Fabrication of barrier-type slab waveguides in Er3+ - doped tellurite
glass by single- and double energy MeV N+ ion implantation
I. Bányásza, Z. Zolnaib, S. Pellic, S. Berneschid,c, G. Nunzi-Contic, M. Friedb, T. Lohnerb, P. Petrikb,
M. Brencic and G. C. Righinic
a Department of Crystal Physics, Wigner Research Centre for Physics, Hungarian Academy of
Sciences, P.O.B. 49, H-1525, Budapest, Hungary
b Research Institute for Technical Physics and Materials Science, Research Centre for Natural
Sciences, Hungarian Academy of Sciences, Budapest, P.O.B. 49, H-1525 Hungary
cMDF-Lab, “Nello Carrara” Institute of Applied Physics, IFAC-CNR, Via Madonna del Piano 10, 50019
Sesto Fiorentino (FI), Italy
d “Enrico Fermi” Center for Study and Research, Piazza del Viminale 2, 00184 Roma, Italy
Ion implantation proved to be a universal technique for producing waveguides in most optical materials. Tellurite glasses
are good hosts of rare-earth elements for the development of fibre and integrated optical amplifiers and lasers covering
all the main telecommunication bands. Er3+- doped tellurite glasses are good candidates for the fabrication of broadband
amplifiers in wavelength division multiplexing around 1.55 μm, as they exhibit large stimulated cross sections and broad
emission bandwidth. Fabrication of channel waveguides in such a material via N+ ion implantation was reported recently.
Parameters of waveguide fabrication in an Er-doped tungsten-tellurite glass via implantation of N+ ions were optimized.
First single-energy implantation at 3.5 MeV with fluences between 1·1016 and 8·1016 ions/cm2 was applied. Waveguide
operation up to 1.5 µm was observed. Then double-energy implantations at a fixed upper energy of 3.5 MeV and lower
energies between 2.5 and 3.0 MeV were performed to suppress leaky modes by increasing barrier width. Improvement of
waveguide characteristics was found by m-line spectroscopy and spectroscopic ellipsometry.
Keywords: planar optical waveguides, ion implantation, two-energy irradiation, Er-doped tungsten-tellurite glass, m-line
spectroscopy, spectroscopic ellipsometry
Ion implantation or irradiation, compared with other methods for optical waveguide fabrication, has some unique
advantages. It proved to be a universal technique for producing waveguides in most optical materials1. It offers better
controllability and reproducibility than other techniques. The first articles reporting fabrication of waveguides by ion
implantation appeared between the end of 1960’s and early 1980’s2 - 17. The first ion implanted waveguides were
produced in 1968 by proton implantation into fused silica glass2. Index produced by irradiation with H+, He+ and N+ ions
have been characterized by several other groups3-5. A detailed review on ion-implanted optical waveguides has been
The possibility to obtain compact Erbium-doped waveguide amplifiers (EDWA) is an important step in the fabrication of
small multi-functional integrated optical devices with higher performance19. To achieve that goal, the choice of the
appropriate material and fabrication process is important. To guarantee high optical gain for shorter physical length in an
EDWA device, active materials with high solubility of rare-earth ions are needed. Tungsten- tellurite oxide glasses meet
this condition. Moreover, with their broad emission band around 1.55 μm and low phonon energy, they are suitable for
gain-flattened optical amplifiers in wavelength-division multiplexer (WDM) applications20. Fabrication of two
dimensional (2D) guiding structures with very narrow width and long length is a must for such applications. However,
fabrication of 2D guiding structures with standard photolithography and diffusion processes (i.e.: ion exchange) is not
feasible in these materials, because of the chemical damage occurring at the surface of the sample during wet etching
Integrated Optics: Devices, Materials, and Technologies XVI, edited by Jean Emmanuel Broquin,
Gualtiero Nunzi Conti, Proc. of SPIE Vol. 8264, 826406 · © 2012 SPIE
CCC code: 0277-786X/12/$18 · doi: 10.1117/12.910706
Proc. of SPIE Vol. 8264 826406-1
Width of well and barrier vs. lower energy.
Iwo-energy N implanted Er: Te-glass. E2 = 3.5 MeV
0.0 242526272.82.93-031 32333.43536
- - well
steps. Therefore, alternative fabrication processes must be used to induce a local 2D guiding effect in rare-earth doped
tellurite glasses such as, for instance, the UV beam and the femtosecond pulsed laser writing21,22. Recently the possibility
of fabricating multimode channel waveguides in an Er3+-doped tungsten-sodium-tellurite glass (labeled as WNT) by
nitrogen ion implantation has been demonstrated,23. In terms of the application of ion implantation for 2D wave guide
formation we note that the use of photoresist to define the guiding structure is impractical, because the ion irradiated
photoresist layer is difficult to remove from the sample surface.
