Monday, December 20, 2010
Horizons of Nanophotonics and Nanoelectronics, Harvard University (Cambridge, MA)
As the volume of internet traffic worldwide explodes and processing demands continually increase, solutions are required to overcome the inherent speed limitations of electronic devices. In particular, there is a need for all-optical devices, with their higher bandwidth and transmission rate, to replace various electronic functions such as routing data between processors and logic operations. We identified TiO2 as a promising yet unexplored material platform for ultrafast, on-chip nonlinear optical devices. TiO2 has a high nonlinear index of refraction (n2), enabling such operations as all-optical switching, logic, and efficient wavelength conversion. Its transparency throughout visible wavelengths makes TiO2 compatible with all telecommunications windows. TiO2âs high linear index of refraction enhances optical confinement down to nanometer-scale dimensions and facilitates tight waveguide bends necessary for dense on-chip integration. By exploring this material, we seek to demonstrate its viability as the next on-chip nonlinear optics platform. Fabricating our advanced nonlinear optical devices begins with evaluating bulk TiO2. We present z-scan measurements of bulk TiO2 (rutile) demonstrating an optical nonlinearity ~30 times that of silica. We fabricate high index (2.4), low loss (<0.4 dB/cm) thin-film planar waveguides using reactive sputtering of titanium metal in an oxygen environment. We pattern our films using electron-beam lithography and use a combination of reaction-ion etching and metal lift-off with chromium to form our structures. We achieve waveguides with 100 nm features and demonstrate visible light propagation. Working at the nanometer-scale is advantageous for nonlinear optical devices. Waveguides having dimensions of several hundred nanometers can strongly confine light and achieve peak intensities and effective nonlinearities an order of magnitude greater than conventional micrometer-scale devices. In such nanometer-scale devices, waveguide dispersion varies significantly with the physical dimensions of the waveguide and can overcome material dispersion. Using this, we control temporal pulse broadening inherent in ultrafast pulse propagation. We discuss TiO2 waveguide design and present optimum dimensions for both the highest effective nonlinearity and the range of attainable group-velocity dispersion for our waveguides. By fabricating high-optical-quality TiO2 nanophotonic waveguides and engineering waveguide properties, we have achieved critical steps towards fabricating all-optical logic devices in this novel medium.