# Domain-Reversed Lithium Niobate Single-Crystal Fibers are Potentially for Efficient Terahertz Wave Generation

Authors: Yalin Lu; Kitt Reinhardt
Journal: Advances in OptoElectronics
 (2008)

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## Abstract

Nonlinear frequency conversion remains one of the dominant approaches to efficiently generate THz waves. Significant material absorption in the THz range is the main factor impeding the progress towards this direction. In this research, a new multicladding nonlinear fiber design was proposed to solve this problem, and as the major experimental effort, periodic domain structure was introduced into lithium niobate single-crystal fibers by electrical poling. The introduced periodic domain structures were nondestructively revealed using a crossly polarized optical microscope and a confocal scanning optical microscope for quality assurance.


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## Body

## 1. Introduction

The  terahertz (THz)  frequency range
(0.1∼30THz) lies in the gap between microwave and infrared of the
electromagnetic spectrum. THz technology lags behind both microwave and
infrared technologies, mainly because of the limitations in both THz generation
and detection. Development of new THz sources has been recently receiving
considerable interest in many applications such as security inspection,
spectroscopy, medical imaging, and sensing. The application requirements for
such THz sources are versatile, and it will be reasonable to classify them according
to the THz sources’ compactness, frequency tunability, emission linewidth, coherence,
and output power.The most common approach used to
generate THz waves is to rectify a femotosecond (fs) laser pulse using an electro-optic (EO) crystal. Efficient
ultrabroad band, single-cycle THz wave generation has been realized in a few
crystals such as ZnTe or GaP at a wavelength around 800 nm, under the condition
of matching both optical and THz pulses’ group velocities [1]. Tradeoff for this approach is that the majority of such EO crystals
have strong material dispersion, which limits the output wave’s bandwidth and
power. On the other extreme, continuous-wave (cw) THz generation has been realized in free-electron lasers and
quantum cascade lasers. The former offers high output power, but they are bulky
and inflexible. The latter, however, provides the potential for good system compactness,
high efficiency, and suitable frequency tunability, but with very limited
output power availability and short wavelength coverage.Using periodically poled nonlinear
optical crystals for efficient THz generation is becoming an alternative
approach. Conventional techniques such as difference frequency generation (DFG),
which uses two laser sources (either nanosecond-pulsed or cw), are attractive in inducing coherent THz waves with a suitable frequency
tunability. Phase matching among the three interactive waves (two optical and
one THz) can be realized by artificially introducing reversed domain structures
(so-called quasiphase matching (QPM) method if periodically or
quasiperiodically poled [2]). Unfortunately, such techniques’ generation
efficiency is low due to the strong absorption of THz waves in those commonly used
nonlinear crystals such as LiNbO3(LN), KTiOPO4(KTP) [3].Optical rectification of fs laser pulses using artificially poled
nonlinear optical crystals is used to generate multicycle or arbitrary wave
forms [4]. When an fs optical
pulse propagates through a poled lithium niobate (PLN) crystal with its second-order
nonlinear susceptibility (χ(2)) reversing its sign between
neighboring domains, a THz nonlinear polarization is generated via DFG or optical
rectification. Due to the group velocity mismatch between optical and THz waves,
the optical pulse will lead the THz by the optical pulse duration τp after a walkoff length lw=cτp/(nTHz−noptical).
If the domain length of the poled nonlinear crystal is comparable to the
walk-off length, each domain in the crystal contributes a half cycle to the
radiated THz field. Similar to the above DFG approach, in this case, high
material absorption to THz waves will be still the major reason for the poor generation
efficiency. Apparently, either a significant improvement on the material’s
transparency over those THz wavelengths or a new device design able to
significantly minimize the THz absorption issue will be pressingly in demand in
order to bring such devices to the more practical side of the potential THz applications.In this article, a new device design relying
on the multicladding nonlinear fiber format (MCNF) will be discussed for potential
efficient THz generation application. This design has the potential to solve
the nonlinear material’s absorption issue over those generated THz waves, and
it maintains the high conversion efficiency that a strongly confined optical
fiber may provide.  To realize such new multicladding
fiber designs, efficient fabrication of the poled nonlinear optical fibers will
be the first and major step, and this will be discussed with details in Section3 after
introducing the device design. In Section 4, those reversed domain structures are nondestructively revealed by both
crossly polarized optical microscope (CPOM) and confocal scanning optical
microscope (CSOM).