Our results on the optimization of waveguide formation in an Er-doped tungsten-tellurite oxide glass via irradiation with
3.5 MeV N+ ions were reported recently24. However, the greatest part of the observed guiding modes in those ion-
implanted waveguides proved to be leaky. To overcome that problem, double irradiation at two energies of the N+ ions
was devised and realized.
2. SRIM SIMULATIONS AND FABRICATION OF THE WAVEGUIDES
Composition of the glass we developed for our experiments was 60TeO2-25WO3-15Na2O-0.5Er2O3 (mol. %) and its
fabrication procedure has been reported elsewhere25. The glass substrates were cut to typical sizes of 1 x 1 x 0.2 cm3 and
carefully polished for the optical measurements.
SRIM26 (Stopping and Range of Ions in Matter) simulations were performed to calculate depth-distribution of the N+ ions
implanted successively at two energies in the Er: Tungsten – tellurite glass sample. Results are shown in Fig. 1.
Two-energy N+ distributions were calculated as simple algebraic sum of the corresponding pairs of single-energy N+
distributions obtained by SRIM. Single-energy irradiation at 3.5 MeV energy results in a narrow N+ ion distribution of a
width of about 0.5 μm (dash-dot-dot-dash curve in Fig. 1). The N+ distribution becomes wider when a two-energy
irradiation at 3.5 and 3.25 MeV is applied (dash-dot- dash curve in Fig. 1). Further decrease of the lower energy
produces even wider N+ ion distributions. A double-peaked N+ ion distribution is obtained when the lower energy is set
to 2.5 MeV. Full width at half maximum of the barrier and well layers vs. lower energy of irradiation is shown in Fig. 2.
Higher energy was kept constant at 3.5 MeV. Beginning with the single-energy irradiation (E1 = 3.5 MeV), it can be
seen that decreasing the lower energy results in a wider barrier layers, but the gain in barrier width can be obtained at the
Fig. 1 Depth-distributions of the implanted N+ ions
in Er: Tungsten – Tellurite glass. Higher energy
was 3.5 MeV in each case. Lower energy is
indicated in the inset.
Fig. 2 Width of the well and barrier layers vs. the
lower energy of irradiation.
Proc. of SPIE Vol. 8264 826406-2
Fig. 3 Measured and fitted ellipsometric spectra of
a planar waveguide, irradiated at single energy of
3.5 MeV with a fluence of 4 x 1016 ions/cm2.
expense of a decrease of well width. Barrier width increases from 0.5 μm to about 0.9 μm at E1 = 2.5 MeV, but well
width decreases from the initial 2.4 μm to about 1.9 μm.
Irradiations were carried out with a collimated beam of N+ ions at various energies, at the Van de Graaff accelerator of
the Wigner Research Centre for Physics, Budapest, Hungary with normal incidence on the sample. Lateral homogeneity
of the irradiation was ensured by defocusing the ion beam with a magnetic quadrupole and by scanning the sample under
a 2mm x 2mm beam. Fluence of each irradiation was set to 2.0· 1016 ions/cm2, so that each waveguide was irradiated
with a total fluence of to 4.0· 1016 ions/cm2. Waveguides with three combinations of double-energy irradiation were
fabricated: 3.5 + 3.5 MeV, 3.5 + 3.0 MeV and 3.5 + 2.5 MeV.
3. EXPERIMENTAL STUDY OF THE WAVEGUIDES
3.1 Ellipsometric measurements
The ion-implanted waveguides were measured with a WOOLLAM M-2000DI spectroscopic ellipsometer (λ = 193-1690
nm). A three-layer optical model was applied in the evaluation of the spectroscopic ellipsometry (SE) data. The first
layer, adjacent to the substrate, represents the stopping region. The second layer is the region that the implanted ions
traverse before they stop. The third layer is a surface roughness film taken into account on basis of effective medium
approximation27. Dielectric functions of the first and second layers were described by the Cauchy dispersion relation.
Parameters of the Cauchy dispersion relations and layer thicknesses were considered as free parameters. We applied the
evaluation software WVASE32 created by J.A. Woollam©, Inc28 for the analysis of the spectroellipsometric data. The
evaluation yielded 7.9 ± 0.1 nm for the thickness of the surface roughness layer and 2.019 ± 0.001 for the refractive
index of the non-implanted glass substrate at λ = 635 nm. Measured and fitted ellipsometric spectra of a planar
waveguide, irradiated at single energy of 3.5 MeV with a fluence of 4 x 1016 ions/cm2 are shown in Fig. 3. Fitted spectra
are very close to the measured ones in this case.
Thicknesses of the individual layers obtained after fitting the model to the measured data are shown in Table 1.
It can be seen that thicknesses of the barrier and well
layers obtained from the ellipsometric measurements are
close to those obtained from SRIM simulations.
