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Submerged waveguide constructed by the implantation of 12C ions in electrooptic
crystals.
Har’el Ilan, Alexander Gumennik, Roei Fathei, and Aharon J. Agranat
Department of Applied Physics,
The Benin School of Engineering and Computer Science,
The Hebrew University of Jerusalem, Jerusalem 93707, Israel.
Igal Shachar and Michael Hass
Department of Particle Physics, Faculty of Physics,
Weizmann Institute of Science, Rehovot 76100, ISRAEL.
Abstract:
Waveguide structures were fabricated in Potassium Lithium Tantalate Niobate crystals by
the implantation of high energy 12C ions. The implantation forms an amorphous layer
with a lowered index of refraction within the depth of the crystal, which serves as the
cladding of the waveguides. Two amorphous layers were fabricated at 18.8 m and 25.6
m below the surface of the crystal by implantation at 30MeV and 40 MeV respectively.
This formed a submerged slab waveguide sandwiched between the two amorphous
layers, and two slab waveguides that were formed between the surface of the crystal and
each of the amorphous layers. Coupling between those waveguides was observed and
investigated, and confinement of the light in the sandwiched waveguide was
demonstrated.
It is well established that the implantation of high energy light ions such as H+ and
He++ in electrooptic crystals forms an amorphous layer beneath the surface of the crystal1.
Such amorphous layers characterized by a lowered refractive index (RI), serve as a
cladding for constructing waveguiding structures in the crystal. In particular it was
shown2 that the bombardment of high energy He++ ions into a Potassium Lithium
Tantalate Niobate (KLTN) crystal results with the formation of a slab waveguide beneath
the surface of the crystal, with an amorphous cladding layer that has a lowered RI up to
4%. The amorphous layer that was formed in the KLTN substrate exhibited thermal
stability after annealing. Thus, the formation of lowered RI regions by selective
implantation can be the basis for a fabrication method of reliable electrooptic devices in
waveguided architectures.
In this paper we report the fabrication of slab waveguides using the implantation of
high energy Carbon ions of 12C isotopes into KLTN crystalline substrates. In particular, a
slab waveguide sandwiched between two amorphous cladding layers positioned well
within the depth of the substrate was fabricated by successive implantation at two
energies.
The interaction between the high energy Carbon ion and the medium in which it
propagates is governed by two distinct mechanisms: electronic stopping and nuclear
stopping3. Upon penetrating the material, during the electronic stopping phase, the high
energy Carbon ion propagates through the crystal without diverting from its original
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direction and gradually slows down. At a sufficiently low energy the cross section for
scattering by the lattice ions increases steeply. In this nuclear stopping phase the
propagating ion tears a lattice ion from its site causing it to become an interstitial ion at a
different site. Thus, the nuclear stopping phase generates a well-defined amorphous
layer4,5 of Frenkel defects6 with an RI that is significantly lower than that of the substrate
crystal.
The implantations were preformed in two substrates of KLTN. All of the samples
where bombarded with a beam of high energy Carbon ions generated by a tandem Van de
Graff (VDG) accelerator. During the implantation the samples where kept at constant
temperature of 25oC. The samples compositions and implantation data is summarized in
Table1. The predicted spatial distribution of the RI was obtained by TRIM simulation and
is presented in Fig. 1 for samples S1 and S2. (The simulations were preformed by TRIM
version SRIM 20037.) The reflection spectrum of the TE modes of the samples was
measured using the prism-coupler setup2, 8, 9 operated at   1.55m .
Table 1
Sample
Tc
1st Energy
[Co]
[MeV]
S1
K1(-)T0.68N0.32O3:Li
-9
30
S2
K1(-)T0.65N0.35O3:Li 11.2
30
S3
K1(-)T0.68N0.32O3:Li
-9
40
* Li concentration is estimate at 0.3% per mol.
Substrate
Dosage
[ions/cm2]
0.63  1015
0.47  1015
1.86  1015
2nd Energy
[MeV]
40
30
Dosage
[ions/cm2]
0.91  1015
1.52  1015
Consider first sample S1. In this sample one amorphous layer was formed by the
bombardment of Carbon ions at 30 MeV. The reflection spectrum of the TE modes
propagating in the waveguide that was formed by the implantation is shown in Fig. 2a.