## 2. The Multicore Nonlinear Fiber Design

Figure1
shows the schematic of the multicladding nonlinear fiber. The main core inside
the design can be a domain-inverted lithium niobate single-crystal fiber (1), while the first (2), the second
(3), and the third (4) claddings can be made from the polyamide matrix materials
with their refractive indices changed by certain dopants. The refractive index
requirements for such designs are as follows: for the optical wave, nO1>nO2,nO3,andnO4; and for the THz wave, nT3>nT2 and nT3>nT4. In this case, the pumping optical wave
will be confined inside the main core (1), and the generated THz wave will be
coupled into and propagates in cladding 3. Geometrically, cladding 2 should be
thin, which should allow the generated THz wave losslessly side-emitted into
cladding 3. In the THz frequency range, LN crystal has a refractive index
around 5.5, and that of those polyamide materials is normally around 2.1. For
the two materials in optical frequencies, their refractive indices are around
2.2 and 1.4, respectively. Apparently, the use of such MCNF design has the
potential to fully eliminate the nonlinear optical material absorption issue
discussed before by separating the generated THz beam from the optical beam
into a different path of propagation. Those cladding materials to be selected,
such as the polyamide materials discussed here, should be highly transparent for
a wide THz frequency range.Schematic of the multicladding nonlinear fiber
design. (a): the cross-section view, and (b): the axial index profile.
(a)(b)The energy
and the momentum conversion laws for generating the THz frequencies via the
DFG, for example, can be described as(1)Eω1=Eω2+EωTHz,K→ω1=K→ω2+K→ωTHz+K→Λ, where E is the
photon energy, K is the wave vector
at each frequency, and KΛ is the grating’s reciprocal vector. The generated THz frequency can be then
determined by (2)ωTHz=cΛ⋅1|nO−nTHz⋅sinΦ|, where Λis the  domain-poled period, nO and nTHz are refractive indices for optical and THz waves,
respectively, and Φis the internal direction of the generated THz
emission, which will be a key parameter when designing the device for realizing
efficient coupling of the generated THz wave from the main core into cladding
3, as shown in Figure 2 before.Figure 2
Schematic of the domain-reversed nonlinear core, the generated THz
wave coupling, and both forward and backward propagation THz waves.Figure3
shows the calculated dependence of forward and backward THz frequencies on the
poled domain periods (Λ) when
using lithium niobate single crystals. Listed Kp,Ks,
and KTHz are wave vectors
for the pump (ω1), the signal (ω2), and the generated THz wave, respectively. Aside
each curve, schematic of the corresponding wave-vector diagram is also shown.
Inset inside Figure 3
shows a relationship of THz frequency versus the internal
emission angle (Φ) at
a fixed poling period around 50 μm using the LiNbO3 crystal. Combining with the general waveguide theories of optical fibers,
simulation results obtained here can be further used to design the device
including both material selection and the dimensional determination of claddings
and the main core. However, this is a separate research effort that will be
published in somewhere else. In the following sections, we mainly report the
fabrication of a periodically poled LiNbO3 single-crystal fiber,
which is the key to further make the MCNF device.Figure 3
The generated THz frequency versus the domain
period inLiNbO3 crystal for both forward and backward propagation
schemes. Kp,Ks,KTHz,KΛ, are wave vectors for the pump (ω1), the signal (ω2), the generated THz wave, and the reciprocal vector
of the grating, respectively. Inset
shows the relationship between the generated THz frequency and the internal
emission angle (this angle is to be used for further coupling the THz wave out
from the main core).