Dispersion of the refractive indices of the bulk, barrier
and well layers obtained with the above described fitting
procedure are presented in Figs. 4, 5 and 6 for the
irradiation energy combinations: 3.5 + 3.5 MeV, 3.5 + 3.0
MeV and 3.5 + 2.5 MeV, respectively.
In the case of single-energy irradiation (Fig. 4) refractive
indices of both the barrier and well layers are higher than
that of the substrate up to λ = 1.2 µm, then barrier
refractive index gets slightly lower than substrate
refractive index (less than 0.01 at 1.6 µm) while well refractive
index remains still some 0.025 higher than that of the substrate.
Another “cross-over” can be seen in the same figure at λ = 0.8 µm, from where the initially higher barrier refractive
index gets lower than that of the well.
Proc. of SPIE Vol. 8264 826406-3
Fig. 6 Refractive indices of the substrate (open circles),
barrier (closed triangles) and well (closed diamonds)
layers vs. the wavelength, obtained from the
ellipsometric model. E1= 2.5 MeV, E2 = 3.5 MeV.
Table 1 Width of the barrier and well layers for three combinations of the two energies of the irradiating ions obtained from
the ellipsometric measurements. Total fluence was 4·1016 ion/cm2 in all the cases. Compare to Fig. 2.
Name of the
A 3.5 3.5 457 ± 4 2405 ± 5
B 3.0 3.5 503 ± 15 2250 ± 15
C 2.5 3.5 760 ± 5 1990 ± 7
Fig. 5 shows the same curves for the double-energy
irradiated waveguide, with E1 = 3.0 MeV, E2 = 3.5 MeV.
Difference of the barrier and well refractive indices at
short wavelength is lower than for the single-energy
irradiated waveguide. Barrier refractive index is higher
than substrate refractive index up to λ = 1.1 µm. Well
refractive index is higher than substrate refractive index up
to 1.6 µm, where they reach a common value around 2.0.
Results for the waveguide irradiated at the E1 = 2.5 MeV,
E2 = 3.5 MeV combination of ion energies are presented in
Fig. 6. Differences between the refractive indices of the
three layers tend decrease with increasing wavelength.
Barrier refractive index is higher than substrate refractive
index up to λ = 1.0 µm, then it gets slightly lower.
It must be emphasized that the fitted dispersion curves
depend on the model used for the fitting procedure. So
they are approximate ones. One has to keep in mind that
real waveguide profile is not a stack of homogeneous layers; it has a smoothly varying refractive index profile. So the
Fig. 4 Refractive indices of the substrate (open
circles), barrier (closed triangles) and well (closed
diamonds) layers vs. the wavelength, obtained from
the ellipsometric model. E1= E2 = 3.5 MeV
Fig. 5 Refractive indices of the substrate (open
circles), barrier (closed triangles) and well (closed
diamonds) layers vs. the wavelength, obtained from
the ellipsometric model. E1= 3.0 MeV, E2 = 3.5 MeV.
Proc. of SPIE Vol. 8264 826406-4
above results can serve only as rough estimations of the guiding properties of the ion beam irradiated waveguides. More
reliable results can be obtained by m-line spectroscopy.
3.2 M-line spectroscopy
In order to characterize the effect of N+ ion implantation in the tungsten-tellurite glasses, we used a semi-automatic
instrument developed at IFAC (“COMPASSO”), based on dark-line spectroscopy. The resolution of the instrument is
around 1·10-4 and 5·10-4 on the effective refractive index and bulk refractive index, respectively. In this case, due to the
lower contrast in the measurement, the resolution was lower, about 5·10-4 and 1·10-3 respectively Due to the high
refractive index of the bulk (around 2.0 at 635 nm), we used a rutile prism to couple the light in the irradiated regions.
First we measured the bulk refractive index in a non-irradiated region of the sample (i.e.: the face of the sample, opposite
to the irradiated one). The results are reported in Fig. 7 a) and b) for the wavelengths of 635 nm and 1550 nm. In the first
case the refractive index of the substrate (nbulk) was 2.040 (± 1·10 –3) while, in the second case, the nbulk value was 1.988
(± 1·10 –3).
Fig. 8 M-line spectra of the planar waveguide irradiated with single energy N+ ions at 3.5 MeV taken at 635 nm (a)
and 1550 nm (b).
Fig. 7 M-line spectra of the non irradiated Er: Tungsten – Tellurite glass taken at 635 nm (a) and 1550 nm (b).
Proc. of SPIE Vol. 8264 826406-5
Fig. 10 M-line spectra of the planar waveguide of the planar waveguide irradiated with double energy N+ ions at
3.5 MeV and 2.5 MeV, taken at 635 nm (a) and 1550 nm (b).
Fig. 9 M-line spectra of the planar waveguide irradiated with double energy N+ ions at 3.5 MeV and 3.0 MeV,
taken at 635 nm (a) and 1550 nm (b).
