## 4 Novel Materials for All Optical Processing Nonlinear Devices
### CMOS Compatible Tantalum Pentoxide Waveguides for Nonlinear Optical Processing  
Tantalum pentoxide (Ta\({}_{2}\)O\({}_{5}\)) is a CMOS compatible material that has been studied in the past for its nonlinear applications at 980 nm and has only very recently been considered as a possible alternative for telecom-wavelength applications. Thanks to its extremely large band-gap value (3.8 eV), no TPA effect is present at the 1550 nm wavelength region, making this material an ideal candidate for nonlinear optical processes in waveguides integrated on a SOI substrate [70; 71; 72]. Here, we briefly review the nonlinear optical properties of low loss Yb:Ta\({}_{2}\)O\({}_{5}\) rib waveguides integrated on the top of a SiO\({}_{2}\) layer reported for the first time in [71].  
Nominally 1 \(\upmu\)m thick Yb:Ta\({}_{2}\)O\({}_{5}\) films were deposited by RF magnetron sputter deposition from a powder-pressed Ta\({}_{2}\)O\({}_{5}\) target doped with 2.5 wt. % of ytterbium oxide onto a four inch silicon substrate with a 2.5 \(\upmu\)m thick thermally-grown silica layer. Waveguides were defined by using standard photolithography and Ion Beam Milling (IBM). Strips of photoresist of widths varying from 1 \(\upmu\)m to 10 \(\upmu\)m in steps of 0.2 \(\upmu\)m were defined in positive photoresist spun onto the deposited Yb:Ta\({}_{2}\)O\({}_{5}\) film. Etching was then performed using an ion beam etch and deposition system. To reduce any stresses and oxygen deficiency introduced during the sputtering and etching processes, the wafer was then annealed for two hours at 600 \({}^{\circ}\)C. Additional details on the fabrication process can be found in [70; 71; 73]. A schematic representation and a SEM image of a fabricated waveguide are shown in Figure 13. Through CW-FWM experiments, the nonlinear parameter Re[\(\gamma\)] was evaluated and by using the simulated effective area we extracted the nonlinear refractive index was determined to be equal to 6.65 \(\times\) 10\({}^{-18}\) m\({}^{2}\)/W, which is of the same order of magnitude as in silicon. Propagation losses of the waveguide were also measured to be 0.19 dB/cm. It is also worth noting that no TPA and FCA effects were observed in the 1550-nm region, even at relatively high power operation (peak power \(>\)20 W), suggesting that this material can be operated at both low and high power levels. In order to test the material stability, transmission experiments with a CW light beam (kept at a constant power level of 2.5 W) were carried out revealing no material degradation even after \(>\)8 h of continuous light feeding. No damage was observed during these measurements, confirming that Ta\({}_{2}\)O\({}_{5}\) can be an ideal alternative for applications requiring high power operation.  
Importantly, it has been shown that Ta\({}_{2}\)O\({}_{5}\) can be used as a host material for many dopants and can therefore be used as a gain material for laser applications [73]. The implications of this for silicon photonics can be significant, since the integration of Ta\({}_{2}\)O\({}_{5}\) on the silicon platform could introduce several functionalities that are vital for the development of fully operational, multifunctional, optical transceivers for next generation optical communications.