z  surfaces and on
the cross sections in the vicinity of the −Z surfaces were
examined and are shown in Figure 6. At first domain-inverted regions nucleated
at the −Z surface
(Figures
6(a) and
6(b)).  The
initially-domain-inverted regions expanded for higher and longer voltage
application, and a domain-inverted grating was formed on the −Z surface
(Figure
6(c)). The inverted region was limited in the vicinity of −Z surface. The
depth of the inverted region was a few microns (Figure 6(d)). The regions
laterally expanded at the −Z surface and
formed a very thin domain-inverted layer (Figures 6(e) and
6(f)). The thickness
of the layer was ~0.5 μm. Besides, next the
initially-inverted regions started expansion toward +Z surface to form immediately
a domain-inverted grating with inverted regions continuing in the whole crystal
thickness (Figures 6(g) and
6(h)). Further voltage application resulted in a
uniformly inverted structure without grating. The domain inversion took place
in the same way under UV irradiation of higher optical intensity up to 2 W/cm2.
Suppression of excess lateral expansion of domain-inverted region on −Z surface is
crucial for forming domain-inverted gratings applicable to QPM devices.


## 4. Formation of Domain-Inverted Gratings

We modified the method to suppress
the excess lateral expansion. Voltage pulse was applied to a crystal with
photoconductive cladding layer on−Z  surface under
UV. The application setup was similar to Figure 3 except that a photoconductive
layer was deposited on the −Z surface. In the UV-irradiated region, the
photoconductive layer serves as a (semi-)conductor, allows charge transfer
associated with domain inversion, and therefore, does not prevent domain
inversion. On the other hand, in the nonirradiated region, it serves as an
insulator and prevents inversion. Therefore, excess lateral expansion of the
inverted region can be suppressed, and the grating structure can be maintained
on −Z surface until
the inverted regions reach +Z surface.Figure 3
Setup for voltage application under spatially
periodic UV light at room temperature.Typical waveforms of the applied voltage and
the accumulated inversion charge.
(a)(b)Figure 5
Typical domain structure formed by voltage
application under periodic UV irradiation.Figure 6
Nucleation and evolution of domain-inverted
regions (a), (c), (e), (g) on−Z surfaces and (b), (d), (f), (h)
in the cross sections. Inverted regions evolved as ((a), (b))→((c), (d)) →((e), (f)) →((g), (h)).
The period of the grating pattern was 15.4 μm.We noticed that a commercially
available photoresist (Clariant,
AZ-P4620) can be used as a
photoconductive material for this purpose. The resistivity of the film was
measured. It was initially∼6×1015(Ωcm), and dropped down to ∼1×1012(Ωcm) by irradiation of UV light with intensity of 1 W/cm2.The
photoresist was spin-coated on−Z surface of
MgO:LiNbO3 crystal of 0.5 mm thickness, and  voltage pulses were applied under various
conditions. The thickness of the photoconductive layer, intensity of the UV
light, duration of the voltage application, and the voltage were chosen within
ranges of 7–10 μm, 0.05–0.5 W/cm2,
50–1000 seconds, and
3.3–3.9 kV,
respectively. The resultant domain structure was examined after etching. When
the thickness was too thin and/or the intensity was too high, suppression of the
lateral expansion was not sufficient. When the thickness was too thick and/or
the intensity was too low, the width of the inverted region was too narrow.
When the duration was too short and/or the voltage was too low, the inverted
region did not reach to +Z face.  When
the duration was too long and/or the voltage was too high, damages took place
in the crystal. A
domain-inverted grating fabricated for the photoconductive layer thickness of
10 μm, the UV intensity of 0.07 W/cm2, the duration of
900 seconds, and the voltage of 3.5 kV, are shown in Figure 7.  The grating period was 15.4 μm and the area was 10×5mm2. Grating structure was
obtained on both −Z face
(Figure 7(a))
and +Z face
(Figure 7(b)) in the same part. The crystal was cut by a dicing saw
and etched again to examine the domain structure on the cross section. The
inverted regions continued through the whole crystal thickness
(Figure 7(c)).
However, there are some points to be improved further. They include that the width of
the grating line on −Z was somewhat
wide, and that the grating lines on +Z were not continuous lines but segmented
lines.Figure 7
Domain-inverted grating formed by voltage
application to crystals with photoconductive cladding under UV irradiation.Fabrication
of domain-inverted gratings in larger area was carried out. A grating with
period of 18μm and area of 25×5mm2 was obtained. In a simple bulk SHG experiment using the crystal, a QPM SH wave
was obtained for a pump wavelength at 1549.5 nm. It agrees well with theoretical
prediction based on a reported Sellmeier equation [1]. The SHG performance will
be characterized in detail and reported elsewhere.


## 5. Conclusion

Domain inversion of MgO:LiNbO3 by voltage application under UV light was characterized, and reduction of the
inversion voltage was shown. Voltage application under UV light with spatial
periodic intensity modulation suggested that it was crucial to suppress excess
lateral expansion on −Z surface for
domain-inverted grating formation. Application of photoconductive cladding
layer on the crystal was proposed and successfully suppressed the expansion.
Domain-inverted gratings with period of 18 μm and area of 25×5mm2 were obtained. The formation method does not require the photo-lithography
process and allows voltage application at room temperature; and therefore it is
quite simple and productive. Experimental work is being continued for higher-quality
grating formation and for application of the gratings to QPM nonlinear-optic
devices.



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*Source: 421054-2008-10-30.xml*