The RI profile perpendicular to the surface was reconstructed from the spectrum by the
method of Chandler and Lama10 and is shown in Fig. 1a. The maximum decrease of the
RI in the amorphous layer that was formed by the implantation relative to the RI in the
KLTN substrate was 3.6%. The amorphous layer was found to be 17.7 m below the
surface of the substrate with a width of 0.6 m , defined as the full width of half
maximum FWHM. Note that the RI profile derived from the modes spectrum is slightly
shifted from the respective profile derived from the TRIM simulation. This is attributed
to the fact that the TRIM simulation uses the density and the composition of the substrate,
but does not take into consideration the specifics of its crystalline structure.
Consider next sample S2. As shown in Table 1 this sample was implanted with ions at
30 MeV and 40 MeV, and therefore has two amorphous layers that were formed at
different depths. This leads to a profile of RI that forms three slab waveguides:
waveguide A immediately beneath the surface of the substrate cladded between the air
and the first amorphous layer, waveguide B submerged in the depth of the substrate
cladded between the two amorphous layers, and waveguide C cladded between the air
and the second amorphous layer. The predicted geometry of the three waveguides as
predicted by the TRIM simulation is presented in Fig. 1b.
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It should be noted that a prism coupler can excite modes that propagate adjacent to the
surface of the crystal. It was a priori expected that the reflection spectrum of the TE
modes shown in Fig. 2b would be the combined spectrum of modes of waveguides A and
C, that are cladded between the surface and the first and the second amorphous layers
respectively. It was also expected that the modes of waveguide A should closely resemble
those of the waveguide that was formed in S1. The latter waveguide is cladded between
the surface of the substrate and the amorphous layer that was formed by ions implanted at
30MeV. It is therefore expected to have the same structure as waveguide A in S2.
By comparing the spectrum of S2 to the spectrum of S1 it was possible to remove the
modes that belong to waveguide A. these modes are indicated by (*) in Fig. 2b. It was
expected a priori that the remaining modes should be those of waveguide C. Furthermore,
it was also expected that the remaining modes should be denser then the modes of
waveguide A because of the latter smaller width. However, the remaining modes (that are
left unmarked in Fig. 2b) are in fact spaced more sparsely than the modes of waveguide
A and therefore belong to a waveguide with a smaller width. The waveguide profile that
was extracted from these modes indicates that these modes belong in fact to waveguide
B, i.e. the waveguide that is buried in the depth of the substrate cladded between the two
amorphous layers. As the prism coupler cannot excite modes that propagate in a buried
waveguide, it is concluded that a coupling occurs between waveguides A and B, enabling
the excitation of modes in waveguide B directly by the prism coupler. The basic mode
(mode 0) of this waveguide is not excited, and modes 1÷7 are excited with an increasing
magnitude due to the fact that the efficiency of the coupling between close adjacent
waveguides increases with the mode number. The fact that the modes of waveguide C
were not observed is due to shadowing of the second (deeper) amorphous layer by the
first (shallower) layer.
The reconstruction of the RI profile in sample S2 is shown in Fig. 1b and compared to
the prediction derived by the TRIM simulation. The first amorphous layer is formed
17.7 m below the surface of the substrate and has a width of 0.6 m (defined as
FWHM) identical to S1, the second amorphous layer was formed 26.1 m below the
surface of the substrate with a width of 0.65 m . The two layers form the cladding for a
buried waveguide with a width of 8.4 m . The peak of the relative decrease of the RI in
the amorphous layers that was formed by the implantation was found to be 2.8% for the
first amorphous layer and 4.8% for the second layer.
In order to substantiate further the occurrence of coupling between waveguides A and
B, sample S3 was prepared by the implantation of Carbon ions at 30 MeV and 40 MeV,
the same energies as S2, but, with a much higher dosage: 1.52  1015 [ions/cm2] and
1.86  1015 [ions/cm2] respectively. The influence of the higher dosage is on the barrier
between those waveguides. Higher dosage of the implantation causes higher decrease of
the RI, and therefore a higher barrier between the waveguides. Indeed the modes
spectrum of S3 (Fig. 2c), was found to be similar to that of S2 with one difference: the
observed modes of the submerged waveguide started at mode 3, whereas in S2 they
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started at mode 1. This clearly indicates that the coupling between waveguide A and B is
governed by the height of the barrier between these waveguides as predicted.
