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.
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.
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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.
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II. EXPERIMENTAL DETAILS
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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
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On sabbatical leave from Benemerita Universidad Autonoma de Puebla, Mexico.
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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
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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.
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ACKNOWLEDGEMENT
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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.
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Fig. 2. Refractive index of TiO2 films deposited at ambient temperature and 350 C.
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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 presence of an anatase polycrystalline phase. Ongoing investigations into the morphology of each film will provide further details as to their structural differences and the influence of crystallite formation on their optical properties.
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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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