Simultaneous multiphoton absorption in rutile (TiO2) across the half-bandgap

Future optical systems require compact, ultra-fast devices capable of switching and logic across a wide range of wavelengths. To realize this goal, ultrafast nonlinearities must be exploited while maintaining manageable linear losses and nonlinear absorption. We present TiO2 as a nonlinear material to meet these needs. TiO2 is highly transparent for wavelengths > 400 nm and possesses both high linear and nonlinear refractive indices. We measurements the nonlinear index and multiphoton absorption in bulk TiO2 (rutile) using the z-scan technique near the half bandgap (800 nm). Using broadband femtosecond pulses, we observe behavior consistent with simultaneous two- and three-photon absorption. We present a theoretical model used to fit our data which includes the effects of simultaneous multiphoton absorption. By tuning our wavelength above and below the half-bandgap energy, we observe the onset of two-photon absorption. By combining our nonlinear refraction and two-photon absorption measurements, we will discuss the merits of TiO2 for all-optical processing.