## 3. Poling the LN Single-Crystal Fiber

Thea-axis-oriented LN single-crystal fibers having
cross-section dimensions ranging from 100 μm to 130 μm and lengths from 10 mm to 50 mm were
grown using the LHPG method [5]. Normally, an a-axis LN single-crystal fiber has an elliptical cross-section with
two ridges as shown in Figure 4(a). Its c-axis
orientation is determined along the short axis of the ellipse and the b-axis along its long axis. This natural
configuration makes it convenient to use contacting electrodes to electrically
pole the fiber, simply for the reason that the applied electric field is
required to be aligned parallel to the c-axis
(the polarization direction). During the poling, the fiber is placed on top of
a copper block and a gold grating is slightly pressed onto the fiber. The gold
grating, which has the predetermined structures, was made on a Φ2" sapphire wafer using a standard
photolithographic  process (Figure 4(b)). Listed d1 and d2 inside the inset are gold electrode widths, which are changeable by designing, in order to adjust
the poled domain period duty cycle. The copper block and gold grating are
connected to each polarity of a high-voltage power supply. LN crystals have a
coercive field approximately 22 kV/mm.
For an LN fiber with a dimension of 120 μm along the c-axis, the applied voltage should be
close to 4 kV. In our experiments,
special measures such as filling with Teflon coatings were taken to prevent the
breakdown of air, simply because of the short electrode space and high voltage
(this is different from puling bulk crystal wafers). Most as-grown
single-crystal fibers present a single-domain structure. However, it is
difficult to discriminate between the +c or −c surfaces
without using destructive methods such as wet etching. In our case, simply
switching the applied electrical field’s polarity and observing the sudden
current change during poling can find the right polarization arrangement.(a) The poling setup used for polinga-axis-grown lithium niobate single-crystal
fiber (inset: the fiber’s elliptic cross-section image). (b) One design of the
top interdigital electrode. d1 and d2 inside the inset are
gold electrodes’ width, which are changeable by design in order to adjust the poled domain period
duty cycle.
(a)(b)Periodic domain structure inside the
LN fiber was then revealed using both a crossly polarized optical microscope
(CPOM) and a confocal scanning optical microscope (CSOM) [6]. The main
reason to use the two methods is because of their nature of being nondestructive to
samples under the measurement. When using the CPOM, the poled LN fiber was
simply put between a crossly polarized light. The method can disclose periodic
domains in a relatively larger view field. Because of this, in this research the
CPOM was mainly used to evaluate both completeness and uniformity of the
periodic domains along the length of the fiber, characteristics that can be regarded
as the poling quality at macroscale level. The CSOM, on the other hand,
provides a means for measuring the electro-optic response to a small ac electrical field modulation. With the
use of lock-in amplification, the sensitivity of this method can be greatly
enhanced, allowing one to obtain high-contrast ferroelectric images using a relatively
small ac field. The linear
relationship between the electro-optic coefficient and the ferroelectric
polarization allows this technique to work over a broad range of field
amplitudes, frequencies, and orientations of electric field, ferroelectric
axis, and light polarization. The CSOM method was used because of its
capability to nondestructively study the LN single-crystal fibers and to
disclose the domain structure details including domain boundary or period duty
circle inside the fibers, characteristics that can be regarded as the poling
quality at the microscale level.In the CSOM, a 632.0 nm HeNe laser is used as a light source.
The beam then passes a linear polarizer and half-wave plate which provides a
capability to select the beam’s polarization direction. The light is focused to
a diffraction-limited spot using a high NA objective. The fiber was mounted on
a three-dimensional piezoelectric scanner. In the measurement, the LN fiber was
tightly clamped by two silver-coated copper plates as electrodes. The soft silver
coating was used for reaching better electric contact. The LN fiber was aligned
with itsb-axis normal to the plates.
The ac voltage was applied onto the
two electrodes variable from 0–100 V,
and, therefore, the ac field can vary
between 0∼7kV/cm.Figure5
shows an image of the periodically poled ferroelectric domains inside the LN
single-crystal fiber under a crossly polarized optical microscope. The
periodicity of the structure was measured to be roughly 20 μm, which is very
close to the designed value for further generating 2.5 THz wavelength. A typically
scanned image of linearly polarized light reflected from the LN fiber’s surface
is shown in Figure 6(a). This is actually an optical image when observed using
a conventional optical microscope. The stripes and dark dots are due to surface
defects and possible surface contaminations. Using this operational mode,
ferroelectric domains or boundaries are invisible. In Figure 5(b), a
simultaneously acquired image of the ac field-modulated signal is presented. The dark and bright areas represent
ferroelectric domains having +c and −c polarizations. The observed domain boundaries are thin and sharp, and are below
the resolution of the CSOM. The image confirms the periodicity of ∼20μm and a period
duty circle of about 1:1. Uniformity  of
the period duty cycle across the fiber length was examined to be quite good.Figure 5
The periodic domains inside a 17 mm long LN single-crystal fiber revealed
by a crossly polarized optical microscope.The confocal scanning optical microscope images under bothdc-mode (a) and ac-mode (b). Uniformity of the period duty cycle across the fiber
length is good.
(a)(b)


## 4. Conclusion

A new multicladding nonlinear fiber
design was proposed to generate THz waves with the potential of reaching high
efficiency. Inside the design, the main core of the fiber will be a
domain-reversed nonlinear crystal such as periodically poled LN crystal and the
multiple claddings are from those THz transparent materials such as polyamides.
This design has the potential to reduce the common material absorption issue
over the THz frequency and keeps
the high efficiency when using the fiber format. As one of the major
experimental efforts, periodic ferroelectric domain structures were
successfully introduced into lithium niobate single-crystal fibers. The poling
completeness and domain uniformity were examined by using a crossly polarized optical microscope. The
periodic domains’ boundary and period duty circle were examined by a confocal scanning optical
microscope.



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*Source: 208458-2008-10-30.xml*