Download Low-Loss TiO2 Planar Waveguides for Nanophotonic

Low-loss TiO2 planar waveguides for
nanophotonic applications
J. D. B. Bradley, C. C. Evans, F. Parsy, K. C. Phillips, R. Senaratne, E. Marti, and E. Mazur
School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, MA, 02138
I. INTRODUCTION
TiO2 thin films are widely used in photocatalysis, optical
coatings, and solar cell applications [1-3]. We show that in
addition to those applications, TiO2’s high refractive index
(~2–2.7) and transparency at visible and near-infrared
wavelengths make it a promising medium for integrated
optical devices. The higher refractive index, in particular,
allows for a greater index contrast with surrounding media,
which leads to tighter light confinement and nanoscale
light-guiding structures. Furthermore, the exceptionally
high nonlinearity of TiO2 [4], coupled with strong
confinement of short pulses, could lead to nonlinear
nanophotonic devices, such as supercontinuum sources or
ultrafast all-optical switches [5,6].
Bulk TiO2 can exist in a variety of stable crystalline
phases, including anatase, brookite, and rutile. The
presence or absence of the various crystalline phases in
thin films, which strongly depends on the deposition
method and conditions, can lead to complex film
morphology [7]. This, in turn, can lead to a large variation
in optical properties, including losses. To realize
nanophotonic devices, optical waveguides with low
propagation losses are required.
In this contribution we present TiO2 planar waveguides
with low losses as a first step towards achieving TiO2based nanophotonic devices. We also discuss the
crystalline structure and optical properties of the films.
index and thickness of the TiO2 films using Raman
spectroscopy
and
variable
angle
spectroscopic
ellipsometry, respectively. We measure the propagation
losses using a variable angle prism coupling setup. An 826nm laser source is coupled via the prism to excite the
fundamental mode. Following excitation, the scattered
light is collected by an optical fiber normal to the sample
surface and the intensity at the fiber output is measured by
a photodiode. We then scan the fiber along the propagation
direction to measure the change in scattered light intensity.
III. RESULTS
Figure 1 shows Raman spectra (λexcitation = 633 nm)
measured for films deposited at ambient temperature (20
ºC) and 350 ºC. Both spectra show peaks centered around
300 cm–1 and 520 cm-1 attributed to the Si substrate. The
film deposited at 350 ºC shows an additional peak at 144
cm–1, consistent with the anatase phase of TiO2 [8]. This
peak is observed in films deposited at 350 ºC and above.
The films deposited at ambient temperature show no
discernable peaks associated with the known crystalline
phases of TiO2. Therefore, we expect those layers to be
primarily amorphous.
intensity (a. u.)
Abstract- We deposit TiO2 planar waveguides on oxidized
silicon substrates by reactive sputtering. The films exhibit
Raman spectra consistent with an amorphous or anatase
phase and have losses as low as 0.4 dB/cm at 826 nm.
141 cm–1
II. EXPERIMENTAL DETAILS
350 °C
We deposit TiO2 films by reactive radio frequency (RF)
sputtering using a metallic Ti target in an Ar and O2
atmosphere. We deposit the layers on 0.5-mm-thick silicon
substrates with a 2.2-µm-thick thermal oxide layer. For the
films presented here, the applied RF sputtering power and
chamber pressure were fixed while the substrate
temperature during deposition was varied.
We investigate the crystalline structure and refractive
20 °C
1
On sabbatical leave from Benemerita Universidad Autonoma de Puebla,
Mexico.
978-1-4244-5369-6/10/$26.00 ©2010 IEEE
100
300
500
700
Raman shift (cm–1)
Fig. 1. Raman spectra of TiO2 films deposited at substrate
temperatures of 20 C and 350 C.
In Fig. 2 we compare the refractive index of 160-nmand 80-nm-thick TiO2 films deposited at ambient
temperature and at 350 ºC, respectively. We applied a
313
refractive index
Cauchy fit to the ellipsometric data, taken over the
wavelength range 600–1100 nm. The refractive index of
the ambient film varies from 2.44 to 2.35. The film
deposited at 350 ºC has a higher refractive index, which is
consistent with single-crystal anatase and varies from 2.54
to 2.40.
IV. CONCLUSIONS
We have deposited TiO2 planar waveguides with
propagation losses as low as 0.4 dB/cm at 826 nm. Low
losses and high refractive index confirm that these TiO2
films are useful for integrated nanophotonic devices.
3.0
ACKNOWLEDGEMENT
2.8
This work was performed in part at the Center for
Nanoscale Systems (CNS) at Harvard University. The
author’s acknowledge the support of the National Science
Foundation under contract ECCS-0901469 and the
National Science and Engineering Center under contract
PHY-0646094.
2.6
350 °C
2.4
20 °C
2.2
REFERENCES
2.0
600
[1]
700
800
900
1000
1100
[2]
w avelength (nm)
[3]
Fig. 2. Refractive index of TiO2 films deposited at ambient
temperature and 350 C.
[4]
In Fig. 3, we show the measured scattered intensity
versus propagation distance of the fundamental transverseelectric (TE) polarized mode in the same two films. By
fitting an exponential decay to each curve, we determined
loss coefficients of 0.4 dB/cm and 3.5 dB/cm for the films
deposited at ambient temperature and 350 ºC, respectively.
We attribute the higher losses in the second film to the
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[6]
[7]
[8]
TiO2 waveguide
intensity (a. u.)
Guided light
20 °C
350 °C
0
0.0
0.5
1.0
1.5
2.0
propagation distance (cm)
Fig. 3. Propagation loss measurements at 826 nm and TE
polarization for TiO2 films deposited at ambient temperature and
350 C. The dashed lines correspond to the fitted exponential loss
values: 0.4 dB/cm and 3.5 dB/cm, respectively. Inset: photograph
of 633 nm light propagation in a TiO2 film deposited at ambient
temperature.
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