PROCEEDINGS ARTICLE | June 1, 2011
Proc. SPIE. 8069, Integrated Photonics: Materials, Devices, and Applications
KEYWORDS: Refractive index, Optical design, Thermal optics, Titanium dioxide, Waveguides, Polymers, Reflectivity, Atomic layer deposition, Temperature metrology, Diffraction gratings
We investigate the design of binary grating structures, e.g. resonance waveguide filters (RWFs), with
subwavelength feature sizes, taking the temperature dependence of different material parameters into account. Our final
goal is to demonstrate devices with athermal operation. We design the binary grating structures to be made in polymer
substrates, such as polycarbonate (PC), due to their potential for low cost, mass fabrication. The high thermal expansion
coefficient (TEC) of polymers, compared to inorganic optical materials, enhances the thermal sensitivity of the grating
structures. The gratings are designed using Fourier Model Method (FMM) by considering both thermal expansion and
thermo-optic effects on the resonance wavelength shift. The fabrication of RWF structures is proposed by e-beam
lithography, creating a master stamp and copying the structures into a polymer substrate by some replication techniques,
followed by an ALD deposition of TiO2. When the resonance waveguide grating RWG is designed for nearly room
temperature operation at a peak wavelength of 633 nm with a full width half maximum FWHM of 3 nm (TM mode
reflectance), the peak wavelength shifts 0.2 nm /50C when only the TEC is taken into account. However, taking into
account also the thermo-optic coefficients TOCs of PC and TiO2, the peak position shifts to 0.4 nm/ 50C on the opposite
side of spectral central wavelength. Thus the overall shift reduces to 0.2 nm /5 0C, illustrating partial athermalization. It
was also observed that thermo-optic coefficient TOC contributed more significantly than TEC effect. The wavelengths
shift was almost linear with respect to temperature for both effects and showed slopes of 0.0673, 0.0422 and 0.02352 for
TOC, TEC and combined effects, respectively.