Direct observation of the light-wave as it propagates through the waveguides was
obtained by illuminating the input plane of the waveguides while observing the light as it
emerges from the output plane using a microscope attached to a digital imaging system.
In Fig. 3a a section of the output plane of S2 is shown while the input plane was
illuminated by the white light source of the microscope. The amorphous layers that were
formed by the implantation are clearly seen at 17.7 m and 26.1 m below the surface of
the crystal. In Fig. 3b the same section of the output plane of S2 is shown while light was
coupled directly into waveguide B that was formed between the two amorphous layers.
The light source was a diode laser operating   670nm that was focused by a tapered
lensed fiber into a spot of less than 5 m in diameter. It can be seen that the light
propagating in the sample is confined between the two amorphous layers that function as
the cladding of waveguide B.
In conclusion, carbon ions are found to be effective projectiles for the formation of
cladding layers within the depth of electrooptic crystals, possessing some inherent
advantages over the use of lighter ions such as H+ and He++. In general, the ratio between
the width of the formed amorphous layer and the depth at which it is formed is inversely
proportional to the mass of the implanted ion. For implantation processes in KLTN, it
was found that for carbon implantation this ratio is approximately 3%, whereas for He it
was found to be2 approximately 10%. Furthermore, a projectile ion with a higher mass,
creates during the nuclear stopping phase a large number of Frenkel defects. This enables
the generation of a large change in the RI with a low dosage of implantation. For
example, it was found that the generation of a cladding layer with a relative RI change of
3.6% required the implantation of carbon with a dosage of 0.63·1015 cm2. Generating a
relative change of 4% of the RI using He++ projectile ions2 required implantation with a
dosage of 1016 cm2. Consequently, the use of carbons as the implanted projectile ion
enables the formation of thin cladding layers well within the depth of the substrate using
a moderate dosage, while maintaining high relative change in the index of refraction.
These results clearly indicate that the implantation of high velocity ions, and in
particular carbon ions can be the basis for a generic fabrication method waveguides in a
KLTN crystalline substrate. The implantation at different energies enables the formation
of well defined thin cladding layers well within the depth of the substrate. The RI barrier
height of these layers can be preprogrammed by governing the dosage of the implantation
process. Exploiting the fact that the KLTN has a strong electrooptic effect together with
the possibility of lateral control of the implantation opens the way for the construction of
optoelectronic circuits with multilevel waveguiding structure.
Acknowledgement: This research was supported in part by Grant No. 2002341 from the
United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel.
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References:
1
D. Kip, Appl. Phys. B 67, 131 (1998).
2
A. Gumennik, A. J. Agranat, I. Shachar, and M. Hass, Applied Physics Letters 87
(2005).
3
Emanuele Rimini, Ion Implantation: Basics to Device Fabrication
(Kluwer Academic, Dordrecht, 1995), Chap. 3.
4
J. Y. C. Wong, L. Zhang, G. Kakarantzas, P. D. Townsend, and P. J.
Chandler, J. Appl. Phys. 71, 49 _1992_.
5
T. Pliska, D. H. Jundt, D. Fluck, and P. Gunter J. Appl. Phys. 77, 6114
(1995).
6
G. H. Kinchin and R. S. Pease, Rep. Prog. Phys. 18, 1 (1955).
7
J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping and Range of
Ions in Solids (Pergamon, New York, 1985), Vol. 1.
8
R. Ulrich and R. Torge, Appl. Opt. 12, 2901 (1973).
9
J. Seligson, Appl. Opt. 26, 2609 (1987).
10
P. J. Chandler and F. L. Lama, Opt. Acta 33, 127 (1986).
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Figures
Fig. 1. The RI profile derived from
the reflection spectrum measured by a
prism coupler setup operated at
  1.55m (solid line), and the
matching prediction computed by
trim (dashed line): (a) sample S1 after
carbon ions implantation at 30MeV;
(b) sample S2 after carbon ions
implantations at 30MeV and 40 MeV.
Fig. 2. TE reflection spectrums
at   1.55m of the waveguides formed
by carbon implantation, (a) sample S1;
(b) sample S2 (the modes of waveguide
A are marked by (*)); (c) sample S3.
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Fig. 3. View of the output plane of
sample S2, (a) while the input plane was
illuminated by the white light source of
the microscope; (b) while light
at   670nm coupled directly into
waveguide B.
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