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Optoelectronics & Communications

A new degree of freedom for silicon integrated optics

Refractive index engineering by subwavelength grating waveguides adds versatility to photonic device component design.
17 April 2012, SPIE Newsroom. DOI: 10.1117/2.1201203.004145

Following a decade of rapid development, integrated silicon photonic circuits now appear poised to have a major impact on optical interconnects and data transport applications, as well as being used for biosensing and spectroscopy. Among other advantages, silicon photonic chips can be manufactured cost effectively using existing CMOS fabrication infrastructure.

Planar optical waveguides are the basic building blocks of integrated photonic circuits, which combine multiple photonic functions on a chip. A conventional waveguide consists of a core and cladding. The core has a higher refractive index, confining light by total internal reflection. The difference in refractive indices between the core and the cladding is a crucial parameter in waveguide design because it determines the core cross sections that can be used for single-mode operation, the mode profiles, and the minimum possible waveguide curve radius.

Traditionally, doped silica waveguides with an index difference ∼10−2 have been used for integrated optics. The low index contrast necessitates waveguide cross-sectional dimensions of several microns and minimum bend radii in the millimeter range. However, a high-refractive-index contrast of ∼2 between silicon and silica allows for tight waveguide bends with radii of the order of only a few microns. As a result, silicon photonic circuits can be made much more compact than their silica counterparts.

In the design of silicon waveguide devices the refractive index of the waveguide core is usually assumed to be fixed, being the material refractive index of silicon. We have recently demonstrated the use of the subwavelength grating (SWG) effect to effectively tune the index of a waveguide core simply by lithographic patterning, thus introducing a new degree of freedom in silicon waveguide design. The use of SWGs with a periodicity less than half the wavelength of light suppresses any diffraction effects. Such SWGs, with spatially averaged refractive index, are already well established in free-space optics.1

A schematic example of refractive index engineering using the SWG effect for a so-called silicon photonic wire waveguide, i.e., a submicron silicon strip waveguide, is shown in Figure 1(a). Here, periodic gaps are etched into a standard silicon photonic wire. An SWG waveguide is formed with an effective core index determined by the SWG duty ratio, which can be defined as the volume ratio of silicon material inside the composite waveguide core. We calculated the dispersion relation of the segmented waveguide and compared it with the dispersion of an equivalent photonic wire waveguide with identical cross-section and a core index of n=2:65—see Figure 1(b)—to confirm theoretically the concept of spatial refractive index averaging: see Figure 1(c). Experimentally, we have observed wave guiding in SWGs with a propagation loss as low as 2.1dB/cm.2

Figure 1. Schematic of a silicon (Si) subwavelength (SWG) waveguide (a). Equivalent wire waveguide with spatially averaged core index, n (b). Calculated dispersion diagrams for the structures in (a) and (b) compared (c). Scanning electron micrograph (SEM) of SWG waveguide crossings (d). SEM of low-loss SWG fiber-chip coupling structure (e). SU-8 polymer: A commonly used photoresist. λ: Wavelength.

We have demonstrated several applications of SWG waveguides, such as highly efficient waveguide crossings—see Figure 1(d)—which enable the design of complex, high-density photonic circuits,3 and photonic wire fiber-chip couplers with extremely low coupling loss:4 see Figure 1(e). Refractive index engineering by SWG patterning can also be used in two dimensions for slab waveguides. We and others have used this in the design of a novel microspectrometer, where an SWG nanostructure fulfills a dual purpose by acting as an effective slab waveguide for diffracted light and as a lateral cladding for a channel waveguide.4 SWG structures have also been incorporated into surface grating fiber-chip couplers to enhance performance and reduce fabrication complexity.5–9

We have recently used SWG waveguides to address another important issue with silicon photonic circuits, namely, the temperature dependence of their optical output signals caused by the comparatively high thermo-optic (TO) material coefficient of silicon (dnSi/dT=1:8×10−4K−1). With SWG waveguides the spatial index averaging effect can be exploited for mitigating the silicon thermo-optic effect by filling the gaps with polymer material of negative TO coefficient for an appropriate grating duty ratio. We demonstrated the effect experimentally.10 We observed athermal waveguide behavior for SWG devices with a composite core consisting of interlaced subwavelength segments of silicon and SU-8 polymer. The latter possessed a thermo-optic material coefficient of dnSU8/dT=−1:1×10−4K−1.

Scanning electron micrographs of three SWG waveguides with different duty cycles, i.e., silicon volume ratios, are shown in Figure 2(a). Plots of the measured and calculated waveguide effective thermo-optic coefficients as a function of SWG duty ratio show that, as the volume ratio of silicon material in the waveguide core increases, the TO coefficients show a sign reversal from negative to positive: see Figure 2(b). In other words, the thermal behavior of the SWG waveguide is dominated by the polymer material for low duty ratios and by silicon for high duty ratios. Athermal waveguide operation is achieved for an SWG duty cycle of 61% for transverse electric (TE) polarization and 85% for transverse magnetic (TM) polarization.

Figure 2. (a) SEM micrographs of SWG waveguides with Si duty ratios of 46, 56, and 66%. (b) Experimental and theoretical results for the effective thermo-optic coefficient of SU-8 clad silicon SWG waveguides as a function of grating duty cycle. (c) SEM of a subwavelength sidewall grating (SWSG) waveguide. TE: Transverse electric polarized light. TM: Transverse magnetic polarized light.

Making SWG waveguides with high duty cycles is challenging. We can circumvent the difficulties by using a subwavelength sidewall grating (SWSG) waveguide: see Figure 2(c). Our recent experimental results show that athermal SWSG waveguides can be designed for both TE and TM polarized light with duty cycles of 50%. We can also use mode profile engineering, as employed for the athermal SWG waveguides, to achieve ultrafast nonlinear all-optical switching.11 Engineering the refractive index of slab waveguides with SWGs may prove to be a suitable method of adapting to integrated optics conventional components such as lenses12, 13 or transmission gratings, which are common in free-space optics.

In summary, refractive index engineering by SWGs has enabled us to develop highly efficient waveguide crossings necessary for complex, high-density photonic circuits. We have demonstrated photonic wire fiber-chip couplers with extremely low coupling loss, and a novel microspectrometer incorporating an SWG nanostructure as a slab waveguide for diffracted light and also as a lateral cladding for a channel waveguide. We have designed temperature-independent silicon-polymer hybrid waveguides through careful control of the relative volume ratios of silicon and polymer materials, both with full gaps or in a waveguide sidewall grating geometry, which is simpler to make. The mode profile engineering can also be used for ultrafast all-optical switching and is a suitable method to adapt lenses and transmission gratings for integrated optics.

Future research on using SWG structures in silicon photonic circuits will focus both on the design of novel individual components with improved performance as well as on higher levels of integration of the components that have already been demonstrated. In fact, since SWG waveguides can be readily combined with conventional silicon photonic wire waveguide devices, we expect to see, in the future, more integrated devices incorporating both refractive index engineered and conventional waveguide components. Athermal SWG waveguides may find applications in multiplexers or ring resonator devices, including chemical sensors.

Jens H. Schmid, Pavel Cheben, Przemek J. Bock, Jean Lapointe, Siegfried Janz, Dan-Xia Xu
Institute for Microstructural Sciences
National Research Council Canada
Ottawa, Canada
Marc Ibrahim, Winnie Ye
Department of Electronics
Carleton University
Ottawa, Canada

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