Then we performed m-line spectroscopy of the three irradiated waveguides at the following wavelengths: 635, 980, 1330
and 1550 nm. We performed three different measurements in different points of each ion implanted planar waveguide.
M-line spectra of the planar waveguide irradiated with single energy N+ ions at 3.5 MeV taken at 635 and 1550 nm are
presented in Fig. 8. One can see at least four modes at 635 nm and one at 1550 nm. Indices of all the observed modes are
below the corresponding bulk refractive index; this is a clear indication of leaky modes.
Double-energy N+ ion irradiation was applied to improve light confinement in the waveguides by making the barrier
layer thicker. M-line spectra of the planar waveguide irradiated with double energy N+ ions at 3.5 MeV and 3.0 MeV,
taken at 635 and 1550 nm are presented in Fig. 9.
No significant differences between the two sets of m-line spectra can be seen. Double-energy irradiated planar
waveguides also work at 1550 nm. However, only slight shifts of the modes could be detected.
Proc. of SPIE Vol. 8264 826406-6
Table 2 Effective refractive indices of the three waveguides measured at λ = 635 nm, 980 nm and 1550 nm.
M-line spectra of the planar waveguide irradiated with double energy N+ ions at 3.5 MeV and 2.5 MeV, taken at 635 and
1550 nm are shown Fig. 10. More modes can be seen at 635 nm, and they seem to be sharper that in the case of the 3.5
MeV + 3.0 MeV energy combination. On the other hand, the mode around 1.5 detected at 1550 nm is not as well defined
as in the previous case. Measured effective refractive indices of the three waveguides are summarized in Table 2.
General trend in the effective refractive index values is a slight decrease with the increasing energy difference of the
Planar waveguides were fabricated in an Er-doped tungsten tellurite glass sample by implantation of single- and double-
energy nitrogen ions at the combinations of 3.5 MeV, 3.5 + 3.0 MeV and 3.5 + 2.5 MeV. Fluences of the irradiations
were the same, 4·1016 ions/cm2. Spectroscopic ellipsometric and m-line spectroscopic measurements of the ion beam
irradiated waveguides revealed that double-energy N+ ion irradiation also resulted in fully operative waveguides up to λ
= 1550 nm. Using a simple ellipsometric model, fitting of the experimental data confirmed that SRIM calculations
predicted correctly the waveguide structure. On the other hand, no significant improvement with respect to the single
energy irradiated waveguides could be found. Only slight decrease of the effective refractive indices with increasing
difference of the irradiation energies was detected. Based on previous experience with the method and the material, it is
hoped that thermal annealing of the ion beam irradiated planar waveguides up moderate temperatures (around 260 C°)
could substantially re-structure waveguide and improve light confinement.
Support from the Hungarian National Research Fund (OTKA-NKTH and OTKA) projects K 68688 and K 101223 as
well as from the bilateral Italian-Hungarian 2010-2012 CNR/MTA project is gratefully acknowledged.
The authors are grateful to Profs. Marco Bettinelli and Adolfo Speghini (Università di Verona, Italy) for providing the
tungsten-tellurite glass samples and to Mr. Roberto Calzolai (IFAC optical shop) for polishing the samples.
Effective refractive index at
635 nm (average value)
Effective refractive index at
980 nm (average value)
Effective refractive index at
1550 nm (average value)
, 0 = 2.0207 (± 5 · 10 –4) Nef
, 0 = 1.9777 (
5 · 10 –4) Nef
, 0 = 1.9571 (± 5 · 10 –4)
, 1 = 2.0097 (± 5 · 10 –4) Nef
, 1 = 1.9543 (
5 · 10 –4)
, 2 = 1.9925 (± 5 · 10 –4) Nef
, 2 = 1.9142 (
5 · 10 –4)
, 3 = 1.9668 (± 5 · 10 –4)
3.5 + 3.0 Nef
, 0 = 2.01818 (± 5 · 10 –4) Nef
, 0 = 1.9750 (
5 · 10 –4) Nef
, 0 = 1.9535 (± 5 · 10 –4)
, 1 = 2.00700 (± 5 · 10 –4) Nef
, 1 = 1.9504 (
5 · 10 –4)
, 2 = 1.98914 (± 5 · 10 –4)
3.5 + 2.5 Nef
, 0 = 2.01728(± 5 · 10 –4) Nef
, 0 = 1.97391 (
5 · 10 –4) Nef
, 0 = 1.95140 (± 5 · 10 –4)
, 1 = 2.00636 (± 5 · 10 –4) Nef
, 1 = 1.94792 (
5 · 10 –4)
, 2 = 1.99104 (± 5 · 10 –4)
, 3 = 1.96745 (± 5 · 10 –4)
Proc. of SPIE Vol. 8264 826406-7
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